Materials for Renewable and Sustainable Energy (2019) 8:6 https://doi.org/10.1007/s40243-019-0144-1

ORIGINAL PAPER

Towards high‑efciency CZTS solar cell through bufer layer optimization

Farjana Akter Jhuma1 · Marshia Zaman Shaily1 · Mohammad Junaebur Rashid1

Received: 18 April 2018 / Accepted: 8 February 2019 / Published online: 13 February 2019 © The Author(s) 2019

Abstract

Cu2ZnSnS4 (CZTS)-based solar cells show a promising performance in the feld of sunlight-based energy production system. To increase the performance of CZTS-based solar cell, bufer layer optimization is still an obstacle. In this work, numerical simulations were performed on structures based on CZTS absorber layer, ZnO window layer, and transparent conducting layer n-ITO with diferent bufer layers using SCAPS-1D software to fnd a suitable bufer layer. sulfde (CdS), sulfde (ZnS) and their alloy cadmium zinc sulfde (Cd­ 0.4Zn0.6S) were used as potential bufer layers to investigate the efect of bufer thickness, absorber thickness and temperature on open-circuit voltage (Voc), short-circuit current (Jsc), fll factor (FF) and efciency (η) of the solar cell. The optimum efciencies using these three bufer layers are around 11.20%. Among these three bufers, ­Cd0.4Zn0.6S is more preferable as CdS sufers from toxicity problem and ZnS shows drastic change in performance parameters. The simulation results can give important guideline for the fabrication of high-efciency CZTS solar cell.

Keywords CZTS · SCAPS-1D · CdS · ZnS · Cd0.4Zn0.6S

Introduction market [4]. On the other hand, CIGS and CdTe ofer high efciencies (around 21% and 21%, respectively) for which The green, clean, renewable and sustainable energy source, they attracted the researchers for the last few years [5]. The sun has a very high potential to meet up the electric- main problem with these two materials is the toxicity of the ity demand of the world and the sunlight can directly be constituent materials cadmium and selenium and also the converted to electricity through solar cell using the princi- less availability of and on earth [6]. To ple known as photovoltaic (PV) efect. To obtain low cost meet up the problems, an alternative material, CZTS has and high-efciency solar cell, researchers are working on drawn the attraction with good advantages over CIGS and many diferent solar materials such as Si, CdTe, Cu(In,Ga) CdTe. During last few years, a good amount of researches Se2 (CIGS), Cu­ 2ZnSnS4 (CZTS), CZTSSe and organic had been done to improve the solar cell efciency using resources [1, 2]. CZTS absorber layer [7]. Wei Wang et al. presented a record The most widely used Si-based solar cell exhibits high cell efciency of around 12.6% using a hydrazine pure solu- conversion efciency (up to 24.5% at the University of New tion approach for ­Cu2ZnSnS4-based solar cell [8]. South Wales) [3]. However, it sufers from low throughput The material CZTS is a quaternary semiconductor with and high cost, therefore, could not afect the world’s energy two diferent crystal structures and . Kester- ite structure shows more stability than stannite structure [9]. * Farjana Akter Jhuma One important advantage of CZTS is that it consists of earth- [email protected] abundant materials which are non-toxic. Thus, CZTS ofers Marshia Zaman Shaily lower cost in comparison with other thin-flm solar cell that [email protected] makes it more promising in PV technology. CZTS shows Mohammad Junaebur Rashid excellent photovoltaic properties such as a direct bandgap [email protected] with bandgap energy of 1.45–1.6 eV and absorption coef- fcient over 1 × 104 cm−1 [10]. CZTS can be fabricated using 1 Department of Electrical and Electronic Engineering, University of Dhaka, Dhaka 1000, Bangladesh

Vol.:(0123456789)1 3 6 Page 2 of 7 Materials for Renewable and Sustainable Energy (2019) 8:6 diferent processes such as sputtering, evaporation, spray Methodology and device structure pyrolysis, electrodeposition, sol–gel technique, etc. [11]. Though CZTS has become a good choice as solar cell SCAPS-1D, a one-dimensional solar cell simulation soft- material, bufer layer optimization is still an issue for its pro- ware developed at the Department of Electronics and Infor- gress. The bufer layer provides band alignment between the mation Systems (EIS), University of Gent, Belgium, was CZTS and window layer and also reduces defects and inter- used for numerically analyzing the solar cell. Up to seven facial strain due to the window layer [12]. Cadmium sulfde diferent layers can be added in the cell defnition panel of (CdS) can be used as a prominent bufer as it improves inter- the software which makes it more suitable for solar cell face with the absorber CZTS and has higher transmission in simulation. Physical properties such as bandgap, electron the blue wavelength region. It has a bandgap of 2.42 eV and afnity, dielectric permittivity, etc., for diferent layers can absorbs photons with wavelength lower than 590 nm cover- be modifed in the layer properties panel which helps to ing 24% of the solar spectrum [13]. However, CdS contains achieve the desired structure. The necessary working point large amount of Cd which is toxic and also produces a large specifcation can be indicated in the action panel. The soft- amount of wastage during the deposition processes. To avoid ware supports grading of all physical parameters and also this problem, an environment friendly material zinc sulfde specifcation of properties of front and back contact. A large (ZnS) can also be used as an alternative bufer layer. ZnS has number of AC and DC electrical measurements including a higher bandgap of 3.5 eV compared to CdS which results short-circuit current density (Jsc), fll factor (FF), open-cir- in less absorption of low-wavelength photons. It also pro- cuit voltage (Voc), conversion efciency (η), quantum ef- duces better interface with CZTS creating a potential barrier ciency (QE), spectral response, generation and recombina- to separate the electron–hole pair [14]. tion profle can be calculated as well as displayed using the Cd1−xZnxS is another bufer layer which is promising as software SCAPS-1D, compared to other simulation software its bandgap can be varied between 2.42 eV (CdS) and 3.5 eV [18]. (ZnS). The bandgap of Cd­ 1−xZnxS for diferent Zn composi- Figure 1 shows the solar cell structure used in this study. tion (x) can be calculated using the following equation which The structure started with soda lime glass substrate. A thin is a modifed Vegard’s law [15]: MoS­ 2 layer of thickness of 100 nm to avoid high series resistance was used over the absorber layer, as this layer can Cd Zn S 2.566 0.041 1.086 2 eV . Eg 1−x x  = + x + x ( ) (1) be experimentally formed by the reaction of Mo back con- tact with sulfur contained in the absorber precursor (ZnS or With increasing Zn concentration, bandgap increases SnS) in the case of CZTS solar cell [19]. The next layer was letting lower wavelength photon through the device. Thus, CZTS absorber layer in which most of the incident photons instead of CdS, Cd­ 1−xZnxS can be used to obtain a more were absorbed to produce electron–hole pairs. CdS or ZnS transparent window in the short-wavelength region, already or ­Cd0.4Zn0.6S bufer layer was used after that to provide the demonstrated by Oladeji et al. [16]. On the other hand, band alignment between CZTS and following window layer. the grain size decreases as Zn concentration increases in Afterwards, less costly and available zinc oxide (ZnO) of ­Cd1−xZnxS [17]. It turns out that for a Zn content of x = 0.6 in ­Cd1−xZnxS, highest current density can be achieved which leads to a signifcant enhancement in the solar cell efciency [15]. Therefore, in this simulation study, 60% Zn is used in Sunlight ­Cd1−xZnxS and the bandgap of 2.98 eV is calculated using Eq. (1). The main objective of this work is to ofer a CZTS solar n-ITO (60 nm) cell with high efciency using a suitable bufer layer. For this reason, three CZTS solar cell device models using three dif- ZnO (80 nm) ferent bufer layers such as CdS, ZnS, and ­Cd0.4Zn0.6S were CdS or ZnS or Cd0.4Zn0.6S (50-150 nm) numerically analyzed using SCAPS-1D to obtain the solar cell performance parameters. Diferent solar cell parameters CZTS (100-2000 nm) Voc, Jsc, fll factor (FF), and efciency (η) were observed and performance analysis was conducted between the three MoS2 (100 nm) structures CdS/CZTS, ZnS/CZTS and ­Cd0.4Zn0.6S/CZTS. Substrate (glass)

Fig. 1 Structure of CZTS thin-flm solar cell

1 3 Materials for Renewable and Sustainable Energy (2019) 8:6 Page 3 of 7 6 thickness of 80 nm was used for the window layer over the Results and discussion bufer layer which enhances light scattering to enable the efcient use of sun light to maximize the number of incident Efect of bufer layer’s thickness photons to the bufer and absorber layers [20]. Finally, a transparent conducting flm n-type indium oxide (n-ITO) To investigate the efect of bufer layer’s thickness, simula- of thickness of 60 nm was used to provide a high mobility tions were done on the structure shown in Fig. 1 with dif- leading to an increase in visible absorption to obtain a lower ferent bufer layers. As already mentioned in Sect. 2, the sheet resistance [21]. bufer layer thickness was varied from 50 to 150 nm with The illumination of light was through the side of n-ITO a fxed CZTS layer of 2000 nm for the frst steps of simu- layer with “Air mass 1.5 global” spectrum with 1000 W/ lation. A single acceptor-type defect with defect density 2 m light power at 25 °C. In this work, the efect of series of 6 × 1016 cm−3 was introduced in the bufer layer [25]. or shunt resistance was not considered. Two types of sin- Figure 2 shows the variation of solar cell parameters such gle layer defect were introduced in the absorber layer and as open-circuit voltage (Voc), short-circuit current density bufer layer for the simulations. The values of the physical (Jsc), fll factor (FF) and efciency (η) with the increase in parameters used in this study are all taken from experimen- bufer layer thickness. In Fig. 2 (for all graphs), the black tal study, diferent literatures or reasonable estimates [15, line shows the variations for ZnS, the red line (behind the 22–25] which are summarized in Table 1. blue line) for CdS and the blue line for Cd­ 0.4Zn0.6S bufer. The observation of CZTS solar cell performances was In Fig. 2a, the Voc vs. bufer layer thickness is shown where performed in three diferent steps. In the frst step, the bufer an increasing trend is found for ZnS bufer after 100 nm layer’s thickness was varied from 50 to 150 nm with equal whereas for CdS and Cd­ 0.4Zn0.6S, bufer layer change is too steps of 25 nm while keeping the absorber layer fxed at small to be found. ZnS bufer layer has higher bandgap with 2000 nm. The optimum thickness of the bufer layer was respect to the other two bufer layer for which it acquires determined and then using it, the performance parameters larger Voc as larger bandgap decreases diode saturation cur- were calculated and presented for each structure. In the rent. As bufer thickness increases, the amount of photons next step, the bufer layer was fxed at its optimum thick- absorbed outside the hole difusion length region increases ness and the CZTS absorber layer was varied for the range which lowers the recombination rate and thus increases Voc of 100–2000 nm with a step size of 400 nm. Then, using [26]. Next, the change in Jsc is shown in Fig. 2b where a the obtained CZTS optimum thickness for their respective decreasing curve is found for ZnS bufer after about 110 nm, bufer layer, the performance parameters were simulated and on the other hand, curves for other two bufers remain almost displayed in this case. Finally, the efect of temperature was fat. When bandgap of a material becomes larger, photons observed for each structure by varying the temperature from can not achieve the required amount of energy to absorb 290 to 350 K with equal steps of 10 K. It is to be mentioned higher wavelength photons and create electron–hole pair and that for each step the other layers thickness and the material so result in lower short-circuit current density. That is why properties were fxed at their respective values.

Table 1 The physical parameters of diferent layer

Parameters MoS2 CZTS CdS ZnS Cd0.4Zn0.6S i-ZnO n-ITO

Thickness (nm) 100 100–2000 50–150 50–150 50–150 80 60 Bandgap (eV) 1.7 1.5 2.4 3.5 2.98 3.3 3.6 Electron afnity (eV) 4.2 4.5 4.5 4.5 4.2 4.6 4.1 Dielectric permittivity 13.6 10 10 10 9.4 9 10 CB efective density of states (cm­ −3) 2.2 × 1018 2.2 × 1018 2.2 × 1018 1.8 × 1018 2.2 × 1018 2.2 × 1018 2.2 × 1018 VB efective density of states ­(cm−3) 1.8 × 1019 1.8 × 1019 1.8 × 1019 1.8 × 1019 1.8 × 1019 1.8 × 1019 1.8 × 1019 Electron thermal velocity (cm s−1) 1 × 107 1 × 107 1 × 107 1 × 107 1 × 107 1 × 107 1 × 107 Hole thermal velocity ­(cm−1) 1 × 107 1 × 107 1 × 107 1 × 107 1 × 107 1 × 107 1 × 107 2 Electron mobility ­(cm /Vs) 100 100 100 100 270 100 50 2 Hole mobility ­(cm /Vs) 25 25 25 25 27 25 75 −3 1 18 15 17 18 19 Shallow uniform donor density, ­ND ­(cm ) 0 1 × 10 1 × 10 5 × 10 1 × 10 1 × 10 1 × 10 −3 16 14 1 Shallow uniform acceptor density, ND ­(cm ) 1 × 10 2 × 10 0 1 × 10 0 0 0 Defect type – Donor Acceptor Acceptor Acceptor – – Defect density ­(cm−3) – 1 × 1013 6 × 1016 6 × 1016 6 × 1016 – –

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Fig. 2 a Voc vs. bufer layer thickness. b Jsc vs. bufer layer thickness. c Fill factor vs. bufer layer thickness. d Efciency vs. bufer layer thick- ness

ZnS bufer layer possesses lower value of Jsc than the two and 11.19%, respectively. The suitable thickness of CdS and other bufer layers [26]. The decrease in short-circuit den- ­Cd0.4Zn0.6S bufer layers is in the range of 50–150 nm, and sity results due to the less production of electron–hole pair for ZnS it is the range of 50–75 nm. as less number of electron–hole pair can reach the absorber layer with increase in bufer layer thickness [22]. In the case Efect of CZTS absorber layer’s thickness of fll factor, the decreasing behavior for ZnS can be seen around 65 nm in Fig. 2c whereas the fll factor for CdS and The efect of CZTS thickness on solar cell performance was ­Cd0.4Zn0.6S do not show any change. The fll factor of ZnS observed through simulations in three diferent structures decreases from 65 nm hereafter due to the efect of series CdS/CZTS, ZnS/CZTS and Cd­ 0.4Zn0.6S/CZTS. For each resistance which increase with thickness and reduces the simulation, the thickness of CZTS was varied from 100 maximum achievable power output [27]. Finally, Fig. 2d to 2000 nm with a fxed bufer layer thickness of 50 nm shows the efciency curves for the diferent bufer layers at a temperature of 300 K. A single donor-type defect which follow the trend of the fll factor. The efciency of the with a defect density of 1 × 1013 cm−3 was introduced in solar cell for ZnS bufer layers falls to almost 1% at 150 nm the CZTS layer [25]. Figure 3a–d shows the variation of thickness whereas for CdS and ­Cd0.4Zn0.6S bufers the ef- open-circuit voltage (Voc), short-circuit current density ciencies decrease slightly. The efciencies are changed due (Jsc), fll factor (FF) and efciency (η), respectively, with to the combined efect of Voc, Jsc and fll factor. Note that increasing absorber layer’s thickness. In each fgure, the the solar cell performance can also decrease due to the addi- black curve shows the variation for ZnS/CZTS structure, tional trapping layers created by the defect, which can trap the red curve for CdS/CZTS and the blue curve indicates the incident photon [28]. The obtained optimum efciencies the ­Cd0.4Zn0.6S/CZTS structure. It is evident from Fig. 3 for CdS, ZnS and ­Cd0.4Zn0.6S bufers are 11.18%, 11.20% that all the parameters such as Voc, Jsc, FF and η are low at

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Fig. 3 a Voc vs. absorber layer thickness. b Jsc vs. absorber layer thickness. c Fill factor vs. absorber layer thickness. d Efciency vs. absorber layer thickness lower thickness of CZTS layer (nearly 250–550 nm) which Efect of working temperature results due to the high recombination of the photo-generated carriers which recombines before reaching the bufer layer/ In this section, the efect of temperature was observed for CZTS layer interface [25]. All the parameters then start to each structure by varying it from 290 K to 350 K. For CdS/ increase with the increase in CZTS thickness because the CZTS, ZnS/CZTS and ­Cd0.4Zn0.6S/CZTS structures, the thicker CZTS layer will absorb more photons and gener- CZTS absorber layer’s thickness was kept fxed at 900 nm, ates more electron–hole pairs [25]. But after a certain thick- 1300 nm and 1700 nm, respectively, while the bufer layer’s ness (900 nm, 1300 nm, and 1700 nm for CdS/CZTS, ZnS/ thickness was fxed at 50 nm for every structure. The altera- CZTS and ­Cd0.4Zn0.6S/CZTS structures, respectively) the tion of open-circuit voltage (Voc), short-circuit current den- performance again starts to deteriorate as the CZTS layer sity (Jsc), fll factor (FF) and efciency (η) with increasing crosses its minority carrier difusion length. The decreas- temperature is shown Fig. 4 where the black, red and blue ing nature can be observed for CdS/CZTS structure clearly lines represent the changes in ZnS/CZTS, CdS/CZTS and whereas for other two structures the decrement is very small Cd­ 0.4Zn0.6S/CZTS structures separately. When the tempera- to be observed. The incident photons absorbed outside the ture increases, there is an increase in the energy of electrons minority carrier difusion length are lost by recombination thus the bandgap of the material decreases. The electrons and thus decreases the solar cell performance. The best ef- gaining extensive energy thus recombine with other holes. ciencies for CdS/CZTS, ZnS/CZTS and ­Cd0.4Zn0.6S/CZTS As a result, recombination rate of internal carrier increases structures were found 10.69% at 900 nm, 10.96% at 1300 nm resulting in decreasing Voc and efciency [29]. This efect and 10.93% at 1700 nm, respectively. can be understood quite well from Fig. 4. In Fig. 4a, Voc decreases almost linearly with increasing temperature. This is due to the reverse saturation current which increases with

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Fig. 4 a Voc vs. temperature. b Jsc vs. temperature. c Fill factor vs. temperature. d Efciency vs. temperature temperature and reduces the saturation current to decrease parameters were observed by varying bufer layer thickness, the open-circuit voltage [22]. The change of Jsc is very small absorber layer thickness and temperature. The bufer layer with increasing temperature shown in Fig. 4b as recombina- thickness for all these three types was found 50 nm to obtain tion process starts to dominate [29]. Whereas the fll fac- high efciency CZTS solar cell. Among them the CdS bufer tor frst increases with temperature as the resistance efect layer used in the CZTS solar cell exhibited higher efciency decreases. But it decreases for higher temperature due to (11.20%). In all cases performance parameters are afected light-induced degradation which can be seen from Fig. 4c signifcantly with the increased bufer layer. Because the [29]. Finally, from Fig. 4d, the efect of increase in tempera- loss of incident photons increases with increased bufer ture on solar cell efciency can be observed which shows a thickness which results in lower short circuit density lead- decreasing behavior due to the combined efect of Voc, Jsc ing to lower efciency of the solar cell. The lower limit of and fll factor. From the simulation, the best efciencies for thickness of CZTS absorber layer was obtained 500 nm to CdS/CZTS, ZnS/CZTS and ­Cd0.4Zn0.6S/CZTS were found avoid the recombination of photogenerated carriers for all 11.20%, 11.47% and 11.58%, respectively, at a temperature three bufer layers. In contrast the higher limit of thickness of 290 K. of CZTS absorber layer were found 1000 nm, 1400 nm and 1800 nm respectively for CdS, ZnS and ­Cd0.4 ­Zn0.6S bufer layers to avoid the efect of minority carrier recombination. Conclusion The best obtained efciency (10.69%) was found for 900 nm thick CZTS layer using CdS bufer layer. The efect of In this work, numerical simulations were performed temperature in CZTS solar cell was also important. In all for CZTS solar cell using three different buffer layers cases (using diferent bufer layers) the efciency decreased such as CdS, ZnS, and Cd­ 0.4Zn0.6S and the performance if we go beyond 290 K, as in higher temperature there is an

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