Measurements

More documents

Recommendations

Info

Chapter 1 Introduction Solar cells provide a nearly inexhaustible, environmentally neutral way to produce electricity. After the first discovery of the photoelectric effect in 1839 by Becquerel [1], the technological breakthrough came in 1954 when D.M. Chapin, C.S. Fuller, and G.L. Pearson first reported of a crystalline silicon based solar cell with a conversion efficiency of η = 6% [2]. Since then a lot of progress, in both the scientific and the technological sense, has been made, and nowadays conversion efficiencies for silicon solar cells of up to 20% for commercial manufacture and above 24% on the laboratory scale have been achieved [3]. However, the costs of photovoltaics are still too high to be competitive with classical electricity production, e.g. coal/oil/gas-fired or nuclear fission powerplants. Thin film solar cells offer a great potential for a reduction of costs as they combine the advantage of low temperature procession, low material consumption, large area producibility as well as the prospect of monolithic series connection to modules [4, 5]. The most promising materials for thin film solar cells are copper-indium-gallium-diselenite (CIGS), cadmium-tellurite (CdTe), and thin film silicon in various modifications. Photovoltaic modules based on amorphous silicon were the first thin film solar cells commercially available and are presently the only thin film devices that have an impact on the photovoltaic world market [5]. However, the conversion efficiencies of solar cell modules based on amorphous silicon are low (η=4-7 % [6]), caused by the presence of defects, tail states, and light induced degradation, known as Staebler-Wronski effect (SWE) [7]. Recently, microcrystalline silicon (µc-Si:H) has attracted interest due to its higher stability against light induced degradation, with the absorption extending into the near infrared, similar to crystalline silicon. First produced as a thin film by Vepřek and Mareček in 1968 [8] using a hydrogen plasma chemical transport technique, it has been shown about 10 years later by Usui and Kikuchi [9] that µc-Si:H can also be prepared using plasma enhanced chemical vapor deposition 1
Chapter 1: Introduction (PECVD), providing compatibility with already well established amorphous thin film technology. In the last few years much progress regarding the preparation, the solar cell performance as well as the understanding of the structural and electronic properties of µc-Si:H has been made. However, there are still tremendous technological and scientific challenges, e.g. the understanding of the interrelation between the solar cell performance and the material properties of µc-Si:H are of great interest. Microcrystalline silicon as referred to in the literature describes a wide range of silicon material rather than a well defined system. In fact, µc-Si:H is a structure modification consisting of varying amounts of microcrystallites, hydrogenated amorphous silicon and voids [10, 11]. Interestingly, it has been shown that not, as one might expect, material with the highest crystalline volume fractions and the largest crystallite size but material prepared close to the transition to amorphous growth yields the highest conversion efficiencies [12, 13]. Obviously, the transition between microcrystalline and amorphous growth is of great importance. Approaching this transition, e.g. by increasing the silane concentration, the structural as well as the optoelectronic properties, e.g. the electronic conductivity, the photosensitivity as well as the spin density, of the µc-Si:H material change significantly [14, 15, 16, 17, 18, 13]. The variation of the amorphous volume content is often accompanied by changes of the compactness of the material. In particular it is generally observed that deposition conditions which lead to the technologically needed high deposition rates, tend to result in a porous structure. Also attempts to grow material with large grain size in order to improve the carrier mobility, frequently result in porous material. Although it has been reported that µc-Si:H is more resistant [13] and highly crystalline material even does not suffer from SWE [19], the presence of crack-like voids makes this material susceptible to in-diffusion of impurities and atmospheric gases which might lead to various metastable and irreversible effects. Earlier investigations on highly crystalline material prepared with chemical transport deposition show that atmospheric gas adsorption and/or oxidation affects the density of surface states, electrical transport and the electron spin density [20]. So far only a few investigations on inand meta-stable effects on recently prepared material exists [21, 22, 23], and the detailed nature of these effects is presently still not understood. The rather complicated structure has major consequences on the electronic structure, e.g. the density of states (DOS) within the band gap. In particular, since there is no well defined structure, the microscopic identification of states observed is complicated as they can be located in the various phases, at boundaries or at interfaces. It is therefore not surprising that there exists no conclusive DOS map and the understanding does in many cases not go beyond a phenomenological description. Thus a study of the density and properties of defect states as a function of the structural composition is of great importance. Electron spin resonance (ESR) is a powerful tool to investigate and identify 2
Page 1 and 2: Forschungszentrum Jülich in der He
Page 4 and 5: Forschungszentrum Jülich GmbH Inst
Page 6 and 7: Kurzfassung In der vorliegenden Arb
Page 8 and 9: Abstract The electronic properties
Page 10 and 11: Contents 1 Introduction 1 2 Fundame
Page 12 and 13: CONTENTS C Abbreviations, Physical
Page 16 and 17: defect states provided that they ar
Page 18 and 19: Chapter 8: In this chapter, the inf
Page 20 and 21: Chapter 2 Fundamentals In this firs
Page 22 and 23: 2.2 Electronic Density of States St
Page 24 and 25: 2.2 Electronic Density of States ta
Page 26 and 27: 2.2 Electronic Density of States Fi
Page 28 and 29: 2.3 Charge Carrier Transport influe
Page 30 and 31: 2.3 Charge Carrier Transport functi
Page 32 and 33: Chapter 3 Sample Preparation and Ch
Page 34 and 35: 3.1 Characterization Methods Figure
Page 36 and 37: 3.1 Characterization Methods Homoge
Page 38 and 39: 3.1 Characterization Methods µ Fig
Page 40 and 41: 3.1 Characterization Methods compar
Page 42 and 43: 3.1 Characterization Methods bution
Page 44 and 45: 3.1 Characterization Methods posite
Page 46 and 47: 3.2 Deposition Technique admixture
Page 48 and 49: 3.3 Sample Preparation 3.3.1 Sample
Page 50 and 51: 3.3 Sample Preparation reverse bias
Page 52 and 53: Chapter 4 Intrinsic Microcrystallin
Page 54 and 55: 4.2 Electrical Conductivity Figure
Page 56 and 57: 4.3 ESR Signals and Paramagnetic St
Page 58 and 59: 4.3 ESR Signals and Paramagnetic St
Page 60 and 61: 4.4 Discussion - Relation between E
Page 62 and 63: 4.4 Discussion - Relation between E
Page 64 and 65:
Chapter 5 N-Type Doped µc-Si:H In
Page 66 and 67:
5.2 Electrical Conductivity Figure
Page 68 and 69:
5.4 Dangling Bond Density Figure 5.
Page 70 and 71:
5.5 Conduction Band-Tail States Fig
Page 72 and 73:
5.6 Discussion concentration evalua
Page 74 and 75:
5.7 Summary Figure 5.7: Schematic b
Page 76 and 77:
Chapter 6 Reversible and Irreversib
Page 78 and 79:
6.1 Metastable Effects in µc-Si:H
Page 80 and 81:
Page 82 and 83:
Page 84 and 85:
Page 86 and 87:
Page 88 and 89:
6.2 Irreversible Oxidation Effects
Page 90 and 91:
Page 92 and 93:
Page 94 and 95:
6.3 On the Origin of Instability Ef
Page 96 and 97:
6.3 On the Origin of Instability Ef
Page 98 and 99:
Chapter 7 Transient Photocurrent Me
Page 100 and 101:
7.2 Transient Photocurrent Measurem
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
7.3 Temperature Dependent Drift Mob
Page 108 and 109:
7.4 Multiple Trapping in Exponentia
Page 110 and 111:
7.5 Discussion Table 7.2: Multiple
Page 112 and 113:
7.5 Discussion 7.5.3 The Meaning of
Page 114 and 115:
Chapter 8 Schematic Density of Stat
Page 116 and 117:
silicon as shown in Fig. 8.1 b, whi
Page 118 and 119:
Chapter 9 Summary In the present wo
Page 120 and 121:
Appendix A Algebraic Description of
Page 122 and 123:
Eq. A.7 then becomes L(t) F = sin(
Page 124 and 125:
Appendix B List of Samples Table B.
Page 126 and 127:
Table B.3: Parameters of n-type µc
Page 128 and 129:
Appendix C Abbreviations, Physical
Page 130 and 131:
µ d Drift mobility µ d,h Hole dri
Page 132 and 133:
Bibliography [1] E. Becquerel. Mém
Page 134 and 135:
BIBLIOGRAPHY [21] D. Will, C. Lerne
Page 136 and 137:
BIBLIOGRAPHY [45] H. Overhof and M.
Page 138 and 139:
BIBLIOGRAPHY [66] P.W. Anderson. Ab
Page 140 and 141:
BIBLIOGRAPHY [93] J.W. Orton and M.
Page 142 and 143:
BIBLIOGRAPHY [121] J.R. Haynes and
Page 144 and 145:
BIBLIOGRAPHY [148] W. Luft and Y.S.
Page 146 and 147:
BIBLIOGRAPHY [170] G.N. Advani, R.
Page 148 and 149:
List of Publications 1. T. Dylla, R
Page 150 and 151:
Acknowledgments At this point, I wa
Page 152 and 153:
Schriften des Forschungszentrums J
Page 154 and 155:
Page 156 and 157:
Page 159:
Forschungszentrum Jülich in der He
show all

Measurements

Create successful ePaper yourself

Delete template?

Save as template?