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7.2 Transient Photocurrent <strong>Measurements</strong> the transients show some interesting features which will be discussed below. In Fig. 7.2 (a) photocurrent transients of sample A are shown. Capacitance measurements have shown that the depletion layer has a width of d w = 1.3 µm and extends to about 1/3 of the real i-layer thickness (see table 7.1). One can observe that, after an increase of the current (t < 2 × 10 −8 s), which is caused by the RC-constant of the measurement circuit, the current starts to decrease with a power-law in time. The decrease starts at earlier times for increasing bias voltage and a ”crossover” of the currents can be observed. The power-law decay is independent of the applied voltage and one does not observe any change in slope corresponding to a transit time, as expected for dispersive transport (compare section 2.3.2). The photocharge (panel (b)) determined by integrating the photocurrent transients approaches an asymptotic value at prolonged times, indicating that Q 0 is still a good estimate of the photoinjected charge. On the other hand, the transits measured on sample B show a quite unexpected behavior. The photocurrent transients and the photocharge are shown in Fig. 7.2 (c, d). For this sample, the depletion layer width extends only to one fourth of the i-layer thickness. The currents shown in panel (c) show a shape that strongly depends on the applied voltage. As for early times (t < 2 × 10 −8 s) the current is determined by the RC rise time of the measurement circuit, the current for e.g. the 1 V transient drops down to almost zero for about 80 ns before it increases again. The current peaks at about 200 ns before it starts to decrease with a power-law decay as observed for specimen A. In the photocharge transient plotted in panel (d) this effect is observable in a plateau at times between t = 2 × 10 −8 s and 1 × 10 −7 s. With increasing voltage the time range of ”zero current” becomes shorter, the effect is less pronounced and disappears for the highest voltage V = 10 V. The ”zero-current” and the plateau observed in the charge transients indicate that within the sample there are regions, where the energy of the externally applied field is smaller than kT and the charge carrier packet has to cross this section by diffusion. As diffusion is a statistical process no current can be observed. At the time the packet has crossed this low electric field region, the charge carrier drift can again be measured in the external circuit. With increasing applied voltage the depletion width increases and therefore the electric field penetrates deeper into the film; the effect becomes less pronounced. Unfortunately, because of the unknown electric field distribution within the sample, the transients can not be readily analyzed to obtain drift-mobilities. Even if the field distribution were known, there are no established techniques for analyzing transients with nonuniform fields to obtain dispersive mobility parameters. The reason for the nonuniform electric field might be a high concentration of free charge carrier in this intentionally undoped material. A series of samples with different compensation levels could provide additional information [143]. 89
Chapter 7: Transient Photocurrent <strong>Measurements</strong> 7.2.2 Uniform Electric Field Distribution Unlike samples A and B, in samples C and D the depletion layer thickness d w extends over the whole i-layer, indicating a uniform distribution of the electric field. Fig. 7.3 (a) and (c) show photocurrent transients taken at T = 250K on specimens C and D for several applied bias voltages. The transient photocurrents were measured for several bias voltages in the range between V=0 V and V=6 V. The transient at 0Visduetothe”built-in” electric field, and indeed such transients have been used to infer the internal fields (see e.g. [176]). Each transient shows two power-law segments, the ”pretransit” with a fairly shallow power-law decay and the ”posttransit” regime with a steep decay. This is a signature of dispersive transport (compare section 2.3.2). The currents, obtained for different voltages, do intersect, which indicates a voltage dependent transit time t τ . In other words, with increasing voltage the change in the power law occurs earlier, resulting in a ”crossover” of the currents. Because the time where the change in the power law occurs is interpreted as t τ , this indicates a decreasing transit time upon increasing V. Fig. 7.3 (b, d) show the transient photocharge Q(t,V) for varying externally applied voltage obtained by integrating the transient photocurrent I(t) (shown in Fig. 7.3 (a, c)). The photocharge at longer times and for higher voltages approach a constant value, denoted as Q 0 , which was used to estimate the total photogenerated charge. The fact, that the charge measurement for the higher voltages approach the same asymptotic value for the total charge of Q 0 =25 pC and Q 0 =20 pC for sample C and D, respectively, indicates that Q 0 is likely a good estimate of the charge of photogenerated holes. It seems important to note, that the difference of Q 0 between both samples is due to different laser intensities and not due to any loss of photoexcited charge carriers within the intrinsic layer. This is different for the 0 V applied voltage. Even though the photocurrent transient has the form expected for time-of-flight, it does not show the correct asymptotic photocharge. In this case holes are indeed trapped, presumably in dangling bonds, before they could reach the collecting electrode. The time required for a charge carrier to reach the collecting electrode is of the order of the deep trapping life time t D , hence not all the photoinjected charge can be collected. In Fig. 7.4 the collected photocharge Q(∞) for sample C and D is plotted versus the externally applied voltage V. The value of Q(∞) was determined after t = 6 ×10 −6 s for both samples; for higher voltages Q(∞) saturates indicating that all the photoinjected charge has been collected. Determining the photoinjected charge from the high voltage transients, the deep trapping mobility-lifetime product µτ h,t can be evaluated using µτ h,t = Q(∞) Q 0 90 d 2 i (V + V int ) , (7.1)
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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
Page 52 and 53: Chapter 4 Intrinsic Microcrystallin
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Page 56 and 57: 4.3 ESR Signals and Paramagnetic St
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Page 88 and 89: 6.2 Irreversible Oxidation Effects
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Page 118 and 119: Chapter 9 Summary In the present wo
Page 120 and 121: Appendix A Algebraic Description of
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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
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