Layered Composite Thin Films for Cost Effective Transparent Organic Solar Cell Electrodes
Layered Composite Thin Films for Cost-Effective Transparent Organic Solar Cell Electrodes.
ABSTRACT
In this dissertation, two main themes are explored. In the first part, some phenomenologies of light transport across the ZnO/Al/ZnO (ZAZ) thin-film multilayer structures are investigated.
The multilayered ZAZ thin-film composite structures are explored for potential applications as transparent electrodes (TEs) alternatives to the costly transparent indium-doped-tin-oxide (ITO) anodes that are used currently in organic solar cells (OSCs), organic light-emitting devices (OLEDs), and a variety of other layered optoelectronic structures.
The transmission characteristics and the optimum interlayer thicknesses of ZAZ thin-film structures were numerically predicted in this part of the project. Computational modeling and simulations were used successfully to study the transmittances (Ts) of the multilayered ZAZ thin-film composite stacks with intermediate aluminum (Al) layer thicknesses between ~ 1 – 100 nm.
Multilayered ZAZ thin-film composite structures with mid-Al layer thicknesses between ~ 1 – 10 nm are shown to have average Ts between ~ 75 – 90%, which decreased further to ~ 63 and 41% for the mid-layer Al thicknesses of 20 and 40 nm, respectively.
A further decrease of the T values down to ~ 35 and 15% was observed for the mid-layer Al thicknesses of 50 and 100 nm, respectively. The actual multilayered ZAZ thin-film composite structures were then successfully synthesized with the predicted optimal interlayer thicknesses and tested accordingly.
These were produced via radio frequency (RF) magnetron sputtering (MS). Both computational modeling and experimental studies examined the effects of Al nanolayers on the TE properties of ZAZ film composites.
The experimental study clarified the role of the Al mid-layer thickness in a multilayered ZAZ thin-film composite with a ???(25 ??)/??/???(25 ??) structure and an optimum mid-layer Al thickness between ~ 1 – 10 nm.
Within this range, the numerical simulations are compared with experimental optical T measurements in multilayered ZAZ thin-film composite structures produced with similar ZnO layer thicknesses and the predicted optimum intermediate Al layer thicknesses between ~ 1 – 10 nm.
The electrical properties of multilayered ZAZ thin-film composite structures were also investigated for structures produced with optimum intermediate Al layer thicknesses. Multilayered ZAZ thin-film composite structures, with resistivity values as low as ~ 3.62×10−4 ??? at average Ts between ~ 85 – 90% (in the visible region of the solar spectrum), were produced.
The results show further that the best multilayered ZAZ thin-film composite structures that were produced have the highest Haacke Figure of Merit (HFoM) of 4.72×10−3 Ω−1 and electrical sheet resistances as low as ~ 7.25 Ω/??.
These transparent conductive properties of multilayered ZAZ thin-film composite structures are shown to be comparable to the performance characteristics of ITO-coated anodes that are used currently in organic solar cells, light-emitting devices, and other electronics and optoelectronic components in passive and active technological systems.
The highest HFoM above was obtained for a multilayered ZAZ thin-film composite structure with an intermediate Al layer thickness of ~ 8 nm. Furthermore, the combined apparent optical bandgap energy of the multilayered ZAZ thin-film composite structures changed from ~ 3.26 to 3.85 ??, an increase of ~ 0.60 ?? for intermediate Al layer thicknesses between ~ 1 – 10 nm.
This optical bandgap energy widening led to shifts in the optical absorption edges to shorter wavelengths in the solar spectrum. Such shifts are shown to be in agreement with the Moss-Burstein effect.
Generally, the structural, optical, and electrical properties of the obtained multilayered ZAZ thin-film composite structures revealed the realistic physics of TEs. These were also comparably in good agreement with the transparent conductive properties of the standard ITO thin film coated substrates.
The second theme of the project explores the effects of contact on charge-carrier transport across the interface between the photoactive organic layer and the TE layer of organic photovoltaic (OPV) solar cell systems. The photo-current-density versus voltage (J-V) characteristics of an OSC as a function of contact height for different contact lengths were studied by numerical modeling.
The obtained results are used to assess the prospects of charge transport and/or collection across the photoactive/TE interfaces of OPV solar cell systems and other electronics/optoelectronic devices and components.
The results show that an optimum contact length above ~ 80% is needed for the organic solar cell to have performance characteristics that resemble closely those of organic solar cell systems with perfect planar interfacial/interlayer contacts.
TABLE OF CONTENTS
Dedication ……………………………. iii
Abstract ……………………………………. iv
List of Publications ……………………… vii
Acknowledgments…………………….. ix
Table of Contents ……………………. xiii
List of Figures ……………………… xvii
List of Tables ………………………………….. xxi
Chapter One
Background and Introduction …………………………. 1
1.1 Photovoltaics and the Challenging Energy Supply Context …………. 1
1.2 Technologies Alternatives to c-Si PV Solar Cells …………… 6
1.2.1 CdTe and CIGS Thin Film PV Generation ………….. 7
1.2.2 Organic PV (OPV) Solar Cell Generation …………. 8
1.2.2.1 Power Conversion Efficiency…………….. 9
1.2.2.1.1 Optical Losses………………………. 9
1.2.2.1.2 Electron-Hole Pair Losses ……………… 10
1.2.2.1.3 Charge carrier transport losses ……….. 11
1.2.2.1.4 Charge carrier collection losses ………….. 11
1.2.2.2 Solar Cell Lifetime …………………………… 12
1.2.2.3 OPV Solar Cell Components and Material Costs …………….. 14
1.3 Background to the Study: Unresolved Issues ………….. 15
1.3.1 Conceptual Question……………………………………… 18
1.3.2 Transparent Conducting Materials Alternatives to ITO ………… 18
1.3.3 Motivation of the Study ………………………………. 20
1.4 Imperativeness and the Challenging TCO Context of ZnO ……… 21
1.4.1 Imperativeness of ZnO as Transparent Electrode ……………………………………. 21
1.4.2 The Challenging TCO Context of ZnO Films ………………………………………… 23
1.4.3 The Emerging Layered Thin Film Composite Electrodes ………………………… 24
1.4.4 Materials Selection …………………………………………………………………………….. 25
1.5 Research Problem ………………………………………………………………………………. 25
1.6 Objectives of the Study……………………………………………………………………….. 26
1.7 General Methodology and Significance of the Work ………………………………. 27
1.8 Dissertation Plan ………………………………………………………………………………… 28
Chapter Two
Literature Review………………………………………………………………………………………… 32
2.1 Introduction ………………………………………………………………………………………. 32
2.1.1 Introduction to PV Technologies ………………………………………………………….. 33
2.2 PV Solar Cell Operation Principle ………………………………………………………… 35
2.2.1 Inorganic PV Solar Cell Systems …………………………………………………………. 35
2.2.2 Organic PV Solar Cell Systems ……………………………………………………………. 40
2.2.2.1 Advances in OPV Technology and OSC Designs ……………………………. 42
2.2.2.2 Organic PV Solar Cell Operational Mechanisms ……………………………… 45
2.2.2.2.1 Organic BHJ Solar Cell Operation Principles ………………………………. 48
2.2.2.2.2 Difference between OPV and Inorganic Solar Cells ……………………… 49
2.2.2.2.3 BHJ Organic PV Solar Cell Structure: How it Works? ………………….. 52
2.3 Transparent Conducting Oxides in OPV Systems …………………………………… 52
2.3.1 Doped Transparent Conducting Oxides ………………………………………………… 54
2.3.2 Multilayered Transparent Conducting Films ………………………………………….. 55
2.3.2.1 ITO-based Multilayered Films ………………………………………………………. 55
2.3.2.2 Multilayered ZmZ Film Composite Electrodes ……………………………….. 60
2.3.2.2.1 Design of Multilayered ZmZ Thin Film Composites …………………….. 60
2.3.2.2.2 Prior Work on Multilayered ZmZ Thin Film Composites ……………… 61
2.3.2.2.3 Multilayered ZnO/Al /ZnO Thin Film Composite Structures …………. 67
Chapter Three
Numerical Modeling of Multilayered ZAZ Thin Film Composite Structures ………. 68
3.1 Introduction ………………………………………………………………………………………. 68
3.2 Numerical Methods ……………………………………………………………………………. 69
3.2.1 Theory of Light Absorption …………………………………………………………………. 69
3.2.2 Light Reflection and Transmission Model …………………………………………….. 71
3.2.3 Numerical Modeling Tool and Procedure ……………………………………………… 74
3.2.3.1 The Modeling Tool………………………………………………………………………. 74
3.2.3.2 Numerical Procedure ……………………………………………………………………. 75
3.3 Results and Discussion ……………………………………………………………………….. 76
3.3.1 Effects of Al Thickness on the Optical Properties of ZAZ Film ……………….. 76
3.3.2 Effects of ZnO Thickness on the Optical Properties of ZAZ Film ……………. 79
3.3.3 Average Optical Properties of Model Multilayered ZAZ Films ……………….. 80
3.4 Concluding Remark ……………………………………………………………………………. 82
Chapter Four
Optical and Electrical Properties of Multilayered ZAZ Thin Film Composites for Applications in Transparent Electrode Coatings ……………………………………………… 83
4.1 Introduction ………………………………………………………………………………………. 83
4.2 Methods ……………………………………………………………………………………………. 85
4.2.1 Simulations of ZAZ with Ultra-thin Mid-Al Layer Thicknesses ………………. 85
4.2.2 Experimental ……………………………………………………………………………………… 85
4.2.2.1 Experimental Procedure ……………………………………………………………….. 85
4.2.2.2 Characterization Techniques …………………………………………………………. 88
4.2.2.2.1 The Crystalline Structure of the Obtained ZAZ Multilayer Films …… 88
4.2.2.2.2 Optical Characterization ……………………………………………………………. 91
4.2.2.2.3 Electrical Characterization ………………………………………………………… 92
4.2.2.2.4 Effective Electrical Properties of ZAZ Multilayer Films ……………….. 94
4.3 Results and Discussion ……………………………………………………………………….. 97
4.3.1 Crystal Properties of ZAZ Multilayer Film Composites ………………………….. 97
4.3.2 Optical Properties of Model ZAZ Multilayer Film ……………………………….. 100
4.3.2.1 Transmittance of ZAZ Multilayer Thin Film Stacks ………………………. 100
4.3.2.2 Reflectance of Multilayered ZAZ Thin Film Stacks ……………………….. 102
4.3.2.3 Comparison of Multilayer ZAZ Simulations with Experiments ……….. 104
4.3.2.3.1 Effects of Al Thickness on the Optical Bandgap of ZnO Films …….. 108
4.3.3 Electrical Properties of ZAZ Multilayer Film Structures ……………………….. 112
4.3.4 Comparison of Model ZAZ with other Multilayer Films ……………………….. 114
4.3.4.1 Other Multilayered ZAZ Thin Film Stacks ……………………………………. 114
4.3.4.2 Effects of Mid-Al on the Electrical Properties of ZAZ Film ……………. 115
4.3.5 Performance of Model ZAZ Multilayer Films ……………………………………… 117
4.4 Concluding Remarks ………………………………………………………………………… 119
Chapter Five
Prospects of Non-ideal Interfacial/Interlayer Contacts in Polymer: Fullerene BHJ Solar Cells ………………………………………………………………………………………………… 120
5.1 Introduction …………………………………………………………………………………….. 120
5.2 Overview of Organic Solar Cell Models ……………………………………………… 121
5.3 Characterization of Solar Cells via Numerical Modeling ………………………. 123
5.4 Solar Cell Numerical Modeling Approach …………………………………………… 128
5.4.1 Basic Drift Diffusion Equations …………………………………………………………. 129
5.4.1.1 The Poisson’s equation……………………………………………………………….. 129
5.4.1.2 Current Continuity Equations ……………………………………………………… 129
5.4.1.3 Boundary Conditions………………………………………………………………….. 131
5.4.2 Generation-Recombination of Excitons and EHPs ……………………………….. 131
5.4.3 Generation and Recombination of Charge Carriers ………………………………. 132
5.4.4 Solving the 2DG Drift-Diffusion Model ……………………………………………… 135
5.4.4.1 Iteration Scheme ………………………………………………………………………… 135
5.4.5 Discretization of the OSC Device ………………………………………………………. 138
5.4.5.1 Discretization of Poisson’s Equation ……………………………………………. 139
5.4.5.2 Discretization of Current Densities ………………………………………………. 140
5.4.5.3 Discretized Equations for Charge Carrier Densities ……………………….. 142
5.4.6 Modeling of Non-ideal Photoactive-Anode Contacts ……………………………. 143
5.5 Results and Discussion ……………………………………………………………………… 145
5.5.1 Effects of contact on the J-V characteristics ………………………………………… 145
5.5.2 Influence of contact on potential and carrier distributions ……………………… 151
5.5.3 Evaluation of the Numerical Results …………………………………………………… 154
5.5.4 Implications and Concluding Remarks ………………………………………………… 156
Chapter Six
Implications, Concluding Remarks and Suggestions for Future Work ……………… 158
6.1 Implications of the Study …………………………………………………………………… 158
6.2 Summary and Concluding Remarks ……………………………………………………. 160
6.3 Suggestions for Future Work ……………………………………………………………… 162
6.3.1 The Multilayered ZAZ Thin Film Composite Electrodes ………………………. 163
6.3.2 The Organic Solar Cell ……………………………………………………………………… 164
References …………………………………………………………………………….. 167
APPENDIX A …………………………………………………………………………… 184
INTRODUCTION
1.1 Photovoltaics and the Challenging Energy Supply Context
Environmentally benign renewable energy resources must provide a good alternative energy source. Renewable energy is produced from natural resources such as sunlight, wind, rivers, biomass, and geothermal sources, which are naturally replenished. Depending on its nature, sunlight is one of the most competitive renewable energy sources. It is a long-life, environmentally indisputable energy source that is conveniently accessible from nearly everywhere on earth.
Some evidence-based figures to give the novelty of photovoltaic (PV) solar cell technology potentials exist. For example, the total energy consumption on earth in 2009 was ~ J,[1] which is equivalent to ~ 1 hour of the annual solar energy supply to the earth. This has been estimated to have a value of ~ J. [1,2] It has been estimated also that, covering ~ 0.2% of the Earth’s land with 10% power conversion efficiency (PCE) solar cells would provide ~ 20 Terawatts of photoelectric power.
This is about two times greater than the total fossil fuel consumption of the world including other numerous fuel types. [3] The sun is, therefore, a highly abundant source of clean energy. 20 10 524 10 2
However, partly due to the high cost, solar energy is not maximally exploited.[1] Even though, as shown in Figure 1.1, several PV technologies are available with different characteristics in terms of PCEs, costs, stability, applicability, and maturity.
These are commonly categorized into three PV generations, namely: generation 1, which involves the mono- and/or polycrystalline silicon (c-Si) based PV solar cells.
REFERENCES
N. Grossiord, M. J. Kroon, R. Andriessen and M. W. P. Blom: Degradation Mechanisms in Organic Photovoltaic Devices. Organic Electronics 13, 432–456, 2012.
T. Ameri, G. Dennler, C. Lungenschmied and J. C. Brabec: Organic Tandem Solar Cells: A Review. Energy Environ. Sci. 2, 347–363, 2009.
S. N. Lewis, G. Crabtree and others: Basic Research Needs for Solar Energy Utilization: Report of the Basic Energy Sciences Workshop on Solar Energy Utilization,” http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf , (U.S.), 2005.
Z. Sun, R. K. Zhang, H. H. Xie, M. Liang, R. H. Du and S. Xue: Influence of 4-N,N-dimethylaminopyridine on the Photovoltaic Performance of Dye-sensitized Solar cells with Poly(ethyleneoxide)/oligo(ethylene glycol) Blend Electrolytes. Electrochimica Acta 56, 7555– 7562, 2011.
R. H. Bossert, C. J. J. Tool, J. A. M. van Roosmalen, C. H. M. Wentink and M. M. J. Vaan de: Thin-film solar cells – Technology Evaluation and Perspectives, 2000.
S. E. Shaheen, D. S. Ginley and G. E. Jabbour: Organic-Based Photovoltaics: Towards Low-Cost Power Generation. MRS Bull. 30, 10-19, 2005.
