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7.5 Discussion Table 7.2: Multiple trapping fitting parameters Multiple trapping Parameter µc-Si:H µc-Si:H a-Si:H sample C sample D ref. [142] Valence band mobility µ 0 [cm 2 /Vs] 1.0 2.3 0.7 Band-tail width ∆E V [meV ] 31 32 45 Attempt-frequency ν [s −1 ] 9× 10 8 5 × 10 9 1 × 10 12 time-of-flight measurements, estimation of drift mobilities is best done in samples for which the depletion width d w is close to the physical layer thickness d i , which corresponds to a nearly uniform electric field across the intrinsic layer during the time-of-flight experiment. As can been seen in table 7.1 this criterion is not always fulfilled for the specimens investigated. In this section, the data of the samples where a standard time-of-flight analysis could be successfully applied will be summarized and discussed in more detail. 7.5.1 Photocurrent and Photocharge Transients The photocurrent and photocharge transients presented in section 7.2.2 show several features expected for a conventional time-of-flight interpretation. First, the charge measurements for the higher voltages approach the same asymptotic value for the total photocharge Q 0 . This indicates that Q 0 is likely a good estimate of the charge of photogenerated holes and that for the higher voltages no loss of photoexcited charge carriers occurs, e.g. due to deep trapping. Plotting the currents on a log-log scale, the photocurrent decays consist of two linear branches, indicating a power-law behavior typical for dispersive transport. Additionally, the current transients exhibit ”transit times” (where the power-law decay steepens markedly) varying from about 500 ns (for 0.5 V) to 70 ns (for 4 V). Even the 0 V transient has the form expected for time-of-flight, but does not have the correct asymptotic photo-charge. It seems reasonable that for 0V bias voltage applied the holes were ultimately trapped by deep levels (not band-tail states) during transit. Plotting the collected photocharge versus the voltages, a deep-trapping mobility-lifetime product for holes µτ h,t of about 1 × 10 −7 cm 2 V could be evaluated. This value is in quite good agreement with earlier reports from Brueggemann et al. [23]; however, Juška and coworkers found a value which is about one magnitude lower than the one reported here [178]. These Hecht plots (see Fig. 7.4) have also been used to determine the internal field caused by the pin structure of the specimen. The obtained values of V int = 0.3 V and 0.4 V are quite reasonable and expected for 97
Chapter 7: Transient Photocurrent <strong>Measurements</strong> a semiconductor device with a band gap of E G = 1.1 eV. Considering the internal electric field, the normalized currents shown in Fig. 7.5 establish an ”envelope” curve, indicating that, for a given time, the drift of holes depends linearly on electric field. 7.5.2 Hole Drift Mobilities The drift mobilities have been evaluated using Eq. 3.9. At room temperature hole drift mobilities of µ d,h ≈ 1cm 2 /Vs are obtained from these results. Additionally, temperature dependent photocurrent transients have been recorded in a range between 100K and 300K. The average drift mobility was evaluated at a particular ”displacement/field” ratio of L/F = 7 × 10 −8 cm 2 /V. The hole drift mobilities derived are much larger than that of a-Si:H, where typical values are about two orders of magnitude smaller [142]. However, a direct experimental comparison at the same value for L/F is not possible, because the transit times t τ in amorphous silicon expected for this L/F value would by far exceed the deep trapping life time τ d . Thus, holes would already be captured by deep traps without any chance of release before they can reach the collecting electrode. Plotting µ d,h in an Arrhenius plot shows that the hole drift mobility is simply activated with an activation energy of E A = 0.13 eV. The physical meaning of E A for multiple trapping in an exponential distribution of traps is still unclear. It has been suggested that E A corresponds to an average energy required for a charge carrier trapped to release it above the mobility edge [100], but does not describe any particular feature, like a band-tail width or the depth of any particular trap. Although electron and hole transport are crucial properties of these films, there are only few conclusive data regarding the drift mobilities of these carriers in µc- Si:H, and even less understanding of the physics governing carrier drift. The main experimental problem has been the high dark conductivity of many of the microcrystalline materials, which militates against conventional photocarrier time-offlight estimation of drift mobilities. Mobility estimates have been reported using time-of-flight on specially compensated samples [143] and also using a novel ”photo-CELIV” approach [179]. The results presented here differ qualitatively from these works performed on different samples of µc-Si:H, which found only a weak temperature-dependence [143, 179] and a strong electric-field dependence of the hole drift mobility [179]. The previous work differs from this both in samples measured and methods applied. However, recent work also performed on one sample prepared under conditions for optimized solar cells agrees in both the magnitude of the hole drift mobility and activation energy of µ d,h [49]. 98
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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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3.3 Sample Preparation reverse bias
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Chapter 4 Intrinsic Microcrystallin
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4.2 Electrical Conductivity Figure
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4.3 ESR Signals and Paramagnetic St
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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 98 and 99: Chapter 7 Transient Photocurrent Me
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 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
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