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Abstract The electronic properties of microcrystalline silicon (µc-Si:H) films have been studied using electron spin resonance (ESR), transient photocurrent time-of-flight (TOF) techniques, and electrical conductivity measurements. Structural properties were determined by Raman spectroscopy. A wide range of structure compositions, from highly crystalline films with no discernable amorphous content, to predominantly amorphous films with no crystalline phase contributions, was investigated. Models and possible explanations concerning the nature and energetic distribution of electronic defects as a function of film composition are discussed. It is shown that the spin density N S in µc-Si:H films is linked strongly to the structure composition of the material. The highest N S is always found for material with the highest crystalline volume fraction. With increasing amorphous content, N S decreases, which is attributed to increasing hydrogen content and improved termination of dangling bonds. Moreover, the amorphous phase content, incorporated between the crystalline columns, appears to act as a passivation layer, leading to more effective termination of unsatisfied bonds at the column boundaries. Both reversible and irreversible changes in the ESR signal and dark conductivity due to atmospheric effects are found in µc-Si:H. These are closely connected to the structure composition, in particular the active surface area. The porous structure of highly crystalline material facilitates in-diffusion of atmospheric gases, which strongly affects the character and/or density of surface states. Two contributing processes have been identified, namely adsorption and oxidation. Both processes lead to an increase of N S . In the case of adsorption the increase is identified as arising from changes of the db2 resonance (g=2.0052), while the intensity of the db1 resonance (g=2.0043) remains constant. With increasing amorphous content the magnitude of both adsorption and oxidation induced changes decreases, which may be linked to the greater compactness of such films. <strong>Measurements</strong> on n-type µc-Si:H films were used as a probe of the density of gap states, confirming that the spin density N S is related to the density of defects. The results confirm that for a wide range of structural compositions, the doping induced Fermi level shift in µc-Si:H is governed by compensation of defect states, v
for doping concentrations up to the dangling bond spin density. At higher concentrations a doping efficiency close to unity was found, confirming that in µc-Si:H the measured spin densities represent the majority of gap states (N S = N DB ). The nature and density of defects is of great importance in determining electronic transport properties. By applying the TOF technique to study pin solar cells based on µc-Si:H, conclusive hole drift mobility data were obtained. Despite the predominant crystallinity of these samples, the temperature-dependence of hole transport is shown to be consistent with multiple-trapping in an exponential distribution of band tail states, behavior that is frequently associated with non-crystalline materials. A valence band tail width of 31 meV, and hole band mobilities of 1 − 2cm 2 /Vs, were estimated from the data. These measurements support the predominance of mobility-edge transport for holes in these microcrystalline films, and extend the range of materials for which an apparently universal band mobility of order 1 cm 2 /Vs is obtained. vi
Page 1 and 2: Forschungszentrum Jülich in der He
Page 4 and 5: Forschungszentrum Jülich GmbH Inst
Page 6 and 7: Kurzfassung In der vorliegenden Arb
Page 10 and 11: Contents 1 Introduction 1 2 Fundame
Page 12 and 13: CONTENTS C Abbreviations, Physical
Page 14 and 15: Chapter 1 Introduction Solar cells
Page 16 and 17: defect states provided that they ar
Page 18 and 19: Chapter 8: In this chapter, the inf
Page 20 and 21: Chapter 2 Fundamentals In this firs
Page 22 and 23: 2.2 Electronic Density of States St
Page 24 and 25: 2.2 Electronic Density of States ta
Page 26 and 27: 2.2 Electronic Density of States Fi
Page 28 and 29: 2.3 Charge Carrier Transport influe
Page 30 and 31: 2.3 Charge Carrier Transport functi
Page 32 and 33: Chapter 3 Sample Preparation and Ch
Page 34 and 35: 3.1 Characterization Methods Figure
Page 36 and 37: 3.1 Characterization Methods Homoge
Page 38 and 39: 3.1 Characterization Methods µ Fig
Page 40 and 41: 3.1 Characterization Methods compar
Page 42 and 43: 3.1 Characterization Methods bution
Page 44 and 45: 3.1 Characterization Methods posite
Page 46 and 47: 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
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4.3 ESR Signals and Paramagnetic St
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4.4 Discussion - Relation between E
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4.4 Discussion - Relation between E
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Chapter 5 N-Type Doped µc-Si:H In
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5.2 Electrical Conductivity Figure
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5.4 Dangling Bond Density Figure 5.
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5.5 Conduction Band-Tail States Fig
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5.6 Discussion concentration evalua
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5.7 Summary Figure 5.7: Schematic b
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Chapter 6 Reversible and Irreversib
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6.1 Metastable Effects in µc-Si:H
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6.2 Irreversible Oxidation Effects
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6.3 On the Origin of Instability Ef
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6.3 On the Origin of Instability Ef
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Chapter 7 Transient Photocurrent Me
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7.2 Transient Photocurrent Measurem
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7.3 Temperature Dependent Drift Mob
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7.4 Multiple Trapping in Exponentia
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7.5 Discussion Table 7.2: Multiple
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7.5 Discussion 7.5.3 The Meaning of
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Chapter 8 Schematic Density of Stat
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silicon as shown in Fig. 8.1 b, whi
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Chapter 9 Summary In the present wo
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Appendix A Algebraic Description of
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Eq. A.7 then becomes L(t) F = sin(
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Appendix B List of Samples Table B.
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Table B.3: Parameters of n-type µc
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Appendix C Abbreviations, Physical
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µ d Drift mobility µ d,h Hole dri
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Bibliography [1] E. Becquerel. Mém
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BIBLIOGRAPHY [21] D. Will, C. Lerne
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BIBLIOGRAPHY [45] H. Overhof and M.
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BIBLIOGRAPHY [66] P.W. Anderson. Ab
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BIBLIOGRAPHY [93] J.W. Orton and M.
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BIBLIOGRAPHY [121] J.R. Haynes and
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BIBLIOGRAPHY [148] W. Luft and Y.S.
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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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Forschungszentrum Jülich in der He
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