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Chapter 7 Transient Photocurrent <strong>Measurements</strong> In the previous Chapters, the density and distribution of defects as well as bandtail states in µc-Si:H were studied. The presence of these localized states strongly influences the charge carrier transport in the material. Time resolved charge transport measurements are a powerful tool, which have been successfully applied to e.g. a-Si:H [97] to study charge transport mechanisms and electronic states associated with the transport. In µc-Si:H there is, of course, a wide range of possible structure compositions giving rise to the possibility that transport properties may also vary considerably. In this section, a study of hole drift mobility measurements performed on material prepared under conditions for optimum solar cell performance will be presented. Various samples, as listed in table 7.1, have been prepared as described in section 3.3.2 and charge carrier transport has been investigated by time-of-flight experiments. The specimens can be divided into two groups. While for samples C and D the depletion width d w extends over the entire i-layer thickness d i , for samples A and B this is not the case. 7.1 Electric Field Distribution The standard time-of-flight analysis requires a uniform distribution of the externally applied field within the specimen. The method relies on the fact, that a typical time-of-flight sample acts as a capacitor, thus Eq. 3.13 applies. Ideally, the film is fully depleted and the depletion layer d w extends over the entire sample thickness d i , indicating a uniformly distributed electric field within the specimen. While this behavior is typically observed for a-Si:H diodes, capacitance measurements performed on microcrystalline silicon diodes suggest that in µc-Si:H this is not always the case [143, 144]. 85
Chapter 7: Transient Photocurrent <strong>Measurements</strong> Table 7.1: Properties of microcrystalline silicon pin diodes used in the TOF experiment. Listed are the Raman intensity ratio IC RS , the i-layer thickness d i, the depletion width d w , the capacitance C(4V,300K), and C geom calculated from the geometry (see section 7.1). Sample IC RS d i d w C(4V,300K) C geom [µm] [µm] [pF] [pF] A 0.71 4.0 1.3 60 20 B 0.70 6.5 1.7 46 13 C 0.60 3.4 3.4 22 22 D 0.61 4.3 4.3 18 18 Depletion Layer Capacitance Capacitance measurements have been performed on samples A-D (see table 7.1) for different applied voltages V and varying temperatures T, as described in section 3.13. The measured values of the capacitance are plotted in Fig. 7.1. Also indicated are the geometrical values C geom calculated using Eq. 3.13 with d w = d i .In Fig. 7.1 (a) the measured values of the capacitance are plotted as a function of the applied voltage, measured at a temperature of 300K. One can observe that, from the characteristic of the capacitance the specimens can be divided into groups. While for sample C and D the capacitance is independent of the applied voltage and agrees very well with the values expected from the geometrical dimensions, samples A and B significantly deviate from this behavior. For samples A and Figure 7.1: Sample capacitance of the samples A-D versus (a) the externally applied voltage and (b) the temperature. The geometrical capacitance C geom shown in the legend has been calculated using Eq. 3.13 and d w = d i . 86
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Forschungszentrum Jülich in der He
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Forschungszentrum Jülich GmbH Inst
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Kurzfassung In der vorliegenden Arb
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Abstract The electronic properties
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Contents 1 Introduction 1 2 Fundame
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CONTENTS C Abbreviations, Physical
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Chapter 1 Introduction Solar cells
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defect states provided that they ar
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Chapter 8: In this chapter, the inf
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Chapter 2 Fundamentals In this firs
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2.2 Electronic Density of States St
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2.2 Electronic Density of States ta
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2.2 Electronic Density of States Fi
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2.3 Charge Carrier Transport influe
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2.3 Charge Carrier Transport functi
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Chapter 3 Sample Preparation and Ch
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3.1 Characterization Methods Figure
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3.1 Characterization Methods Homoge
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3.1 Characterization Methods µ Fig
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3.1 Characterization Methods compar
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3.1 Characterization Methods bution
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3.1 Characterization Methods posite
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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 88 and 89: 6.2 Irreversible Oxidation Effects
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 100 and 101: 7.2 Transient Photocurrent Measurem
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.
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List of Publications 1. T. Dylla, R
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Acknowledgments At this point, I wa
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Schriften des Forschungszentrums J
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