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7.2 Transient Photocurrent Measurements B the capacitance decreases with increasing specimen thickness d i , as expected from Eq. 3.13 (see table 7.1). However, for both samples the measured values are higher than one would expect from the geometry of the structure. With increasing applied reverse voltage the capacitance decreases, indicating an increasing depletion layer width. The capacitance also decreases with decreasing temperature (see Fig. 7.1 (b)). However, even for the highest voltage of V = 12 V or the lowest temperature, the capacitance of samples A and B deviates considerably from C geom . The excess of C indicates that the depletion width d w (see table 7.1) does not extend over the whole sample thickness and the electric field is concentrated in only a fraction of the sample. From the capacitance measurements the specimen investigated can be divided into two groups. For samples C and D the depletion width d w is close to the physical layer thickness d i . This criterion corresponds to a nearly uniform electric field across the intrinsic layer during the time-of-flight experiment. On the other hand, sample A, B do not satisfy this criterion. The shortage of the depletion width, compared to the i-layer thickness, indicates that the electric field is not uniformly distributed and rather concentrated to only a fraction of the sample. 7.2 Transient Photocurrent Measurements Transient photocurrent measurements were applied to all samples listed in table 7.1, in order to determine the properties of the drift of holes. The first part of this section deals with the transients taken of specimens A and B. Since a detailed analysis of these transients failed due to the unknown electric field distribution, results will be discussed qualitatively only. In the second part photocurrent transients of specimens C and D are presented. As shown above, for both samples the electric field, applied during in the TOF experiment, is uniformly distributed across the i-layer and the transients could be analyzed in terms of a standard time-of-flight analysis. 7.2.1 Non-Uniform Electric Field Distribution Although the electric field distribution within samples A and B is not uniform, which is a basic requirement for a TOF experiment to be suitable, transient photocurrent measurements were also done on these samples. In Fig. 7.2 photocurrent and photocharge transients for different applied voltages measured for specimens A and B are shown. All transient currents were taken at a temperature of 300K; the photocharge transients were determined by integrating the currents. The samples have been illuminated through the n-layer using an excitation wavelength of 87
Chapter 7: Transient Photocurrent Measurements Figure 7.2: Transient photocurrent and photocharge measured for (a, b) sample A and sample B are shown (a, b) and (c, d), respectively. The transients were taken at a temperature of T = 300K for different applied voltages at room temperature. λ=500 nm (E Photon =2.48eV). For samples A and B with a Raman crystallinity of IC RS ∼0.7 the absorption coefficient is α ≈5.5×104 cm −1 , which corresponds to an absorption depth in the intrinsic layer of about 0.16 µm [175]. While the electrons, generated very close to the n-layer, are swept away very quickly (within nanoseconds), the photogenerated holes have to traverse most of the thickness of the intrinsic layer before reaching the p-layer. Thus nearly the entire photocurrent corresponds to hole motion (see section 3.1.4 for a detailed description). As discussed above, for both samples A and B the depletion layer width d w is considerably lower than the real i-layer thickness d i (see table 7.1). This indicates a nonuniform distribution of the electric field within the specimen and, therefore, unsurprisingly the transients do not show any typical shape, i.e. neither Gaussian nor dispersive transport behavior (compare section 2.3.2 and Fig. 2.4). In fact, an interpretation of the transients in terms of the properties of charge carrier transport is impossible, because of the unknown electric field distribution. However, 88
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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
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3.3 Sample Preparation 3.3.1 Sample
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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
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Page 64 and 65: Chapter 5 N-Type Doped µc-Si:H In
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Page 68 and 69: 5.4 Dangling Bond Density Figure 5.
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Page 74 and 75: 5.7 Summary Figure 5.7: Schematic b
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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
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Page 106 and 107: 7.3 Temperature Dependent Drift Mob
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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
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Acknowledgments At this point, I wa
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Schriften des Forschungszentrums J
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