Developing high efficiency Cu2ZnSnS4 (CZTS) thin
film solar cells by sputtering
Chang Yan
A THESIS SUBMITTED FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
School of Photovoltaic and Renewable Energy Engineering
Faculty of Engineering
August 2016
Table of Content Table of Content...... i Acknowledgement...... iv Abstract...... vii List of Abbreviations...... ix Chapter 1 Introduction...... 1 1.1 Thin film solar cells...... 2 1.2 Motivation of this study and thesis outline...... 5 Chapter 2 Backgrounds...... 9 2.1 Structure of kesterite compound (CZTS)...... 9 2.2 Optical properties...... 11 2.2 electrical properties and defects...... 12 2.3 Secondary phases...... 13 2.4 Kesterite reaction equilibrium...... 16 2.5 Heterojunction Interface....... 17 2.6 Band tailing issue....... 20 2.7 Sputtering technology...... 22 2.8 Analysis and Characterization Technology....... 24 2.8.1 X-ray diffraction (XRD)...... 24 2.8.2 Raman spectroscopy...... 25 2.8.3 Scanning electron microscopy (SEM)...... 27 2.8.4 Transmission electron microscopy (TEM)...... 28 Chapter 3 Composition effects on CZTS absorber properties and device performance...31 3.1 Introduction...... 31 3.2 Composition effects for CZTS from metallic precursors & two-zone sulfurization process...... 32 3.2.1 Experimental...... 32 3.2.2 Results and discussions...... 33 3.2.3 Conclusion...... 44 3.3 Composition effects on CZTS from sulfur contained precursor and one-zone sulfurization process...... 45 3.3.1 Experimental...... 45 3.3.2 Results and discussions...... 45 3.3.3 Conclusion...... 51 i 3.4 Boost Voc of sulfide kesterite solar cells via a double CZTS layer stacks with different Cu content...... 51 3.4.1 Motivation...... 51 3.4.2 Experimental...... 52 3.4.3 Results and discussions...... 54 3.4.4 Conclusions...... 64 Chapter 4 Effects of sulfurization and post-treatment onnanoscale microstructure and chemistry of Cu2ZnSnS4/CdS interface...... 66 4.1 Introduction...... 66 4.2 Experimental...... 67 4.3 Results and discussion...... 69 4.3.1 Interface microstructure investigation...... 69 4.3.2 Interface chemistry investigation...... 76 4.3.3 Device characteristics...... 85 4.4 conclusions...... 95 Chapter 5 CZTS/buffer interface engineering...... 97
5.1 Band alignments of different buffer layers (CdS, Zn(O,S) and In2S3) with Cu2ZnSnS4 ...... 97 5.1.1 Introduction...... 97 5.1.2 Experimental...... 98 5.1.3 Results and discussions...... 99 5.2 Boosting efficiency of CZTS solar cells using the In/Cd-based hybrid buffers...... 109 5.2.1 Motivation...... 109 5.2.2 Experimental...... 110 5.2.2 Results and discussions...... 112 5.2.4 Conclusions...... 124
Chapter 6 Beyond 10% efficient sulfide kesterite Cu2ZnxCd1-xSnS4 solar cell: role of cadmium alloying...... 125 6.1 Introduction...... 125 6.2 Experimental...... 126 6.2.1 Film synthesis...... 126 6.2.2 Device fabrication...... 127 6.2.3 Characterization...... 127 6.3 Results and discussions...... 128 6.4 Conclusion...... 143
ii Chapter 7 Summary and future work...... 145 7.1 Summary of results...... 145 7.2 7.6% record efficiency for standard CZTS solar cell with total area over 1cm2.....148 7.3 Future Work...... 151 7.3.1 Heterojunction interface engineering...... 151 7.3.2 Optimization bulk....... 151 Author’s Publications...... 153 Reference...... 157
iii Acknowledgement
Before beginning this thesis, I would like to acknowledge the following people who enabled me to complete my thesis.
First and foremost, I would like to express my sincere gratitude to my supervisor,
Professor Martin Green, for his kindly guidance and support. Professor Green enabled me to see the big picture of my research and allocated plenty of resources facilitating the completion of this thesis. Not only his broad knowledge helps me to understand research more deeply, his comforting calm character also supports me emotionally.
Great thanks to my joint-supervisor Dr Xiaojing Hao. It is her who enabled my Ph.D to be carried out at UNSW Australia. I still remember, very frequently, our have long and heated discussions about every corner of kesterite solar cells research, from which I have learnt, benefited and grown. Special thanks to her encouragement and great assistance in helping me to attend conferences and to visit famous collaboration labs which has broadened my research horizon and improved my soft power as well. Her academic zeal and diligence set up a good model for me, which encourages and motivates me to discover the truth underlying kesterite and push efficiency forwards.
I would also give my gratitude to Dr Fangyang Liu, who is actually my “quasi- supervisor”. Thank you for bringing me to the world of photovoltaics, laying the groundwork for the basic fundamental knowledge about solar cells. All my first starts were with you, first depositions of Mo and thin film, first digestion of the literature, first presentation, first fabrication of a working device, first I-V curve and other measurements. The fact that you are the co-author of almost all my publications has already proved how important you are to my research. All the photos during sports and trips built up a good brotherhood. Thanks so much for your mentorship and friendship.
iv Besides, I would like to appreciate the researchers who have given me great scientific help and insights in my research. Thanks Dr Jialiang Huang for complicated
TEM characterization, especially obtaining very nice HRTEM images in Chapter 4, Dr.
Steve Johnston from National Renewable Energy Laboratory (NREL) for Temperature dependent J-V measurement and capacitance measurement in Chapter 6, Dr Xiaoming
Wen and Kaiwen Sun for their expertise on photoluminescence (PL) measurements,
Aobo Pu for his assistance in the data analysis , model building and fitting, particularly band tailing parameter fitting in Chapter 6, Fangyang Liu for his insight knowledge and helping in writing and data analysis in Chapter 4, Dr Charlie Kong, Katie Levick and
Sean Lim for FIB and TEM training, Dr Anne Rich for Raman training, Dr Yu Wang for his XRD expertise, Dorothy Yu for ICP measurements, Dr Bill Gong for XPS measurements, Dr Anton Tadich, Bruce Cowie and Australian Synchrotron for enabling
Near Edge X-ray Absorption Fine Structure (NEXAFS) Measurements and gaining knowledge on the band alignment in Chapter 5, David Mitchell and Gilberto Casillas
Garcia at the University of Wollongong (UOW) Electron Microscopy Centre for the use of facilities and their assistance, Patrick Campbell and Tom Puzzer for maintaining good condition of sputtering machine, Berhard Vogl for maintaining the sulfurization furnace, Alan Yee for training all the basic solar cell characterizations, Dr Hiroki
Sugimoto (Solar Frontier), Dr Homare Hiroi (Solar Frontier), Dr Ingrid Repins (NREL)
Dr Charlotte Platzer-Björkman and Dr Guy Brammertz (IMEC) for kindly discussion and guidance on topic of kesterite.
I acknowledge Jian Chen for his help in training me on deposition tools. I still remember our memorable time and experiments together at the Bay St lab, and fighting to midnight, as well as the buddy for each other when we are single dogs. Thanks to
Ning Song who is more like my big sister, for taking care of me in experiments and in v daily life. Thanks to Kaiwen Sun and Fangzhou for fighting together for the sake of breaking the record. Thanks also go to other research friends: Andrea Crovetto, Lei Shi,
Hongtao Cui, Xiaolei Liu, Wei Li, Aobo Pu, Xu Liu, Jongsung Park, Heng Sun, Fajun
Ma, Dun Li, Li Wang, Ziheng Liu, Rui Sheng, Ran Chen, Wei Zhang, Yajie Jiang,
Claire Disney, Hui Jiang, Ziyun Lin, Jingnan Tong and Qingshan Ma for their kindly assistance in research.
Last but not least, thanks to all my friends and families for their continuous support and encouragement when I was down and frustrated about my research or even doubt about the meaning of life. I thank Na Bao, Ziyu Chen, Xiang Ji, Xin Zhao, Xin
Peng, Hongwei Wang, Hong Liu, Kaile Sun, Xinyue Cheng, Zhen Yang, Xun Lu,
Fenglong Wang and Jay Zheng for their support and happy times together. Particular thanks give to my Mom Aiyan Wu and Dad Hongqi Yan. Without all your love and support, neither this thesis nor myself could ever exist.
vi Abstract
Kesterite Cu2ZnSnS4 (CZTS) has demonstrated high potential and value as a new promising absorber material for thin film photovoltaics recently. It possesses the merits of earth abundant element constituent, facilitating its low-cost and large-scale production. Moreover, as a high bandgap material (Eg~1.5eV), CZTS is able to be combined with other photovoltaic cells with lower bandgap (like Si) for tandem solar cells, enabling higher power conversion efficiency beyond the single junction solar cell
Shockley-Queisser efficiency limit. To be widely deployed in either case, the development of high efficiency single junction CZTS solar cells is the first priority.
This thesis aims to investigate, understand and address the present efficiency-limiting issues for the sake of boosting the efficiency of CZTS solar cells.
Firstly, the effect of CZTS composition has been carefully studied. The Cu content can greatly affect the microstructure of CZTS grains and, the electrical property of CZTS film such as carrier concentration and minority carrier lifetime, thereby influencing the CZTS solar cell efficiency.
Secondly, the effects of sulfurization annealing atmosphere and post-heat treatment have been studied. The “epitaxial” CdS/CZTS interface and moderate Cd diffusion effects were discovered, found to play critical roles in improving the cell efficiency.
Thirdly, the conduction band alignments of different buffer materials with CZTS have been carefully investigated. Conduction band offset (CBO) at the CdS/CZTS hetero-interface was found to be unfavourably “cliff-like” whereas CBO of In2S3/CZTS was confirmed to be “spike-like”. Based on these results, the CdS/In2S3 hybrid buffers
vii with different stacking sequences were studied, among which CdS/In2S3/CZTS shows the best cell performance.
Last but not least, Cd alloying with CZTS has been studied. Cd alloyed CZTS
(i.e. CZCTS (Cu2ZnxCd1-xSnS4)) can effectively boost energy conversion efficiency to over 10% (active area). The improved efficiency is believed to arise from better quality of the CZCTS absorber, i.e. larger grains, longer minority carrier lifetime and reduced band tailing issues.
Based on above combined processing strategies, a full-sized 7.6% efficient
CZTS solar cell (with total area over 1.0 cm2) has been achieved (certified by NREL), setting a new world record for such a standard CZTS solar cell.
viii List of Abbreviations
PV Photovoltaic
TF Thin Film
CZTS Cu2ZnSnS4
CIGS CuInxGa(1-x)Se2
CZTSe Cu2ZnSnSe4
CZTSSe Cu2ZnSn(S,Se)4
SLG Soda-lime glass
XRD X-ray diffraction
TEM Transmission electron microscopy
SCR Space charge region
SEM Scanning electron microscopy
AFM Atomic force microscopy
SE Secondary electrons
BSE Back-scattered electrons
EDS Energy dispersive spectroscopy
PL Photoluminescence
Eg Band gap energy
STEM Scanning transmission electron microscopy
FWHM Full width at half maximum
T Temperature
ix J-V Current density-voltage
EQE External quantum efficiency
RTP Rapid thermal processing
AM Air Mass
Voc Open circuit voltage
Jsc Short circuit current
FF Fill Factor
PCE Power conversion efficiency
J0 Dark saturation current density
A Ideality factor
Wd Depletion width
IJ Carrier life time
Rs Series resistance
Gs Shunt conductance
CBD Chemical bath deposition
ITO Tin doped In2O3
DC Direct current
RF Radio frequency
x Chapter 1 Introduction
The global warming and energy crisis are believed to be two of the most challenging issues for future human society. Developing renewable energy is one of the most effective ways to deal with these issues. Among the numerous renewable energies, photovoltaics (PV), which can directly convert sunlight energy to electricity, has been considered as the most favourable and promising one due to its merits of abundance and readily availability, non-pollution, zero carbon emission, and etc.. Within the PV field, silicon solar cell dominates over 90% market due to its relatively more mature lab and industrial fabrication process as well as high efficiency (the previous highest light to electricity conversion efficiency of mono-grain silicon solar cell is above 25%, which is developed by UNSW[1] and recently it has been refreshed to 26.33%[2]). Unfortunately, silicon is an indirect bandgap light absorbing material, which means absorbing light requires the help of phonons, resulting in low light absorbing coefficient, namely, less efficient absorbing of light. Usually several hundred microns thick high purity silicon was needed to fabricate high efficiency Si solar cells. The relative high-cost and pollution for producing high purity silicon would inevitably hinder the development of
Si solar cell, thus giving some opportunity to thin film solar cells. Besides, the light weight and flexibility characters makes thin film solar cell can be readily used in flexible solar cell market and building integrated photovoltaics (BIPV) markets.
Meanwhile, Tandem cells by stacking materials with different bandgaps with the highest on top and lowest at the bottom, seems to be the most promising and effective way to further push the current photovoltaic field forwards. Tandem cells can be from either high band gap thin film technology combined with bottom Si cell configuration or
1 several thin film solar cells with matched band gaps [3]. In order to achieve these, developing efficient high band gap thin film solar cells is of great importance.
This chapter demonstrates the basis of thin film solar cells and illuminates the reason why we are developing Cu2ZnSnS4 (CZTS) thin film photovoltaic technology.
1.1 Thin film solar cells
Thin film solar cell is a physical device that is made of one or more thin film materials for converting light power directly into electricity. The following feature of thin film technology make it interesting for photovoltaic application.[4]
1) Large numbers of technologies such as physical, chemical or plasma based are
readily available for thin film deposition.
2) A variety of the microstructure can be achieved such as: epitaxy,
monocrystalline, enabling certain preferred crystalline orientation, amorphous,
and etc..
3) Easy achievement of multicomponent doping, alloying, surface/grain boundary
passivation, graded bandgap/composition, and etc..
4) Easy surface engineering like light trapping and type inversion
5) Thin-film processing is usually material-saving process and environmental
friendly.
Generally, a good candidate for thin film solar cell absorber possesses the following characteristics:
2 1) A direct band gap of 1.1~1.5 eV with absorption coefficient higher than 106 m-1,
within the range required for maximum theoretical conversion efficiency for
single junction solar cell.
2) High external quantum efficiency (EQE), long collection length and low
recombination velocity.
3) Enabling the formation of a low defect junction with a compatible material and a
favourable band alignment.
Thin film solar cells based on the absorber materials of amorphous Si, GaAs, CdTe,
Cu2S, Cu2O, InP, Zn3P2, CuInxGa1-xSe2 (CIGS) or hybrid organic-inorganic perovskite have been developed[4]. Among them, the GaAs, InP and CdTe based solar cells possess very high efficiency and are considered as the perfect material for photovoltaic application. However, they are either too expensive or toxic. Meanwhile amorphous Si and hybrid organic-inorganic perovskite solar cells suffer from severe degradation problem. Therefore, CIGS based solar cell with efficiency of 22.7%[5] is recognized the most promising one due to the following reasons:
A) This quaternary compound material enables band gap tuning using either metal or chalcogenide substitutions, which will facilitate optimization of absorber band gap with respect to solar spectrum for achieving high efficiency.
B) As the grain boundary can be well passivated, the requirement of thin film quality is reduced, enabling some high-throughput and/or low-cost device fabrication processes (e.g. roll-to-roll process).
C) Thin film compounds also enables the flexible high efficiency solar cells, allowing the expansion of PV market (like building integrated photovoltaics (BIPV)). 3 The device configuration of CIGS material based solar cell is shown in Figure 1-1.
Generally, a ~1ȝm thick molybdenum (Mo) layer is deposited on the SLG serving as the back contact due to the facts that (1) it has suitable electrical & mechanical properties;
(2) The formation of Mo(Se,S) between Mo and CIGS can improve adhesion & ohmic contact, and (3) its growing orientation is beneficial for absorber quality[6]. On top of the Mo layer is the CIGS absorber layer, namely aforementioned p-type thin-film materials, which is used for absorbing the injected photons and generating the electron- hole pairs. In order to form a p-n junction, an n-type semiconductor material with band gap between 2.0eV~3.6eV is applied as the buffer layer. The function of this buffer layer is to form a good p-n junction with p-type absorber material and protect the junction from detrimental chemical reactions as well as mechanical damage[6].Usually,
Cadmium sulphide (CdS) is the most suitable buffer for CIGS solar cells[7]. On top of
CdS, an ultra-thin high resistance intrinsic ZnO is deposited for the sake of reducing device shunting problem. Then a transparent conductive layer (TCO) was deposited enabling the most of the incident light reaching the underlying p-n junction whilst providing the sufficient lateral conductivity allowing the photo-generated carriers collection without much resistance loss. Finally, a patterned metal grid is deposited on top of ITO for the sake of outputting the current.
4
Figure 1-1 Conventional device configurations for CIGS thin film solar cells.
1.2 Motivation of this study and thesis outline
High efficiency thin film photovoltaics like GaAs, CdTe or CIGS are consider as the future flagship solar cells. However, the toxicity of GaAs and CdTe limits their widespread terrestrial application[4]. In terms of CIGS, elemental scarcity places a big risk for its future application. Specifically, the abundance of indium (In) in the earth crust is estimated to be 0.005ppm[8], and its metallurgy mineral availability is much lower. What is worse, the booming of the flat panel display industry and widespread of touch screen applications have posed a big consuming load for In usage. In this scenario, the use of In inevitably undermines large scale deployment of CIGS solar cells.
Therefore, it is imperative to identify an alternative thin film material containing only non-toxic and earth-abundant material whilst possessing a good device performance for future large scale PV application.
5 Fortunately, kesterite Cu2ZnSnS4 (CZTS) has been found and considered as one of the most promising thin film candidates owing to the following characteristics.
1) Non-toxic element constituent.
2) Earth-abundant element constituent. Abundance of Cu, Zn, Sn are 25, 71, 5.5
ppm respectively, two order of magnitude higher than that of In.[8]
3) Directly band gap with absorption coefficient higher than 106 m-1.
4) Tunable band gap by alloying with other elements like Se, Ge, Ag, etc..
5) Similar crystalline structure to that of high efficiency CIGS, enabling high
efficiency potential.
In recent years, the kesterite solar cells have been developed rapidly, with 9.1% efficiency pure sulfide CZTS achieved by Toyota [5] whereas 12.6% efficiency Se alloyed CZTS (CZTSSe) developed by IBM [9], demonstrating its high efficiency potential. However, the efficiency of CZTS is still much lower than its counterpart
CIGS with record efficiency over 22%[10]. For the sake of possible industrialisation, the efficiency of CZTS solar cells must be improved. The aim of this thesis is to understand the key factors limiting CZTS device efficiency and to develop high efficiency CZTS solar cells by optimizing the processing parameters, addressing heterojunction interface recombination and bulk recombination issues.
The main chapter descriptions and the outline of this thesis are presented as follows:
Chapter 1 briefly introduces the thin film solar cell technology and demonstrates the motivation of fabricating high efficiency CZTS solar cells.
6 Chapter 2 reviews the state of the art kesterite solar cells, starting from kesterite material properties such as crystal structure, optical and electrical properties, followed by the summary of the current issues for further improvement in device efficiency, and finally the experimental details of device fabrication and characterization employed in this thesis.
Chapter 3 demonstrates the effect of CZTS composition on performance of CZTS solar cell. Cu/Sn ratio was found to play a significant role in determining the crystal quality and electrical property of absorber. Based on these findings, a double CZTS layer with different Cu content was designed and corresponding device was fabricated showing large Voc improvement. By optimization of the composition, over 7% efficient CZTS solar cells have been demonstrated.
Chapter 4 investigates the effects of sulfurization annealing and post-heat treatment on nanoscale microstructure and chemistry of Cu2ZnSnS4/CdS interface, which is proved crucial for the further efficiency improvement to beyond 8%.
Chapter 5 studied the band alignment of different buffer layers (CdS, Zn(O,S) and In2S3) with Cu2ZnSnS4. Due to the more favorable In2S3/CZTS band alignment and importance of CdS layer to CZTS, a In/Cd hybrid buffer has been developed to further boost the efficiency.
Chapter 6 illustrates the alloying of Cd with CZTS to reduce the band-tailing problem of CZTS solar cells. Beyond 10% efficient Cd-alloyed CZTS solar cells was obtained.
7 Chapter 7 briefly introduces the record 7.6% efficient standard CZTS solar cell with total cell area of 1.067cm2, based on aforementioned developed optimization strategies.
Some suggestions and outlooks for future work are also proposed.
8
Chapter 2 Backgrounds
2.1 Structure of kesterite compound (CZTS)
The formation of multicomponent compound semiconductor can be regards as a series of cation mutations (substitution) in which the whole compound remains neutral and total valence state is unchanged. For instance one +1 ion and +3 ion can replace two
+2 ions or one +2 and +4 can replace two +3 ions. The schematic of revolution pathway from binary compounds to quaternary compounds is demonstrated in Figure 2-1.
Figure 2-1 Schematic of quaternary kesterite evolution pathway: from binary to
ternary and quaternary[11].
The original binary compound processes the hexagonal wurtzite and/or cubic zinc blend structure which adopts tetrahedral configuration. The ternary compounds can be generated via substituting a pair of two +2 ion with one +1 and one +3 ions. Two different cations ordering may be resulted from the same process: the chalcopyrite
(lowest energy) and CuAu structure. All these structure follows local charge neutrality
9 rules. That is to say each anion is coordinated with half +1 and half +3 ions. Compared to their parent binary compounds, the ternary compounds show flexible properties due to their chemical and structural freedom. For instance, the band gap of CuInS2 (1.5eV) is much lower than ZnS (3.6eV) and hence CuInS2 becomes a popular photovoltaic absorber material.
Half +2 and half +4 ion can replace one +3 ion from ternary compound, forming quaternary compound. A similar tetrahedral crystal structure of quaternary compound can be heritage from their ternary parent materials (see Figure 2-2). As the complexity of the system ascends to three lattice sites for the quaternary compound material, the possibility for cation ordering is increased accordingly. But, in order to follow the law of local charge neutrality, the number of thermodynamically accessible energy phases is limited. Three new structures can be generated: kesterite arising from chalcopyrite and stannite and PMCA from CuAu-like structure.
10 Figure 2-2 Schematic diagram of quaternary kesterite evolution pathway: from
binary to ternary and quaternary[12].
2.2 Optical properties
In the perspective of the optical properties, CZTS is highly suitable for single junction solar cells. According to the Shockley-Queisser limit (SQL) theory[13], the theoretical maximum conversion efficiency for band gap of 1.5eV should be ~ 33%.
Moreover, CZTS is a direct band gap material having light absorption coefficient higher than 104cm-1[14] (See figure 2-3), indicating 1-2 ȝm absorber should be able to absorb over 90% of incident light regardless of reflection loss[8].
Figure 2-3 Light absorption coefficient of CZTS thin film (The inset demonstrates the
normalized (Įhv)2 vs. hv curve, showing the band gap of 1.53eV.)
11 2.2 electrical properties and defects
In terms of electrical properties, the CZTS is quite similar to its counterpart
CIGS. Firstly, the point defects of CZTS give p-type semiconductor CZTS with carrier concentration of 1016~1017 cm-3, close to that of high efficiency CIGS solar cells[15].
According to first-principle calculations[16], there are three types of defects involving vacancies (VCu,VZn,VSn,VS), antisite defects (CuZn, ZnCu,CuSn. SnCu ,etc.) and interstitial defect (for instance Cui, Zni, Sni). Some point defects enable the p-type doping of CZTS like: VCu, VZn, CuZn etc, whilst some leads to n-type doping for example ZnCu, SnZn, Cui, and etc.. Some are deep level traps like VS, and Sn related defects as illustrated in
Figure 2-4[16].
Figure 2-4 The ionization levels of intrinsic point defects in the bandgap of Cu2ZnSnS4
[16].
The formation energy of all p-type point defects is lower than that of all n-type doping point defects hence the CZTS always shows the p-type conductivity rather than n-type [16]. In addition, VCu is a shallow level acceptor which is beneficial for doping 12 of CZTS whereas VSn is deep level defect with relatively low formation energy. These could be the reasons why the reported high efficiency CZTS solar cells have Cu-poor composition which facilitates the formation of VCu and comparably equal to have more
Sn in CZTS system thus suppressing the formation of VSn.
Figure 2-5 The defect formation energy vs. Fermi energy according to first principle
calculations of Cu2ZnSnS4[12].
The mobility of CZTS is below 1cm2/Vs, which is much lower than that of CIGS, causing the lower conductivity of CZTS absorber[17].
2.3 Secondary phases
Single phase CZTS absorber is required for high efficiency kesterite solar cells.
However, the synthesis of single phase CZTS is quite challenging due to frequent presence of secondary phases. This is because the stable phase region for CZTS is 13 narrower than that of its counterpart CIGS according to the thermodynamic stability calculation [12]. Figure 2-6 demonstrates chemical potential schema illustrating the potential stable region for CZTS.
Figure 2-6 Chemical potential schematic illustrating the potential stable region for
CZTS[12].
The small black area in Figure 2-6 is the region for stable single phase CZTS. It is obvious that the precise control of composition is very important for preventing the secondary phases. Take the Zn as an example: the deficit of Zn will lead to the formation of Cu2SnS3, whereas too much Zn will result in coexistence of ZnS secondary phase. Table 2-1 illustrates some possible coexisted secondary phases within CZTS and their corresponding band gaps.
Table 2-1 Possible coexisted secondary phases within CZTS and their corresponding
band gaps [18].
14 Compound Secondary phases Band gap (eV)
Cu2ZnSnS4 Cu2S 1.2
ZnS 3.6
SnS 1.0~1.3
SnS2 2.5
Cu2SnS3 1.0
Actually the presence of secondary phase has been reported deleterious to the solar cell efficiency. For instance, the existence of high conductive Cu-S phase especially within the junction region will shunt the device. The presence of low band gap secondary phases like SnS and Cu2SnS3 will introduce the deep electronic states in the band gap, decreasing the activation energy and thereby reducing the Voc[18]. The presence of too much high resistance of ZnS will lead to high resistivity of absorber and therefore decreasing the Jsc and FF[19].
In order to avoid the formation of detrimental secondary phases and suppress the deep level defects, empirically, the composition required for high efficiency CZTS solar cells is usually Cu-poor and Zn-rich ([Cu]/[Zn]+[Sn]=~0.8 and [Zn]/[Sn]=1.2~1.3), as indicated in Figure 2-7.
15
Figure 2-7 The elemental ratio (Cu/(Zn + Sn) and Zn/Sn) distribution of kesterite based solar cells with different conversion efficiency[20].
Owing to the highly importance and close correlation between the composition and the CZTS device efficiency, in the first experimental part (chapter 3), the effect of
CZTS compositions on the properties of CZTS thin films and corresponding device performances will be investigated in details.
2.4 Kesterite reaction equilibrium
Usually, CZTS thin films are synthesised at a high temperature of 550°C~600°C, which provides a sufficient energy for solid-state chemical reactions, grains growth, migration of defects to grain surfaces/boundaries, and etc.. At such high temperature, the equilibrium reactions for CZTS can be described by equation (2.1) and (2.2) listed as follows[21]:
οହହ̱ι (ሻ (2.1ݏሺܼܵܶܥ ሻ ܵ݊ܵሺ݃ሻ ͳȀʹܵଶ ርۛۛۛۛۛۛۛሮݏሻ ܼ݊ܵሺݏݑʹܵሺܥ
16 ܵ݊ܵሺݏሻ ՞ ܵ݊ܵሺ݃ሻ (2.2)
Chemical equilibria are actually dynamical at the smallest level of atom and molecular. Note that at equilibrium, both forward and backward reactions take place simultaneously at the same reaction rate. It means, owing to the chemical equilibrium, tiny amount of CZTS is always undergone both formation and decomposition processes, and therefore, all the volatile gas must be supplied in excess amount during the whole annealing/sulfurization process, for the sake of keeping the reaction towards the CZTS side. Herein, copper and zinc sulfide are not volatile but SnS and S are[22]. Alex et al.
[23] first introduced SnS during the sulfurization process to maintain chemical equilibrium favourable for the formation of CZTS and suppress the decomposition of
CZTS, resulting CZTS efficiency improvement from 0.02% to 5.4%.
Owing to the high importance in controlling equilibrium by providing combined
SnS and S atmosphere for CZTS, in chapter 4 of this thesis, we also introduced the SnS atmosphere during high temperature sulfurization process, and carefully studied the
CZTS surface and CZTS/CdS interface. The epitaxy CZTS/CdS interface with low defects density is confirmed and attribute to the SnS atmosphere during sulfurization, leading to a 8.7% efficiency CZTS solar cell[24].
2.5 Heterojunction Interface.
The key feature for a heterojunction is that the bandgap and electron affinity of the p/n semiconductor are generally different, leading to the fact that there will be discontinuities in both the conduction band and valence band connection. Therefore the alignment of the energy bands at the heterojunction interface plays a crucial role for solar cell efficiency[25].
17 A “spike-like” conduction band offset (CBO) is considered as favourable band line-up. However, the CBM of the n-type layer cannot be significantly higher than that of the p-type absorber because this will lead to a large energy barrier blocking the photo-generated electrons from absorber to the n-type buffer[26]. As a result, if the
CBO is higher than the spike 0.4eV, the FF and Jsc will decrease sharply. Therefore a spike-like CBO within the range of 0~ 0.4 eV is believed to be the optimal one.
“Cliff-like” CBO, where the position of the conduction band minimum of buffer is lower than that of the p-type absorber, is considered as undesirable band alignment.
The reason is that under illumination the electrons at buffer can be easily recombined with holes at absorber at their hetero-interface, leading to high interface recombination velocity, especially severe when there is high defect density at the interface, decreasing the Voc sharply. Meanwhile, if under forward bias, there will be a formed energy barrier blocking the flow of electrons, which reduce the FF[26]. The band alignment schema for both spike and cliff conformations are illustrated in Figure 2-8.
18 Figure 2-8 Schematics of energy band diagram of a spike-like conformation(left) and a cliff-like confirmation (right)[26].
Now we scrutinized the band alignment at current CZTS/CdS hetero-interface. As the kesterite CZTS is evolved from chalcopyrite CIGS, CZTS solar cells has inherited the configuration of CIGS solar cells, where CdS is adopted as the “standard” buffer layer. The conduction band alignment of CdS/CIGS is “spike-like” with a desired band offset of 0.2-0.3 eV, which facilitates the high performance CIGS solar cells, especially high Voc[25]. However, the type of CBO is still unresolved when it comes to the case of CdS/CZTS based cells. Theoretically, the CBO of CdS/CZTS has been calculated to be negative (i.e. cliff-like), while the reported experimental values vary widely. For example, Richard et al. report a spike-like CBO of +0.41eV [27] whereas others studies have measured cliff-like CBO value of -0.06eV[28], -0.33eV[29], -0.34eV[30]. As the band alignment has been found to be very sensitive to the interface of CdS/CZTS, the differences in the reported experimental CBO of CdS/CZTS could be due to the variation in the surface condition of the CZTS absorber and/or any treatment prior to the deposition of CdS buffer. Further investigation is required to investigate and resolve this issue. On the other hand, if the CBO of CdS/CZTS is indeed cliff-like, it is imperative to identify alternative buffer material, which yields an optimal band alignment with
CZTS (small spike-like CBO of 0.1-0.2eV). However, very few experimental attempts have been made to measure band alignment at the interface of Cd free buffer layers and
CZTS (pure sulfide).
Therefore, in the beginning of Chapter 5, the band alignment of three different buffer layer alternatives, i.e. CdS, Zn(O,S) and In2S3, with CZTS has been carefully measured. Due to the results of spike-like CBO of CZTS/In2S3 whereas cliff-like CBO 19 of CZTS/CdS, in the latter part of this chapter, we proposed and investigated the In/Cd hybrid buffers based CZTS for further boosting the efficiency of CZTS solar cells.
2.6 Band tailing issue.
The band tailing issue for kesterite involves with spatial variation in the conduction and valence band, namely, a superposition of change in band gap and electrostatic potential[31]. The band tailing problem means the presence of high density of tailing states that will largely limit the Voc.
Researcher at IBM first found the band tailing issue in their high efficiency kesterite solar cells[31]. They believed the observations of the slow decay in the IQE near the band gap, PL peak shift to a lower value compared to Eg, as well as the broadened PL peak are attributed to the band tailing issue. Figure 2-9 demonstrates the tailing state schema[31], which involves two parts: 1) band gap fluctuation and 2) the electrostatic fluctuation. The former is caused by the compositional inhomogeneity whereas the latter can be attributed to the high density of charged defects.
20
Figure 2-9 Schemas of bandgap fluctuation (a) and electrostatic potential
fluctuations (b)[18].
The time-resolved photoluminescence (TRPL) measurements on CZTS samples at 4k suggested the minority lifetime can be boosted by three orders of magnitude higher than that at room temperature, due to that the electrons are trapped in the wells of the conduction band fluctuation, which can be attributed to the charged bulk defects like
- + donor-acceptor pair of [CuZn +ZnCu ][31].
- + The band tailing problem is caused by large density of [CuZn +ZnCu ] donor- acceptor pair, which is attributed to the Cu/Zn neighbourhood nature in periodic table as well as similar atomic radius. Therefore, Chapter 6 presents the successful trial of suppressing band tailing issues by alloying Cd into CZTS (partially substituting Zn with
Cd to form CZCTS). Consequently, beyond 10% efficiency has been achieved for
CZCTS solar cell device. 21 2.7 Sputtering technology
Sputtering deposition is one of the physical vapour deposition techniques allowing the deposition of high quality thin film and large-scale up-scaling production.
In the fundamental set-up of a sputtering system, the target and substrate are positioned facing each other and connected to the cathode and anode. Note that wall must connect to the anode as well for grounding. Before the sputtering, the chamber is evacuated using a pump system to a low background pressure (~10-7 Torr). After that, an inert gas, usually argon, is introduce into the system, maintaining at an operating pressure (0.5 mT~50 mT). Then, by applying a high voltage between the cathode and anode, a proportion of the Ar atom will ionize forming the Ar+ plasma. Owing the charge of Ar+, under the electrical field formed between cathode and anode, Ar ions are accelerated towards and hit the targets, causing ejecting of atoms from the target surface. These ejected atoms will travel within the chamber and end up on all surfaces including the substrates[32].
There are two kinds of power supply that can be applied to the cathode and anode. For a conductive/metallic target, a direct current (DC) power supply can be used usually with high deposition rate. In terms of an insulating or semiconductor target, a radio frequency (RF) current power supply with high frequency switching voltage can be utilized to eliminate the electrical breakdown and arc discharge caused by acuminated charge at target surface. Note that a magnetron sputtering is to use magnets in sputtering system to increase the deposition rate. The magnetic field created by the magnets near targets, confining the electrons and forcing them to do multiple collisions to the targets surface. As a result, the plasma intensity has been enhanced (see Figure 2-
10)[33]. 22
Figure 2-10 Schematic of electron confinement around the targets in a magnetron sputtering system[33].
Comparing to other deposition methods, sputtering technology has the following advantages:
1. Easy control to the composition over a large area, especially, the
deposited film has similar compositions with the target;
2. A high deposition rate;
3. A better adhesion on substrate;
4. Ability to sputter high melting point material,
5. Compatible with reactive gas,
6. Enabling epitaxy growth.
Therefore, sputtering has become the mainstream technology for thin film solar cell devices fabrication.
23 2.8 Analysis and Characterization Technology.
As a quaternary compounds, CZTS possesses high complexity like multiple composition and various secondary phases, etc. Therefore, the careful and comprehensive characterizations are of great importance. Usually, several techniques must be used together to measure the quality and properties of CZTS films. X-ray diffraction (XRD) together with Raman spectroscopy can be used in order to identify the crystal structure and secondary phases. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy
(EDS) are used together for morphology and elemental distribution measurements.
2.8.1 X-ray diffraction (XRD)
X-ray diffraction is a powerful tool to clarify the structure property and presence of secondary phases via a non-destructive reflection angel scan. Specifically, a monochromatic X-ray beam is focused on and scanned the sample at different angles.
Then a XRD pattern will be generated reflecting the structure of the materials based on the Bragg’s law[34].
ndO 2sinT Equation 1.1
Where n represents the order of the reflection, Ȝ is the wavelength of the X-ray, and ș stands for the incident angle.
Based on the fundamental of Bragg’s law, the X-ray diffractometer is invented as illustrated in Figure 2-11.XRD pattern is obtained by sweeping the specimen surface with the X-ray at different angles, when the angle between the sample and the incident
X-ray is in accordance with the Bragg angle ș, the strong interference signals reflecting the crystal structure (reflection peak) is detected. The reflection intensity is expressed as 24 a function of the angle and reflection peak forming a reflection intensity curve call X- ray diffraction pattern.
Figure 2-11 Schema of ș/2ș diffraction geometry[34].
2.8.2 Raman spectroscopy
Raman spectroscopy is a spectroscopic technology utilized to measure vibrational, rotational modes in a material system. It provides a sensitive, fast as well as non-destructive approach for characterizing the crystallinity of the film. Moreover, the
Raman spectrum reflects the symmetry of the crystal structure, the masses and charges of the constituent elements, and the strength of the chemical bonds. Thus, Raman spectroscopy is reliable to identify the secondary phases and the order parameter inside
25 the crystal[35]. The schematic representation of Raman spectroscopy is shown in Figure
2-12.
Figure 2-12 Schema of experimental configuration in a Raman system[35].
The CZTS samples were measured using a Renishaw inVia Raman microscope
(with excitation wavelength of 325, 442 and 514 nm). During the measurements, the laser beam is focused on the sample surface via a traditional optical microscope with a
50× objective. The power of the laser source is limited to 1mW to eliminate the potential decomposition of the CZTS film under high intensity laser caused by over- heating. Before measuring the sample, a single crystal Si wafer is utilized as the calibration reference (520cm-1 Raman shift with 514 nm excitation source).
26 2.8.3 Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) is a kind of electron microscope that generates images of a sample by scanning it with a focused beam of electrons. SEMs are able to magnify an object from around several times all the way up to 300,000 times.
The schema of a SEM is illustrated in Figure 2-13. The source generates the X-rays, which then pass the condenser lens. The condenser lens controls the quantity and intensity of the electron beam approaching to the specimen, whereas the focusing lens force the electron beam focusing on the samples. Finally, the electron beam reach the top layer of specimen, interacting with the surface atoms, consequently ejecting electrons from the sample which can be detected and captured by the detector. The types of signals generated by SEM are secondary electrons (SE), back-scattered electrons (BSE) and characteristic X-rays, and etc.[36]. When doing the SEM scans, the secondary electrons are emitted from the top atoms generating an image, in which, the contrast is a function of the sample surface morphology. The interaction of the electron beam with surface atom will lead to the shell transitions. As a result, X-rays are emitted and captured by the detector. Generally, the detection and measurement of the X-rays facilitate the quantitatively elemental analysis, namely EDS, which enable a rapid elemental analysis of composition with the certain penetration depth. Usually, the penetration depths are determined by the accelerating voltage. Higher accelerating voltage means larger and deeper interaction volumes. Often an 18kV accelerating voltage will be used, considering that the thickness of the sample is ~1ȝm.
27
Figure 2-13 the schematic representation of SEM[37].
All the surface morphologies of the thin films were characterized by an ultra- high resolution field-emission SEM (FESEM), FEI NanoSEM 450. The images were taken in immersion mode at 5 kV with a working distance of ~5 mm. The EDS data are collected using 18kV accelerating voltage and working distance of 8 mm.
2.8.4 Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) is a microscopy technology in which a beam of electrons, generated at source and accelerated by high voltage, pass through an ultra-thin specimen, and interact with it. TEM is an advanced microscopy technique which can be utilized not only for imaging the microstructure (like SEM), but also for
28 distinguishing phases. There are 3 ways of interaction: scattered electrons, elastically scattered electrons, and inelastically scattered electrons. The thickness of the specimen has a converse relationship with the intensity of the transmitted electrons[38]. There are different operation modes. For instance, a bright field, mode is utilized to image grains and crystalline defects within the specimen. The dark field mode uses Bragg diffracted electrons to image the region from which they originated and thereby is useful to identify defects or impurities. The main function of the electron diffraction is to tell and measure the crystallographic structure of the sample. The selected area electron diffraction pattern is obtained via placing an aperture in the image area of the interest.
Figure 2-14 shows a schematic outline of TEM.
Figure 2-14 The schematic outline of TEM[39].
29 High-resolution transmission electron microscopy (HRTEM) is an imaging mode of the TEM technology which enables directly imaging for the atomic structure. It is a very powerful tool to investigate properties of the specimen at the nano-scale or atomic-scale. The specimen can be metals, semiconductor, ceramics, and etc.. Often, the
HRTEM can be utilized as high resolution scanning TEM (STEM) which enables the identification of the phase distribution within the specimen. The contrast of a HRTEM image comes from the interference in the image plane of the electron wave with itself.
This interaction of the electron wave with the crystallographic structure of the specimen is sophisticated. However, a qualitative idea of the interaction can be obtained. Each imaging electron interacts independently with the specimen. Above the specimen, the wave of an electron can be approximated as a plane wave incident on the sample surface.
When it penetrates the sample, it is attracted by the positive atomic potentials of the atom cores, and channels along the atom columns of the crystallographic lattice[40].
Simultaneously, the interaction between the electron wave at different atom columns follows the Bragg law to achieve diffraction. The physics of electron scattering and electron microscope image formation are well known to enable accurate simulation of electron microscope images[41].
In this thesis, a FEI Tecnai G2 operating at 200 kV is utilized. The system is equipped with a field emission gun, a Gatan CCD camera and an energy dispersive spectroscopy (EDS) detector. The TEM specimens were prepared with a focused ion beam system (FIB, xT Nova NanoLab 200).
30 Chapter 3 Composition effects on CZTS absorber properties and device performance
3.1 Introduction
As Cu2ZnSnS4 is a quaternary compound, it has very complicated phase space that makes the formation of both defects and secondary phases difficult to be controlled.
Figure 3-1 Pseudo-ternary phase diagram of CZTS system, the very small central area is where CZTS forms (left, a), band gap values for different secondary phase (right)[42, 43].
According to the Pseudo-ternary phase diagram of CZTS system in Figure 3-1, the single phase CZTS can only be formed in a rather small compositional area close to its stoichiometric composition (i.e Cu:Zn:Sn:S=2:1:1:4). The deviation of composition may lead to co-existence of second phases such as: ZnS, Cu2S, SnS, SnS2, Cu2SnS3, and etc. As these secondary phases possess different optical and electrical properties, some are benign to CZTS while others are detrimental. Thus, the composition of CZTS plays a vital role in determining the amount and distribution of impurity phases.
In the first part of this chapter the deposition of Zn/(Cu&Sn) metallic precursor followed by a two-zone tube furnace sulfurization process was adopted to synthesize 31 CZTS films with three different compositions, based on which corresponding solar cells have been made. The composition effect using metallic precursor route has been investigated in detail. In the second part, the deposition of metal sulfide precursor followed by rapid thermal sulfurization annealing process has been developed and how the composition influences the CZTS properties and corresponding device performance have been illuminated. Based on the findings and knowledge acquired in the second part, a double CZTS layers stack with different compositions has been developed to further boost the Voc and efficiency of CZTS devices.
As the composition can affect the optical and electrical properties and thereby the efficiency of the CZTS solar cells, the identification of appropriate CZTS composition (i.e. Cu/Sn and Zn/Sn) is of highest priority.
3.2 Composition effects for CZTS from metallic precursors & two-zone sulfurization process
3.2.1 Experimental
The metallic precursor of Zn/(Cu & Sn) are achieved by depositing Zn (with direct current (DC) power supply) followed by co-depositing Cu (with radio-frequency
(RF) power supply) and Sn (with DC power supply) on Mo-coated soda-lime glass using a magnetron sputter system (AJA International, Inc., model ATC-2200). The purity of the Cu, Zn and Sn are all 4 N. Both Zn and Sn are deposited at 0.37 W/cm2 whereas Cu is deposited at around 1 W/cm2. The pressure of Ar is 1.5 mTorr and the substrate rotates at 30 rpm without intentional heating. The CZTS layers are formed via the sulfurization of metallic precursors at 560–575 °C for one hour in a two-zone furnace (OTF-1200 MTI). The CZTS devices are fabricated using typical CIGS/CZTS
32 solar cell configuration. The CdS buffer layer is deposited by chemical bath deposition
(CBD) method by using cadmium sulfate (3CdSO4·8H2O), thiourea (H2NCSNH2) and
25–28% ammonium hydroxide as reagents. The reactor containing these precursor and
CZTS film was put into a water bath system with 80 °C and constant stirring and after the reaction 8 min, an orange color thin CdS layer was formed onto the CZTS films.
The i-ZnO and AZO layers are prepared by RF sputtering. Evaporated Al grids serve as the top contact.
3.2.2 Results and discussions
The composition ratio of metallic precursor and corresponding sulfurized CZTS film measured by inductively coupled plasma mass spectrometry (ICPMS) is illustrated in Fig. 1a. By varying the copper power density from 0.987 to 1.181 W/cm2, three metallic precursors are obtained with different copper content from low to high labeled
A, B and C, respectively. Elements Sn and Zn are more likely to be lost during the sulfurization process due to the high volatility of Sn (SnS) and Zn,[44] whereas elemental Cu is less likely to be lost.
Figure. 3-2 a) Composition ratio of metallic precursors and sulphurised CZTS films determined by ICPMS b) corresponding ternary composition diagram. The black, red
33 and green color represent sample A, B and C, respectively. Solid symbols stand for the precursors while the hollow symbols represent annealed samples. The red arrow shows the composition changing trend after sulfurization.
Therefore, in order to get the desired CZTS composition (Cu/(Zn & Sn) = 0.8–
0.9, Zn/Sn = 1.2–1.3)[8], the Cu/Sn ratio of precursor is designed in the range of 1.4–
1.9 (below two) and Zn/Sn ratio in the range of 1.1–1.25.
As expected, the elemental ratio changes greatly after sulfurization. The precursor experiences a major loss of Sn. Interestingly, the loss of Sn is highly correlated to the Cu content and it seems that Cu helps to capture Sn and retain the ratio of Cu/Sn at two during the sulfurization process. As CZTS can be formed by direct reaction of binary compound (reaction 3.1) and/or via ternary compounds (reaction
3.2)[23, 45], Cu is able to capture Sn via the formation of either Cu2ZnSnS4 or Cu2SnS3 compound during the sulfurization process where the ratio of Cu/Sn is two. The excess
Sn may be lost as SnS due to its high volatility[23]. According to ternary composition diagram (Figure. 3-2b), the fact that compositions move to the near opposite direction of tin sulfide after sulfurization (red arrow in Fig. 3-2b) also clearly confirms the loss of tin during high temperature sulfurization process.
Cu2S SnS ZnS S2 o Cu2ZnSnS4 (3.1)
Cu2SnS3 ZnS S2 o Cu2ZnSnS4 (3.2) Compared to Sn loss, the loss of Zn does not seem to be severe, which may be due to the metallic stacking arrangement that Zn has been positioned as the bottom layer. As a consequence of different extents of Sn loss, the Zn/Sn ratio of sulfurized CZTS films
34 ranges from 1.8 to 1.3 and Cu/(Zn & Sn) ratio varies from 0.72 to 0.85. Generally, the compositions of all the films are copper-poor and zinc-rich.
Figure 3-3 demonstrates X-ray diffraction patterns of sulfurized CZTS films. All three samples can be readily indexed to kesterite Cu2ZnSnS4 (JCPDS no. 00-026-0575) with (112) plane the preferred orientation. The strong peaks at 2ș=40.6° and 74° belong to the Mo back contact. Besides, XRD peaks assigned to MoS2 can also be found at 2ș
= 33.8°[46], indicating a MoS2 layer was formed at the Mo/CZTS interface, which has been considered as a current blocking layer, deteriorating the device performance. There are no other impurities such as CuS, SnS2, SnS present within the detect limitation of
XRD. However, impurities like Cu2SnS3 and ZnS cannot be clearly distinguished owing to their similar crystal structure to CZTS [26].
Figure. 3-3 XRD patterns of sulphurised CZTS films with different compositions.
35 The full width at half maximum (FWHM) value of the (112) plane of CZTS films with different compositions is displayed in the right-hand corner inset of Figure 3-
3. Sample A (with low Cu-content and high Zn-content) showed sharp peaks and a low
FWHM value. This may be due to the grain size of sample A being larger than that of samples B and C[46]. In addition, less crystalline defects (such as dislocations) presented in low Cu and high Zn samples could also contribute to this change.
Raman scattering characterization was carried out on the surface of CZTS samples with different composition, as shown in Figure 3-4. All three samples show sharp peaks at 338 cm-1 as well as peaks at 287 cm-1, together with XRD data, confirming the formation of kesterite CZTS. No impurity peaks, such as CuS/Cu2-
-1 -1 -1 xS/Cu2S (at 475cm )[47-49], Sn2S3 (at 304cm )[50], SnS2 (at 315cm )[50], SnS
(220cm-1)[51] were present. A shoulder peak at approximately 353 cm-1 can be attributed to ZnS and/or Cu2SnS3[52].
36 Figure 3-4 Raman spectra of sulphurised CZTS films with different compositions.
The fact that the Cu/Sn ratio of all the samples are close to two and no copper sulfide or tin sulfide could be found according to XRD and Raman data indicates that all of the copper reacts with excess tin without yielding a Cu-S phase, and any excess tin is left out of the films without any trace by-products (tin sulfide).
Figure 3-5(a-c) present the top view SEM images of CZTS samples with different compositions. All three samples display the rough, loose as well as inhomogeneous surface morphology. It is notable that some pinholes with diameters ranging from 0.1-0.3 ȝm can be observed on the surfaces. These pinholes may be the escape pathways of Sn or SnS during sulfurization process resulting from the process of loss of tin or tin sulphide. Both large grains (size of 0.4 - 0.6 ȝm) with triangular shape and small grains with round shape (size of tens of nanometers) were presented. No SnS2 platelets, which has a hexagonal morphology and can be easily distinguished on the surface, were found on the surfaces of the films[53].
37 Figure. 3-5 SEM images of top-viewed CZTS thin films with different compositions
(a,b,c) and cross sectional views of corresponding CZTS devices (A,B,C).
Cross-sectional SEM images of the CZTS devices are considered as a useful tool in order to have an idea of the grain size and uniformity of the CZTS films. The corresponding cross-sectional views of our CZTS devices with different compositions are shown in Figure 3-5(A-C). Typical structure was utilized to fabricate CZTS cells
(Mo/CZTS/CdS/i-ZnO/AZO). According to these images, the AZO layers are of 0.6-0.7
ȝm, which is thicker than that of reported high efficiency kesterite thin film solar cells[54]. This is aimed to make the resistance of our TCO below 100 /square. The thickness of the Mo back contact is approximately 1.1-1.2 ȝm with columnar grains. Of all images, the darkening of top parts of Mo or smaller particles within the upper part of
Mo are due to the sulphurisation of Mo[55], forming MoS2 layers with thickness of 0.3-
0.4 ȝm. The formation of a MoS2 layer was also supported by the aforementioned XRD data. Between the AZO and MoS2 layers are CZTS/CdS/i-ZnO layers, i-ZnO layer and
CdS layers cannot be clearly identified in the SEM images due to their low thickness 38 (50-70 nm). The thicknesses of CZTS layer are ranged between 0.9 and 1.1 ȝm and both large grains (hundreds nm) and small grains (tens of nm) coexist along the depth profile.
The average grain size in the upper layers is larger than that in the lower layers. This may result from the growth sequence in which grains in the upper positions grow first and become larger grains during the sulfurization process as proposed by Fairbrother et al.[44] . Such bilayer CZTS structure with different grain size has also been observed in some reports using metallic[48] and metal sulfide precursors[55].
The sample A (high Zn content and low Cu content) demonstrates larger grains
(size of 0.5-0.6 ȝm) than that of sample B and C, consistent with trend of FWHM values extracted from XRD data. It is widely believed that Cu-S(e) phase facilitates the crystals growth process and grains with low copper content are smaller than that with high copper content for CIGS process[56, 57]. Some believed this principle can be also applied to CZTS(e)[57, 58]. However, it was found that the sample with lowest Cu/(Sn
& Zn) ratio get larger gains than samples with higher Cu/(Sn & Zn) ratio, contradicting with Cu-assisted crystal growth phenomenon in CIGS/CIS filed. In 2013, scientists at
NREL compared the CZTSe growth under copper-rich conditions with zinc-rich (copper poor) conditions by the evaporation method. Interestingly, the grain size of zinc-rich
CZTSe films is as large as copper-rich ones, both demonstrating columnar micro-scale grains. They concluded that Cu-rich and Zn-rich growth routes share equal promise for high efficiency kesterite solar cells[59]. As all three samples are identical with only difference in zinc content, it is speculated that high zinc content or excess zinc sulfide may facilitate the grain growth in the sulfurization annealing process. This may help to explain that large grains can be frequently found in zinc-rich and copper-poor samples[8,
60, 61].
39 The solar cell performances of CZTS devices with different absorber compositions are illustrated in Table 1 and corresponding J-V curves are shown in
Fig.3-6. As expected, sample A with high zinc content and larger grain size demonstrates the best conversion efficiency (2.59%) with all device performance indices (i.e. Voc, Jsc and Fill Factor) superior to the others. The FF of the best device was only 46.4%, which is quite low compared to high efficiency CZTS cells (with FF around 60%) [7, 9, 13]. According to the light J-V curve, the Rs and Rsh have been calculated (illustrated in Table 1). Obviously the low FF of device A is due to shunting and high series resistance, which may be caused by pinholes observed in SEM images of CZTS surface and by thick MoS2 (0.3 - 0.4 ȝm), respectively. Other samples suffer a more severe shunting problem and high series resistance, leading to lower FF (merely
27-28%). According to dark J-V curves, sample A also shows better diode characteristic than those of sample B and C.
Table 3-1 CZTS device performances with different compositions.
Sample Voc(mV) Jsc(mA/cm2) FF Eff(%) Rs(cm2)Rsh(cm2) A 572 9.76 46.4 2.59 18.5 819.2 B 373 7.11 28.1 0.75 42.3 67.7 C 206 3.76 27.4 0.21 47.2 66.3
40 Figure 3-6 a) light and b) dark J-V curves of CZTS devices with different compositions.
Figure 3-7 demonstrates the External Quantum Efficiency (EQE) curves of the
CZTS device with different composition. The Jsc values calculated from EQE data are consistent with those measured from J-V curves. All the EQE data peaked at around 530 nm and the large decrease in the blue region is due to the absorption of i-ZnO/AZO
(with band gap around 3.4 eV significantly absorbing light <380 nm)[62] and CdS (with band gap around 2.4 eV significantly absorbing light <520 nm). Recently, Winkler et al. reported nearly 10% improvement in Jsc and efficiency by decreasing and optimizing the thickness of TCO and CdS layers, achieving 12% efficient CZTSSe solar cell[63].
In terms of wavelengths larger than 530 nm, the EQE value decays at two different slopes. Sample A demonstrates better characteristics. The slopes at long wavelength
(>700 nm) are quite different, indicating that the band gap value of the three samples may be different. In order to illuminate on this question, the graph of hQÂln(1-EQE)2 vs. photon energy was plotted and is shown in Figure 3-8 [64]. The estimated band gap value for sample A, B and C are 1.52 eV, 1.5 eV and 1.48 eV, respectively, consistent with band gaps value of reported CZTS [65, 66].
41 Figure 3-7 EQE curves of CZTS devices with different compositions.
Except for the infrared region, Sample A shows an overall higher absorption, followed by samples B and C, whilst the EQE curve of A is more squared than that of B and C. This may be due to sample A having a longer depletion width and/or sample A having a longer lifetime /diffusion length. Recently, Sugimoto reported that minority life time of CZTS highly depends on the Cu/Sn ratio, and CZTS film with a Cu/Sn ratio of 1.95 - 2.0 demonstrates life time merely 0 – 1 ns[67]. In this regard, as the Cu/Sn ratio of all the samples (A,B,C) is around 2.0 in this study, the large difference in EQE is more probably owing to differences in depletion width.
Figure 3-8 a) h҃Âln(1-EQE)2 vs. Photon energy curve to estimate band gaps, b) ln(1-
EQE)2 vs. absorption coefficient curve of CZTS with different compositions
It was reported by Scragg et al. that a simple way to estimate the depletion width of low life time CZTS is by using EQE data and the absorption coefficient[68].
Specifically, the absorption (EQE) of a semiconductor is described by the Gartner
42 equation, which is valid when there is no recombination in the space charge region or at the surface and the ideality factors of all samples are similar.
exp(DW ) EQE 1 (3.3) 1DLn
Where Į is the optical absorption coefficient of CZTS, W is the width of the depletion region and Ln is the minority (electrons) diffusion length of CZTS. If the lifetime of the CZTS is long, for instance, several nanoseconds or even longer or the diffusion length is non-zero, the –ln(1-EQE) vs. absorption coefficient should be nonlinear. The lifetime of our CZTS samples is supposed to be merely 0-1 ns according to Sugimoto’s report on the correlation of lifetime vs. the Cu/Sn ratio, implying very short minority diffusion length. Therefore, if assuming Ln equal to zero, namely, that all of the electrons are collected within the depletion width, equation (3.3) can be simplified as
EQE 1 exp(DW ) (3.4)
Utilizing equation (3.4) and the published absorption coefficient Į for bulk crystalline CZTS[69], the graph of –ln(1-EQE) vs. absorption coefficient has been plotted in Fig 7b. By linearly fitting the data, the slope gives an estimation of the depletion width. The scatter of points for different samples clearly follows a linear relationship. The depletion widths of samples A, B and C are estimated to be 114±2 nm,
51±2 nm and 24±1 nm, respectively. The sample with the lowest Cu and highest Zn content demonstrates the largest depletion width, which facilitates the collection of the free carriers and hence improves the short circuit current, especially for low minority lifetime/ low minority diffusion length samples. In contrast, compared to the depletion
43 width of 0.45 ȝm for high efficiency (19.9%) CIGS solar cells[70], the depletion width in this study is still too low.
3.2.3 Conclusion
Mo/Zn/(Cu & Sn) precursors with different copper content were used to fabricate CZTS thin films and corresponding solar cell devices. During the sulfurization process, the loss of tin correlates the copper content, namely, copper captures tin with a
Cu/Sn = 2 in terms of this Zn/(Cu & Sn) precursor structure. The escape pathway that tin species lose during sulfurization has been observed in surface SEM images.
Formations of kesterite are confirmed by XRD and Raman and MoS2 layer are also detected by XRD as well as observed in SEM cross-sectional images. It is notable that
CZTS with low copper content and high zinc content yields the largest grains. It is speculated that besides high Cu content, higher Zn content can also facilitate the crystalline growth, generating large grains. The best solar cell with low Cu content and high Zn content in this paper is 2.59% and its device related characteristics (Voc, Jsc and FF) are all superior to those of samples with higher Cu content and lower Zn content. One explanation for the improved device performance should be a larger depletion width (114±2 nm) than those of others. But still, this value is too low; depletion width of our CZTS device still needs to be expanded in order to get higher efficiency.
44 3.3 Composition effects on CZTS from sulfur contained precursor and one-zone sulfurization process
3.3.1 Experimental
A magnetron sputter system (AJA International, Inc., model ATC-2200) was utilized to simultaneously co-deposit ZnS (RF power supply), SnS (RF power supplyand Cu (DC power supply) on Mo-coated soda-lime glass as the precursor film.
The pressure of Ar is maintained around 1.5 mTorr and the substrate rotates at 30 rpm without intentional heating. Then the precursors were sent to a Rapid thermal annealing furnace (AS-One 100) annealing/sulfurization in S containing atmosphere for several minutes at 550~580°C. The CZTS devices are fabricated using typical Mo/CZTS/CdS/i-
ZO/ITO/Al solar cell configuration. The CdS buffer layer is deposited by mixing cadmium sulfate (3CdSO4·8H2O), thiourea (H2NCSNH2) and 28% ammonium hydroxide in a chemical bath system. The i-ZnO and ITO layers are deposited by RF sputtering. Evaporated Al grids serve as the top contact.
3.3.2 Results and discussions
We have deposited the Cu/SnS/ZnS precursor with different metallic compositions, mainly differentiated in Cu content, marked as A,B,C, which have been displayed in Table 3-2.
Table 3-2 Composition for different deposited precursors.
Sample Cu/Sn Zn/Sn Cu/Zn Cu/(Zn+Sn)
A 1.69 1.20 1.41 0.77 B 1.85 1.27 1.45 0.81 C 1.98 1.32 1.50 0.85
45 Figure 3-9 SEM top-view images of CZTS with different compositions.
The top view SEM images have been demonstrated in Figure 3-9. It can be clearly seen that the sample with lower Cu/Sn possess larger grain. The grain length of sample A can be extending as long as 2 um, whereas the sample C with higher Cu/Sn content only has the grain with size of several hundred nanometers. The author speculates that during the sulfurization more Sn loss will be experienced with low
Besides, accompany with the loss of tin process is the volume expansion and crystal growth process, giving more space for the latter two process and allowing large grain formed.
Figure3-10 Shows the J-V curves of the CZTS devices with different Cu contents and the Table 3-3 demonstrates their corresponding specific performance parameters.
46
Figure 3-10 Light J-V curves of CZTS with different compositions.
Table 3-3 Composition for different precursors.
2 2 2 Sample Voc(mV) Jsc(mA/cm ) FF(%) PEC(%) Rs,L( cm ) Rsh,L( cm )
A 554 19.02 41.39 4.36 2.8 108
B 663 18.18 56.84 6.85 2.56 428
C 653 15.82 59.28 6.13 1.58 410
Device with lowest Cu content shows the highest Jsc, however, the Voc and FF are much lower than the others, leading to a much lower overall efficiency. The decrease in both Voc and FF should be attributed to a more severe shunting problem, which is quite common for sample with low Cu/Sn. In terms of device with Cu/Sn=1.85, the highest Voc (663mV) and efficiency (6.85%) has been achieved which can be attributed to less shunting problem compared to that with the lowest Cu content. The device with highest Cu content gives the best FF which results from decreased series resistance (Rs), however, sharply decreased Jsc, leading to an overall decreased
47 efficiency. For the sake of deeply understanding the variations of Jsc and FF in different samples, further characterizations have to be conducted.
First and foremost, in order to understand more details on Jsc variation, the EQE data had been collected, as shown in Figure 3-11. The integrated EQE for all samples are well matched with their Jsc extracted from J-V curves. Sample A and B demonstrate overall higher absorption. In the meanwhile the EQE curves of A and B seem more squared than that of C in terms of long wavelength in the visible region. This indicates sample A and B (lower Cu content) have a longer collection length which results from a longer depletion region width and/or a longer diffusion length or longer lifetime.
Figure 3-11 EQE curves of CZTS devices with different compositions.
To further shed light upon EQE trends, the time-resolved photoluminescence
(TRPL) measurements were conducted on devices with different Cu content, as show in
Figure 3-12. The TRPL decay curves could be well fitted by a bi-exponential decay function I(t)= A1 exp(-t/Tau1) + A2 exp(-t/Tau2). And the effective minority lifetime can
48 be obtained by the equation Taueff= (A1 Tau1+A2 Tau2)/(A1+A2).[71] Via this method, the effective lifetime for sample A, B, and C are calculated to be 14.5ns, 11.0ns, and
4.9ns, respectively. This should be one major factor leading to low EQE at long wavelength of sample C and resulting much lower Jsc compared to that of sample A and
B. Our observation is consistent with the results from solar frontier that minority lifetime of CZTS devices highly depends on the Cu/Sn ratio and the lower the Cu/Sn content, the higher the minority lifetime[67].
Figure 3-12 TRPL curves of CZTS with different compositions.
Capacitance-Voltage (C-V) measurements were conducted at a frequency of 100 kHz. We assume that CZTS solar cell consists of one-sided step junction, for example a n+p junction and the number of defects that measured by C-V is far lower than the free carrier density as well as the CZTS is not a heavily compensated material. If the material is heavily compensated, the C-V data should be dependent on the frequency.
49 According to a Mott-Schottky analysis, equation (3.5), a plot of A2/C2 vs. the applied bias should have a linear region where the slope is the function of the C.C. of the CZTS ,
2 A 2(V Vbi ) 2 (3.5) C qH 0H S N A
where A is the device area, Vbi is the built-in potential, C is the capacitance, H0 and HS are the vacuum permittivity and the relative permittivity of CZTS (here 7 is used according to ref [60]), respectively.
According to equation (3.6) and (3.7), the C-V profiling figure can be plotted.
H H A X 0 S (3.6) d C
2 N d (X d ) 2 2 (3.7) qH 0H S d(A / C ) / dV
Where Xd is the distance of model compacitor İ0 is the vacuum permittivity and
İs is the relative permittivity of CZTS, q is the elementary charge of an electron, A is the device area, C is the capacitance, V is the applied bias voltage. The C.C. can be readily extracted when the bias voltage is 0 V [72, 73].
The depletion width (Wd) and carrier concentration (C.C.) values for different samples are estimated from the C-V measurements, as illustrated in the Table 3-4. The doping of CZTS and Wd of the corresponding devices are highly dependent on the Cu content. Low Cu/Sn ratio possesses the lower doping and longer depletion width, which is consistent with previous electrical property of CZTS single crystals[74]. This may well explain why the sample C has a lower Rs. It should at least partially be attributed to its higher Cu content, which yields higher doping of CZTS and thereby higher
50 conductivity of CZTS layer, consequently, decreasing the Rs and FF of the corresponding device. Besides, another reason for the decreased EQE at long wavelength for sample C is the shortened Wd comparing to those of sample A and B.
Table 3-4 Electrical properties of CZTS solar cells with different compositions.
-3 Sample C.C.(cm ) Wd (nm) A 2.2×1016 272 B 7.0×1016 117 C 8.5×1016 82
3.3.3 Conclusion
For CZTS from sulfide precursor plus rapid thermal annealing process, Cu content plays a vital role in determining the microstructure, quality (lifetime), and electrical property of CZTS thin films, thus greatly deciding the efficiency of their corresponding devices. Specifically, the CZTS samples with lower Cu content possess larger grain size, longer minority lifetime and lower doping of CZTS layer and vice versa. However, devices with serious Cu deficiency, for instance in the case of Cu/Sn lower than 1.7, will suffer from severe shunting problem. Therefore, it is the Cu/Sn at a compromised middle value range (1.8~1.9) yields the best device efficiency.
3.4 Boost Voc of sulfide kesterite solar cells via a double CZTS layer stacks with different Cu content
3.4.1 Motivation
One of the key effective recipes allowing the realization of high Voc and efficiency of CIGSe solar cells is the conduction band grading with back heavily P doped for the benefits of a) generating a field force to push the minority carrier flowing from back to the front; b) extending the effective diffusion length and c) passivating the
51 back interface[75]. Recent Voc and efficiency improvement for sulfide CuInxGa1-xS2 solar cell is also involved with suitable/severe conduction band grading[76]. Another example is the back surface field (BSF) applied in Si solar cells which aims to make the back/bottom heavily doped acting as a rear pushing force to push the minority carrier to the front (depletion area), thus passivating the back surface[77]. Meanwhile, the
+ configuration of P at the bottom with graded doping helps to enlarge the fermi level splitting between p-type absorber and its n-type partner upon illumination on the device.
Herein based on our finding that doping can be easily tuned by changing the Cu content, a structure with double CZTS layers with higher P doping at the bottom and lower doping at the top via rapid heating method was tried and corresponding solar cell devices were fabricated.
3.4.2 Experimental
The basic fabrication method for our CZTS process has been described in previous section. Briefly, a magnetron sputtering system (AJA International, Inc., model
ATC-2200) was used to co-sputter Cu/ZnS/SnS precursors on Mo-coated soda lime glass, which were then transferred for sulfurization treatment using a Rapid Thermal
Processor (AS-One 100) in a combined sulfur and SnS containing atmosphere. Note that the ramping rate is 9°C/min for the single sulfurization annealing process whereas
30 °C/min for the second sulfurization annealing process. CdS buffer layers were deposited by the chemical bath deposition (CBD) method. Specifically, cadmium sulfate (3CdSO4Â8H2O), excess thiourea (H2NCSNH2) and ammonium hydroxide were mixed with a base environment in the glass reactor for CdS, with details shown elsewhere[78]. The 60 nm intrinsic ZnO (i-ZnO) and 220 nm ITO films were deposited
52 by RF sputtering sequentially. Al grids were used as the top contact and the total area of the final cells was 0.45 cm2, defined by mechanical scribing.
The scanning electron microscope (SEM) images were obtained with a FEI
Nova NanoSEM 230 FESEM under 3 kV accelerating voltage. The compositions were measured by An Inductively Coupled Plasma (PerkinElmer quadrapole Nexion ICPMS).
Note that all the composition mentioned in this manuscript refers to the composition of the whole precursor. Cross-sectional transmission electron microscopy (TEM) specimens were prepared by a focused ion beam (FIB) system (the xT Nova NanoLab
200). Note that a ~20 nm Au layer and a ~1 ȝm Pt layer were deposited consecutively on top of specimens before cutting process for the sake of preventing the ion damage.
JEOL JEM-ARM200F (200kV) aberration-corrected scanning transmission electron microscope (STEM) equipped with energy dispersive X-ray spectroscopy (EDS) system was used for acquiring HAADF (high-angle annular dark-field) image and EDS mappings as well as linescans, respectively. Raman spectra are acquired with the
Renishaw in via Raman Microscope by using 514 nm (green) Ar+ laser. The excitation wavelength of our steady state PL is 405 nm with power of 2.5 mW and beam diameter of 0.9mm.
The J-V curves were measured using a solar simulator (PV measurements Inc.
Oriel model 94023A) under standard condition (AM 1.5 and 100mW/cm2) calibrated with a standard Si reference cell. External quantum ef¿ciency (EQE) data were collected by a QEX10 spectral response system (PV measurements, Inc.) calibrated by the National Institute of Standards and Technology (NIST)-certified reference Si. This system uses monochromatic light chopped at 120 Hz. The capacitance spectroscopy was
53 carried out using an impedance analyzer at a frequency of 100 kHz with a DC bias voltage sweeping from -1.5 to 0.5 V.
3.4.3 Results and discussions
For Silicon, the doping type is achieved by diffusing certain amount of group III or group V elements into the Si bulk and the diffused amount determines the doping level/ majority carrier density. Different from that of Si, the carrier concentration of
CZTS largely depends on Cu content, specifically Cu/Sn ratio [74, 79]. The C-V measurements were conducted (25 °C and frequency of 100 kHz) for CZTS devices with different Cu/Sn ratios fabricated in our labs, and Table 3-5 illustrates the corresponding data of device performance and electrical properties. Samples with lower
Cu content (Cu/Sn ratio) demonstrate lower carrier concentration (C.C.) and larger depletion width (Wd) and vice versa. This observation is consistent with the reports from Nagaoka [79] and Bishop[74]. Besides, considering the converse relationship between lifetime and majority carrier concentration, the fact that CZTS samples with lower Cu/Sn ratio yield higher minority lifetime reported by Sugimoto et al., indirectly indicates lower Cu/Sn should have lower doping level[80].
Table 3-5 Device performance and corresponding electrical properties for CZTS with
different Cu/Sn ratios.
Zn/Sn Median efficiency C.C. W (nm) Cu/Sn d Voc (cm-3)
1.85 1.27 655 7~8% 6.5×1016 113
1.95 1.32 650 6~7% 9.0×1016 82
2.08 1.33 500 1~2% 7.0×1017 64
54 Now that the Cu/Sn ratio can be used to control the majority doping level, two
CZTS layers with different Cu/Sn ratios are stacked with CZTS with high Cu content at bottom (Cu/Sn=2.0, 2.15, 2.30 respectively with thickness of 300nm) and low Cu content at the top (Cu/Sn=1.80 with thickness of 600nm). It is notable that due to the high diffusivity nature of copper, two sulfurization annealing processes were utilized to sulfurize the bottom CZTS and top CZTS individually, similar to that reported in Shin
Tajima’s paper [81]. Different from previously method that uses the lower annealing temperature to achieve doping difference, a faster heating ramping rate (30°C/s) are utilized herein to realize the doping difference. Figure 3-13 shows the surface morphology of the bottom CZTS layers with different Cu/Sn ratios. In the case of Cu/Sn
(C/T)=2.0, the surface of CZTS is quite smooth. However, when it come to the cases of
C/T=2.15 and C/T=2.3, large dark grains dislocated on the CZTS surface can be observed. According the EDS measurement, the large and dark grains mostly contain
Cu and S elements. During the sulfurization process the excess copper in the copper- rich samples (C/T=2.15 and 2.3) would react with S forming Cu-S phase on the surface.
Meanwhile, strong Raman peaks at around 475cm-1 (see Figure 3-14) double confirm the existence of Cu-S phase on top of CZTS layers. Besides, no MoS2 peaks (typically at ~407cm-1, 381cm-1) can be found [82, 83] in the Raman spectra for all the samples, indicating the very thin CZTS bottom layers (~300nm) are densely packed and well cover the MoS2 without large voids or cracks.
55 C/T=2.0 C/T=2.15 C/T=2.3
Figure 3-13. Top view SEM images of CZTS bottom layers with various Cu/Sn ratios.
Figure 3-14. Raman spectra of CZTS bottom layer with different Cu/Sn ratios.
Figure 3-15 demonstrates the representative J-V curves of double-layer CZTS devices and a reference single-layer CZTS device (noted as reference sample), and the corresponding device performance parameters are summarized in Table 3-6.
56
Figure 3-15 (a) The representative J-V curves of double-layer CZTS devices and a
reference single layer CZTS device, and (b) the Voc distributions of reference device
and double-layer CZTS device with bottom C/T=2.15.
Table 3-6 Device performances of double-layer CZTS and reference cells.
2 2 2 sample Voc(mV) Jsc(mA/cm ) FF(%) Efficiency(%) RSL(cm ) RSHL(cm )
Bottom 714 15.8 57.9 6.53 3.56 689 C/T=2.0
Bottom 734 15.4 67.0 7.57 1.86 730 C/T=2.15
Bottom 672 14.3 64.6 6.21 2.16 723 C/T=2.3
Single layer 665 20.0 57.3 7.62 3.94 473 CZTS ref
Compared to the reference device, all devices from the double layer CZTS achieve increased Voc, FF, whilst decreased Jsc. The double layer CZTS device with bottom C/T=2.15 and top C/T=1.8 shows the highest Voc (734 mV) and FF (67%), which may be owing to the appropriate Cu content difference in the double CZTS layer stacks. Figure 2(b) compares the Voc distribution between reference sample and double layer samples with bottom C/T=2.15. The Voc of C/T=2.15 sample is mainly distributed in the range of 730~740mV with the highest of 745mV, which is 70mV~80mV higher
57 than those of the reference sample. It is speculated the double P+/P CZTS layer structure or eventually possible vertically-graded Cu gradient has exerted some benefits to device performance. In order to illuminate on this issue, TEM characterization was conducted on double-layer CZTS device with bottom C/T=2.15. Figure 3-16 shows the HAADF image (3-16(a)) with corresponding EDS (energy dispersive spectroscopy) line scan data (3-16(b)) of the double-layer CZTS device along red arrow in Figure 3-16(a). The
EDS mapping data was processed by PCA (Principal Component Analysis) and phase analysis to determine the covariance of different elements, where different signals that vary in a similar fashion can be picked up as one single phase. By combining the PCA processed image and HAADF image together as shown in Figure 3-16(a), different functional layers in the CZTS device can be clearly seen and marked. It is notable that lots of voids or porous microstructures are formed in the vicinity of the interface of the double CZTS layers. In the reference CZTS sample, voids are usually found at the
CZTS/Mo interface which is believed to be due to the reaction between CZTS and Mo leading to formation of MoS2 and decomposition of CZTS[84]. The formation of the voids at the interface of double CZTS layers should be attributed to the dual sulfurization annealing process. On one hand, the top CZTS precursor is sputtered on a non-uniform rough surface of bottom CZTS layer. One the other hand, the diffusion velocities of different elements such as Cu, Zn, Sn are very different, resulting in
Kirkendall effect[81, 85]. As the consequence, the voids are easily formed during the second annealing process. Actually, when we tried processing the two precursor stacks with only one sulfurization annealing process, no voids were observed at the middle of
CZTS. However, all the Voc, Jsc and efficiency in this case are similar to those of single layer CZTS reference. Hence, the voids at the interface of double layers are
58 detrimental for current transportation and thereby contributing to the decreased Jsc of the double-layer CZTS devices.
Referring to the vertical elemental line scan in Figure 3-16b), Cu gradient can hardly be identified. However, in terms of the average Cu/Sn ratio, the average copper content of bottom CZTS layer is 3% higher than that of the top CZTS layer. Note that the spot size of applied EDS detection is smaller than 2nm and the resolution of EDS scan is within 1nm. Compared to the designed Cu content difference in the precursor
(top C/T=1.8 and bottom C/T=2.15), the final Cu content difference in CZTS absorber detected by EDS is very small due to the strong copper diffusion during the second sulfurization annealing process.
Figure 3-16 (a) HAADF image combined with the PCA processed EDS mapping image
and (b) EDS line scan along the red arrow of the C/T=2.15 sample shown in (a).
Figure 3-17 demonstrates the carrier concentration (C.C) depth profile of double-layer CZTS films and Table 3-7 summarizes the corresponding C.C. and Wd calculated from C-V measurement (Frequency of 100kHz). When bias voltage is 0V, the C.C. reflects the properties of CZTS near depletion region, namely, the properties of
59 very top CZTS layer. It seems the C.C. and Wd of the double layered CZTS device can be greatly influenced by bottom C/T ratio, indicating the fact that there are indeed significant diffusion of Cu from bottom to top CZTS, increasing the doping level of top
CZTS layer. However top CZTS layer with bottom C/T=2.0 and C/T=2.15 samples show lower C.C. than that of single layer CZTS reference sample, suggesting the doping of very top CZTS still remains in a low level and Cu is not homogeneously vertically distributed from bottom to top. In other words, there should be vertical Cu gradients within the double layer CZTS stacks. The carrier profiles of the two layer devices are compared with that of single layer device. We found that the C.C. and Wd derived from single layer CZTS is similar to those of double layer CZTS (with
C/T=2.15 and 2.3), suggesting the surface C/T value of double layer CZTS with
C/T=2.15~2.3 is around 1.85.
Figure 3-17. C.C depth profile of double-layer CZTS films obtained by C-V
measurement.
Table 3-7 Electrical property of CZTS with double and single layered CZTS.
60 bottom Top C.C. Wd (nm) Cu/Sn Cu/Sn (cm-3)
2.3 1.8 7×1016 102
2.15 1.8 6×1016 122
2.0 1.8 3×1016 184 Single layer 1.85 6.5×1016 113
Another PCA processed EDS mapping image of the C/T=2.15 sample further reveals that some Cu-S secondary phase exists in the bottom CZTS layers (see Figure 3-
18). In Figure 3-13, Cu-S phase was observed to form within the bottom CZTS layer after first sulfurization annealing process. This Cu-S phase at the surface of bottom
CZTS layer might be consumed by the growth of the top CZTS layer during the second sulfurization process. However, it is obviously noted that not all the Cu-S phase at bottom layer is consumed and some still remain at the back of bottom CZTS layer. This suggests the bottom layer is still Cu saturated or Cu-rich, maintaining bottom CZTS layer as the high doping (P+) layer. The 300nm P+ CZTS bottom layer is relatively more conductive, enabling Ohmic contact between CZTS layer and MoS2/Mo layer, and thereby reducing the series resistance (Rs). Besides, the double-layer CZTS absorber with two sulfurization annealing process seems to have a lower chance of forming shunting pathways compared to the single CZTS layer reference, resulting in a higher shunt resistance (RSH). Therefore the FF of double-layer CZTS is higher than that of single layer reference.
61
Figure 3-18. PCA processed EDS mapping image of double layer CZTS device with
bottom C/T=2.15.
In order to further identify the reasons causing the Jsc decrement, EQE curves of double-layer CZTS and reference sample are provided in Figure 3-19a). It can be concluded that the Jsc is deteriorated in two aspects. Firstly, The highest point of EQE of double-layer CZTS is lower than that of reference sample and in the meanwhile EQE curves decline faster after reaching their peaks (at ~520nm) or the curve shape at long wavelength region is not as square as that of reference sample. This is mainly because of the aforementioned voids between the double CZTS layers, reducing the effective light absorption volume of the absorber. In addition, the high doping bottom CZTS layer would act as an absorption dead layer with very low minority lifetime. Therefore the total EQE was reduced with the long wavelength part more quickly dropped.
Secondly, the EQE of double-layer CZTS demonstrated slimmer feature than that of single-layer CZTS reference. Their Egopt (optical band gap) was calculated by using the rear inflection of the EQE curves (the peak of the dEQE/dȜ) near the band edges[31]. It is notable that the Egopt of double-layer CZTS (1.66eV) is much higher 62 than that of single-layer CZTS reference (1.55eV), which should be another key factor leading to the severely decreased Jsc. The 1.66eV optical band gap of double-layer
CZTS deduced from EQE seems to be the highest among the reported for pure sulfide
CZTS which is usually within the range of 1.4~1.6eV [65, 80, 81, 86]. For the sake of further studying the band gap change phenomenon of double-layer CZTS, we conducted steady state photoluminescence (PL) characterization, which is believed to reflect more effective electrical band gap. As suspected, the PL peak of C/T=2.15 sample (~1.36eV) is slightly higher than that of single layer reference (~1.32eV), which is believed to be another reason for increased Voc. However, the difference of PL peak between double- layer CZTS and single-layer CZTS reference is not as large as their difference in the optical band gap. That is to say, Egopt of double-layer CZTS blue shifts to a much higher position relative to PL peak, compared to that of single-layer reference. This kind of pronounced shift, together with broadened steady state PL peak as well as slower decay of EQE below Egopt, suggests the double-layer CZTS devices suffer from much more severe band tailing problem compared to single-layer reference. The band tailing problem would enlarge the optical band gap, narrowing the EQE absorption thus reducing the current. This kind of band tailing usually results from either bandgap or electrostatic potential fluctuations in the CZTS layer[31], which is mainly caused by the
- + + 2+ [ZnCu and CuZn ] defects complex due to similar radius of Cu and Zn ions and low
- + formation energy of the [ZnCu and CuZn ] cluster[12]. Herein the double-layer CZTS having Cu gradients from bottom to top CZTS layers, together with fast ramping rate during sulfurization annealing[87], would enable a large number of active Cu atoms within the double CZTS layers and lead to easier formation of antisite defects such as
- + ZnCu and CuZn etc. This is likely the main reason for the observed severe band tailing
63 problem. Note that there is a high energy shoulder at around 1.5eV in the PL spectrum, which may be attributed to the band to band emission of CZTS. This shoulder usually takes place when the temperature of the CZTS has been elevated or the samples experience a large laser flux.[88]
Figure 3-19 (a) EQE curves of double-layer CZTS and reference sample with dash lines
marked the corresponding Egopt, (b) steady state photoluminescence spectra with dash
line marked PL peak positions.
3.4.4 Conclusions
This section has demonstrated an effective method to increase Voc and FF of
CZTS solar cells by stacking two CZTS layers with higher doped level at bottom and lower doped one at the top by RTP sulfurization process. Average Voc boost of
70mV~80mV can be achieved owing to the Cu gradient and blue shifted PL peak. The
Cu gradient cannot be directly measured using the high resolution EDS vertical linescan in the TEM characterization. However, it can be indirectly confirmed by shallow doping of the top CZTS region according to C-V measurement. In terms of double-layer CZTS devices, the decreased Jsc can be attributed to the voids at the interface of double CZTS layers, high doping of bottom CZTS layer and severe band tailing. The double CZTS
64 stacks shows its promise to boost Voc and FF, however, requires further optimization to reduce the Jsc loss.
65 Chapter 4 Effects of sulfurization and post-treatment on nanoscale microstructure and chemistry of Cu2ZnSnS4/CdS interface
4.1 Introduction
Good progress has been made over the past few years and the highest power conversion efficiency (PCE) of 9.1% for CZTS [5] and 12.6% for Se-incorporated
CZTS (CZTSSe) [9] solar cells have been achieved, demonstrating substantial commercial promise. However, this efficiency is still far below that of well-developed
CIGSe solar cells. The main device performance loss is caused by a low open circuit voltage when compared to the band gap of the absorber layer (i. e. a large Voc deficit).
The decisive differences between CIGSe and CZTS devices lie in the position of the p-n junction and the conduction band offset. For high efficiency CIGSe solar cells, the overall composition of the CIGSe absorber (p-type) is slightly Cu-deficient, with a thin and even more Cu-deficient surface layer corresponding to the “ordered vacancy compound Cu(In,Ga)3Se5 (OVC, weakly n-type)”. This inverted surface indicates the formation of a buried p-n homojunction a few nanometers below the physical
CIGSe/CdS heterojunction [89]. For CZTS there is no such inversion of conduction type at the surface region and therefore the p-n junction is located at the CZTS/CdS interface. Another important difference is the band alignment of the absorber/buffer heterojunction. According to simulation, a small positive conduction band offset of 0-
0.4 eV is optimal, where the thermal energy of the electrons at normal operating temperature is high enough to overcome the barrier without a significant voltage drop.
The conduction band offset is positive for CIGSe/CdS, but negative for CZTS/CdS[90]
66 which can dramatically increase the interface recombination via interface defects. Both the p-n junction position at the CZTS/CdS interface and negative conduction band offset lead to a much more pronounced dependence of the electrical characteristics of the device on the quality of the CZTS/CdS interface due to interface-related and tunneling-assisted recombination. Therefore, in order to achieve high efficiency CZTS devices, it is essential to understand the correlations between interfacial properties of
CZTS/CdS and film growth processes as well as associated device performance.
In this following section, the nanoscale microstructure and chemistry of the
CZTS/CdS interfaces from three absorber fabrication processes (Process I: sulfurization in sulfur only atmosphere; Process II: sulfurization in sulfur only atmosphere followed by air annealing; Process III: sulfurization in a combined sulfur and SnS atmosphere) are investigated first by transmission electron microscopy (TEM), which reveals the secrets and well-built bridge between different annealing atmosphere/post treatment and then their effects on corresponding device performance are revealed. The epitaxial growth of CdS by chemical bath deposition (CBD) directly onto well-structured CZTS
(Process II and III) is observed for the first time to the best of authors’ knowledge, which is one crucial feature (secret one) for minimum interface recombination and high efficiency CZTS solar cells. Cd diffusion is found to be another crucial feature (secret two) for high efficiency, which can widen depletion width and improve lifetime. In addition, the effects of further treatments such as HCl etching on interface properties and device performance were also studied.
4.2 Experimental
A typical CZTS solar cell configuration of Mo/CZTS/buffer/i-ZO/ITO/Al was used in this study. CZTS absorbers were prepared by the sulfurization of co-sputtered
67 Cu/ZnS/SnS precursors by a magnetron sputtering system (AJA International, Inc., model ATC-2200) on Mo-coated soda lime glass substrates. The sulfurization process was performed in a sulfur only (Process I) atmosphere at a temperature of 560 oC with heating rate of 15 oC/min for 5 minutes. In order to passivate the defects, air annealing treatment (300 oC, 1-2 minute) on absorber from sulfur-only sulfurization was employed (Process II). The sulfurization is also performed in a combined sulfur and
SnS (Process III) atmosphere at 560 oC with heating rate of 15 oC/min for 5 minutes.
Absorbers from Process III were also air-annealed to further reduce defects. The average composition of the CZTS absorbers is controlled to be Cu-poor and Zn-rich
(atomic ratio Cu/Sn=1.80 and Zn/Sn=1.15 measured by energy dispersive X-ray spectroscopy). HCl solution etching (0.01 mol/L, 10 seconds) was employed on some absorbers at room temperature. Buffer layers of CdS with thickness of 50 nm were prepared by chemical bath deposition (CBD) method. Specifically, cadmium sulfate
(3CdSO4 8H2O), excess thiourea (H2NCSNH2) and ammonium hydroxide were mixed with a base environment for CdS deposition, with details shown elsewhere.[78] The 50 nm intrinsic ZnO (i-ZnO) and 300 nm ITO films were deposited by RF sputtering. Al grids by evaporation were used as the top contact. The final step in the device fabrication sequence is the deposition of an antireflection coating (110 nm MgF2). The total area of the final cells is 0.45 cm2 defined by mechanical scribing.
The microstructure and elemental distribution across the CZTS and buffer interface were measured by JEOL JEM-ARM200F (200kV) aberration-corrected scanning transmission electron microscope (STEM) equipped with energy dispersive X- ray spectroscopy (EDAX) system. Time-resolved photoluminescence (TRPL) measurements were performed on devices using the time-correlated single photon
68 counting (TCSPC) technique (Microtime200, Picoquant) at a wavelength of 800 nm and room temperature. The excitation is a 467 nm laser with tunable repetition. The J-V curves of CZTS solar cells were measured using a solar simulator (Newport) with
AM1.5 illumination and intensity of 100 mW/cm2 calibrated with a standard Si reference cell. Capacitance spectroscopy was carried out using an impedance analyzer at a frequency of 100 kHz with a DC voltage bias sweeping from -1.5 to 0.5 V. External quantum ef¿ciency (EQE) data were collected by a QEX10 spectral response system
(PV measurements, Inc.) calibrated by the National Institute of Standards and
Technology (NIST)-certified reference Si photodiode. This system uses monochromatic light chopped at frequency of 120 Hz.
4.3 Results and discussion
4.3.1 Interface microstructure investigation
The fact that the p-n junction is located directly at the interface of the CZTS absorber and CdS buffer indicates the significance of interfacial microstructure and chemistry in CZTS device performance. Figure 4-1(a) shows a typical high-resolution
TEM image from CZTS/CdS heterointerface taken along the CZTS [021] zone axis, where CZTS is prepared from sulfurization in sulfur-only atmosphere (Process I). In the
CdS region, it can be seen that CdS layer consists of grains or domains with small size.
Extensive stacking faults and twins are clearly present which lead to the streaking in fast
Fourier transform (FFT) pattern as show in Figure 4-1 (b). Close examination of the lattice of CdS domains, shows an [ABCABC]-type of atomic stacking sequence along the (111) close-packed plane of cubic CdS. Therefore, the cubic zincblende structure is predominant in the CdS films, which is also supported by the FFT pattern indexing. The lattice planes in one domain are straight and parallel suggesting that there is not much
69 lattice strain in the CdS. For the CZTS region, no lattice defects such as stacking faults and dislocations in the CZTS matrix, along with sharp diffraction dots without satellite spots in the corresponding FFT pattern as shown in Fig.1(c) indicate a perfect local crystal quality. In contrast, for the CZTS/CdS interface, it is notable that there are lots of defects along the interface which are believed to arise from the compensation of the lattice mismatch between CBD-CdS and underlying CZTS surface. But there are still a few coherent parts of the interface, implying this mismatch should mainly be caused by the surface defects of CZTS. These interface defects usually act as recombination centres in the space charge region, which is harmful to Voc and FF of the device and therefore leads to low efficiency.
(a) (b)
(c)
Figure 4-1(a) High-resolution TEM image along CZTS [021] showing the CZTS/CdS
heterojunction. CZTS is prepared by Process I. (b) and (c) are FFT spectra taken from CdS and
CZTS regions, respectively.
One efficient way to reduce/eliminate the heterointerfacial crystalline defects is through defect annealing. Herein, an air annealing treatment was employed on the
CZTS absorber (Process II). Figure 4-2 shows a typical high-resolution TEM image taken from the interface of CdS and air annealed CZTS (Process II). It is indicative that 70 CdS/CZTS heterointerface is coherent and nearly free of any lattice defects along the interface, revealing a sort of epitaxial growth of CdS on CZTS. The atomic stacking sequence in CdS matrix following the [ABCABC]-type of the (111) plane of cubic CdS is clearly present in Figure 4-2(b). The lattice spacing is measured to be 0.336±0.001 nm which agrees well with the theoretical value of the (111) plane of cubic CdS.
Therefore, it can be concluded that the epitaxial relationship for CBD-CdS on CZTS is
(111) CdS || (112) CZTS and (002) CdS || (200) CZTS. This is quite similar with the epitaxial relation of CdS on chalcopyrite CIGSe based materials [91-94] which have similar crystal structure with kesterite CZTS. The relative orientation of these two types of lattices is shown by the parallelogram on FFT pattern in Fig. 2(c). The coincidence of
(111) and (002) reflections from the CdS with the (112) and (200) reflections from
CZTS indicates that the lattice constant match between cubic CdS and kesterite CZTS is sufficiently close to permit epitaxial growth of CBD-CdS on CZTS. The heterointerface is therefore coherent without lattice defects once the surface defects of the CZTS are eliminated by air annealing treatment. Note that the epitaxy is achieved at a relatively low substrate temperature (75oC) and not blocked by oxides formed on the CZTS surface during air annealing. This is because the surface oxide layer can be largely removed by etching in an ammonia based solution [95] in the initial stage of the CBD process, followed by atom-by-atom based growth [96]. Air annealing has been reported to be able to reduce defect sites and recombination at the interface [97, 98]. Different from previous studies, in this work, it was found that air annealing can actually eliminate surface defects and facilitate the epitaxial growth of CdS after the removal of oxide layer by ammonia solution, during the early stages of the CBD process, forming a coherent heterointerface without interfacial defects. An epitaxial junction was reported
71 by S. Tajima et al. [99] based on electron diffraction analysis on the CZTS/CBD-CdS
o heterojunction upon annealing at 300 C in N2 atmosphere, where the improved photovoltaic performance was believed to be due to reduced recombination at the heterointerface. Here it is firstly observed that the cubic CdS buffer layer can epitaxially grow on kesterite CZTS directly by CBD.
(a) (b)
(c)
Figure 4-2 (a) High-resolution TEM image along CZTS [021] showing the epitaxial growth of
CBD-CdS (CZTS is prepared by Process II). (b) the lattice image of the CBD-CdS. (c) the FFT
spectrum taken from the whole region of (a).
Another way to solve the surface defect problem is by optimising the growth conditions, in situ to yield defect-free surfaces. The advantage of this route is that it can avoid the potentially detrimental effect of post air annealing. For instance, oxides formed during air annealing cannot be completely removed by ammonia solution etching [95]. These residues may be unfavourable to carrier transportation and thereby
Jsc of the device due to a high resistivity, although they could passivate defects, improving Voc and FF. According to a previous report [23, 84], surface defects can be related to surface instability from decomposition of CZTS under high temperature processing conditions (> 500oC) and sulfurization in atmospheres of sufficiently high
72 partial pressures of combined SnS and sulfur can prevent the surface decomposition reaction and therefore avoid the loss of Sn, the presence of secondary phases (such as
ZnS and Cu2-xS) as well as a highly defective surface. Therefore the interface of CdS and CZTS from sulfurization in a combined SnS and sulfur atmosphere (Process III) was further investigated. Fig.4-3(a) shows the high resolution TEM image taken from the interface of CdS and CZTS sulfurized in a combined SnS and sulfur atmosphere.
The epitaxial growth of CdS on CZTS is clearly observed from the coherent interface, free of any lattice defects. The FFT pattern in Figure 4-3(b) again reveals cubic CdS and its epitaxial relationship with CZTS being (111) CdS || (112) CZTS and (002) CdS ||
(200) CZTS. In addition, it should be remarked that the well-defined CdS lattice fringes do not extend through the entire CdS layer, showing some dislocations and stacking faults several nanometers away from the interface (see Figure 4-3(c)). This means a tendency of degradation of epitaxial growth with deposition time, which can be explained by the transition of CdS growth mode from an atom-by-atom mechanism to a colloidal growth mechanism [93, 96]. A similar behaviour also has been observed in the case of CIGSe[93]. In CBD processing, when the deposition time is beyond about only
2 mins, the colloids can form in the bulk of the solution with the appearance of the solution color becoming greenish and then turning to yellow. The inclusion of colloids in the growing layer may lead to the increasing degradation over time. Actually, the
CBD CdS is polycrystalline in regions far away from the interface in both samples of
Figure 4-2 and Figure 4-3 due to the colloidal growth mechanism for most of the deposition time. The crystallinity of CBD-CdS can be improved by annealing the
CZTS/CdS stack, making the epitaxy of the CdS in a larger scale possible, which should be beneficial to improve carrier transportation and Jsc due to the reduced density of
73 grain boundaries. But the effects of the polycrystalline nature of CBD-CdS on device physics and performance need further investigation considering the interaction between
CdS and CZTS during the post annealing (this is not the focus of this work).
(a) (b)
(c)
Figure 4-3(a) High-resolution TEM image along CZTS [021] showing the epitaxial growth of
CBD-CdS. CZTS is prepared by Process III. (b) The FFT spectrum taken from the whole region
of (a). (c) The lattice image of the CBD-CdS showing stacking faults.
In addition, we have also observed another feature, i.e. the propagation of twin boundaries (lamellar twins) from CZTS into the CdS layer, as shown in Figure 4-4. The twins directly thread from kesterite CZTS into cubic CdS owing to their very small difference in lattice constant, making twin propagation energetically favourable.
Therefore, this twin boundary penetrating from CZTS into CdS can be also regarded as a direct consequence of CdS epitaxy on CZTS. It should be noted that the lamellar twins are not believed to create deep levels in cubic photovoltaic materials such as CIGSe because they are free of dangling bonds or wrong bonds. [100] Due to the similarity between CZTS and CIGSe, the twins may not be detrimental to the electronic properties and performance of the CZTS-based devices.
74
Figure 4-4 High-resolution TEM image showing the twin boundary (lamellar) in CZTS
propagating into CBD-CdS. The inset shows the FFT spectrum from twins in CZTS.
From the above analysis, one can see that the sulfurization in combined SnS and sulfur atmosphere yields a CZTS absorber with a well-structured surface, largely free of surface lattice defects compared with that from sulfurization in sulfur-only atmosphere with a highly defective surface, enabling the epitaxial growth of CBD-CdS. The epitaxy of CdS can also be achieved after reduction of these surface defects by post air annealing of the CZTS absorber obtained from sulfurization in sulfur-only atmosphere and subsequent etching by ammonia based solution in the initial stage of CBD. The epitaxy in both cases is observed to persist only for a few nanometers, but is sufficient to eliminate the interfacial defects which cause significant interface recombination and deterioration in the electronic quality of device. However, device performance of device in the former case is much better than the latter case due to differences in the interface chemistry which will be discussed below.
75 4.3.2 Interface chemistry investigation
The compositional analysis was performed using the micro-EDS in combination with TEM to investigate the chemical nature of the interface. Figure 4-5 shows the cross-sectional bright field TEM images and EDX line scan profiles of the CZTS/CdS interface regions. For CZTS from sulfur-only atmosphere sulfurization, the CZTS/CdS interface is very fuzzy with lot of small scale domains (dark dots) in the interfacial region (see Figure 4-5(a)) and shows a quite large transitional interface region (about 40 nm) (see Figure 4-5(b)). The presence of the signals of Cu and Zn, together with Sn in the CdS layer, is observed, suggesting an intermixing of CZTS and CdS due to the poor surface uniformity at the nanoscale. In addition, it is evident that there are two
“shoulders” of Cd signal in the CZTS side of the interface (at distance 30-40 nm and
40-45 nm, respectively), which also could be considered as the consequence of intermixing of CZTS and CdS phases. In order to further confirm the phase intermixing, phase mapping, generated by “Smart Phase Mapping” software in EDS system
(a) (b)
76 (c) (d)
(e) (f)
Figure 4-5 Cross-sectional CZTS/CdS interface TEM images and EDS line scan profiles for
Process I (a) and (b), Process II (c) and (d) and Process III (e) and (f), respectively.
according to elemental distributions (as shown in Figure 4-6) and associated spectra, was employed. A phase map in Fig. 4-7(a) taken from a typical cross-sectional
CZTS/CdS interface TEM image in Fig. 4-7(b) clearly shows the considerable phase intermixing of CZTS and CdS which implies that the small scale domains in the interfacial region in Fig. 4-7(a) could be assigned to CZTS phase.
77
Figure 4-6 EDS elemental mapping of the CZTS/CBD interfacial region. CZTS is
prepared by Process I.
This phase intermixing makes it hard to confirm the interface position and investigate the diffusion behavior between CZTS and CdS. However, it is unlikely that the surface defects would block Cd diffusion into the CZTS absorber. Actually, the previous EDS and XPS compositional profile results [101] demonstrate a substantial diffusion of Cd into the CZTS absorber. It should be noted that in the cases of 78 CdS/CdTe [102] and CIGSe/CdS [103] solar cells intermixing is well accepted and believed to enhance the cell performance due to the reduced recombination current at the interface by phase mixing. Recently, Werner et al. reported that CdS around small grains in the porous part of the DMSO solution-processed CZTSSe absorber could passivate the surface and reduce recombination [104]. In this case, the phase intermixing of CZTS and CdS is caused by the highly rough (at the nanoscale) and defective surface. Owing to the high similarity of CZTS with CIGSe and CdTe it may be helpful to achieve efficient device fabrication using absorber with poor surface quality.
After air annealing (and an ammonia solution etching in the initial stage of
CBD), the CZTS/CdS interface becomes more abrupt (Figure 4-5(c)) with a narrower transition interfacial region (only about 20 nm estimated from Figure 4-5(d)). The intermixing behavior seems less apparent from the cross-sectional morphology and EDS compositional profile due to the modified surface by air annealing and ammonia solution etching during CBD. It is observed that the Cu signal decreases much faster than those of Zn and Sn in the CZTS/CdS interface region, which implies that Cu from
CZTS may diffuse into the CBD-CdS layer or be leached out preferentially by ammonia based solution [105] during the CBD process, forming a Cu depletion on the CZTS surface. But there is no obvious evidence of Cd diffusion from CBD CdS layers into
CZTS from the EDS line scan measurement. The weak diffusion of Cd into CZTS can be related to the oxygen incorporation and oxide passivation layer formed in the CZTS surface region that cannot be removed completely by the following ammonia based solution etching [97] during the initial stage of CBD process.
79 For CZTS prepared by combined SnS and sulfur atmosphere sulfurization,
CZTS/CdS interface in Figure 4-5(e) is also more abrupt than that in Figure 4-5(a), and the interfacial region is very “clean” without the “dark” domains due to excellent nanoscale microstructure of the CZTS surface. The width of the interfacial transition region is about 20nm estimated from Figure 4-5(f). It is also observed that the Cu signal drops faster than the Zn and Sn signals in the interfacial region. For Cd signal, the signal decreases much more slowly in the CZTS side than the CdS side of the interface, which is different from the feature of the “shoulder” present in the case of sulfur-only sulfurization. And even in the CZTS matrix close to the interfacial transition region, a
CdS signal is still clearly observable. From the abrupt and “clean” interface, this behavior should not be caused by phases intermixing of CZTS and CdS. Moreover, according to the phase mapping in Figure 4-7(c) and corresponding cross-sectional
CZTS/CdS interface TEM image in Figure 4-7(d), one can find that the intermixing of
CZTS phase and CdS phase in this case is much less evident than the case in Figure 4-
7(a) and (b). Therefore, this behavior can be reasonably assigned to the Cd diffusion into CZTS during the CBD process, and maybe interpreted as the occupation of Cd for vacant Cu sites in the Cu-poor and Zn-rich CZTS matrix (which favors the formation of
Cu vacancies [106]) according to the first-principles calculations [107] and Sardashti’s report [95]. In CIGSe solar cells, a similar behavior at the CIGSe/CdS interface has been reported by many groups [92, 108], and is believed to be the key factor to the success of the CBD-CdS process [109].
80 (a) (b) ZnO ITO CdS
ZnS CZTS
(c) (d) ITO ZnO
CdS
CZTS
Figure 4-7 Cross-sectional CZTS/CdS interface TEM images and the corresponding phase maps
for Process I ((a) and (b), respectively) and III ((c) and (d), respectively).
Similar to the role of OVC, Cd on Cu sites would act as substitutional donors, and therefore can result in n-type absorber surface, facilitating a more effective buried homojunction [110]. Besides, Cd diffusion has been reported to occur much more easily in OVC than in CIGSe due to the extremely Cu deficient nature of the surface making it easy to substitute Cd for Cu vacancies [91]. In this CZTS sample with Cu-poor and Zn- rich average composition, according to the XPS measurement in Figure 4-8(a), a more
Cu-deficient and Zn-sufficient composition in the surface region with width of approximately 20 nm can be observed. This width coincides well with the Cd diffusion depth suggested by the EDS profile analysis described above. It is thus believed that the diffusion depth of Cd atoms is related to the depletion of Cu level in the front surface
81 region of CZTS. Here it is important to remark that sulfurization in sulfur-only atmosphere can also yield CZTS with more Cu-deficient and Zn-sufficient surface than the bulk as shown in Figure 4-8(b) and these characteristics are maintained after air annealing as shown in Figure4-8(c). These results indicate that the Cd diffusion behavior strongly depends on the nature of the absorber surface, which may affect device performance significantly.
Figure 4-8 XPS depth profiles of CZTS absorbers from Process I (a), II (b) and III (c), revealing more Cu-deficient and Zn-sufficient composition in the surface region than the bulk (Cu/Sn = 1.80 and Zn/Sn = 1.15). For example, the surface composition tested by XPS after 10 seconds etching are Cu/Sn = 0.76 and Zn/Sn = 1.28 for Process I,
Cu/Sn = 0.60 and Zn/Sn = 1.33 for Process II, and Cu/Sn = 0.54 and Zn/Sn= 1.53 for
Process III, respectively. The etching is performed using 3 keV Ar ion beam with reference etching rate of 0.36 nm/s for Ta2O5.
It was also observed the presence of few precipitates in-between the CZTS grains in the absorber from Figure 4-7. Combined with the EDS elemental mapping
Figure4-6 which shows a highly Cu- and Sn-poor and Zn-rich composition, these precipitates correspond to a ZnS secondary phase. Actually, from the low magnification
TEM and EDS analysis, ZnS secondary phase in-between the CZTS grains and at the back contact region is evident for CZTS absorbers from both sulfur-only sulfurization 82 (Figure 4-9) and combined SnS & sulfur sulfurization (Figure 4-10). The segregated
ZnS phase may simply act as a “dark“ space, reducing the effective volume[60] and even help for grain boundary passivation, due to similar crystal structures and lattice parameters with CZTS [111], which therefore will not do harm to device. But if on the front of absorber, it would block the current transport [112] and has been reported to be detrimental to device performance [113]. Unfortunately, the migration of ZnS to the front of the absorber is observed from the cross-sectional CZTS/CdS interface TEM image (Figure 4-11(a)) and the EDS line scan profile (Figure 4-11(b)) when employing air annealing on absorbers from process III in order to further reduce/eliminate the possible interfacial defects. This behaviour indicates a redistribution of Zn, different from the case of CIGSe: a redistribution of Cu after air annealing[114], meaning the difference in elemental migration during air annealing between CZTS and CIGSe.
Besides, this behaviour cannot be observed by comparison of Process I (Figure 4-5(a) and (b)) and II (Figure 4-5(c) and (d)), suggesting that the effectiveness of air-annealing depends on the nature of CZTS absorber.
(a) (b) ITO
CdS+ZnO
CZTS
MoS2
Mo
83 (c)
Figure 4-9 Cross-sectional CZTS device TEM images (a), the corresponding phase map
(b) and individual phase distribution (c). CZTS is prepared by Process I.
(a)
84 (b)
(c)
Figure 4-10 Cross-sectional CZTS device TEM images (a), ZnS and ZnO phases distribution (b) and the corresponding phase map (c). CZTS is prepared by Process III.
(a)
Figure 4-11 Cross-sectional CZTS/CdS interface TEM images (a) and EDS line scan profile (b) for sample from Process III and air annealing.
4.3.3 Device characteristics
Figure 4-12 shows the current density-voltage (J-V) curves of the typical CZTS solar cells from above three processes. And the device electrical parameters are collected and presented in Table 4-1. It can be observed that the Voc and FF are quite
85 low with the highest ideality factor A and reverse saturation current (J0) for the device processed from sulfur-only atmosphere sulfurization, which should be caused by severe interface recombination based on the above TEM interfacial microstructure analysis.
The efficiency in this case is only 6.16%. After passivation by air annealing, Voc is significantly boosted by almost 100 mV and FF is also improved from 60.46% to ~
65.91%, accompanied by a reduction in A and J0. This can be explained by the decreased surface defects by passivation and the epitaxial growth of CBD-CdS minimizing the interfacial defects. But Jsc shows slight decrease after air annealing which can be attributed to oxygen incorporation and the residual oxide passivation materials hindering carrier transportation and raising series resistance of the device. The efficiency shows a substantial increase to 7.13%, in agreement with the recent report by
Jin et al [115]. It should be mentioned that the duration of the air annealing in this experiment is only 1-2 min, which is much shorter than the case of CIGSe (20-30 min), because longer duration (especially > 10 min) is found to lead to degradation in Voc and
FF. The reasons of this difference maybe from the elemental immigration during air annealing as discussed above but still need further investigation. For absorber prepared by combined sulfur and SnS atmosphere sulfurization with high surface quality enabling epitaxial growth of CBD-CdS directly and avoiding interfacial defects, the device shows even higher Voc and FF, reaching 666.7 mV and 67.51%, respectively. Besides this device process the significant decreased A and J0, indicating that the recombination at depletion region (p-n interface) had been greatly reduced due to our forehead- observed epitaxy interface and the this interface recombination account for the largest proportion of all the possible recombination which should be the efficiency limiting factor at current stage Moreover the Jsc of 19.47 mA/cm2 is also the largest value
86 because this route is free from the adverse effects from surface passivation by air annealing. So the device yields the highest efficiency of 8.76%.
(a) (b)
(c) (d)
Figure 4-12 (a) Current density - voltage (J-V), (b) External quantum efficiency (EQE, 0 bias),
(c) Time-resolved photoluminescence and (d) Carrier concentration depth profiles for devices
fabricated from Process I, II and III. These data are derived from Capacitance- Voltage C-V
measurement.
From the external quantum efficiency (EQE) curves in Fig. 6(b), it is found that the
EQE value of device from Process II is a little bit lower than that from Process I in the whole light spectrum independent of wavelength, meaning that the slight lower Jsc for device from Process II than device from Process I is mainly due to the higher series resistance. However, for Process III, it is obviously observed that the device shows 87 better long wavelength response, which reveals the source of the Jsc improvement. The enhancement in the long wavelength region is most likely caused by longer collection length from a longer minority carrier diffusion length (higher mobility-lifetime product) and/or deeper penetration of the depletion range into the absorber.
The time-resolved photoluminescence (TRPL) measurements were performed on finished devices as shown in Figure 4-12(c) to check the minority lifetime. Before
TRPL measurements, photoluminescence (PL) spectra were also recorded as shown in
Figure 4-13 and demonstrate the weakest PL intensity for device from Process I and strongest PL intensity for device from Process III.
Figure 4-13 Room temperature photoluminescence (PL) spectra of the finished devices
from process I, II and III.
The lifetime is estimated from the TRPL traces to be 5.8 ns, 9.8 ns, and 12.4 ns for devices from process I, II and III, respectively, according to a previously reported method [116]. This indicates that air annealing can effectively passivate the surface defects and improve the lifetime, obviously reducing interface recombination, which is responsible for the higher Voc and FF of the device from Process II than that from
88 Process I. Another remarkable observation is that the device from Process III shows a more significant increase in lifetime compared with a device from Process II, explaining its further higher Voc and FF, although in both cases CdS epitaxially grows along CZTS.
This characteristic implies that probably there are other mechanisms that further reduce recombination. It is believed that the diffusion of Cd into the CZTS surface region may contribute to the further improvement in lifetime. Cd on Cu sites would act as
+ substitutional donors CdCu , which makes the inversion of the surface forming a buried homojunction possible. It would reduce the recombination at the interface and yield a long lifetime. This may reasonably explain the higher lifetime for device from Process
III with strong Cd diffusion than device from Process II without evident CdS diffusion, although it is also possible for Cd occupying Zn sites [95, 99]. Cd diffusion is expected to be facilitated by occupying Cu vacancies that may be more energetically favored at and near the surface [97] other than Zn sites for CZTS with a Cu-poorer and Zn-richer surface composition relative to the bulk, because it is quite possible that Cu depleted nature present natural vacant sites for the occupation by Cd. Besides, according to the theoretical calculations, [117] Cu depleted surface termination is energetically favorable and does not introduce any deep gap state which could be detrimental to solar cell performance. Therefore, a Cu-depleted and Zn-rich surface in these samples seems to be
+ beneficial to CdCu donor formation and device performance. It should be pointed out that the lifetime and efficiency is still lower than well-behaved CIGSe solar cells. In
CIGSe solar cells, Cd diffuses into the CIGSe by occupying Cu vacancies, which forms
+ donor states (CdCu ) enhancing the n-doping level of the OVC layer [110] other than solely creating a homojunction like in CZTS solar cells. The highly n-doped surface from OVC enhanced by Cd substitution for Cu could push the depletion region deeper
89 into absorber, enabling a more efficient buried homojunction. Jiang et al. [118] have identified that the p/n boundary is located 30–80 nm away from the CIGSe/CdS interface and the built-in electric field terminates at the CIGSe/CdS interface. According to first-principle calculations [119], the OVC and inversion region will be more difficult to be formed in Cu-poor and Zn-rich CZTS. Therefore, the carrier concentration in the inversion region should not be high enough to provide a strong built-in electric field like in CIGSe device. This suggests that a feasible way of further improving lifetime is to enhance the carrier concentration inversion region, such as indium (In) incorporation
+ forming an InCu donor state. Actually, the CZTS [101] and CZTSSe [120] devices with indium diffusion from indium-containing buffer into the absorber surface region have demonstrated higher Voc.
(a) (b) (c)
Figure 4-14 SEM images of the surface morphology of CZTS absorbers from Process I
(a), II (b) and III (c).
From the capacitance-voltage (C-V) profiles in Figure 4-13(d), the depletion width of devices from Process I (119 nm) and III (125 nm) are very close, larger than that from Process II (107 nm), while the doping density levels of three devices look similar (4.0 - 5.5 x 1016). Together with the above EDS profile analysis, this result can be well linked to the Cd diffusion behaviour. For device showing considerable CdS diffusion into absorber, the depletion width is wider than that showing the absent or
90 + very weak Cd diffusion. Cd diffusion into the absorber, forming donor (CdCu ) sites, could increase the carrier concentration of the whole n-type region, and thereby leads to longer depletion width. Considering that these three samples have a similar doping density, average composition, grain size (lateral as shown in Figure 4-14 and longitudinal as shown in Figure 4-15) and crystallinity (from above TEM), it is reasonable to assume that the electron mobility values in three samples are very close.
Therefore, the device from Process III has the largest collection length (depletion width
+ minority diffusion length) due to having the longest lifetime, explaining the best long wavelength EQE response. The device from Process II has a longer diffusion length but suffers from a shorter depletion width compared with the device from Process I, making little difference in collection length between them. The EQE for the device from Process
II is even slightly lower in the whole light spectrum than that from Process I, which should be related to the highly resistive oxides formed during air annealing and cannot be completely removed during the following CBD process.
A) B)
Figure 4-15 Bright field TEM images of the cross-sectional morphology of CZTS
absorbers from Process I (a) and III (c).
91 Table 4-1. Device characteristics of the CZTS solar cells with various processes. In order to
make data comparable with other groups such as IBM[121], the series resistance under light
(RS,L), ideality factor A and reverse saturation current (J0) are determined using the Sites'
method [122].
Rs,L Voc Jsc FF Efficiency J0 Lifetime Wd Process (ȍ A (mV) (mA/cm2) (%) (%) (mA/cm2) (ns) (nm) cm2)
I 553.6 18.40 60.46 6.16 1.05 2.16 8.3 x 10-4 5.8 119
II 649.1 17.58 65.91 7.52 1.26 2.01 1.6 x 10-4 9.8 107
III 666.7 19.47 67.51 8.76 1.03 1.67 3.1 x 10-6 12.4 125
Since air annealing forms oxides that suppresses Cd diffusion and leads to short depletion width, HCl etching removes the formed oxides and should be beneficial to depletion width widening. From the C-V profile in Figure 4-16(a), the depletion width reaches as high as 156 nm after HCl etching. This can be responsible for the enhancement in Jsc (19.30 mA/cm2) as shown in Figure 4-16(b) from the improvement of long wavelength collection as shown for the EQE in Figure 4-16(c). But HCl etching also removes the passivation effects from air annealing (and ZnS too), leading to the large drop of Voc 649.1 mV to 550.6 mV and FF from 65.91% to 53.04%. Therefore, the efficiency actually decreases to only 5.64%. Besides, as discussed previously in Fig.
4-11, in order to further reduce/eliminate possible interfacial defects, CZTS absorbers from Process III were also treated by air annealing which however leads to the ZnS segregation at the surface region of CZTS. In this case, the produced Jsc of device is only 17.19 mA/cm2 as shown in Figure 4-17 (a) due to the loss in the whole spectrum from EQE in Figure 4-17(b), and FF drops from 67.51% to only 54.67% owning to the blocked current transportation by the segregated ZnS secondary phase on the surface
92 region of the CZTS. The Voc also shows some decrease to 634.9 mV and the efficiency is only 5.97%. Although HCl etching could remove ZnS secondary phase segregated at the surface region together with oxides from air annealing, which widens the depletion width from 110 nm to 167 nm (longest one in this work) estimated from the C-V profiles in Figure 4-17 (c) and boosts the Jsc to the value as high as 20.75 mA/cm2 by significant improvement in long wavelength collection, the Voc shows a remarkable decline to the value as low as 527.3 mV and FF is also very low (only 52.68%). This suggests HCl etching may damage CZTS surface structure and introduce interfacial defects leading to severe recombination, and so the efficiency is still quite low (only
5.76%). Therefore a surface recovery, repairing or reconstruction process to improve the
CZTS/CdS interface is needed. For example, it has been reported recently that treatment using aqueous Na2S solution can reconstruct the kesterite Cu2ZnSnSe4 surface after etching ZnSe by a oxidation route[123]. Similar processes might be suitable for kesterite Cu2ZnSnS4 and will be studied in the future. Anyway, from all of the above discussion, one can easily find that CZTS/CdS interface properties and device performance are extremely sensitive to the preparation and post-treatment processes and should be controlled very carefully based on interfacial microstructural and chemical investigation.
93 (a) (b)
(c)
Figure 4-16 (a) Capacitance-Voltage C-V, (b) Current density-voltage (J-V) and (c)
external quantum efficiency (EQE) curves for device fabricated from Process II
followed by HCl etching. The J-V and EQE data of device fabricated from Process II
(without HCl etching) are also provided for comparison.
(a) (b)
(c)
94
Figure 4-17 (a) Current density-voltage (J-V), (b) External quantum efficiency (EQE)
and (c) Capacitance-Voltage C-Vcurves for device fabricated from Process III followed
air annealing and HCl etching. The J-V and EQE data of device fabricated from Process
II (without air annealing and HCl etching) are also provided for comparison.
4.4 conclusions
Epitaxy of CBD-CdS was observed on CZTS without surface defects by sulfurisation in combined SnS & S atmosphere, or with defects but passivated by air annealing. This is a key feature for high minority lifetime due to a low interfacial defect density and thereby low recombination rate. TEM analysis also revealed that CBD-CdS grows with the epitaxial relationship of (111) cubic CdS || (112) CZTS. Cd diffusion into CZTS surface region was observed, but this behaviour is absent, or much weaker, for the case of air annealed CZTS. Cd diffusion behaviour is believed to be another significant feature for realizing high lifetime by forming CdCu+ donor sites and buried homojunction. For CZTS from sulfurization in a sulfur only atmosphere with highly defective surface, devices show very low Voc and FF. Air annealing can effectively passivate these defects, facilitating epitaxial growth of CdS and improving minority carrier lifetime, and therefore boost Voc and FF significantly. But it would block the diffusion of Cd and lead to a narrow depletion width, and therefore is detrimental to Jsc.
95 For CZTS from sulfurization in combined SnS and sulfur atmosphere with a well- structured surface, both epitaxial growth of CBD-CdS and Cd diffusion are visible, with the corresponding device showing the highest minority lifetime and efficiency of 8.76%.
This work provides the nanoscale microstructural and chemical investigation of CBD-
CdS and CZTS with different processes and reveals the importance of controlling the
CZTS/CdS interface, suggesting ways (e.g., epitaxial growth of CBD-CdS and Cd diffusion) to fabricate high efficiency kesterite photovoltaic devices.
96
Chapter 5 CZTS/buffer interface engineering
5.1 Band alignments of different buffer layers (CdS, Zn(O,S) and In2S3) with Cu2ZnSnS4
5.1.1 Introduction
One key issue that is believed to limit the performance of CZTS solar cells is the large open circuit voltage deficit (around 0.6 eV), likely resulting from a non-optimal conduction band alignment between the p-type CZTS absorber and the traditional n- type CdS buffer layer. As the kesterite material CZTS was evolved from chalcopyrite
CIGS, CZTS solar cells device has inherited the configuration of CIGS solar cells, where CdS is adopted as the “standard” buffer layer. The conduction band alignment of
CdS/CIGS is “spike-like” with a desired band offset of 0.2-0.3 eV, which facilitates the high efficiency of a CIGS solar cell, especially for high Voc[25]. However, the type of
CBO is still unresolved when it comes to the case of CdS/CZTS based cells.
Theoretically, the CBO of CdS/CZTS has been calculated to be negative (i.e. cliff-like), while the reported experimental values vary widely. For example, Richard et al. report a spike-like CBO of +0.41eV [27] whereas others studies have measured cliff-like CBO value of -0.06eV[28], -0.33eV[29], -0.34eV[30]. As the band alignment has been found to be very sensitive to the interface of CdS/CZTS, the differences in the reported experimental CBO of CdS/CZTS could be due to the variation in the surface of the
CZTS absorber and/or any treatment prior to the CdS buffer deposition. Further investigation is required to investigate and resolve this issue. On the other hand, if the
CBO of CdS/CZTS is indeed cliff-like, it is imperative to identify alternative buffer
97 materials which yield an optimal band alignment with CZTS (small spike-like CBO of
0.1-0.2eV). However, very few experimental attempts have been made to measure band alignment at the interface of Cd-free buffer layers and CZTS.
Here, the band alignment of three different buffer layer materials, CdS, Zn(O,S) and In2S3 , with CZTS was examined. The valence band maximum (VBM) and VBO are determined by XPS. The CBOs are estimated by two different methods. Firstly, an indirect approach involving the use of the known band gaps (Eg) of the materials if together with the VBOs probed by XPS, and secondly by a direct measurement of the conduction band minimum (CBM) by NEXAFS. The CBO between CdS and CZTS is confirmed to be cliff-like, whereas those at Zn(O,S)/CZTS interface and In2S3/CZTS are spike-like. Finally, the photovoltaic device performance of CZTS with these three different buffer materials are presented and discussed.
5.1.2 Experimental
CZTS absorbers on Molybdenum were synthesized by method mentioned in previous section. A chemical bath deposition (CBD) method was utilized to prepare the buffer layers (CdS, Zn(O,S), In2S3)[78]. For all of these buffers, minor hydroxides may coexist[124]. To fabricate a complete device, i-ZnO (intrinsic ZnO) and aluminium- doped ZnO (AZO) films were deposited by RF sputtering. Silver paste (Sigma Aldrich) was used for the top contacts.
The XPS measurements were carried out using an ESCALAB250Xi (Thermo
Scientific, UK) under ultra-high vacuum (better than 2×10-9 mbar). The x-ray source was mono-chromated Al KĮ (hQ = 1486.68 eV) with binding energy scale calibrated using the Au4f7/2 core level (EB = 84.00 eV) of a gold standard in electrical contact with
98 the sample. Depth profiling data was achieved by mildly sputtering the buffer/CZTS surface with 1 keV Ar+ ions over an etching area of 2.5mm×2.5mm[30]. Near Edge X- ray Absorption Fine Structure (NEXAFS) spectra of the Sulfur L2,3 edge were measured at the Soft X-ray Spectroscopy beamline at the Australian Synchrotron. The NEXAFS measurements were obtained by measuring the total fluorescence yield (TFY) from the samples for pure CdS, Zn(O,S) and CZTS. The photon energies at the beamline were calibrated using an in-situ photoemission measurement of a Au4f7/2 reference sample, with the total energy resolution of the beamline estimated to be around 50 meV[125].
The J-V curves of the fabricated CZTS solar cells with different buffer layers were measured using a solar simulator calibrated with a standard Si reference cell. A QEX10 spectral response system (PV Measurements, Inc) was used to measure the external quantum ef¿ciency (EQE).
5.1.3 Results and discussions
The elemental depth profiles of the different buffer materials grown on CZTS are illustrated in Figure 5-1. Within CdS and In2S3, elemental oxygen was detected, indicating both of them are not pure sulfide but with minor oxide and/or hydroxide, which is common for buffer layers synthesized by the CBD method[124]. For Zn(O,S), the O/(S+O) ratio is found to be 0.4-0.5. The band gap of Zn(O,S) with a similar composition (S/Zn) is around 3.4eV[126]. The metal ions in all of three different buffer layers, especially at interface, are seen to gradually diffuse in the CZTS layer.
Meanwhile, the metal cations of CZTS, to a lesser extent, diffuse to the interface and even into the buffer layer. This kind of inter-diffusion help blur the interface and facilitate the conjoining of the buffer and absorber layers.
99
Figure 5-1 The XPS composition profile and Cd3d5 peak position (Binding energy), Sn3d5
peak position, Cu2p3 peak position and Zn2p3 peak position as a function of sputtering time.
According composition profile, the blue, red and purple area can be regarded as bulk CdS,
interface and bulk CZTS area respectively.
100 Figure 5-2 The XPS composition profile and Zn2p3 peak position (Binding energy), Sn3d5
peak position and Cu2p3 peak position as a function of sputtering time. According composition
profile, the green, red and blue area can be regarded as bulk Zn(O,S), interface and bulk CZTS
area respectively.
101 Figure 5-3 The XPS composition profile and In3d5 peak position (Binding energy), Sn3d5 peak
position, Cu2p3 peak position and Zn2p3 peak position as a function of sputtering time.
According composition profile, the red, blue and purple area can be regarded as bulk In2S3,
interface and bulk CZTS area respectively.
According to Refs[30, 127], the VBO can be estimated via the formula:
b a VBO EVB EVB Vbb (5.1)
a b Where EVB and EVB represent the energy positions of the valence band edges of
bulk CZTS and bulk buffer, respectively and Vbb is the band bending[30]. The Vbb value can be readily obtained by the following formula:
a a b b Vbb (ECL ECL (i)) (ECL (i) ECL ) (5.2)
102 a b Here, ECL and ECL are the core level energies of two selected elements in the bulk
a b region of CZTS (a) and buffer (b), whilst ECL (i) and ECL (i) represents the corresponding core level energies measured at the interface. If assuming the convention that binding energy below the Fermi level is negative, the VBM is negative and consequently, the value of the VBO should be negative if the valence band edge of the buffer layer is
lower than that of CZTS. (See Ref[30] for further details of the calculation of Vbb ).
Figure 5-4: Normalized valence band (VB) data of different buffer/CZTS
heterojunctions measured by XPS. Binding energies are measured with respect to the
Fermi energy (EF).
Valence band data of CZTS and the different buffer materials measured by XPS are normalized and displayed in Figure 5-4. The valence band maximum (VBM) positions of the buffer and CZTS are estimated by a linear extrapolation approach[128] ,
a b yielding EVB and EVB . The VBM positions (relative to EF) for CdS, Zn(O,S), In2S3 and
CZTS are found to be (-1.72±0.04) eV, (-1.47±0.04) eV, (-0.85±0.05) eV and (-
0.46±0.04) eV, respectively. The VBM values for CdS and CZTS are in good agreement with previous reports[30]. 103
Figure 5-5: Normalized Sulfur L2,3 NEXAFS data of pure CdS, Zn(O,S) and CZTS. The
linear extrapolations required to determine the absorption edge onset of the different
materials and the corresponding energy schema are displayed in the inset.
By applying Equation (2) to the XPS measurements of the core levels, the
average band bending Vbb for CdS-CZTS, Zn(O,S)-CZTS and In2S3-CZTS is estimated to be (0.12±0.1) eV, (0.03±0.1) eV and (0.20±0.1) eV, respectively. The value for the
VBO is then directly obtained using equation (5.1), and the CBO can be readily calculated using the VBO values via Equation (5.3):
b a CBO Eg Eg VBO (5.3) a b Where Eg and Eg correspond to the band gaps of pure CZTS and buffer materials.
All of the measured CBO, VBO and Vbb values are summarized in Table 5-1.
In order to further confirm the CBO derived from the XPS measurements, the
CBM of CdS, Zn(O,S) and CZTS using the NEXAFS data was directly measured, and the results are shown in Figure 5-5. For each materials, the CBM value can be estimated from the absorption onset energy of the sulfur L2,3 edge using a linear extrapolation method[129]. From the results, it is found that the CBM of CdS is slightly lower (- 104 0.3eV) than that of CZTS whereas the CBM of Zn(O,S) is higher (0.84eV) than that of
CZTS.
With the direct CBM values now available, the CBO can also be calculated using the following formula:
b a CBO ECB ECB Vbb (5.4) a b Where ECB and ECB are the energy positions of the conduction band edges of the
CZTS and buffer, respectively. According to Equation (5.4), we can utilize the CBM
values (obtained from the NEXAFS data) and Vbb values (obtained from XPS data) to calculate the CBO. A summary of all the relevant band energies is listed in Table 5-1.
Table 5-1 Band alignment data of CZTS with different buffers (all unit are measured in
eV, ¨VBM=VBM(Buffer)-VBM(CZTS) )
Buffer ¨VBM(XPS) ¨CBM(NEX Vbb(XPS) VBO(XPS) Eg CBO(XPS) CBO(NEXAF
AFS) S)
CdS -1.26±0.04 -0.30±0.10 0.12±0.10 -1.14±0.10 2.4 -0.24±0.10 -0.18±0.10
Zn(O,S) -1.01±0.04 0.84±0.10 0.03±0.10 -0.98±0.10 3.4 0.92±0.10 0.87±0.10
In2S3 -0.39±0.05 NA 0.20±0.10 -0.19±0.10 2.1 0.41±0.10 NA
A schematic of the estimated band alignment between the buffers and CZTS absorber is plotted in Figure 5-6. Here it is notable that the CBO of CdS/CZTS is cliff- like, consistent with previous reports [30, 130], whereas CBO of Zn(O,S)/CZTS and
In2S3/CZTS are spike-like.
105
Figure 5-6: Schema of the band alignment of different buffers and CZTS. Values of the
VBO, CBO and Eg are indicated. The numbers in red represent the values of the CBO
determined from the NEXAFS data.
For the sake of further understanding how the band alignment can influence the device performance, J-V and EQE curves of the completed CZTS photovoltaic devices with various buffer materials were measured and shown in Figure 5-7. The detailed device parameters are also summarized in Table 5-2. It is interesting that the device with an In2S3 buffer demonstrated a higher open circuit voltage Voc than that using CdS.
The different heterojunction (CBO) types of In2S3/CZTS and CdS/CZTS could cause the Voc difference. According to p-n junction band alignment simulation by Minemoto
106 et al18 the reasonably small spike-like CBO (0-0.4eV) inhibits interface recombination, especially recombination between electrons in the conduction band of the buffer and holes in the valence band of CZTS. In contrast, the cliff-like CBO facilitates this kind of recombination, especially in the presence of interface defects and/or a high interface recombination velocity, resulting in a significantly decreased Voc[26].
Table 5-2 CZTS device performances with different buffers.
Buffer Voc(mV) Jsc(mA/cm2) FF Eff(%)
CdS 470 8.93 35.7 1.50
Zn(O,S) 76 0 0 0
In2S3 590 2.77 22.4 0.37
In terms of the Jsc and FF, the CdS/CZTS device with a cliff-like CBO is significantly superior to the In2S3/CZTS device with spike-like CBO. According to simulations by Minemoto et al[26], in the case of CBO values higher than 0.4 eV, Jsc and FF decrease more abruptly and dramatically compared to those with CBO values in the range of -0.7~0.4 eV. This is largely due to the formation of a high barrier against photo-generated electrons. Compared with the desired optimal CBO in the range of 0.0-
0.3 eV[124], the spike-like CBO value of (0.41±0.10 eV) obtained for In2S3/CZTS would result in such a barrier.
107
Figure 5-7: (a) J-V curves and (b) EQE curves for CZTS devices with different
buffers.
The EQE data helps to throw further light upon the CBO issue. According to EQE of In2S3 in Figure 4(b), the majority current contribution to Jsc comes from the blue part of the spectrum while the EQE in the yellow and red regions is significantly lower. It seems that photon-generated holes in In2S3 can readily contribute to the current.
However, photon-generated electrons in CZTS are greatly suppressed. This can be well explained by the estimated band alignment between In2S3 and CZTS, where holes generated in In2S3 can readily flow to the CZTS due to the negative VBO, whereas the majority of electrons generated in CZTS are blocked owing to a high barrier. As the
CBO becomes larger in the case of using a Zn(O,S) buffer, the CBO of around 0.9 eV is
108 so high that it blocks all of the photo-generated electrons from CZTS. This, together with less absorption of light due to the high band gap of Zn(O,S), leads to no current and thereby no measurable efficiency for the device with a Zn(O,S) buffer.
5.2 Boosting efficiency of CZTS solar cells using the In/Cd-based hybrid buffers
5.2.1 Motivation
The main reason responsible for the high Voc deficit in CZTS solar cells is believed to be the charge pair recombination in both the CZTS bulk and at the
CZTS/CdS interface.
As discussed in the previous section, it has been found that the CBO of
CZTS/CdS is undesirable cliff-like whereas that of CZTS/In2S3 is favourably spike-like, suggesting that an In2S3 buffer is a promising solution to deal with the interface problem.
Meanwhile the CdS layer is believed to be essential for the high performance in CIGS and/or kesterite solar cells, because the diffusion of Cd into the CIGS or CZTS absorber during the chemical bath deposition (CBD) processing of CdS, yielding anti-site CdCu
(donor defects) which forms a type conversion at the p-n junction interface, or an in-situ homojunction[109, 131]. Therefore, the hybrid buffer obtained by combining In2S3 and a CdS buffer may also be a promising choice for CZTS solar cells because on the one hand the conduction band alignment can be favourable at the CZTS/In2S3 interface and on the other hand, Cd can diffuse through the thin In2S3 layer into the CZTS.
Recently Solar Frontier and IBM have boosted the Voc of kesterite devices, with the highest efficiency of both CZTS (9.2% for submodule) [132] and CZTSSe (12.7%
2 for 0.45cm cell) [72] solar cells achieved by applying the CdS/In2S3 hybrid buffer, or 109 double emitter with a stacking sequence of CZTS(Se)/CdS/In2S3. As for CZTSSe, researchers at IBM believe the improvement results from increased carrier concentration
(C.C.) of CZTSSe by In doping[72]. In terms of CZTS, the researchers at Solar Frontier claim that the improvement results from some element inter-diffusion and/or improved intrinsic ZnO buffer interface[132]. In order to obtain further insights on this issue and test our hypothesis, we fabricated CZTS devices with CdS, In2S3 and hybrid buffers with different buffer stacking sequences (see Figure 5-8). Our results show that the proposed buffer (CZTS/In2S3/CdS) can effectively boost Voc and efficiency of pure sulfide CZTS solar cells.
5.2.2 Experimental
A typical CZTS solar cell configuration was used in this study, i.e. Mo (~1 ȝm)
/CZTS (~900 nm)/buffer In2S3 (40~80 nm) and/or CdS (~50 nm) /IZO (~60 nm) /ITO
(~200 nm) /Ag. A magnetron sputtering system (AJA International, Inc., model ATC-
2200) was used to co-sputter Cu/ZnS/SnS precursors on Mo-coated soda lime glass.
These precursors were then subject to sulfurization using a Rapid Thermal Processor
(AS-One 100) in a sulfur containing atmosphere. Buffer layers of CdS and In2S3 were prepared by the CBD method. Specifically, cadmium sulfate (3CdSO4Â8H2O), excess thiourea (H2NCSNH2) and ammonium hydroxide were mixed with a base environment in the glass reactor for CdS, with details shown elsewhere [16]. In terms of In2S3 deposition, the precursor solution consists of thioacetamide (CH3CSNH2), indium chloride (InCl3), and ethylic acid in an acid environment. During the CBD In2S3 process,
3+ H2S bubbles were generated and reacted with In forming In2S3, which compete with the In3+ hydrolysis process[17]. Therefore, minor amount of hydroxides may coexist in the film. The hybrid buffers were then subject to a RTP annealing at 200~300 °C. The
110 intrinsic ZnO (i-ZnO) and ITO films were deposited by RF sputtering. Ag grids were used as the top contact and the total area of the final cells were 0.3~0.4 cm2, defined by mechanical scribing.
The scanning electron microscope (SEM) images were obtained with a FEI
Nova NanoSEM 230 FESEM under 3 kV accelerating voltage. Cross-sectional
Transmission electron microscopy (TEM) specimens were prepared by a focused ion beam system (the xT Nova NanoLab 200). Note that a ~100 nm Au layer and a ~1 ȝm
Pt layer were deposited consecutively on top of specimens before cutting process for the sake of preventing the ion damage. Then the specimens were ex-situ lifted out by a glass needle tip and attached to an Au grid (200 mesh). Microstructure and elemental distribution of the sample were analyzed using a FEI Tecnai G2 20 TEM (operated at
200 kV) equipped with an energy dispersive spectroscopy (EDS) detector. The
ESCALAB250Xi (Thermo Scientific, UK) was utilized to conduct X-ray photoelectron spectroscopy (XPS) measurements in which the x-ray source was monochromated Al
K-alpha (1486.68 eV). Depth profiling data were achieved by mildly sputtering the surfaces of CZTS devices using different buffer with 1 keV Ar+ ions over an etching area of 2.5 x 2.5 mm.
The J-V curves of CZTS solar cells with different buffers were measured using a solar simulator (Darkstar) calibrated with a standard Si reference cell. External quantum ef¿ciency (EQE) data were collected by a QEX10 spectral response system (PV measurements, Inc.) calibrated by the National Institute of Standards and Technology
(NIST)-certified reference Si and Ge photodiodes. This system uses monochromatic light chopped at 120 Hz and equipped with a DC white bias light without any light filter.
111 The capacitance spectroscopy was carried out using an impedance analyzer at a frequency of 100 kHz with a DC bias voltage sweeping from -1.5 to 0.5 V.
5.2.2 Results and discussions
Four buffer configurations were designed, i.e. CZTS/CdS, CZTS/In2S3,
CZTS/CdS/In2S3 and CZTS/In2S3/CdS. Their corresponding schema is displayed in
Figure 5-8.
Figure 5-8. Schema of CZTS with different buffers studied in this work.
The devices with hybrid buffers (CdS/In2S3 and In2S3/CdS) were examined by
TEM, with corresponding bright field TEM images shown in Figures 5-9a & d, High-
Angle Annular Dark-Field (HAADF) images in Figures 5-9b&e and EDS line scans corresponding to the red arrow in the HAADF image in Figures 5-9c&f. EDS line scans,
Figures 5-10b & 5-10e and EDS mapping (see supporting information) were used to identify the chemical composition and distribution of different layers. As can be seen in
Figures 5-9a&d, the grain size of CZTS is several hundred nanometers. A cross- sectional SEM image of the device was also obtained to further clarify the grain size of
CZTS absorber, see Figure 5-11. The In2S3 layer consists of nanoparticles and has a porous texture, whereas the CdS layer is much darker and denser as a continuous film.
112 According to the HAADF images (Figures 5-9b&e), several layers of different contrast covered the CZTS, reflecting phases with different average atomic weight. The HAADF image contrast depends on the average atomic weight of materials if the TEM specimen has uniform thickness, that is, material with a higher average atomic number appear brighter in the HAADF image, which is opposite to that in the bright field image[133].
The brightest parts on the top and bottom are Au and Mo, respectively. It is notable that
In2S3 layer displays a darker contrast compared to CdS layer. This is due to not only the lower average atom weight of In2S3 than that of CdS but also the porosity nature of
In2S3 layer. The porosity of In2S3 can be confirmed via the contrast difference within both the bright field image and the HAADF image. During the CBD deposition of In2S3
H2S gas was generated [134], which could make the In2S3 layer porous and lower in density.
Figure 5-9. Bright field TEM images a) d), HAADF images b) e) and corresponding
EDS elemental line scan c) f) of device utilizing CdS/In2S3 and In2S3/CdS hybrid
113 buffers respectively. The EDS line scan data were collected along the red arrow direction in HAADF images accordingly.
Figure 5-10. EDS mapping for devices with CdS/In2S3 hybrid buffer, the CZTS/CdS
/In2S3/ i-ZnO / ITO structure can be clearly identified.
114 Figure 5-11 Cross-sectional SEM image of CZTS devices with CdS/In2S3 hybrid buffer.
EDS and XPS elemental profile data of the long tails of the In signal in the CdS layer and Cd signal in the In2S3 layer, in Figure 5-12, confirms the elemental inter- diffusion between these two buffer layers. Similarly, the elemental inter-diffusion within the p-n heterojunction of the buffer/CZTS can also be observed in the XPS depth profile data. The metallic element in the buffer (either In or Cd) demonstrates a substantial diffusion into the CZTS absorber when the buffer is adjacent to CZTS.
Figure 5-12 XPS elemental profile of CZTS devices with hybrid buffer: a)
CZTS/CdS/In2S3, b) CZTS/In2S3/CdS.
115 Figure 5-13a demonstrates the distribution of Voc of the CZTS devices with different buffers. Previous publication by IBM and Solar Frontier indicated that
CdS/In2S3 hybrid buffer can effectively boost the Voc of a CZTSSe solar cell by
70mV[72, 132]. In contrast, this pure sulfide CZTS device with CdS/In2S3 buffer does not yield a clear enhancement in Voc. In fact, it is the CZTS with reverse buffer sequence structure of In2S3/CdS that gives ~70 mV Voc increment compared to that with CdS buffer. A CZTS device with the In2S3 buffer alone can also yield such a Voc enhancement of ~70 mV.
Figure 5-13b illustrates the light and dark J-V curves of CZTS devices with different buffers and their corresponding device performance parameters are summarized in Table 5-13. In the case of CZTS device with In2S3 buffer, despite of the
~70 mV Voc boost, both the FF and Jsc suffer a dramatic decrease. The decrease in the
FF may be attributed to the fact that the In2S3 layer is not densely packed, introducing some sort of shunting path. Unlike a CZTS device with In2S3 buffer, a CZTS device with a hybrid buffer of In2S3/CdS not only demonstrates a boost in Voc but also in Jsc, with the FF maintaining at the same level as that with a CdS buffer, thus increasing the efficiency by around 20%. In the case of a CZTS device with hybrid buffer of
CdS/In2S3, both the FF and Jsc drop slightly, leading to a decreased efficiency. It is notable that the crossover points of dark and light J-V curves (indicated by hollow circles in Figure 5-13b) of all the buffers containing In2S3 (i.e. devices with In2S3, hybrid CdS/In2S3 and hybrid In2S3/CdS buffers) are significantly lower than that with the CdS buffer, indicating there might be a barrier for hole and/or electron transportation[8]. It is believed that the low crossover points are caused by a thick MoS2 layer (around 200 nm) at the back contact, blocking the transportation of holes. Herein,
116 the CZTS absorbers were synthesized and processed in the same batch and so it is more likely that the presence of an In2S3 layer or In2S3 induced doping of In into CZTS results in some barrier to carrier transport. Besides, the dark J-V curves of CZTS devices containing an In2S3 buffer are much more flat at high forward voltages compared with that with a CdS buffer, suggesting there may exist a higher dark Rs
(Series resistance) and/or Vbi (built-in potential).
Figure 5-13. Voc distribution and light/dark J-V curves of CZTS devices with different buffers.
117 Table 5-3 Performance parameters of CZTS devices with various buffers. Voc Jsc FF Efficiency Buffer (mV) (mA/cm2) (%) (%)
CdS 641 15.9 53.7 5.47
In2S3 716 12.1 30.2 2.62
CdS/In2S3 651 13.7 46.6 4.16
In2S3/CdS 714 17.6 52.7 6.62
Based on the aforementioned discussions of devices having different buffer layers, it is speculated there are two possible reasons that may account for the observed boost in Voc. On one hand, the In2S3/CZTS interface possesses a more favorable spike- like CBO compared with the cliff-like CBO at the CdS/CZTS interface. This speculation is based on our previous work [90], in which the CBO of In2S3/CZTS is estimated to be spike-like, whilst that of CdS/CZTS is cliff-like. When there is a severe interface defect states induced recombination at the buffer/CZTS heterojunction, the spike-like CBO is more beneficial to achieve higher Voc [26]. This can also explain why the improvement in the Voc can be obtained only on the condition of a p-n interface formed between In2S3 and CZTS. On the other hand, In dopants could form n- type doping in CdS and p-type doping in kesterite, by substituting Cd in CdS, forming
InCd anti-sites and Sn in kesterite forming InSn anti-sites. This would make CdS more n- tpye whilst CZTS would be more p-type, as has been reported by others [72, 135, 136].
Indium ions diffusing from the In2S3 buffer into and subsequent doping of the CZTS, leads to an increased C.C. in CZTS, and thereby improves the Voc according to equation (5.5) [137],
kT § 'n(N 'p) · kT V ln¨ A 1¸ v ln N (5.5) OC ¨ 2 ¸ A q © ni ¹ q
118 where k is Boltzmann’s constant, q is electronic charge and T is temperature. NA and ni represent the C.C. of the acceptor and the intrinsic carrier density, whereas ¨n and ¨p are the photo-generated electron and hole densities, respectively.
In order to further investigate and clarify the reason for the pronounced Voc increment, Capacitance-Voltage (C-V) measurements were conducted at a frequency of
100 kHz. According to a Mott-Schottky analysis, equation (5.6), a plot of A2/C2 vs. the applied bias should have a linear region where the slope is the function of the C.C. of the CZTS based on the assumption that the CZTS solar cell consists of one-sided step junction, for example a n+p junction,
2 A 2(V Vbi ) 2 (5.6) C qH 0H S N A
where A is the device area, Vbi is the built-in potential, C is the capacitance, H0 and HS are the vacuum permittivity and the relative permittivity of CZTS (here 7 is used according to ref [60]), respectively.
According to euqations (5.7) and (5.8), the C-V profiling figure can be plotted as in Figure 5-14b.
H H A X 0 S (5.7) d C
2 N d (X d ) 2 2 (5.8) qH 0H S d(A / C ) / dV
Where Xd is the distance of model compacitor İ0 is the vacuum permittivity and
İs is the relative permittivity of CZTS, q is the elementary charge of an electron, A is the device area, C is the capacitance, V is the applied bias voltage. The C.C. can be readily
119 extracted when the bias voltage is 0 V [72, 73]. These electrical parameters are summarized in Table 5-4.
Table 5-4 Electrical parameters extracted from C-V measurements for CZTS devices with various buffers.
Buffer CdS In2S3 CdS/In2S3 In2S3/CdS
C.C of 9E16 3E17 5E16 1E18 absorber
(cm-3)
In terms of the electrical parameters of CZTS devices with an In2S3 buffer, the
C.C. of the CZTS increases by almost one order of magnitude over that with a CdS buffer due to the diffussion and doping of In into the CZTS. On one hand, the enhancement in C.C. of CZTS leads to an increase in Voc. Similarly, the In2S3 buffer was reported to effectively increase the C.C. of CIGS. The Voc of a CIGS/In2S3 device was also boosted compared to those of CIGS device with CdS buffer [138].
In terms of CZTS devices with a hybrid CdS/In2S3 buffer, the In ions also diffuse from In2S3 into the CdS buffer (see Figure 5-9c), which could form n-type anti- site InCd [72] and thereby increase the C.C. of CdS. However, there is less In in the
CZTS of a CdS/In2S3 hybrid buffer compared with that of an In2S3 buffer. The dense
CdS layer between the CZTS and In2S3 seems to serve as some sort of element diffusion barrier layer according to the EDS line scans in Figure 5-9c, suggesting that the indium does not effectively diffuse into CZTS compared to the situation in which the In2S3 is directly adjacent to the CZTS. Hence, the C.C. of CZTS with a hybrid CdS/In2S3 buffer remains almost the same as that with a CdS buffer, as does the Voc of the CZTS device with a CdS/In2S3 buffer. 120 When the In2S3 layer is positioned between CdS and CZTS layers, indium ions can effectively diffuse into and dope both of the neighbouring materials, CdS and CZTS
(see Figure 5-9f). In this case, the C.C. of CZTS was found to increase by one order of magnitude compared to that with a CdS buffer, contributing to a pronounced Voc boost.
Although all of the C-V data presented here indicates that the increased C.C. of CZTS should be one of the reasons contributing to the improvement in Voc, however, the increased C.C. of different buffers or buffer stacks can not fully explain the Voc increment. For instance, CZTS with a CdS/In2S3 buffer has much lower C.C., but a slightly higher Voc, compared to that with a CdS buffer. Therefore, we believe that both the optimized band alignment at the CZTS/buffer interface and increased C.C. of CZTS by In doping, contribute to the observed boost in the Voc.
121 Figure 5-14 The plot of A2/C2 vs. Applied Bias a) and C-V profile curves b) of CZTS devices using different buffers.
In addition to the demonstrated improvement in Voc, the Jsc of CZTS devices with an In2S3/CdS hybrid buffer was also enhanced compared to that with a CdS buffer.
In order to better understand the improvement in Jsc, external quantum efficiency
(EQE) characterizations with and without bias light were carried out, as shown in
Figure 5-15. Since EQE represents the ratio between the numbers of generated electrons to the number of incident photons, the Jsc can be estimated by integrating the EQE over the entire spectrum. The Jsc from the J-V curve and from the integrated EQE with and without bias light (30 mW/cm2), are demonstrated in Table 5-5. The bias light here is normal white light. As for the CZTS device with a CdS buffer, the EQE under bias light is slightly lower than that without the bias light over the whole spectrum. Previously, electrodeposited CZTS devices with a CdS buffer were found to have a large drop in
EQE under white bias due to the significant recombination in the space change region
[139]. In terms of In2S3 and CdS/In2S3 buffers, the EQE with bias light is higher in the blue part of spectrum (wavelengths <500 nm) whilst it is lower over the rest of the spectrum part (wavelengths >500 nm) compared with that without bias light. Note that the Jsc estimated from the EQE with and without bias light are consistent with the Jsc measured from the J-V curve. In contrast, unlike CZTS devices with other buffers, a
CZTS device with a hybrid In2S3/CdS buffer shows a significant increment over the entire spectrum. The Jsc from the integrated EQE (under white bias light) is substantially lower than that measured from the J-V curve. It seems that the In2S3 associated interface to the CZTS is photoactive with an influence on the defects/recombination, which could contribute to the difference in Jsc between EQE and
122 J-V measurements and thereby need further investigation. Compared to the CZTS device with a CdS buffer, the main improvement in Jsc of CZTS device with a hybrid
In2S3/CdS buffer results from the better absorption at short wavelengths (300-500 nm).
Figure 5-15. EQE with white bias light (30mW/cm2) and without bias light (under dark) of CZTS devices with different buffers.
Table 5-5 Jsc and integrated EQE with and without white bias light (30mW/cm2) for
CZTS devices with different buffers.
Jsc Integrated EQE Integrated EQE with Bias w/o Bias light light Buffer (mA/cm2) (mA/cm2) (mA/cm2)
CdS 15.9 15.5 14.9
In2S3 12.1 12.8 10.1
123 CdS/In2S3 13.6 13.3 13.2
In2S3/CdS 17.6 9.6 14.3
5.2.4 Conclusions
This section has demonstrated an effective method to increase the Voc and hence the efficiency of CZTS solar cells. A substantial boost in the Voc, or reduction in the Voc deficit, can be achieved using In2S3 and In2S3/CdS hybrid buffers. Both the optimized conduction band alignment and increased C.C. of the absorber contribute to the Voc boost. As for the CZTS device with an In2S3 buffer, the increment in Voc does not lead to a boost in efficiency, due to a decreased FF and Jsc. Higher efficiency (6.6%) pure sulfide CZTS solar cells can be achieved when a hybrid In2S3/CdS buffer is utilized. The improvement in efficiency is not only due to an increased Voc, but also significantly enhanced Jsc compared to that with a CdS buffer. The EQE of a CZTS device with an In2S3/CdS hybrid buffer was found to highly depend upon the presence of a bias light.
124 Chapter 6 Beyond 10% efficient sulfide kesterite Cu2ZnxCd1- xSnS4 solar cell: role of cadmium alloying
6.1 Introduction
Besides the unfavorable p-n junction band alignment issue, band tailing and defects within the bulk are identified as another two of the dominant mechanisms responsible for its high Voc deficit [12, 31, 106]. The [CuZn+ + ZnCu-] antisite defect cluster is believed to be the culprit for band tailing issue due to the low formation energy of Cu/Zn antisite as a consequence of their neighboring position in the periodic table as well as their similar atomic radius[31, 106]. High density of Cu/Zn antisite defects shifts the PL peak to a lower energy, thereby leading to a pronounced mismatch between optical and electrical bandgap (here the former refers to the energy of the absorption edge while the latter refers to the PL peak position). Mismatch is over 100 meV compared to 30~40meV for CIGS[31]. Therefore, if Cu/Zn defects clusters can be suppressed or eliminated, the efficiency gap between kesterite and CIGS is expected to be reduced.
One effective way to reduce the quantity or suppress the formation of Cu/Zn defect is partially substitute either Cu or Zn with other atoms possessing very different atomic radius. For instance, Talia et al[140] reported that the band tailing problem may be relieved by substituting Cu with Ag which has much larger atomic radius compared to that of Cu or Zn. They found that the band tailing problem was greatly suppressed by introducing 10% Ag into the CZTSe layer, leading to an efficiency improvement to 9.7% compared to 9.0% for their control CZTSe device. Another successful trial was to partially replace Zn with Cd reported by Su et al.[141]. The efficiency of Cd-alloyed
125 Cu2Zn0.6Cd0.4SnS4 (CZCTS) has been enhanced significantly from 5.3% (control CZTS device) to 9.2%, indicating Cd alloying has exerted a huge beneficial impact on efficiency improvement. However, it is still unclear why and how the Cd/Zn substitution contributes to such a significant efficiency increment and what is the role that Cd is playing in CZCTS solar cells.
Herein, an over 10% efficient sulfide CZCTS solar cell is fabricated using the combined sputtering and chemical bath deposition (CBD) method. The role of Cd in the high efficiency CZCTS solar cell has been carefully investigated. The band tailing problem is confirmed to be relieved by Cd alloying, indicated by the reduced difference between electrical and optical bandgap (Eg), decreased charged defect density and the relief of fermi level pinning. Besides, it is ZnxCd1-xS secondary phases rather than ZnS phases are present at the top and bottom of the absorber. Temperature dependent J-V data indicate interface recombination is still dominant in the current CZCTS device due to unfavorable conduction band offset at the CZCTS/CdS heterojunction interface although the desired epitaxial CdS-CZCTS heterojunction interface can be directly observed.
6.2 Experimental
6.2.1 Film synthesis
A magnetron sputtering system (AJA International, Inc., model ATC-2200) was utilized to deposit Cu/ZnS/SnS materials (co-sputtering) on Mo-coated soda lime glass substrate and then a CdS layer was chemical bath deposited on top of sputtered precursor. The Cd containing precursor stacks were sulfurized using Rapid Thermal
Processor (AS-One 100) within a combined sulfur and SnS atmosphere at 560 Ԩ for
126 several minutes. The composition of CZTS precursor is Cu-poor and Zn-rich
(Cu/Sn=1.8, Zn/Sn=1.3, or (Zn+Cd)/Sn=1.3, Cd/(Cd+Zn)=0.4).
6.2.2 Device fabrication
The conventional cadmium sulfide buffer layer was deposited by CBD method as reported in previous section. After that, a thin i-ZnO (~50nm) and ITO (~220nm) layers were deposited using the AJA RF power sputtering. Evaporated Al patterns were applied as the top contact for the device. Finally, a ~100nm MgF2 was evaporated as the
Anti-reflection coating (ARC). The total area of the final cells is ~0.22 cm2 defined by mechanical scribing.
6.2.3 Characterization
The microstructure and elemental distribution across the CZCTS and buffer interface were carefully examined using JEOL JEM-ARM200F (200kV) aberration- corrected scanning transmission electron microscope (STEM) equipped with energy dispersive X-ray spectroscopy (EDAX) system. Time-resolved photoluminescence (TR-
PL) characterizations were performed on the finished devices using the time-correlated single photon counting (TCSPC) technique (Microtime200, Picoquant), with excitation wavelength of 640nm. The XPS measurements were conducted utilizing an
ESCALAB250Xi (Thermo Scientific, UK) under ultra-high vacuum (better than 2h10-
9 millibars). The X-ray source was mono-chromated Al Ka (hv ˙1486.68 eV) with binding energy scale calibrated using carbon reference. Depth profiling data were obtained by mildly sputtering the buffer/CZTS surface with 1 keV Ar ions over an etching area of 2.5×2.5 mm2. The J-V curves were performed using a solar simulator
(Newport) with AM1.5G illumination (100mW/cm2) calibrated with a standard Si reference. External quantum ef¿ciency (EQE) measurements were conducted by
127 utilizing a QEX10 spectral response system (PV measurements, Inc.) calibrated by the
National Institute of Standards and Technology (NIST)-certified reference Si and Ge photodiodes. C-V measurements were carried out using an 175 mV at a frequency of 10 kHz with a DC bias voltage sweeping from -1.0 to 0.8 V and Drive level capacitance profiling (DLCP) is performed using 10 kHz ac excitation with amplitude from 15 to
215 mV.
6.3 Results and discussions
According to previous report, a Cd/(Zn+Cd)=0.4 ratio, which is the optimal ratio of the highest efficiency CZCTS is adopted. Different from the sol-gel method reported in ref[141] where Cd is homogeneously mixed with other cation sources in the precursor, we used the method of sulfurizing chemical bath deposited CdS on top of co- sputtered Cu/ZnS/SnS purcursor to realize Cd alloying. The first step is to confirm whether CdS has been consumed thoroughly during the sulfurization process (560°C for several mins) and if Cd atom is distributed homogeneously within the absorber film.
Figure 6-1(a) demonstrates the top view SEM image.
128
Figure 6-1. (a) SEM top-view image for CZCTS absorber on Mo glass. (b) TEM image of
CZCTS solar cell device. (c) EDS mapping for the corresponding CZCTS devices (the top left shows HAADF image of the corresponding mapping regions; and the rest are the corresponding
elements mapping distribution images.) of CZCTS film. After sulfurization, it seems there are still some minor amount of CdS
(displayed as white tiny dots in Figure 6-1(a))[142] at the surface of the CZCTS film.
The large grain with horizontal grain size up to 4 ȝm can be observed which is much larger than the previously reported CZTS grains without Cd incorporation [83, 86]. This observation indicates that Cd alloying facilitates grain growth, which is consistent with
129 Su’s reports[141]. Figure 6-1(b) displays the bright field TEM image of CZCTS film, showing that the large grains vertically extend from the bottom directly to the top of the absorber. According to STEM elemental mapping images (see Figure 6-1(c)), Cd is distributed homogeneously within the absorber film, confirming that Cd, initially at the surface of precursor, well diffuses and reacts to form the absorber bulk upon high temperature sulfurization process. Usually, ZnS secondary phase is segregated at the top and bottom of the CZTS absorber layer[60]. In contrast, herein, not only Zn and S signal but also Cd signals are found homogeneously presented within the secondary phase areas both at top and bottom of the absorber (See figure 6-1C), suggesting the secondary phases are ZnxCd1-xS rather than ZnS. Further detailed EDS line scan along with these secondary phases (See Figure 6-2), the atomic ratio of Cd/(Zn+Cd) is ~0.4, identical to that in CZCTS grains and the designed value for whole CZCTS absorber film. Besides, the fact that the diffraction peak of the (112) plane in X-ray diffraction
(XRD) patterns shifts from 28.4°(CZTS) to 28.2°(CZCTS) (see Figure 6-3) and Raman peak (see Figure 6-4) blue shifts from 338 cm-1 to 332 cm-1, together with above- discussed STEM elemental mapping confirms that the Cd has homogeneously alloyed with CZTS, forming a solid-solution phase of CZCTS.
Figure 6-2 EDX elemental linescan profile taken along the red lines. 130
Figure 6-3. XRD patterns of CZTS and CZCTS films on Mo glass substrate. The inset
shows their detailed (112) peaks.
Figure 6-4. Raman spectra of CZTS and CZCTS films on Mo-coated glass substrate.
The corresponding CZCTS (Cu2Zn0.6Cd0.4SnS4) device was fabricated to investigate the influence of partially Cd substitution for Zn on final device performance.
131 Figure 6-5(a) shows the J-V curves of the best performing CZCTS solar cell in this work and its CZTS reference with similar Cu/Sn composition under standard AM1.5 illumination. The corresponding detailed device performance parameters are demonstrated in Table 1. This best CZCTS solar cell shows active area efficiency of
10.30% (the total area efficiency of 9.81%), with the Voc of 610 mV, Jsc (active area) of 24.0 mA/cm2, and FF of 70.6%. Note that the active area Jsc value measured from J-
V is consistent with that integrated from EQE, showing as the red curve in Figure 6-5(b).
To the best of the author‘s knowledge, this reported efficiency of CZCTS is the highest among the pure sulfide kesterite solar cells. Figure 6-6 demonstrates several CZCTS J-
V curves with efficiency over 9% to confirm the reproducibility of this CZCTS device performance. Compared with CZTS reference device with similar absorber composition, the improved efficiency of CZCTS device can be attributed to much lower Voc deficit
(Eg/q-Voc), higher Jsc and significantly increased FF. To be more specific, according to
EQE, the major improvement in Jsc arises from extended spectrum absorption in the wavelength range of 820nm~920nm due to decreased optical band gap of CZCTS. The
FF increment may result from reduced series resistance and less shunting according to the calculated light series resistance(RS,L), light series conductance(GS,L) using site’s method[122], as shown in Table 6-1. For CZTS solar cell, highly resistive ZnS phases are usually aggregated at the top or bottom of CZTS layer, contributing to the high Rs of the CZTS device[143]. With Cd alloying the CZTS, the high resistance ZnS is alloyed with Cd as well, forming more conductive ZnxCd1-xS[144], together with reduced voids, which are believed to contribute to the reduction of the Rs correspondingly.
132
Figure 6-5 (a) J-V curves and (b) EQE curves of CZTS and CZCTS devices, (c) normalized
Tauc plots created from UV-VIS transmission and reflectance measurements (top), and normalized PL spectra(bottom) of CZTS and CZCTS thin films.
Figure 6-6 Light J-V curves of several different CZCTS cells.
In addition to the change in the dominant secondary phase, when it comes to the microstructure, CZCTS device shows less voids at the absorber/MoS2 interface compared to that in CZTS control sample (See Figure 6-7 and 6-8) which may contribute to the better dark shunt conductance (Gs,D) of CZCTS device. To better 133 understand the reduced Voc deficit, ideality factor (A) and reverse saturation current (J0) were calculated according to the Sites’s method[122]. The use of site’s approach enables the direct comparison between these calculated parameters with IBM’s ones[9].
The J0 of CZCTS device has decreased by two orders of magnitude compared to that of
CZTS, indicating much less recombination with Cd alloying, taking place at the hetero- junction interface and/or within CZCTS bulk. Note that the values of RS,L, GS,L ,A, J0, of
CZCTS device are closed to those of 12.6% efficient champion Se alloyed CZTSSe solar cell[9]. This, together with enlarged grain size, suggests Cd alloying may have similar effect comparable to Se alloying on kesterite device characteristics.
Figure 6-7. Annular bright-field imaging of CZCTS device.
134 Figure 6-8. Annular bright-field imaging of reference CZTS device.
Table 6-1. Device characteristics of the CZTS and CZCTS solar cells.
Absorber Voc Jsc FF Eff(%) Eg/q-Voc RS,L Gs,L Gs,D A J0
(mV) (mA/cm2) (%) (mV) ( cm2) (mS cm-2) (mS cm-2) (A/cm2)
CZTS 647 20.6 53.6 7.14 853 0.98 5.32 1.89 2.9 4.0×10-5
CZCTS 610 24.0 70.6 10.3 770 0.45 1.18 0.10 1.6 1.1×10-7
To examine if Cd alloy helps to reduce the band tailing caused by Cu/Zn antisites, firstly, the optical band gap derived from Tauc plots [145](see detailed light absorption coefficienct data in Figure 6-9) and “electrical bandgap“ estimated from PL peak, (illustrated in Figure 6-5c) were compared. The difference between the optical band gap and PL peak, namely, ¨(Eg-PL peak), is found to decrease from ~170meV to
~70 meV after partially Cd substitution for Zn. Meanwhile, in the normalized EQE close to the spectral regime around Eg, the EQE curve of CZCTS drops more steeply compared to that of CZTS (see Figure 6-10). Moreover, in order to further illuminate the band tailing issue, following the models described in ref[31], the detailed band gap fluctuation standard deviation ıg and electrostatic potential fluctuation amplitude Ȗopt were fitted and estimated, as illustrated in Table 6-2. Note that both the bandgap fluctuation and electrostatic potential fluctuation amplitude are dramatically reduced.
Particularly the decrease in fluctuation of electrostatic potential suggests the number of charged defects (most likely the CuZn+ZnCu defects cluster) has been significantly reduced [31]. Moreover, reduction of bandgap fluctuation amplitude further implies that
Zn related detrimental defect cluster [2CuZn+SnZn] has been suppressed as well[146].
These results as discussed above indicate the band tailing problem, one of the major
135 culprit for Voc deficit, has been significantly suppressed by proportional alloying Cd into the CZTS.
Figure 6-9. Light absorption coefficient of CZTS and CZCTS thin films.
136
Figure 6-10 Normalized EQE curves of CZTS and CZCTS devices, we found that wavelength changes 111 nm when EQE dropped from 0.7 to 0.1 for CZTS, compared to that of 99nm for CZCTS, demonstrating the CZCTS have a more steep edge at around
Eg.
Table 6-2. The optical and electrical fluctuation characteristics of the CZTS and CZCTS
samples.
Absorber Optical PL Peak ¨(Eg-PL ıg (meV) Ȗopt (meV)
Eg (eV) (eV) peak) (mV)
CZTS 1.54 1.37 ~170 ~130 ~32
CZCTS 1.38 1.31 ~70 ~90 ~23
137 A typical reported issue of CZTS materials is its short minority carrier lifetime, which is about one to two orders of magnitude lower compared with its CIGS counterpart[147]. To further evaluate the impact of Cd-alloying on the minority carrier lifetime of the CZCTS absorber, the time-resolved photoluminescence (TRPL) measurement was performed on their corresponding final devices, as shown in Figure 6-
11(a). The TRPL curve of CZCTS shows significantly slower decay compared to that of
CZTS. By a biexponetial function model fitting[71], the effective minority carrier lifetime of CZCTS and CZTS are estimated to be 10.8 ns and 4.1 ns, respectively, indicating that Cd incorporation improves the lifetime of the CZCTS device. It is speculated that the observed lifetime improvement should be attributed to both the better microstructure like larger grain size and reduced defects at heterojunction interface and/or within the bulk absorber. In order to better understand the electrical properties and the nature of the defects at junction, capacitance-voltage profiling (C-V) and drive level capacitance profiling (DLCP) measurements have been conducted, as show in Figure 6-11(b). DLCP is more sensitive to bulk rather than the interface, thus, are believed to be more reflection on the free carrier of the absorber[148]. According to the DLCP, after Cd alloying, the carrier concentration (at 0 bias) has been decreased from ~4.4×1016 cm-3 down to ~1.4×1016 cm-3. Admittedly the Cu/Sn ratio may also influence doping density of CZTS and CZCTS thin films [74, 80], however, due to their similar Cu/Sn ratio, we believe the lower free carrier density of CZCTS than its CZTS reference is mainly caused by Cd alloying. As a result of decreased free carrier density, the depletion width (Wd) of CZCTS has enlarged correspondingly from ~160nm to
~400nm. This enlarged Wd of CZCTS, together with its improved lifetime lead to a longer carrier collection length and thereby enhanced Jsc of CZCTS devices with Cd
138 partially substitution for Zn. The FF benefits from the enlarged Wd as well. Usually, a short Wd will lead to the voltage-dependent collection efficiency (VDCE) problem as the current is more sensitive to and dependent on the bias voltage. As a result, the light
J-V curve seems to suffer more shunting problem, showing a small Rsh,L or larger Gs,L, which is obvious for our CZTS reference sample and has also been observed for kesterite solar cells by other groups [140, 149]. With the incorporation of Cd, the VDCE problem has been greatly suppressed via the enlarged Wd, therefore improving the Gs,L as well as the FF of the CZCTS device. Generally, the DCLP is sensitive to the bulk defects whereas C-V profile is more sensitive to the interface defects[148]. Referring back to the champion 12.6% kesterite solar cell, its Ncv is much larger than its NDL, indicating the large number of interface defects[9]. Interestingly, both the CZCTS and reference CZTS devices show a close shape and position between their DLCP curve and
CV curve, suggesting their junction interface may have relatively low defects density.
To shine further lights upon the quality of the junction interface, the microstructure of the CZCTS/CdS hetero-interface was carefully studied by TEM. The right part in Figure 6-11c shows the High-resolution TEM (HRTEM) image at CZCTS and CdS heterointerface taken along CZCTS [112] zone axis. The “epitaxial growth“ of
CdS on the CZCTS is observed according to the corresponding Fast Fourier Transform
(FFT) pattern of CdS and CZCTS lattice fringes, as shown in Figure 6-11c left. The crystalline orientation relationship between CdS and CZTS was confirmed to be
CdS[111]//CZCTS[112]. It is believed that the sort of “epitaxy” nature of the heterojunction helps to reduce the crystalline defects at the interface. Besides, the epitaxy CZTS/CdS interfaces have been observed as well in our previously work[24].
139
Figure 6-11. (a) Time resolved photoluminescence (b) C-V and DLCP characteristic of CZTS
and CZCTS solar cells (c) Right image show HRTEM image at CdS/CZCTS interface, whilst
left image show FFT patterns of the corresponding CdS and CZCTS.
As CZCTS not only reduces band tailing of the bulk but also demonstrates an epitaxial interface with CdS, it is interesting to know the dominant recombination for
CZCTS solar cells. The temperature dependent J-V characterization was carried out on our champion CZCTS and reference CZTS devices as shown in Fig. 6-12(a). By linear extrapolation of the Voc (near room temperature range) to the 0K intercept, activation energy (EA) of the main recombination process can be readily estimated[150]. Normally, for high efficiency CIGS solar cell, where main recombination is dominated by band to band SRH recombination within the depletion region rather than heterojunction 140 interface, the EA matches with its Eg[151]. However, activation energies of both CZTS and CZCTS devices are far below its Eg, suggesting the Cd alloying has not brought substantial improvement in the heterojunction interface, despite of an over 0.1 eV decrease in band gap between CZCTS and CZTS. The main recombination for this 10% efficient CZCTS solar cell is still dominant at heterojunction interface. According to the temperature dependent efficiency curve (Figure 6-12b), the efficiencies of both the
CZTS and CZCTS devices collapse at low temperature, owing to the sharply increase in
Rs (showing inset of Figure 6-12b)) as a result of carrier freezing-out effect. This kind of freeze-out effect can be attributed to the absence of a shallow acceptor in the kesterite bulk[152]. According to the density function theory (DFT) calculations for Cu2CdSnS4, the accepter CuCd still has much lower formation energy than that of Cu vacancy (VCu), whilst the calculated acceptor level of CuCd is around 0.13eV above the valance band, much deeper compared to that of VCu (0.01eV)[153]. This helps to explain why this
CZCTS is still lack of shallow acceptor, suffering from the carrier free-out problem.
For the sake of gaining more information on the band alignment of CZCTS/CdS heterojunction interface, X-ray Photoelectron Spectroscopy (XPS) measurements including core-level spectroscopy and valence band (VB) spectroscopy have been conducted, as shown in Figure 6-12c). According to the VB data, the fermi level (EF) of
CZCTS is ~0.2eV higher above the Valence Band Maxium (VBM). The EF of kesterite have been constantly measured at around 0.4eV above the VBM[90, 124] with free hole concentration~1016 cm-3, showing a big discrepancy compared with the ~0.1 eV predicted by classic semiconductor theory[154], owing to fermi pinning problem caused by serious band tailing[140]. However, CZCTS has a on par or even lower free carrier density than that of CZTS, but much closer EF and VBM positions than those of
141 CZTS reference and reported high efficiency CZTSSe solar cells [124, 140], indicating the fermi pinning problem caused by the severe band tailing and compensation problem in CZTS has been alleviated by Cd alloying, consistent with our aforementioned observations. By an indirect method including the measurements of XPS core-levels of different cation elements in absorber and buffer as well as XPS bulk valence band spectra[30], the VBO can be readily estimated. Then the CBO can be calculated combining the VBO and Eg, enabling the band alignment schema of CZCTS/CdS interface (see Figure 6-12d.) The CBO at CZCTS/CdS interface is calculated to be -
(0.16s0.1) eV, indicating a "cliff"-like CBO of the heterojunction interface which is undesirable for efficient heterojunction solar cells. This value is quiet close to the previously reported value of 0.2~0.3 “cliff” for CZTS/CdS heterojunction interface [29,
90], indicating the decrease in kesterite bandgap after Cd alloying is mainly attributed to raising the position of VBM rather than lowering the position of CBM, which does not exert significant improvement in the heterojunction band alignments for CZTS/CdS interface. This unfavorable “cliff-like” band alignment consists with the large discrepancy between EA and Eg in the temperature-dependent Voc measurements, indicating there is a huge space for further efficiency improvement by optimizing band alignment at the CZCTS/CdS interface.
142
Figure 6-12. (a) Voc curves of CZTS and CZCTS devices as a function of temperature.
(b)Temperature-dependent efficiency and dark series resistance curves for CZTS and
CZCTS devices. (d) Schematic of the band alignment at CZCTS/CdS interface.
6.4 Conclusion
In conclusion, this chapter illustrated that over 10% efficiency pure sulfide kesterite solar cell can be achieved by partially alloying Cd into CZTS. The incorporation of Cd in CZTS can enlarge the grain size, alter the secondary phase from
ZnS to ZnxCd1-xS and reduce the voids at absorber/Mo interface. More importantly, the band tailing problem, correlated with high Voc deficits, can be significantly supressed by Cd substitution for Zn which is confirmed by reduced mismatch between PL peak 143 and optical band gap, decreased charged defects density, and the observation of reduced pinning of fermi level. Moreover by introducing Cd, the minority carrier lifetime can be increased and free carrier density decreased, benefiting the Voc and FF improvements.
Despite the epitaxy nature at CZCTS/CdS interface, the recombination of current
CZCTS device is still dominant at heterojunction interface, where the band alignment is found to be unfavourable cliff-like. Optimizations on the heterojunction band alignment is expected to further boost the efficiency of Cd alloyed kesterite solar cells, demonstrating the high efficiency potential for developing the state-of-the-art kesterite photovoltaic solar cells.
144 Chapter 7 Summary and future work
7.1 Summary of results
The main objective of this Ph.D. thesis was to fabricate high efficiency CZTS solar cells. In order to tackle this task, by identifying the efficiency-determining factors or efficiency-limiting issues, my work has been focused on the four major factors which play crucial roles in the efficiency improvement: the composition of CZTS, sulfurization annealing atmosphere, band alignment at heterojunction interface and band tailing issue.
Firstly, composition of CZTS absorber is of the first priority from both the material aspect and the device performance aspect. Cu content plays a vital role in determining the microstructure, quality (lifetime), and electrical property of CZTS thin film and thereby greatly deciding the efficiency of corresponding solar cell devices.
Specifically, the CZTS samples with lower Cu content possess larger grain size, longer minority lifetime and lower doping of CZTS layer and vice versa. However, device with serious Cu deficiency, for instance Cu/Sn lower than 1.7, will suffer from severe shunting problems. Therefore, it is the Cu/Sn at a compromised middle value range
(1.8~1.9), that yields the best device efficiency of beyond ~7%.
Due to the kesterite equilibrium and volatile nature of SnS, a SnS atmosphere must be provided. It was found that “epitaxial” CBD-CdS was observed on CZTS without surface crystalline defects by sulfurization in combined SnS & S atmosphere, or with defects but passivated by air annealing. This is a key feature for high minority lifetime due to a low interfacial defect density and thereby low recombination rate.
TEM analysis also revealed that CBD-CdS grows with the epitaxial relationship of (111)
145 cubic CdS || (112) CZTS. Cd diffusion into CZTS surface region was observed. Cd diffusion behaviour is believed to be another significant feature for realizing high lifetime by possible buried homojunction. For CZTS from sulfurization in a combined
SnS and sulfur atmosphere with a well-structured surface, both epitaxial growth of
CBD-CdS and Cd diffusion are visible, with the corresponding device showing the highest minority lifetime and efficiency of 8.8%.
The band alignment at the p-n junction for heterojunction solar cell is of great importance. This work first examines the band alignment of three different n-type buffers, i.e., CdS, Zn(O,S), and In2S3 on p-type Cu2ZnSnS4 (CZTS) utilizing X-ray
Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure
(NEXAFS) Measurements. The CBO of the CdS/CZTS heterojunction was found to be cliff-like with CBO(XPS)ௗ=ௗí0.24ௗ±ௗ0.10ௗeV and CBO(NEXAFS)ௗ=ௗí0.18ௗ±ௗ0.10ௗeV, whereas those of Zn(O,S) and In2S3 were found to be spike-like with
CBO(XPS)ௗ=ௗ0.92ௗ±ௗ0.10ௗeV and CBO(NEXAFS)ௗ=ௗ0.87ௗ±ௗ0.10ௗeV for Zn(O,S)/CZTS and CBO(XPS)ௗ=ௗ0.41ௗ±ௗ0.10ௗeV for In2S3/CZTS, respectively. The CZTS photovoltaic device using the spike-like In2S3 buffer was found to yield a higher open circuit voltage
(Voc) than that using the cliff-like CdS buffer. However, the CBO of In2S3/CZTS is slightly higher than the optimum level and thus acts to block the flow of light-generated electrons, significantly reducing the short circuit current (Jsc) and Fill Factor (FF) and thereby limiting the efficiency. Therefore, in the second part of this thesis we have tried using an In/Cd hybrid buffer for boosting the efficiency of CZTS. We found that a Voc of over 710 mV was possible by utilizing either In2S3 buffer or In2S3/CdS hybrid buffer.
The improvement in Voc mainly results from (i) an increased carrier concentration in
CZTS due to In-doping when the In2S3 buffer is adjacent to the CZTS absorber and (ii)
146 favourable conduction band alignments at the CZTS/buffer interface. However, devices with In2S3 buffers suffer from having a lower Fill Factor (FF) and short circuit current density (Jsc) when compared with those with CdS buffers. In contrast devices with
In2S3/CdS hybrid buffers not only yield an enhancement in Voc, but also give the same level of FF and an even higher Jsc relative to CdS buffers, thereby increasing the efficiency from 5.5% to 6.6%. These hybrid In2S3/CdS buffers provide a promising way to reduce the Voc deficit and further boost the efficiency of pure sulfide CZTS solar cells.
In the following chapter, cation substitution of Cd for Zn was tried to relieve the band tailing issue and boost the efficiency for pure sulfide kesterite. Fortunately, an over 10% efficiency pure sulfide kesterite solar cell was achieved by partially alloying
Cd into CZTS. The incorporation of Cd in CZTS can enlarge the grain size, alter the secondary phase from ZnS to ZnxCd1-xS and reduce the voids at absorber/Mo interface.
More importantly, the band tailing problem, correlated with high Voc deficits, can be significantly supressed by Cd substitution for Zn which is confirmed by reduced mismatch between PL peak and optical band gap, decreased charged defect density, and the observation of reduced pinning of fermi level. Moreover by introducing Cd, the minority carrier lifetime can be increased and free carrier density decreased, benefiting the Voc and FF improvements. Despite the epitaxy nature at the CZCTS/CdS interface, the recombination of current CZCTS device is still dominant at this heterojunction interface, where the band alignment is found to be unfavourably cliff-like.
Optimizations of the heterojunction band alignment is expected to further boost the efficiency of Cd alloyed kesterite solar cells, demonstrating the high efficiency potential for developing the state-of-the-art kesterite photovoltaic.
147 7.2 7.6% record efficiency for standard CZTS solar cell with total area
2 over 1cm
Based on the findings in this thesis and by optimization of composition, sulfurization annealing atmosphere, and etc., a 7.6% efficiency CZTS standard solar cell with total area of 1.067cm2 has been fabricated. The efficiency was certified by
National Renewable Energy Laboratory (NREL) using standard (AM1.5 temperature
25°C, etc) condition for cell certification. The certified I-V curve and EQE curved along with their performance parameter are illustrated in Figure 7-1 and Figure 7-2, respectively.
Figure 7-1 Certified I-V curve by NREL. The specific area, temperature and other
device performances are provided.
148
Figure 7-2 Certified EQE curve by NREL.
Table 7-1 demonstrates the specific device performance characteristics of this champion standard CZTS solar cell and 12.6% efficient CZTSSe solar cell reported by
IBM. Note that the light series resistivity(RS,L), light shunt conductance (GS,L) ideality factor (A) and reverse saturation current (J0) were calculated according to the Sites’s method[122]. The use of Site’s approach enables the direct comparison between calculated parameters and IBM’s [9]. Compared to the Se alloyed CZTS champion cell, the current 7.6% CZTS device suffers from low Voc or high Voc deficit, which can be partially attributed to high recombination (high J0) and high ideality factor. Meanwhile, the higher series resistance and high shunt conductance leads to lower FF of CZTS compared to that of CZTSSe.
Table 7-1. Device characteristics of the CZTS and CZTSSe solar cells.
149 Absorber Voc Jsc FF Eff Eg/q-Voc RS,L GS,L A J0
(mV) (mA/cm2) (%) (%) (mV) ( cm2) (mS cm-2) (A/cm2)
CZTS 658.5 20.43 56.68 7.63 842 1.92 5.32 3.61 1.7×10-2
CZTSSe 513.4 35.2 69.8 12.6 617 0.72 1.45 1.45 7.0×10-8 Meanwhile, the higher series resistance and high shunt conductance leads to lower FF of CZTS compared to that of CZTSSe.
Figure 7-3 Voc of champion CZTS device as a function of temperature.
It is clear the high Voc deficit can be attributed to severe recombination. It is interesting to understand the dominant recombination for 7.6% champion CZTS solar cells. The temperature dependent J-V characterization was carried out as shown in Fig.
4(a). By linear extrapolation of the Voc (near room temperature range) to the 0K intercept, activation energy (EA) of the main recombination process can be readily estimated[150]. The obtained activation energies for CZTS is only 0.9V, far lower than its optical band gap 1.5eV. This indicates that the recombination process is
150 heterojunction interface dominated. Further optimization to reduce the p-n junction interface recombination is of the first priority.
7.3 Future Work
7.3.1 Heterojunction interface engineering
As a high band gap chalcogenide material, one of the major problems for efficiency improvement is an unfavourable conduction band alignment with the conventional CdS buffer. Therefore we need to explore an alternative buffer (better if
Cd free) or buffer series which can tune the position of the CBM by changing the composition of the solid solution buffer for instance, ZnxCd1-xS, Zn(O,S), ZnxSn1-xO, and ZnxMg1-xO, etc.. In addition to the conduction band alignment, we also need to reduce the defects density at heterojunction interface, which acts as the recombination catalyst at the p-n hetero-interface. Meanwhile an ultra-thin passivation layer can be inserted at the interface, for instance, a very thin passivation material like Al2O3, SnOx or ZnS. Last but not least, to reduce the possibility for interface recombination, lowering the valence band minimum by alloying Ag or O at the front surface of CZTS or decreasing the hole carrier density of the front surface area is desirable.
7.3.2 Optimization of bulk.
Large mismatch of optical band gap and electrical band gap caused by severe band tailing inevitably reduce the effective band gap of the CZTS, hence limiting the
Voc and efficiency. Ag alloying or other elemental alloying and optimization of the sulfurization process can be tried to reduce the band tailing. Moreover very few experimental and characterization knowledge is available regarding the defects level and density for sulfide CZTS. Hence, advanced characterization technique will be
151 needed to gain more understanding of these defects. Meanwhile, we need to find some effective ways to passivate these defects, and achieve a longer lifetime for CZTS.
Moreover, alloying with other elements like Ge or Ag to help in creating a notch-like band grading (similar to that in CIGS) will facilitate the minority carrier collection and further boost the efficiency.
152 Author’s Publications
Journal Publications
C. Yan, F. Liu, K. Sun, et al., Boosting the efficiency of pure sulfide CZTS solar cells using the In/Cd-based hybrid buffers. Solar Energy Materials and Solar Cells, 2015. 144: p. 700.
C Yan, F Liu, N Song, et al., Band alignments of different buffer layers (CdS, Zn(O,S), and In2S3) on Cu2ZnSnS4, Applied Physics Letters, 2014, 104:p. 173901.
C Yan, J Chen, F Liu, et al., Kesterite Cu2ZnSnS4 solar cell from sputtered Zn/(Cu&Sn) metal stack precursors, Journal of Alloys and Compounds, 2014, 610:p.486.
C. Yan, K. Sun, F. Liu, et al., Boost Voc of pure sulfide kesterite solar cell via a double
CZTS layer stacks. Solar Energy Materials and Solar Cells, 2016, Accepted
C. Yan, K. Sun, F. Liu, et al., 10% efficient sulfide kesterite Cu2ZnxCd1-xSnS4 solar cell:
Role of cadmium alloying. , 2016, In preparation
K. Sun*, C. Yan* (* co-first author), F. Liu, et al., Over 9% Efficient Kesterite
Cu2ZnSnS4 Solar Cell Fabricated by Using Zn1–xCdxS Buffer Layer. Advanced Energy
Materials, 2016. 6: p. 1600046.
F. Liu*, C. Yan* (* co-first author), J. Huang, et al., Nanoscale Microstructure and
Chemistry of Cu2ZnSnS4/CdS Interface in Kesterite Cu2ZnSnS4 Solar Cells. Advanced
Energy Materials, 2016. 6: p. 1600706.
Z. Tong, C. Yan, Z. Su, et al., Effects of potassium doping on solution processed kesterite Cu2ZnSnS4 thin film solar cells. Applied Physics Letters, 2015, 105:p. 223903
153 K. Sun, F. Liu, C. Yan, et al., Influence of sodium incorporation on kesterite Cu2ZnSnS4 solar cells fabricated on stainless steel substrates. Solar Energy Materials and Solar
Cells, 2016, 157: p. 565.
L. Zhao, Y. Di, C. Yan, et al., In situ growth of SnS absorbing layer by reactive sputtering for thin film solar cells. RSC Advances, 2016, 6: p. 4108.
W. Li, J. Chen, C. Yan, et al., The effect of ZnS segregation on Zn-rich CZTS thin film solar cells. Journal of Alloys and Compounds, 2015, 632: p. 178.
J. Chen, W. Li, C. Yan, et al., Studies of compositional dependent Cu2Zn(GexSn1íx)S4 thin films prepared by sulfurizing sputtered metallic precursors. Journal of Alloys and
Compounds, 2015, 621: p. 154.
W. Li, J. Chen, C. Yan, et al., Transmission electron microscopy analysis for the process of crystallization of film from sputtered Zn/CuSn precursor. Nanotechnology,
2014, 25: p. 195701.
K. Sun, Z. Su, C. Yan, et al., Flexible Cu2ZnSnS4 solar cells based on successive ionic layer adsorption and reaction method. RSC Advances, 2014, 4: p. 17703.
Z. Tong, K. Zhang, K. Sun, C. Yan, et al., Modification of absorber quality and Mo- back contact by a thin Bi intermediate layer for kesterite Cu2ZnSnS4 solar cells. Solar
Energy Materials and Solar Cells, 2016, 144: p. 537.
K. Zhang, Z. Su, L. Zhao, C. Yan, et al., Improving the conversion efficiency of
Cu2ZnSnS4 solar cell by low pressure sulfurization. Applied Physics Letters, 2014,
104:p. 141101.
154 F. Liu, K. Sun, W. Li, C. Yan, et al., Enhancing the Cu2ZnSnS4 solar cell efficiency by back contact modification: Inserting a thin TiB2 intermediate layer at Cu2ZnSnS4/Mo interface. Applied Physics Letters, 2014, 104:p. 051105.
X. Liu, F. Zhou, N. Song, J. Huang, C. Yan, et al., Exploring the application of metastable wurtzite nanocrystals in pure-sulfide Cu2ZnSnS4 solar cells by forming nearly micron-sized large grains. Journal of Materials Chemistry A, 2015, 3:p. 23185.
X. Liu, J. Huang, F. Zhou, F. Liu, K. Sun, C. Yan, et al., Understanding the key factors of enhancing phase and compositional controllability for 6% efficient pure-sulfide
Cu2ZnSnS4 solar cells prepared from quaternary wurtzite nanocrystals. Chemistry of
Materials, 2016, 28:p. 3649.
F. Zhou, F. Zeng, X. Liu, F. Liu, N. Song, C. Yan, et al., Improvement of Jsc in a
Cu2ZnSnS4 Solar Cell by Using a Thin Carbon Intermediate Layer at the Cu2ZnSnS4/Mo
Interface.ACS applied materials & interfaces, 2015, 7:p. 22868.
H. Cui, X. Liu, F. Liu, X. Hao, N. Song, C. Yan, Boosting Cu2ZnSnS4 solar cells efficiency by a thin Ag intermediate layer between absorber and back contact. Applied
Physics Letters, 2014, 4:p. 041115.
F. Liu, F. Zeng, N. Song, L. Jiang, Z. Han, Z. Su, C. Yan, et al., Kesterite Cu2ZnSn (S,
Se)4 Solar Cells with beyond 8% Efficiency by a Sol–Gel and Selenization Process.ACS applied materials & interfaces, 2015, 7:p. 14376.
F. Liu, S. Shen, F. Zhou, N. Song, X. Wen, J. Stride, K. Sun, C. Yan, X. Hao, Kesterite
Cu2ZnSnS4 thin film solar cells by a facile DMF-based solution coating process.Journal of Materials Chemistry C, 2015, 3:p. 10783.
155 Conference
th C. Yan, F. Liu, K. Sun, et al., CdS, In2S3 and their hybrid buffer for CZTS solar cell. 5
European Kesterite Workshop, Tallinn, 2014
C. Yan, F. Liu, K. Sun, et al., 6.7% efficient CZTS solar cell by sputtering technique.
Australian Centre for Advanced Photovoltaics (ACAP) Conference, Sydney, 2014
C. Yan, F. Liu, K. Sun, et al., Boost Voc of pure sulfide kesterite solar cell via a double
CZTS layer stacks. 6th European Kesterite Workshop, Newcastle, 2015
C. Yan, K. Sun, F. Liu, et al., Strategies towards high performance Cu2ZnSnS4 (CZTS)
-based solar cells. Australian Centre for Advanced Photovoltaics (ACAP) Conference,
Brisbane, 2015
J. Chen, C. Yan, W. Li, et al., Cu2ZnSnS4 thin film solar cell fabricated by magnetron sputtering and sulfurization. MRS Proceedings, 2014, 1638
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