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Development of Crystalline Germanium for Thermophotovoltaics and High-Efficiency Multi-Junction Solar Cells

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Development of Crystalline Germanium for Thermophotovoltaics and High-Efficiency Multi-Junction Solar Cells Development of Crystalline Germanium for Thermophotovoltaics and High-Efficiency Multi-Junction Solar Cells Dissertation zur Erlangung des akademischen Grades des Doktor der Naturwissenschaften (Dr. rer. nat.) an der Universität Konstanz Fakultät für Physik vorgelegt von Jara Fernández Fraunhofer Institut für Solare Energiesysteme Freiburg 2010 Dissertation der Universität Konstanz Tag der mündlichen Prüfung: 29.04.2011 Referent: Prof. Dr. Gerhard Willeke Referent: Prof. Dr. Thomas Dekorsy iii Acknowledgements Working as a PhD student at Fraunhofer ISE in Freiburg was a very rewarding and challenging experience for me. During all these years, many people have supported me in this endeavour. I can hardly imagine the success of this work without these people. Here is a small tribute to all them: First of all, I want to thank Prof. Gerhard Willeke for accepting me as one of his students, for his valuable advice and inspiring discussions, for his patience in the proofreading and for the support and encouragement during the last months. Prof. Thomas Dekorsy and Prof. Peter Nielaba from the Universty of Konstanz were also mem- bers of my supervisory panel, for which I am grateful. I want to thank Dr. Andreas Bett and Dr. Frank Dimroth for offering me valuable advice and for allowing me great freedom in my research. I am grateful to Dr. Stefan Glunz and Dr. Stefan Rein for answering many questions about silicon and characterization measurements. My special thanks go out to Dominic Suwito, who supported me in the passivation technology, characterization, mathematical simulation and for sharing with me a fascination for germanium. I also take this opportunity to express my special gratitude toward Dr. Eduard Oliva for explaining to me all about solar cell technology. It was a pleasure to work with the III-V technology group. I would especially like to thank Elvira Fehrenbacher, Rüdiger Löckenhof, Swita Wassie and Ranka Koch for their crucial support in cell technology and the great and motivating work atmosphere. I am grateful to Elke Wesler and Rene Kellenbenz for the growth of the cells, Benjamin George for his absorption measurements and Martin Hermle and Simon Philipps for their help in the simulations. I want also to thank Dr Marc Hoffman and Dr Stefan Janz, who helped me with the plasma technology, Dr.Thomas Roth, Dr.Stefan Diez and Phillip Rosenits for the characterization support and Jan Nekarda for the laser fired contacts. I want to thank Raymond Hoheisel, Gerald Siefer, Elisabeth Schäffer, Tobias Gandy and Michael Schachtner for the solar cells measure- ments, Luciana Meinking and Wesley Dopkins for the proofreading and Gerald Siefer and Ray- mond Hoheisel for the German proofreading. I am also grateful to Alejandro Datas from the Instituto de Energia Solar in Madrid for the co- working and for answering the many questions about the TPV system. I would like to thank my workmates for creating a very good atmosphere: Daniel Stetter, Marc Steiner, Tobias Rösener, Henning Helmers, Fabian Eltermann and Johannes Schubert. My DAAD friends in Freiburg; Agnès Millet, Carlo Catoni, Katerina Marcekova, Almir Maljevic, Tamirace Fakhoury, Roza Umarova, and Anna Novokhatko for their friendship and support. My friends in Spain; Laura Alcober, Ana Bribián, Edurne Gállego, Javier Hernández, Clara Pau- les, Marta Villegas and Coral Merchán for always being there and for believing in me. Lucía Alvarez, Guadalupe Asensio, Maria Alba, Juan Miguel del Río, Isabel Fernández, Juan Pablo Ferrer, Michael Schossow, Teresa Orellana, Mark Schumann, Laura Serrano and Alberto Soria for the great moments we shared in Freiburg, for supporting me and believing in me. Finally, I wish to thank my parents and my sister Marta for their constant support and encourage- ment. Freiburg, October 2010 iv v Table of contents Acknowledgments iii 1 Introduction 1 1.1 Motivation 1 1.2 Thesis outline 5 2 Germanium (Ge) 7 2.1 Introduction 7 2.2 Basics of Ge 9 2.2.1 Supply and crystal growth 9 2.2.2 Band structure 10 2.3 Optical properties 11 2.3.1 Electron-hole absorption 11 2.3.2 Free-carrier absorption and related phenomena 12 2.4 Electrical properties 14 2.4.1 Efective mass 14 2.4.2 Carrier densities 15 2.4.3 Conductivity and mobility 17 2.5 Theory of carrier lifetime 20 2.5.1 Generation and recombination 20 2.5.2 Carrier recombination mechanism 21 2.5.3 Effective lifetime and separation of bulk lifetime 26 3 Lifetime measurements 29 3.1 Introduction 29 3.2 Microwave-detected photoconductance decay technique 29 3.2.1 Measurement principle 29 3.2.2 Transient decay study of minority carrier lifetime in Ge 31 3.2.3 Differential and absolute lifetime 40 4 Ge surface passivation 43 4.1 Introduction 43 4.1.1 Fundamentals of surface passivation 43 4.1.2 State of the art passivation layers for Ge surfaces 45 4.2 Plasma enhanced chemical vapour deposition (PECVD) 46 4.2.1 Basics of plasma and plasma excitation 46 4.2.2 PECVD with the STS and SI reactors 49 4.2.3 PECVD with the AK400 reactor 50 vi 4.3 Optimization of Ge surface passivation using amorphous silicon and silicon nitride STS/SI 51 4.3.1 Wet chemical etching 51 4.3.2 Plasma cleaning 52 4.3.3 Plasma deposition of amorphous silicon and silicon nitride 54 4.4 Optimization of Ge surface passivation using amorphous silicon carbide AK400 54 4.4.1 Plasma cleaning 54 4.4.2 Plasma deposition of amorphous silicon carbide 55 4.5 Passivation mechanisms 57 4.5.1 Comparison between the passivation layers and the influence of annealing on p-Ge 57 4.5.2 Comparison between the passivation layers and the influence of annealing on i-Ge 59 4.6 Characterization of passivation layers 61 4.6.1 Effective lifetime and separation of bulk lifetime 61 4.6.2 Surface recombination velocity 64 4.7 Impact of intrinsic limits 65 4.8 Diffusion length in p-Ge 68 5 Ge thermophotovoltaic (TPV) cells 71 5.1 Introduction 71 5.2 Solar TPV system 72 5.3 Principles of Ge TPV cell 75 5.4 Development of back side technology 78 5.5 One-dimensional simulations using PC1D 81 5.5.1 Optical parameters 81 5.5.2 Analysis of the infrared mirror 86 5.5.3 Electrical parameters 89 5.5.4 Electrical passivation of Ge TPV cells 91 5.6 Effect of the back side passivation 94 5.7 Ge cells under TPV condition 96 6 Ge substrate optimization in triple-junction solar cells under space conditions 101 6.1 Introduction 101 6.2 Back side technology optimization 102 6.3 Optimal parameters for space conditions 103 6.3.1 Ideal case 104 6.3.2 Analysis of the cell parameters using PC1D 106 vii 6.4 Results and discussion 111 6.4.1 Passivation layer 111 6.4.2 Infrared mirror 114 7 Summary and outlook 117 8 Deutsche Zusammenfassung 121 9 Appendix: Characterization methods 125 9.1 Reflectometry 125 9.2 Quantum efficiency measurements 126 9.3 IV Measurements. MuSim set up 127 10 List of symbols, acronyms and constants 129 11 List of publications 133 12 Bibliographie 135 viii Introduction 1 1 Introduction This work explores the potential of germanium single-junction solar cells for thermophotovoltaic (TPV) application and the potential of germanium bottom cells for high-efficiency triple-junction solar cells. This chapter encourages the necessity of studying the germanium material and provides the outline of the thesis. 1.1 Motivation The generation of electricity by means of the photovoltaic effect is one of the most promising methods to generate clean and renewable energy [1]. Most research in this field focuses on reducing the cost of photovoltaic energy generation in order to make this technique more at- tractive for the commercial market. Two main strategies can be followed in order to fulfill this requirement. The first strategy is to reduce the cost of the solar cell and module without losing too much of their efficiency. This can be obtained by using cheap substrates. The mainstream material used in the industry is multicrystalline silicon. Another possibility is to use thinner silicon substrates or other material systems like amorphous silicon, CdTe, CIGS or even or- ganic materials. The second strategy is to increase the solar cell conversion efficiency without increasing the cost too much. The research described in this thesis corresponds to this second method. Several research groups focus on obtaining high conversion efficiency using high quality monocrystalline silicon. A remarkable achievement was obtained by means of a sophisticated technology [2]. However, single silicon solar cells absorb only a limited part of the sun spec- trum. The energy of photons with high energy is partly lost due to thermalization, whereas photons with low energy are not absorbed at all. The solar spectrum can be more profitably exploited using multiple stacking of solar cells with increasing band gap energies. This struc- ture reduces the transmission and thermalization losses using a low band gap bottom solar cell and a high band gap top solar cell, respectively (see Figure 1-1). To find the most suitable combination of triple-junction solar cells which makes the best use of the solar spectrum, a simulation using EtaOpt was carried out. This program, developed at Fraunhofer ISE [3], uses the detailed balance method, first introduced by Shockley and Queis- ser [4], to evaluate the theoretical efficiencies of different solar cells design. Figure 1-2 shows how the conversion efficiency of a triple-junction solar cell varies with the band gap energies of the three individual junctions [5]. The global maximum (efficiency higher than 60.5 %) under AM1.5d spectrum at 500 x 1000 W /m2 and 298 K is represented in Figure 1-2 with black dots whereas the grey dots mark efficiencies of 59.0%.
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