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4.4 Discussion - Relation between ESR- and Structural Properties tures. As observed in Fig. 4.5 (b), N S increases as a function of T S , independent of the structure composition. Infrared studies have shown that with increasing T S , the amount of hydrogen incorporated in the material decreases significantly [13], which can be ascribed to an aggravated desorption of hydrogen during the growth process. The increasing N S , which is accompanied by a shift of g to higher values, indicates that the increasing spin density is associated with an increasing number of db 2 states. Because the bonded hydrogen is located mainly at the column boundaries, which passivates dangling bonds, one can conclude that the desorption leads to un-terminated db 2 states residing at the column boundaries. The question whether the ESR signal for both materials, prepared at high T S or high amorphous phase content, is a result of the same paramagnetic states cannot be decided. In this context it is interesting that even fully amorphous material prepared at the microcrystalline/amorphous transition shows the resonance at g=2.0052, rather than the expected value of g=2.0055. Also, the observed line width of the db 2 resonance is essentially the same as observed in the highly crystalline regime. The differences of the g-value, compared to that observed in a-Si:H might be a result of a different medium range order in this so-called ”polymorphous”, ”protocrystalline”, or ”edge-material” [163]. Speculatively, one might conclude that the degree of disorder around the defects is comparable to that found around the paramagnetic states located at the column boundaries. To answer this question remains a challenge for future investigations. Similarly to a-Si:H, in the microcrystalline regime (IC RS = 0.85 − 0.4), the observed spin density correlates nicely with the energy conversion efficiency of the solar cells, containing the same absorber layer. Independent of the particular deposition process, it was found that not, as one might have expected, material with the highest crystalline volume fractions, but material prepared close to the transition to amorphous growth yields the highest solar cell conversion efficiencies. The performance of the solar cell increases slightly with increasing amorphous content from IC RS = 0.71 − 0.5. On the other hand, material with the highest crystalline volume fraction leads to very poor conversion efficiencies. Apparently, the highest crystallinity and the largest grain size is only obtained at the cost of a poor defect passivation. By contrast, the increasing amorphous phase content incorporated between the crystalline columns, is a highly efficient passivation layer and the increasing hydrogen content leads to a better termination of surface states. A critical relation between N S and solar cell performance is also found for the variation of the substrate temperature. Again, material which shows the highest solar cell conversion efficiency is found to have the lowest spin density. It is likely, that the optimum of T S is because of this hydrogen desorption problem, just as for a-Si:H deposition. 49
Chapter 4: Intrinsic Microcrystalline Silicon 4.5 Summary In the previous section, the results of the study on paramagnetic defects in undoped µc-Si:H were presented. For structure compositions ranging from highly crystalline to fully amorphous, the highest N S is always found for material with the highest IC RS . While the preparation within this regime leads to the highest crystallinity and the largest grain sizes, this can be achieved only at the cost of a poor defect passivation. The increasing SCduring the process leads to a better termination of the surface states caused by the increasing hydrogen content. On the other hand, additional amorphous phase content, incorporated between the crystalline columns, acts as a passivation layer and leads to a better termination of surface states. Very low N S are found for material close to or beyond the transition to amorphous growth (see Fig. 4.5 (a)). For HWCVD material a strong dependence of the spin density N S on the deposition temperature T S was found. Generally, the increasing spin density, caused by either low SC or high T S during the deposition process leads to an increasing contribution of the db 2 resonance. On the other hand, the g-value shifts to higher values with increasing amorphous content. Surprisingly, even material prepared close to the structural transition, with no discernable crystalline signal in the Raman spectra, does not show the typical a-Si:H value of g=2.0055. In fact, the ESR signal is clearly dominated by the resonance at g=2.0052. Only for material prepared at yet higher silane concentrations can g=2.0055 be detected (see Fig. 4.8). Whether the differences of the g-value compared to that observed in a-Si:H are the result of a different medium range order [61] cannot be determined from these data and remains e a task for future investigation. A detailed study of material prepared at transition between microcrystalline and amorphous growth using TEM and ESR, could provide further information on how changes of the structure, in particular the medium range order, affect the position of the resonance line observed in ESR. This might also provide further details concerning the nature of defects in µc-Si:H. 50
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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
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
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Page 28 and 29: 2.3 Charge Carrier Transport influe
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Page 32 and 33: Chapter 3 Sample Preparation and Ch
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Page 36 and 37: 3.1 Characterization Methods Homoge
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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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Page 60 and 61: 4.4 Discussion - Relation between E
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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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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