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BARIUM OXIDE AS AN INTERMEDIATE LAYER FOR

POLYMER TANDEM SOLAR CELL

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Zhehui Li

May, 2013

BARIUM OXIDE AS AN INTERMEDIATE LAYER FOR

POLYMER TANDEM SOLAR CELL

Zhehui Li

Thesis

Approved Accepted

______Advisor Department Chair Dr. Xiong Gong Dr. Robert Weiss

______Committee Member Dean of the College Dr. Alamgir Karim Dr. Stephen Z. D. Cheng

______Committee Member Dean of the Graduate School Dr. Yu Zhu Dr. George R. Newkome

______Date

ACKNOWLEDGEMENTS

I would like to thank first and foremost my research advisor Dr. Xiong Gong for his patience, encouragement, and guidance throughout the course of this research. Also, my gratefulness is given to all the group members: Mr. Tingbin Yang, Mr. Hangxing Wang, Ms. Xilan Liu, Mr. He Ren, Mr. Wei Zhang, Mr. Chao Yi, Mr. Bohao Li, Mr. Kai Wang, Ms. Chang Liu for their warm caring about my life and helpful comments and suggestions on research. Finally, I would like to express my deepest gratitude to my parents, Mr. Jun Li and Ms. Xiaoyan Xu for their love and support.

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ABSTRACT

Polymer solar cells (PSCs), a member of organic solar cell family, have attracted increasing research interest. PSCs possess significant advantages over their inorganic solar cell counter parts: mechanical flexibility, light weight, low expense, and the potential to achieve roll-to-roll large-scale production. Tandem solar cells, in which two solar cells are linked to take more use of the solar energy, were fabricated with each solution processed layer using Bulk Heterojunction (BHJ) materials comprising semiconducting polymers and fullerene derivatives. For years, tremendous efforts have been put in seeking for an efficient intermediate layer to successfully connect two sub-cells together in the tandem structure. Even though many kinds of intermediate layers such as ZnO/MoO3 etc. have been explored, most of them suffer from the low conductivity or complicated manipulation disadvantages. Barium oxide (BaO) is both high conductive and wide bandgap n-type semiconductor. We successfully fabricated the polymer tandem solar cell using all thermal vacuum deposition fashion with BaO/Ag/MoO3 as an intermediate layer. VOC of the tandem structure is the sum of the component cells demonstrating our proposed intermediated layer can efficiently connect sub-cells with no potential/energy loss.

TABLE OF CONTENTS

Page

CHAPTER

I. INTRODUCTION 1

1.1. General Background 1

1.2. Polymer Solar Cells 4

1.2.1. Working Principle 4

1.2.2. Device Geometry 8

1.2.3. High Performance: Three Essential Parameters 10

1.3. Polymer Tandem Solar Cells 12

1.3.1. General Background 13 1.3.2. Efficient Single Cells 15

1.3.3. Efficient Intermediate Layer 17

1.3.4. Tandem Polymer Solar Cell Characterization 21

1.3.5. Processing Issues of the Tandem Structure. 22

II.. EXPERIMENT 23

2.1. Materials Preparation 23

2.2. Device Fabrication Procedures 24

2.3. Calibration and Characterization 26

2.4. UV-Vis Absorption Spectrum 26

2.5. Atomic Force Microscopy (AFM) Observation 26

2.6. Sol-gel ZnO Nanoparticles Preparation 26

III. RESULTS 28

3.1. Energy Levels 28

3.2. Performance Investigation 29

3.2.1. Current Density-Voltage (J-V) Characteristics 29

3.2.2. UV-Vis Absorption 33 3.2.3.Atomic Force Micrometer (AFM) Images Observation 35

3.3. Comparison of the Intermediate Layers 36

IV. DISCUSSION, CONCLUSIONS AND OUTLOOK 40

REFERENCES 44

LIST OF FIGURES

Figure Page 1.1 Number of scientific publications contributing to the subject of ‘polymer solar cell(s) 3

1.2 (a) The unit cell of silicon; (b) Simplified energy band diagram for a semiconductor 5

1.3 (a) Bulk Heterojunction (BHJ) Structure of the active layer in polymer solar cell, (b) working principle of polymer solar cells 7

1.4 Photoinduced process in the D-A system. (a) photoinduced charge transfer in a forward direction;(b) exciton recombination happens on a time scale of nm; (c) charge transfer in a back direction 8

1.5 (a) The conventional device structure; (b) Bulk heterjunction configuration in organic solar cells 9

1.6 The organic solar cells with (a) conventional geometry and (b) inverted geometry 9

1.7 Current-Voltage Characteristics of a polymer solar cell under illumination (red line) and in the dark (black line) 10

1.8 (a) Typical tandem solar cell device geometry and (b) simplified procedure of the stacking process of two sub-cells 14

1.9 The current-voltage characteristics of two sub-cells under illumination. The front cell delivered more photocurrent than the bottom cell 16

1.10 Basic principle of an organic tandem solar cell using an intermediate layer. The arrows indicate the hole currents and the electron currents. ETL denotes the electron transport layer and HTL indicates the hole transport layer. 18

1.11 Simplified energy level diagram of the metal and n-type semiconductor (a) before contact and (b) band bending in the Ohmic contact 18

1.12 Schematic energy level diagram at open circuit of a double heterojunction solar cell with highly doped layers as recombination contact 19

1.13 Dark Current Density verse Voltage (J-V) characteristics of a tandem cell before and after light illumination. 20

2.1 Molecular structures of PCPDTBT, P3HT and PCBM respectively 24

2.2 Polymer tandem solar cell geometries with (a) PCPDTBT:PCBM as an upper layer and (b) P3HT:PCBM as an upper layer 25

3.1 Energy levels of the single cell composed of bulk heterojunction polymer blends (a) P3HT: PCBM and (b) PCPDTBT: PCBM. 28

3.2 The energy levels of the tandem solar cells composing the upper polymer layer of (a) P3HT : PCBM and (b) PCPDTBT:PCBM. 29

3.3 The current density-voltage (J-V) characteristics of single reference cells using identical P3HT:PCBM polymer systems and tandem cell. (a) J-V curves under illumination and (b) in dark. 31

3.4 The current density-voltage (J-V) characteristics of single reference cells using P3HT:PCBM and PCPDTBT:PCBM and the tandem cell. J-V curves (a)under illumination and (b) in dark. 33

3.5 UV-Vis absorption spectra of a PCPDTBT:PCBM bulk heterojunction composite film, a P3HT:PCBM bulk heterojunction composite film, and a bilayer of the two as relevant to the tandem device structure. a.u. optical density. 34

3.6 AFM images of (a) MoO3 surface morphology of BaO/Ag/MoO3 intermediate layer and (b) PEDOT:PSS on ITO coated glass substrate. The islands observed are due to surface roughness. Note that the islands distribution is more intensive for (b), indicating the roughness is higher for PEDOT:PSS. 35

3.7 AFM height profiles of ZnO nanoparticles under the condition of (a) fast annealing and (b) slow annealing 36

3.8 The intermediate layer composed of ZnO/MoO3. (a) Current Density-Voltage (J-V) Characteristics and (b) Sol-gel preparation of ZnO nanoparticles 37

3.9 The electronic performance of BaO/Ag/MoO3 intermediate layer. (a) Diode property of BaO/MoO3 P-N junction; (b) Conductive property of BaO/Ag/MoO3 intermediate layer 38

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3.10 AFM images of two types of intermediate layers. (a) ZnO/MoO3 and (b) BaO/Ag/MoO3 on ITO coated glass substrate. The islands observed are due to surface roughness 39

LIST OF TABLES

Table Page

1. Photovoltaic performance of the single reference cell P3HT : PCBM and the corresponding tandem cell. 31

2. Photovoltaic performance of the single reference cells P3HT : PCBM/PCPDTBT : PCBM and the corresponding tandem cell. 33

CHAPTER I

INTRODUCTION

1.1. General Background

Nowadays, environmental pollution and resource depletion are the problems that need to be solved urgently. Due to heavily environmental pollution brought by the widely used traditional energy sources such as oil and gasoline, people are seeking for an environmentally-friendly alternative energy source. Harvesting nature energy to generate power is regarded as one of the promising methods. In this spirit, solar energy is one of the best available alternatives, for its embedded nature of both clean and unlimited.

The photovoltaic effect in Silicon (Si) was first proposed in 1954 in Bell

Laboratory and the power conversion efficiency (PCE) was reported reaching 6%.1

Since then, the inorganic-based solar cells including but not limited to Si such as

GaAs, CdTe, CIFGS have been intensively explored. In the past decade, the

technology of PV is mushrooming at an annual rate of 48% and gradually

commercialized.2 Despite the fact that the inorganic photovoltaic has been booming quite fast, it only takes account for less than 0.1% of the energy demand world widely.

Unfortunately, there are many embedded disadvantages of inorganic PV responsible

for its bottleneck of development. On the one hand, the Silicon processing consumes

large quantities of acid and much poisonous waste is disposed into the environment; 1

On the other hand, the installation of Silicon based photovoltaic (PV) expense

is as high as 1500usd/m2, which inevitably hinders its wide application. To

circumvent those issues, researchers are looking for a better solar cell in regards of

pollution-free and low-cost.

No doubt inspired by discovering the ultra-fast photo induced charge transfer

in 19923, collaborative efforts by interdisciplinary researchers in the fields of synthetic chemistry and optical physics have been put in the study of organic solar

cells. Based on organic materials, this new member of PV is regarded as a promising alternative because it is low processing expense, light weight, and could be fabricated in a continuous fashion. In this way, the organic solar cell can be implemented on flexible substrate carrying the possibility of achieving roll-to-roll printing technique.4

Polymer photovoltaic is not a precise definition but typically considered as a generation of OPV, and it means applying semiconducting conjugated polymers5 as

active materials within the thin film PVs. The first highly conductive polymer was

reported in 1977.6 The highly chemically doped polyacetylene can form a new class of conducting polymers, and the electrical conductivity property can be systematically

and continuous varied over a large magnitude. Another notable event in the polymer

solar revolution occurs at 2000 when Heeger, MacDiarmid and Shirakawa were

nominated Nobel Laureates in recognition of their outstanding contribution in

‘discovery and development of conducting polymers’. Figure1.1 displays an overview of the current development tendency of polymer solar cells.7

Figure 1.1 Number of scientific publications contributing to the subject of ‘polymer solar cell(s)’. Search done through ISI, Web of Science, 2007 The principal working mechanism of polymer solar cells is: First, the

conjugated polymer with localized π electrons can absorb sunlight and forms a

coulombically bound pair of electron-hole named exciton; Second, this pair of

electron-hole is dissociated at bulk hetrojucntion interface into free charge carriers

and those carriers transport through active layer and finally reach the electrodes. The detailed working principle and the device geometry will be disclosed later.

To date, thanks to the tremendous efforts by interdisciplinary researchers, the power conversion efficiency of a single polymer solar cell has been pushed to a value which is competent with their inorganic counterparts. Furthermore, the theoretic value of OPV is around 20% and pushing the single cell to 10% has become a reality through thoughtful design of electron-donor polymer and careful device fabrication.

Besides using better materials to fabricate the solar cell, another logical thinking is to modifying the device structure. In this spirit, the newly created tandem solar cell was proposed.

The tandem solar cell where two sub-cells are connected in series through an

intermediate layer is one of the most commonly employed tandem structure. The first

reported two terminal organic tandem solar cell was proposed by Hiramoto et al.8

Previously, people are focusing on the small molecular to make sub-cells, because the

applied dry coating fashion is an easy way to stack different sub-cells together.

Nowadays, thanks to the development of polymer chemistry, tremendous types of

polymers with good electrical conductivity have been successfully synthesized. This

breakthrough has overcome the choice limitation of available material and more work

was emphasized on the polymer based tandem solar cells afterwards. It was Kawano

et al.9 that demonstrated the first polymer based tandem solar cells.

The intermediate layer in the tandem solar cells has been very attractive to numerous investigators since it is key point to connect sub-cells successfully. Several types

of intermediate layers in either evaporation or solution processed fashion were

explored to connect two sub-cells, and its modification is never overstated. I would

disclose the deep-in knowledge about the intermediate layer later.

1.2. Polymer Solar Cells

1.2.1. Working Principle

Polymer solar cells (PSCs) is an important member of the OPV family,

distinguished by utilizing the π-conjugated polymer as an active component. To better

understand the working principle in the PSC, we can simply compare it with the

inorganic solar cell, Si based photovoltaic, to be specific. Crystal Si possesses a

diamond lattice structure, with each silicon covalent bonded to another four silicon

4 atoms. The pure crystal Si is commonly regarded as a semiconductor material, and the

Fermi level is located at the middle of the valance band and conduction band. Figure

1.2 shows the one silicon lattice and the simplified band structure of pure silicon.10

Figure 1.2 (a) The unit cell of silicon; (b) Simplified energy band diagram for a semiconductor.10 As we know, electric conductivity is proportional to the concentration of mobile charge carriers and therefore the electric conductivity for pure silicon is comparatively low. To overcome this issue, one commonly used method is to add dopants, also called impurities, into the pure Si. The electric conductivity of the semiconductor is considerable increased after being doped. The element chosen to be a dopant usually possesses or lacks an extra electron compared to silicon. Undoped silicon carries the equal number of electrons and holes and it is called ‘intrinsic’ silicon, and dopant will generate an excess of either electron or hole. Hence, there are two types of doped silicon: n-type silicon, and ‘n’ refers to negatively charged carriers

(electron); p-type silicon, and ‘p’ refers to positively charged carriers (hole). When the n and p-type silicon is connected together, the ‘p-n junction’ is formed. This ‘p-n’ junction is a key point and a platform for energy conversion and exciton generation in the inorganic PV. Same case with organic solar cells, the importance of donor-acceptor interface can never be overstated. Similarly, the organic solar cells 5

also possess a donor-acceptor (D-A) interface like ‘p-n junction’ in inorganic photovoltaic. The state-of-art active layer structure is called Bulk Heterojunction

(BHJ) structure which was first reported in 199511. This active layer is commonly

consisted of two materials, namely: an electron donor material and an electron acceptor material. Usually, the conjugated polymer serves as an electron donor and a fullerene derivative as an electron acceptor. Blending the donor materials with the acceptor materials together prepared by dissolving them in the common solvent and spin cast to form a BHJ structure is a good way to enhance the interfacial area and to

break photoexcited excitons into free charge carriers. Poly (3-hexylthiophene) (P3HT)

is one of the commercially available donor materials and 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 (PCBM) is acceptor material .

When shining the light to the active layer, an electron of the donor material will absorb a photon and be excited from the Highest Occupied Molecular Orbital

(HOMO) level to the Lowest Unoccupied Molecular Orbital (LUMO) level, leaving a hole in the HOMO level. Because of the small dielectric constant of organic materials, this pair of electron and hole (called exciton) is tightly coulombically bound. At the interface of donor and acceptor, driven by the difference of electron affinity, this pair of electron and hole is dissociated. Afterwards, the free electron and hole transport though the bulk and reach the respective electrodes. To be concluded, the process of conversion of light into electricity by PSC can be described by the following steps12: 1.

Absorption of photon leads to the formation of an exciton; 2. Excion is dissociated at

the ‘D-A’ interface; 3. Free charge carriers transport through the bulk volume; 4. Free

charge carriers accumulate at electrodes, respectively. The bulk heterojunction (BHJ)

configuration of the active layer and working principle of OPV are schematically

shown in Figure 1.3

Figure 1.3 (a) Bulk Heterojunction (BHJ) Structure of the active layer in polymer solar cell, (b) working principle of polymer solar cells. Copyright © 2010 Elsevier Ltd. There are two critical steps determining how efficiently the device can

convert solar energy to electrical energy, one is the efficient excition dissociation and

the other is the efficient charge transport through bulk active layer. The ultrafast

photophysical studies demonstrate that the photoinduced charge transfer in the D-A blends happens on a time scale of 50fs.13 For efficient photovoltaic devices, the created charges need to be transported to the appropriate electrodes within the exciton life time. Figure 1.4 schematically shows the comparison between photoinduced charge transfer and its competing processes like photoluminescence and back transfer which usually happen on the time scale larger than ns.7

Figure 1.4 Photoinduced process in the D-A system. (a) photoinduced charge transfer in a forward direction;(b) exciton recombination happens on a time scale of nm; (c) charge transfer in a back direction. Copyright © Springer-Verlag Berlin Heidelberg.

1.2.2. Device Geometry

The bilayer structure of solar cell was first proposed by C. W. Tang in

1986.14 In the first reported bilayer structure, two layers of small molecules are layer-by-layer vacuum deposited in the vertical direction on the indium tin oxide( ITO) coated glass substrate. The device is finalized by the thermal deposition of back electrode, namely Ag. In such a device, only the exciton created within the distance of

10-20nm from the interface can be efficiently dissociated, and the thicknesses of two active layers are heavily limited. To circumvent those issues, the Bulk Heterojunction

(BHJ) structure was proposed.11 As mentioned before, Bulk Heterojunction is a blend

of the donor and acceptor components in a bulk volume. The advantages of BHJ over

bilayer structure is twofold: First, in this nano-scale interpenetrating network, each

donor-acceptor (D-A) interface is within 10-20nm length scale to guarantee efficient exciton dissociation; Second, BHJ can tremendously increase orders of magnitude of

the interfacial area favoring more exciton generation. The idea of the conventional 8 device structure and BHJ configuration of active layer are schematically displayed in

Figure 1.5.

Figure 1.5 (a) the conventional device structure; (b) Bulk Heterojunction configuration in polymer active layer. Copyright© 2001, Kirchberg in Tirol, Österreich In general, there are two geometries existing for a single cell in terms of the functions of electrodes. The conventional structure and the inverted structures are schematically shown in the Figure 1.6. In the normal geometry, the device is built up by stacking the buffer layers and the active layer in sequence on top of ITO ( a high work-function metal ) and finally covered with a vacuum deposited layer of Al ( a typical low work-function metal ). This conventional structure suffers from the poor stability issue because of the easily oxidized Al. Also, Al is hard to achieve roll-to-roll large-scale printing mass production. To overcome those shortcomings, the alternative inverted device was proposed 15,16, where the two electrodes are in the opposite positions.

The two electrodes can be further modified by introducing buffer layers on

the ITO side and the back metal side. The Hole Transportation Layer (HTL) and

Electron Transportation Layer (ETL) are two kinds of buffer layers commonly employed to selectively transport charge carriers. HTL plays a role of selectively transporting holes and blocking electrons and ETL selectively transporting electrons and blocking holes. Poly (3,4-ethylene dioxythiophene): (polystyrene sulfonic acid)

17 18 PEDOT:PSS and MoO3 are two types of HTL that widely used to improve the

charge extraction in the anode side.

1.2.3. High Performance: Three Essential Parameters

The current-voltage characteristics of a solar cell under illumination (red line)

and in the dark (black line) are shown in Figure 1.7.19

There are three critical parameters for solar cell efficiency: Open Circuit

Voltage (Voc), Short Circuit Current Density (Jsc), and Fill Factor (FF). The

photovoltaic power conversion efficiency of a solar cell is determined in the following

formula.19

2 Here, Pin is the incident light power density standardized at 1000W/m , and

Impp and Vmpp are the current and voltage at the maximum power point.

Open Circuit Voltage: The open circuit voltage is the point where the current-voltage characteristics under illumination intersect with the vertical coordinates. As mentioned before, one of the two important steps towards efficient solar cells is that the created charges need to be efficiently transported to the electrodes. The driving force for this process is the gradient in the chemical potentials of electrons and holes built up in the donor-acceptor junction, namely built-in potential. This gradient is determined by the difference between the HOMO level of the donor and LUMO of the LUMO level of the acceptor. An agreement has been met that the open circuit voltage is given by this built-in potential.20 However, Voc is not

only related to energy levels of the used materials but also their interfaces, and

therefore the Voc of the real device would vary case by case. The open-circuit voltage

of a conjugated polymer–PCBM solar cell is estimated in the following formula.21

Voc = (1/e)(EDonorHOMO – EPCBMLUMO) – 0.3 V

Short Circuit Current Density: The short circuit current (Isc) is the point

where the current-voltage characteristics under illumination intersect with the

horizontal coordinates, and Short Circuit Current Density ( Jsc) is Isc divided by 11

electrode area. It has been demonstrated that Jsc is correlated to the absorption of the

active layer and exciton dissociation efficiency, which requires that the materials have

a better matched absorption with the solar spectrum and high dielectric constant,

respectively.

In the ideal case, Isc is determined by the product of photoinduced charge carrier density and the charge carrier mobility within the organic semiconductor.

Isc =neµE

n; density of charge carriers

e; elementary charge

µ; charge carrier mobility

E; electric field

Fill Factor: Telling from the current-voltage characteristics in Figure 1.7, Fill

Factor (FF) is determined by the ratio between the area of the yellow rectangle and

the area of rectangle with grey border. That is to say, the ratio between VMPP *IMPP (or

the maximum power) and Voc*Isc is called the fill factor. In the respect of device

physics, charge carriers reaching the electrodes can determine the fill factor, when the

built-in potential is lowered towards the open circuit voltage. Fill factor can also be

calculated in the following formula:

P VJ× FF =MAX = MPP MPP VJoc×× sc VJ oc sc

Fill factor is a very important parameter to achieve solar cell high performance, and it is

considerably influenced by the series resistances and finite conductivity of the ITO covered

substrate of solar cell.22 12

1.3. Polymer Tandem Solar Cells

1.3.1. General Background

In general, the tandem solar cells can be classified into three categories: 1.

Small molecular tandem solar cell; 2. Hybrid tandem solar cell; 3. Fully-solution

processed tandem solar cell.

Since the discovery of ultrafast photoinduced charge transfer, researchers

have been intensively explored how to push upwards the efficiency of solar cells. It

was predicted in 2009 that it is possible the PCE can be over 10% analyzed from the

theoretically standing point, however, the PCE was staying around 5% at that time.23

Nowadays, the PCE for a single cell has been reported as high as 13%.

One bottleneck of further increasing PCE generates from the inherent nature of a single cell: limited by the band-gap, a semiconductor can’t make the best use of every energetic photon in the solar spectrum. People have known a while that the way to broaden the solar spectrum absorption is to make a tandem solar cell. In a tandem solar cell, two or more single cells are absorbing in a complementary wavelength range are stacked together. The most commonly employed device structure is a two terminal tandem cell where two sub-cells are connected in series by an interconnecting layer. Figure 1.8 shows the typical tandem solar cell and the simplified procedure of the stacking process.

In 1990, the tandem solar cell was first proposed where the small molecules are employed as an active layer in the two sub-cells.8 In this spirit, the construction of a tandem structure can be easily manipulated by dry-coating method. However, due to the limited choice of small molecular donor materials which carry considerable different absorption profiles, the hybrid tandem solar cells are further explored.24 In the hybrid tandem solar cell, the bottom sub-cell is processed from polymers from solution process, while the top cell is still through the thermal deposition of small molecules. Driven by the prospect that it is possible to achieve roll-to-roll large scale production through printing technology, the fully solution processed tandem solar cell is further explored. Not until in 2006 did Kawano et al.9 developed the first fully solution processed tandem cells using two identical polymer Bulk Heterojunction

(BHJ) sub-cells. Generally, an ideal tandem structure would utilize a large band-gap cell as the front cell and the low band-gap cell as the bottom cell. It was in the same year, Hadipour et al. first fabricated a tandem solar cell employing a low band-gap polymer and a large band-gap polymer at the same time, and PCE reported 0.57%.25

A milestone of the PCE improvement in the tandem solar cell occurs at the year of

2007 when Kim et al. demonstrated the efficiency of all solution processed polymer

tandem solar cell can be over 6%.26 The further calculation suggests that tandem solar cell with more than 15% power conversion efficiency is feasible.27 However, due to

several issue such as the still inefficient utilization of solar spectrum, large series

resistance, and thermal energy loss, the highest efficiency reported so far is slight

larger than 7%.28

To circumvent those issues and achieve high performance, several criteria

should be considered, for instance, sub-cells with minimum absorption overlap, an

efficient intermediate layer, and compatible fabrication process. Many research efforts

so far have been put into the new polymer design for active layer, and one big

problem remaining is how to make a good intermediate layer that can successfully

connect two sub-cells with the minimum energy loss. In addition, it is more attractive

if the intermediate layer can be both efficient and cheap, so that the fabrication

expense would be low down.

1.3.2. Efficient Single Cells

In the case that two sub-cells are connected, the three critical parameters of

the whole device, namely Open Circuit Voltage (VOC), Short Current Density (JSC) ,

Fill Factor( FF) would inevitably differ from the single cells. It has been demonstrated

that in the ideal case assuming no potential loss in the intermediate layer or device

fabrication derivation from the standard procedure, the VOC of the tandem solar cell is

the sum of the two sub-cells: VOC=VFront + VBottom. On the other hand, the total generated photocurrent will be constant throughout the device. it is also believed that

JSC of the tandem cell is limited by the smallest JSC going through the component cell on the condition that the fill factor of the two sub-cells are the same.29 When in a more realistic case where FF of the two component cells are not identical, JSC of the tandem solar cell is more dominated by the cell having higher FF. To be concluded, the general relationship between VOC and JSC of the tandem solar cell and the component sub-cells is displayed in the following formulas.

JTandem = JBottom + JTop

VTandem = VBottom + VTop

Because of the narrow solar spectrum absorption of a single cell limits PCE, it is critical to employ two sub-cells with complementary solar spectrum absorption in a tandem solar cell. Due to the embedded nature of the intermediate layer and its non-conductive nature in a 2-terminal tandem cell, characterization of two sub-cells independently in a tandem structure is impossible. However, the simulation statistics demonstrates that the bottom cell (Figure 1.9) 29 generates much more photocurrent than the top cell under short circuit condition.

Besides materials’ selection for active layer, the thickness of the thin film need to be well tuned in a way that more solar photon can be absorbed in the bottom cell as much as possible to balance the JSC in both top and bottom cells.

1.3.3. Efficient Intermediate Layer

The importance of employing an intermediate layer is never overstated.

Fabrication of the sub-cells in series without a separation layer in between them will cause the formation of an inverse bulk heterojunction (BHJ) between the donor layer of the top cell and the acceptor of the bottom layer. Hence, the critical step in making a good tandem solar cell is to make an efficient intermediate layer. An inefficient intermediate layer brings about potential loss leading to the VOC of the tandem solar cell is not equal to the sum of the VOC of the component cells. There are three main requirements for such a recombination contact: First, it has to ensure that the electrons from the first sub cell and the holes from the second sub cell meet at the same energy level. Therefore a splitting of the electron and hole quasi-Fermi levels has to be avoided. Second, the recombination contact has to be highly transparent to avoid absorption and reflection reducing the power conversion efficiency and disturbing the current matching in the tandem solar cell. Third, it should be compatible to future mass production processes.30 Figure 1.10 shows the basic principle of an organic tandem solar cell using an intermediate layer.30

Figure 1.10 Basic principle of an organic tandem solar cell using an intermediate layer. The arrows indicate the hole currents and the electron currents. ETL denotes the electron transport layer and HTL indicates the hole transport layer. ITO is the conductive transparent indium tin oxide bottom contact. Copyright © 2010, American Institute of Physics

In solid-state physics, the work function is determined by the minimum energy

needed to remove an electron from a solid to a point outside the solid surface (or

energy which is needed to move an electron from the Fermi level into

vacuum).31When the work function of a metal is smaller than the fermi level of a

semiconductor, then after these two materials contact, the electron will flow from the

metal into the semiconductor leading to the metal positively charged (Figure 1. 11).

Under this condition, there is no barrier for the charge transfer, and we call this

contact Ohmic contact.

Figure 1.11． Simplified energy level diagram of the metal and n-type semiconductor (a) before contact and (b) band bending in the Ohmic contact. A possible approach to the desired intermediate layer is to make use of thick metal layers. If the layers are thick enough, a closed metal layer is formed which acts

as an Ohmic contact. This approach has been introduced in polymer solar cells,

because the mental can prevent the underlying layer from dissolving during the

spin-casting of the second solar cell.32,33 The disadvantage of this approach is the high

absorbance and reflectance of the metal layer resulting in the losses and unbalanced

absorption in the sub-cells. An alternative approach is to use the highly doped

semiconducting layers (n-type and p-type) as an intermediate layer (Figure 1.12)30.

From this energy level diagram, we can observe that the doped semiconductor layers form Ohmic contact with back and front cells. This ohmic contact can efficiently extract electrons and holes from the two sub-cells. Typical

materials for n-type semiconductor are solution processed TiOX and ZnO2, and

p-type semiconductors are poly (ethylenediox-ythiophene) doped with poly

(styrenesulfonate) (PEDOT: PSS), MOO3, and WO3.34 However, these materials

still need to be modified to serve as an efficient part of intermediate layer. One big

issue of the olution-processed ZnO in as an n-type intermediate layer lies in the

inherent nature of its solution process that can’t withstand the top polymer sub-cell.

Besides efficiently extracting electrons and holes from the two component

cells, the intermediate layer should also act as an efficient recombination zone for the

charge carriers. A thin layer of mental material is a promising candidate to serve this

purpose given the good conductivity and suitable working function.

The rectification of the current-voltage characteristics in dark has also been

studied recently. Rectification concept originally comes from rectifier device. A

rectifier can convert alternating current (AC) to direct current (DC), and this process is

known as rectification. In the polymer solar cell, rectification represents how many

orders of magnitude of the current at the most positive bias voltage higher than the

current at the most negative bias voltage for the current-voltage characteristics in the

dark.

To date, more and more work is focusing on how to improve the rectification

of the current-voltage characteristics, but merely no breakthrough has been make in

this area. Some reports have demonstrated that the UV illumination can help to increase the rectification of the current-voltage characteristics in dark (Figure 1.13).35

1.3.4. Tandem Polymer Solar Cell Characterization.

Due to the embedded nature of the interlayer and its non-conductive nature

of a two-terminal tandem cell, characterization of sub-cells independently in a tandem

structure is impossible. Therefore, the way to characterize tandem cells is most based

on the knowledge of a single cell. For a single cell, poly (3-hexylthiophene) (P3HT) is

one of the most studied polymer BHJ solar cell applications. Important qualities of the

thin film include the surface morphology and the degree of crystallinity domain are

very necessary in the study of power conversion efficiency. The film morphology can

be further modified through thermal annealing leading to better order within the P3HT

and mixing of the blend.36 As a critical parameter, the degree of phase separation can

be studied by Atomic Force Microscopy (AFM) and Transmission Electron

Microscopy (TEM).37 In addition, the high-resolution cross-section TEM can reveal

the layer by layer feature in the tandem solar cell. The Power Conversion Efficiency

(PCE), Open Circuit Voltage (VOC), Short Current Density (JSC), and Fill Factor (FF)

21 can be detected in a way that the device is test under the solar simulator and analyzed by Keithley 236 source measurement.

1.3.5. Processing Issues of the Tandem Structure

One of the major challenges in fabricating polymer tandem solar cells is layer-by-layer solution procedure without washing away the layer underneath. Initial efforts on the hybrid tandem solar cell structure fabrication utilized a thermal deposition method to build up the bottom cell, and the fully solution processed tandem cells are even more challenging. One approach is to use orthogonal solvents so that solution processing of one layer does not affect the underlying layer. Most polymers for solar cell application are soluble in chlorinated organic solvents such as dichlorobenzene (DCB), chloroform (CB) etc. Therefore, finding a solvent for the top polymer layer that does not dissolve the underlying polymer layer is very difficult.

One concern raised by Sista et al is that the use of low boiling point solvent (the solvent drying process is fast) for the upper polymer layer can largely prevent damage to the underlying polymer layer.

CHAPTER II

EXPERIMENT

2.1. Materials Preparation

2.1.1 Substrate: Conductive and transparent indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 V and surface roughness ~2nm have been purchased. Rectangular pieces of the correct size for the experiments will be cut especially for solar cell device fabrication.

2.1.2. PEDOT:PSS: the doped p-type soluble semiconductor PEDOT:PSS (VP AI

4083 from H. C. Stark, PEDOT4083) has been purchased from Heraeus Precious

Metals GmbH & Co.. The material has been stored in the refrigerator.

2.1.3. Barium oxide: The barium oxide (BaO) has been purchased from

Sigma-Aldrich. It is a white powder with the density of 5.72 g/mL at 25 °C(lit.).

2.1.4. Solvent: For achieving optimum performance, we used chlorobenzene (CB) as the solvent for upper polymer layer and dichlorobenzene (DCB) as the solvent for underlying polymer layer.

2.1.5. PCPDTBT: PCBM and P3HT:PCBM ratio and concentration: The best device performance is achieved when the mixed solution has PCPDTBT:PCBM ratio of

1.0:3.0 with a concentration of 0.7wt % PCPDTBT plus PCBM 2.5wt% in

Chlorobenzene (CB), and P3HT:PCBM ratio of 1.0 : 0.8 with a concentration of 1wt%

P3HT plus PCBM 0.8wt% in a mixed solvent composed of 97% Dichorobenzene

(DCB) and 3% 1,8-diiodooctane (DIO). The molecular structures of the active materials: PCPDTBT, P3HT, PCBM are schematically shown in figure 2.1.38

O S N N

S S n S n

Figure 2.1 Molecular structures of PCPDTBT, P3HT, PCBM respectively 2.2. Device Fabrication Procedures

2.2.1. Substrate clearance: The ITO-coated glass substrate was cleaned with detergent, then ultrasonicated in acetone and isopropyl, and subsequently dried in an oven overnight.

2.2.2. Device Fabrication Procedure: Polymer tandem cells were prepared according to the following procedure: The initially cleaned ITO-coated glass substrate

was UV-Ozone treated for 20 mins and hold for another 20 mins. Conducting

poly(3,4-ethylenedioxylenethiophene)-polystylene sulfonic acid (PEDOT:PSS) was

spin-cast (4000 rpm, 30s) with thickness ~40 nm from aqueous solution (after passing

a 0.45 µm filter). The substrate was dried for 10 minutes at 150˚C in air, and then

after cooling down to room temperature, moved into a glove box for spin-casting of

the photoactive layer. The dichlorobenzene (DCB) solution comprised of P3HT

(1wt%) plus PCBM (0.8wt%) was then spin-cast at 1200 rpm with thickness ~150 nm

on top of the PEDOT layer to become the first charge separation layer of the tandem

cell. Then the substrate was placed on a hot plate, thermal annealed at 80˚C for

15mins to further increase the degree of crystallinity. Afterwards, the substrate was

pumped down in vacuum (~10-6 torr), and a thickness of 5nm barium oxide (BaO),

2.5nm Ag, 5nm molybdenum oxide (MOO3) was thermal deposited at the rate of

0.1A/s in sequence. After finishing the deposition of the intermediate layer, the

substrate was transferred into the glove-box to spin coating another polymer system of

PCPDTBT: PCBM at the spin rate of 1600rpm/s with thickness~100nm. Then the

substrate was thermal annealing at 80˚C for 5 mins. Finally, the device was pumped

down in vacuum (~10-6 torr) again, and a ~100 nm Al electrode was deposited on top.

The deposited Al electrode area defines an active area of the devices as 4.5 mm2.

Therefore, the structure of the polymer tandem solar cell is ITO/40 nm PEDOT/150

nm P3HT: PCBM /5 nm BaO / 2.5nm Ag / 5 nm MOO3 / 100nm PCPDTBT:PSS/5nm

Ca/100 nm Al.

Another tandem solar cell device was fabricated with the upper polymer system using P3HT:PCBM, and the fabrication procedure is as in a similar fashion as the former stated one. The device geometry with P3HT:PCBM and PCPDTBT:

PCBM as upper polymer system are schematically shown in Figure 2.2.

Figure 2.2. Polymer tandem solar cell geometry with ( a) PCPDTBT:PCBM as an upper layer and (b) P3HT:PCBM as an upper layer.

2.3. Calibration and Characterization

For calibration of the solar simulator, the solar spectrum was carefully minimized using an AM 1.5G filter, and then the light intensity was calibrated using calibrated standard silicon solar cells. Current density-voltage characteristics were measured with a Keithley 236 source measurement unit.

2.4. UV-Vis Absorption Spectrum

The absorption spectrum of the reference single cells and the corresponding tandem cells were recorded by using a spectrometer (Hitachi U-3900 PC).

2.5. Atomic Force Microscopy (AFM) Observation

The quality of the buffer layer and the intermediate layer including the

surface roughness were checked by Atomic Force Microscopy (AFM). The AFM was used in tapping model. The AFM images were taken on a surface area of 5.0µm *

5.0µm. The instrument settings were a scan rate of 0.996 Hertz with a set-point of

330mV. At least two images weretaken from separate locations on each sample to ensure that they are representative. The AFM images will be analyzed using the

Nanoscope Analysis software.

2.6. Sol-gel ZnO Nanoparticals Preparation

The fabrication of sol-gel processed ZnO nanoparticle films with different surface morphologies were made from spin coating the same precursor solution but annealing under different conditions. The precursor solution, consisting of 0.75M zinc

acetate dihydrate and 0.75M monoethanolamine in 2-methoxyethanol was first

spun-coated onto indium tin oxide (ITO) substrates at 4000rpm for 40s. For fast

annealing treatment, the substrate was immediately placed onto a hotplate that was

preheated at 250 oC for 5min. For the slow annealing treatment, the spin-coated

substrate was first placed onto a hotplate that was initially at room temperature while

it was still not completely dry. The temperature was then raised at a ramping rate of

50 oC/min to 250 oC and the substrates were subsequently removed from the hot plate

when the final temperature was reached.

CHAPTER III

RESULTS

3.1. Energy Levels

Because of the embedded nature of the tandem solar cells, it’s impossible to investigate the two sub-cells in the tandem structure independently. Hence, we fabricated the two reference cells independently with the geometries: (a) ITO/PEDOT:

PSS/P3HT: PCBM/Ca/Al. (b) ITO/PEDOT: PSS/PCPDTBT: PCBM /Ca/Al. The

28 energy levels of the single reference cells are schematically shown in Figure 3.1.

Figure 3.1 Energy levels of single cells composed of bulk polymer blends (a) P3HT:PCBM and (b) PCPDTBT:PCBM. As for the tandem solar cell, both two types of tandem solar cells use polymer system of P3HT: PCBM as the bottom cell while different materials for upper cell, the proposed tandem solar cell device geometries are:

(a) ITO/PEDOT:PSS/P3HT:PCBM/BaO/Ag/MOO3/P3HT:PCBM/Ca/Al.

(b) ITO/PEDOT:PSS/P3HT:PCBM/BaO/Ag/MOO3/PCPDTBT:PCBM/Ca/Al.

Figure 3.2 is the energy-level diagram showing the HOMO and LUMO energies of each of the component materials in a tandem structure.

Figure 3.2 The energy levels of the tandem solar cells composing the upper polymer layer of (a) P3HT:PCBM and (b) PCPDTBT:PCBM.

3.2. Performance Investigation 3.2.1. Current Density-Voltage (J-V) Characteristics

(a) ITO/PEDOT: PSS/P3HT:PCBM/BaO/Ag/MOO3/P3HT:PCBM/Ca/Al.

The Current Density- Voltage (J-V) characteristics under Air Mass 1.5

Global (AM1.5G) illumination and in dark of the reference cell P3HT:PCBM and the

tandem structure with two identical sub-cells P3HT:PCBM are shown in Figure 3.3.

The photovoltaic performances are summarized in Table 2.

30 a

Figure 3.3 The Current Density-Voltage (J-V) characteristics of single reference cells using P3HT:PCBM and tandem cell fabricated using the identical reference cells. (a) J-V curves under illumination and (b) in dark.

Table 1. Photovoltaic performance of the single cells and the corresponding tandem cell. 2 Device PCE (%) VOC (V) Jsc ( mA /cm ) FF (%) P3HT : PCBM 2.31 0.60 6.93 55.4 Tandem 1.00 1.20 1.78 47.0

(b) ITO/PEDOT: PSS/P3HT:PCBM/BaO/Ag/MOO3/PCPDTBT : PCBM/Ca/Al.

The Current Density –Voltage (J-V) Characteristics of single solar cells and the tandem solar cell with P3HT: PCBM and PCPDTBT: PCBM polymer systems are shown in Figure 3.4. The characterization was under Air Mass 1.5 global (AM1.5G) 32 illumination. The photovoltaic performance of the single cells and tandem cells are summarized in Table 2.

1 Figure 3.4 The current density-voltage (J-V) characteristics of single reference cells using P3HT:PCBM and PCPDTBT:PCBM and tandem cell fabricated using the same polymer system. (a) J-V curves under illumination and (b) in dark.

Table 2. Photovoltaic performance of the single reference cells and the corresponding tandem cell.

Device PCE (%) VOC (V) JSC (mA FF (%)

/cm2)

P3HT : PCBM 2.31 0.60 6.93 55.4

PCPDTBT :PCBM 2.38 0.65 7.61 48.2

Tandem 1.06 0.95 2.70 41.6

3.2.2. UV-Vis Absorption The absorption spectrum of a film of the bulk heterojunction composite of each sub

cells containing P3HT:PCBM and PCPDTBT:PCBM polymer systems, respectively,

and the bilayer tandem cell composed of P3HT:PCBM/PCPDTBT:PCBM is shown in

Figure 3.5.

P3HT:PCBM 1.5 PCPDTBT:PCBM P3HT:PCBM/PCPDTBT:PCBM

1.0

0.5 Absorbance (a.u.)

0.0 300 350 400 450 500 550 600 650 700 750 800 850 Wavelength (nm)

Figure 3.5 Absorption spectra of a PCPDTBT:PCBM bulk heterojunction composite film, a P3HT:PCBM bulk heterojunction composite film, and a bilayer of the two as relevant to the tandem device structure. a.u., optical density. The PCPDTBT:PCBM polymer system has weak absorption in the visible spectral range but has two strong bands: one in the near-infrared (near-IR) between

700 and 850 nm resulted from the interband p-p* transition of the PCPDTBT and one in the ultraviolet (UV) arising primarily from the HOMO-LUMO transition of the

PCBM. The absorption of the P3HT:PCBM film falls in the complementary apart of the PCPDTBT:PCBM spectrum and covers the visible spectral range. The electronic absorption spectrum of the tandem structure can be just described as a superposition of the two complementary composites absorption spectra. In addition, the

PEDOT:PSS and intermediate layers have negligible absorption in the tandem device

structure.

3.2.3. Atomic Force Micrometer (AFM) Images Observation

Figure 3.6 illustrates the Atomic Force Micrometer (AFM) tapping mode height images of the intermediate layer BaO/Ag/MOO3 and Hole Transportation

Layer PEDOT: PSS coated on the ITO substrate. The islands observed are due to

surface roughness and it is clear to see that the distribution of the up-and-down trend

is more intense on the PEDOT:PSS surface. The calculated room-mean-square

roughness of the intermediate layer and PEDOT:PSS are 0.99nm and 1.72nm,

respectively.

Figure 3.6 AFM images of (a) MOO3 surface morphology of BaO/Ag/MOO3 intermediate layer and (b) PEDOT:PSS on ITO coated glass substrate. The islands observed are due to surface roughness. Note that the islands distribution is more intensive for (b), indicating the roughness is higher for PEDOT:PSS.

3.3. Comparison of the Intermediate layers Figure 3.7 shows AFM height images of sol-gel ZnO nanoparticles fabricated on a

ITO coated substrate. It can be seen that the size of ZnO is significantly greater under the fast annealing condition. The calculated Root-Mean-Square roughnesses of the

ZnO nanoparticles under fast-annealing and slow-annealing are 3.82nm and 1.39nm, respectively.

Figure 3.7 AFM height profiles of ZnO nanoparticles under the condition of (a) fast annealing and (b) slow annealing We fabricated the intermediate layer containing an electron transportation layer sol-gel ZnO nanoparticles and a hole transportation layer thermal deposited

MoO3. The ZnO thin films are prepared in two annealing fashions namely, fast

annealing and slow annealing. The electronic performance of the intermediate layer

and the procedures are illustrated in Figure 3.8.

Figure 3.8 The intermediate layer composed of ZnO/MoO3 (a) Current Density-Voltage (J-V) Characteristics and (b) Sol-gel preparation of ZnO nanoparticles.

As we can tell from the Figure 3.8, the current under the slow annealing condition shows a better diode rectification within the sweep voltage from -1V to 1V.

However, when the applied bias was extended as large as 2V, the current under

negative bias is just slightly smaller than the current in the forward bias. The obvious

diode rectification performance different in the low sweep voltage may tentatively

due to the different surface roughness of the ZnO thin film. As the surface get rougher,

there are more defects on the surface leading to the electron traps. In that case, under a

low bias voltage, the current density versus voltage performance is quite different.

When comes to a larger bias, the ZnO and MoO3 intrinsic property would dominate

the diode performance and annealing method does not have much influence.

Figure 3.9 illustrates the BaO/MoO3 P-N junction current density versus

voltage performance and the conductivity property of the intermediate layer

BaO/Ag/MoO3. The diode performance of BaO/MoO3 is comparable to the

ZnO/MoO3 with a slightly decrease in the negative bias voltage. After inserting an 38 electron hole recombination layer Ag in between BaO and MoO3, the intermediate layer shows a very good electronic conductivity as illustrated in Figure 3.9 (b).

Figure 3.9 The electronic performance of BaO/Ag/MoO3 intermediate layer. (a) Diode property of BaO/MoO3 P-N junction; (b) Conductive property of BaO/Ag/MoO3 intermediate layer.

To further investigate the quality of these two intermediate layers, we checked the surface morphology of them and make comparison (Figure 3.10).The calculated root-mean-square roughness ZnO/MoO3 is 1.16nm which is slightly rougher than the surface of BaO/Ag/MoO3.

Figure 3.10 AFM images of two types of intermediate layers (a) ZnO/MoO3 and (b) BaO/Ag/MoO3 on ITO coated glass substrate. The islands observed are due to surface roughness.

CHAPTER IV DISCUSSION, CONCLUSIONS AND OUTLOOK

From Figure 3.3 and Table 1, we can clearly observe that VOC of the tandem solar cell double the VOC of the sub cells. However, the Jsc is comparatively low

compared to the single reference cell. This is tentatively attributed to the non-fully

absorption of the solar spectrum due to the identical sub cells.

The PCPDTBT:PCBM polymer system has weak absorption in the visible

spectral range but has two strong bands: one in the near-infrared (near-IR) between

700 and 850 nm one in the ultraviolet (UV) . The absorption of the P3HT:PCBM film

falls in the “hole” in the PCPDTBT:PCBM spectrum and covers the visible spectral

range. From Table 2 we can see that the VOC of the tandem cell is 0.95, which is

almost 85% of the sum of VOC of the two component cells. The 15% of VOC loss

indicates that there is some potential loss in this layer-by-layer construction. Some factors can be responsible to the potential loss/energy loss such as the thermalization of carriers and electron traps at the interface. From the J-V curves under illumination of the tandem cell shown in Figure 3.4, we noted that there is a significant hump, so-called‘S-shape’, near VOC.

It has been demonstrated by researchers that this ‘S-shape’ should be

related to an interfacial barrier for charge transport.39 In the theoretical standing point

of energy levels of photoelectronic materials: when shining the light, the exciton 41

generated and then set apart at donor-acceptor (D-A) interface in the front cell. The electron will transport from the D-A interface through the bulk of the

acceptor material PCBM and reaches the interface between bulk hetrojunction (BHJ)

polymer layer and BaO. LUMO levels offset of P3HT and BaO is approximately 3eV.

Regarding Ag/MoO3, since MoO3 is hole-transport semiconductor, when the hole

transports in a direction from MoO3 to Ag, this is merely no contact barrier for hole.

Finally, the electron and hole combines in the recombination area where Ag is serving at this purpose.

To be concluded, one possible reason of the energy loss may locate at the interface between the polymer blends and intermediate layer. The electron may traps at the interfacial defects and could not be wiped out by the sweep voltage, then part of the solar energy may be converted to thermal energy leading to a significant potential/

energy loss. We noted that despite the fact the PCPDTBT has almost identical

photovoltaic performance in terms of PCE, JSC, FF is severally lower than P3HT. Fill factor is determined by charge carriers reaching the electrode, and it denotes the competition process between charge carrier recombination and transportation. That is to say, in a single cell, the lower FF, the higher series resistance the cell would have.

Furthermore, in the tandem cell, the overall photovoltaic performance is limited by the component cell possessing lower FF.

From the J-V curves in Figure 3.3 and Figure 3.4, Jsc of the tandem structure is always lower than that of the reference cells. The reason is tentatively attributed to

41 the enhanced series resistance and contact barrier of the tandem cell compared to the reference single cells.

Interfacial quality is another critical point in deciding the performance of the solar cell and we demonstrated the importance by AFM observation (Figure 3.6). For many commonly employed single cells, active polymer layers are spin casted directly on the surface of PEDOT :PSS and acceptable performance is reached. Our results proved that our proposed intermediate layer at least has the compatible surface quality with PEDOT :PSS. The smoother surface can favor the spread of polymer solution and form a uniform thin firm after dying.

To be concluded, the key feature of the polymer solar cells is an efficient recombination contact at the interface between the solar cells in stack, and our proposed structure BaO/Ag/MoO3 proves to be an efficient intermediate layer.

(1) BaO is a high conductive, wide band-gap n-type semiconductor.

Compared to the sol-gel processed ZnO as part of the intermediate layer,

the fully thermal evaporation fashion of our proposed layer is both

continuous and convenient.

(2) The concept of metal clusters sandwiched between n-type and p-type

semiconductors is promising in the intermediate layer construction.

(3) This proposed intermediate layer can efficient connect two sub cells

together with diminished potential loss.

The present laboratory scale effort will conclude with completion of certain tasks and collection of certain data and that is not presently available, including:

(1) Trying to use other polymer systems associated with BaO/Ag/MoO3

intermediate layer to fabricate tandem solar cell.

(2) Fabricating the tandem solar cell with an inverted structure with opposite

electrodes compared to the conventional geometry.

(3) The interfacial modification of the intermediate layer, including

diminishing electron traps and better energy alignment.

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