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7.3 Temperature Dependent Drift Mobility assumption of an uniformly distributed internal field, made above, is valid. At this temperature, the envelope declines as a power-law (t −1+α ) where α=0.70 and α=0.67 are the dispersion parameter for sample C and D, respectively. From the ”pretransit” regime the individual transits break at increasingly earlier times with increasing voltage. These breaks can be used to obtain a rough estimate of the transit times. For e.g. sample C the current transients exhibit ”transit times” varying from about 500 ns (for 0.5 V) to 70 ns (for 4V). For earlier times there is a noticeable increase of the photocurrent above this envelope, especially for the lower voltages. As already discussed above the absorption depth of the laser light was about 160 nm, which is about 5% of the sample thickness; electron motion is thus responsible for about 5% of the total photocharge. The excess current between 2 × 10 −8 s and 6 × 10 −8 s is consistent with a rapid collection of this charge of electrons. To summarize: Allowing to correct the externally applied voltage by a voltage due to the internal field the photocurrent transients show typical features expected for conventional time-of-flight interpretation. (i) Each transient can be divided in a ”pretransit” with shallow and ”posttransit” region with fairly steep power-law decay. (ii) The current transients exhibit ”transit times” increasing with decreasing applied electric field. (iii) The charge measurement for the high voltages approach the same asymptotic value for the total photocharge, indicating that Q 0 is likely a good estimate of the charge of photogenerated holes. 7.3 Temperature Dependent Drift Mobility To get a closer insight into the processes determining the transport, transient photocurrents were measured in a range of temperatures between 125K and 300K. Fig. 7.6 displays, as an example, the transient photocurrent of sample C taken at V = 1V for several temperatures T in the range between 300K and 200K. The photocurrent transients show typical signs for temperature activated hole drift mobilities µ h,T . With decreasing temperature a declining magnitude of the current and a shift of the ”kink” to higher values can be observed. The photocharge transients Q(t), determined by integrating the currents I(t), approach the same asymptotic value for all temperatures. This indicates that the photogeneration quantum efficiency is independent of temperature and deep trapping is negligible in the measured temperature range. The dashed line in Fig. 7.6 (b) indicates the value of half the total collected photocharge Q 0 /2. The times, where the photocharge transits cross this line can be interpreted as the time where half the charge has been collected (see section 3.1.4.3 for a detailed discussion). This value will be used as an estimate for the transit time t τ used to calculate hole drift mobilities. 93
Chapter 7: Transient Photocurrent <strong>Measurements</strong> Figure 7.6: (a) Temperature dependence of the transient photocurrent of sample C and (b) of the transient photocharge calculated by integrating the corresponding currents in panel (a). In Fig. 7.7 the temperature dependence for the average drift mobility of holes determined for sample C and D is illustrated. This average drift mobility is calculated as µ d,h = L Ft τ (7.2) where t τ is the transit time for a particular ratio of a hole displacement L and the electric field F. Within this work L F = d 2 2(V + V int ) = 7 × 10−8 cm 2 /V (7.3) was used. Note, that for dispersive transport systems, drift mobilities for different materials must be compared at a specific value of L/F [131]. The straight line is a fit to the data of sample D, for which the measurement had the least scatter. However, sample C and D have essentially the same average hole drift mobility. The drift mobilities are simply activated with an activation energy of E A = 0.13eV. 7.4 Multiple Trapping in Exponential Band-Tails For sample C and D the model of multiple trapping in an exponential band-tail [100, 101, 98, 102, 103] was applied to the drift mobility data presented in section 7.2.2. The basic features of the model, which has been successfully applied to amorphous semiconductors, are discussed in section 2.4 and appendix A. In the multiple trapping model electronic states are simply divided into transport states (where the charge carriers are mobile) and traps, which simply immobilize the 94
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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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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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