Rahul Dewan - Jacobs University

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1. INTRODUCTION thinner than the absorption length of the material. Over the last decade, several concepts have been proposed to enhance the absorption of light in thin-film silicon solar cells. Most of the concepts are based on random nanotexturing of the contact layers of the solar cells. Conversion efficiencies higher than 10% have been demonstrated for amorphous and thin-film microcrystalline silicon solar cells by introducing randomly textured interfaces in the solar cell [6, 7]. For micromorph (amorphous and microcrystalline silicon) tandem solar cells stable efficiencies of up to 11.9% have been demonstrated using textured interfaces [8]. The introduction of textured interfaces reduces reflection losses and enhances light scattering and diffraction within the device. Due to multiple reflections within the silicon layer the optical path length of the incident light is greatly enhanced. This leads to a significantly enhanced short circuit current and quantum efficiency in the red and infrared part of the optical spectrum. 1.2 Outline of the Thesis In order to understand the optical propagation within such thin-film devices, it is imperative to use numerical methods and solve the Maxwell’s equations rigorously. The finite difference time domain method, finite element method, or rigorous coupled wave analysis are commonly used to simulate the near-field and far-field wave propagation in such devices [9–13]. By considering the near field optics, the nanotexturing process for efficient solar cells can be understood and optimized. Within the scope of this thesis, the optical enhancement and losses of microcrystalline thin-film silicon solar cells with periodic surface textures were investigated. Beginning with a simplified grating structure, the investigation in this work sequentially moved onto investigating the influence of 3-dimensional pyramid textures on the optical wave propagation in the thin-film devices. The different design parameters and methods are discussed in the different chapters of this thesis. This thesis has been organized in such a way that each of the chapters can be read as stand alone chapters as well. The structure of the thesis is briefly described in the following: In chapter 2, the key parameters that entail the study of thin-film silicon solar cells are described. The optical properties of the different materials in the stratified thin-film solar cell stack are discussed. Understanding these properties and how these optical properties interact together in a solar cell is essential for optimization towards efficient solar cells. Theoretical limits on the absorption enhancement and efficiency of solar cells are derived, which serve as a benchmark for comparing the results in the later chapters. The commonly used light confinement techniques in research and production are also described. In chapter 3, the algorithms for the numerical methods used in this study are described. A brief recap on the fundamentals of electromagnetics theory was presented 2
1.2. Outline of the Thesis which preceded the description of the Maxwell’s solvers methods, namely finite difference time domain and rigorous coupled wave analysis. In chapters 4-7, the simulation results from this study are presented and discussed. Each of the chapters begin with a description of the simulation model used for computation in that chapter. The results from every chapter were discussed independently of the others. The finite difference time domain method was utilized for the computation in chapters 4, 5 and 7. And for chapter 6, the computation method was rigorous coupled wave analysis. In these chapters, the influence of the period and height of the surface texture on the quantum efficiency and short circuit current of the solar cell was investigated. The trends in the optical behavior were understood by using theories of optics and subwavelength optics. In chapter 8, the important results from the preceding chapters were summarized. An outlook to future directions of research is also discussed. 3
Page 1: OPTICS IN THIN-FILM SILICON SOLAR C
Page 4 and 5: CONTENTS 4.5 Summary . . . . . . .
Page 9 and 10: Chapter 2 Fundamental Concepts Unde
Page 11 and 12: 2.1. Characteristic Parameters of S
Page 19 and 20: 2.2. Thin-film silicon solar cells
Page 21 and 22: 2.2. Thin-film silicon solar cells
Page 23 and 24: 2.3. Solar Cell Material Properties
Page 25 and 26: 2.3. Solar Cell Material Properties
Page 27 and 28: 2.4. Light Confinement in Thin-film
Page 29 and 30: 2.4. Light Confinement in Thin-film
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4.3. Solar Cells with Integrated Gr
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2 4.3. Solar Cells with Integrated
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4.4. Toward an Optimal Grating Stru
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Chapter 5 µc-Si Solar Cells with T
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5.2. Simulation Results sists of a
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5.2. Simulation Results a significa
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2 2 5.2. Simulation Results ] S h o
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5.3. Parasitic Absorption Losses do
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5.4. Summary on a smooth substrate
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6. SIMPLER AND FASTER METHOD FOR PE
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7. µC-SI SOLAR CELLS WITH 3-DIMENS
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7. µC-SI SOLAR CELLS WITHFig 3-DIM
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Chapter 8 Summary and Outlook 8.1 S
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8.2. Further Areas to Investigate 8
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8.2. Further Areas to Investigate m
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8.2. Further Areas to Investigate Q
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BIBLIOGRAPHY [9] A. Lin and J. Phil
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BIBLIOGRAPHY Photovoltaic Solar Ene
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BIBLIOGRAPHY [57] J. I. Owen, J. H
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BIBLIOGRAPHY [79] K. Yee, “Numeri
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BIBLIOGRAPHY [102] C. Heine and R.
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BIBLIOGRAPHY [124] H. W. Deckman, C
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BIBLIOGRAPHY nanoparticles,” Jour
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LIST OF SYMBOLS AND ACRONYMS Ag Sil
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10. I. Vasilev, R. Dewan, M. Marink
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Acknowledgement • I would like to
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Rahul Dewan - Jacobs University

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