Surface and bulk passivation of multicrystalline silicon solar cells by ...

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77 5.2 Objective An electronic model is utilized to understand the nature of defect clusters, their formation mechanism, their effects on solar cell performance and to assess the limitation on cell performance posed by defect clusters. New gettering/passivation techniques to achieve further improvements in cell efficiency and effectively getter defect clusters are evaluated. 5.3 Characterization of Defect Clusters Multicrystalline Si used for commercial solar cells is grown either as ribbons or cast in a crucible and then sliced into wafers by wire sawing. The PV industry has accepted two basic measures to lower the cost of mc-Si substrates. The first is to utilize substrates with high impurity content, which result from the use of cheaper, lower-grade feedstock (consisting of tops and tails, off-spec rejects from the microelectronic industry). The PV industry has compromised cleanliness of the growth process. Typically, the as-grown material has high concentrations of C and/or O. It also contains transition metal impurities (such as Fe, Cr) in levels reaching 1014cm-3, which are detrimental to the minority-carrier lifetime. Typically, the average minority-carrier lifetime of as-grown material is < 10 μs. The second measure involves the use of much higher crystal growth speeds as compared to conventional crystal growth, resulting in higher thermal stresses accompanied by high densities of defects in asgrown form. Typically, the as-grown material has an average defect density of about 5x105cm-2. A unique feature of current mc-Si wafers is that they contain "defect clusters"— crystal defects that clump together forming extended defect regions, which remain separated from each other.
78 Clustering of defects can be seen from the defect mαρ of a commercial 4.25-in x 4.25-in mc-Si wafer as shown in Figure 5.1. This map was generated by a commercial instrument called GT-PVSCAN, and shows that the majority of the wafer has a low (and much of it nearly zero) dislocation density [110]. The average value of the dislocation density is about 4 x 10 s cm -2 . However, the presence of defect clusters can be seen as dark regions in this otherwise very low defect density material. These regions can have defect density as high as 10 7cm-2. An inset in the figure shows a magnified region of a large defect cluster (identified in the defect mαρ by arrows). A detailed structure of a defect cluster can be seen after etching the wafer [1 1 1]. Figure 5.1 A defect map of a commercial mc-Si wafer showing clustering of defects as dark regions. The inset shows a magnified region indicated by arrows [112].
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Copyright Warning & Restrictions Th
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ABSTRACT SURFACE AND BULK PASSIVATI
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SURFACE AND BULK PASSIVATION OF MUL
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APPROVAL PAGE SURFACE AND BULK PASS
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Chuan Li, B.L. Sopori, P. Rupnowski
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ACKNOWLEDGEMENT The work presented
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TABLE OF CONTENTS (Continued) Chapt
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LIST OF TABLES Table Page 2.1 Posit
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LIST OF FIGURES (Continued) Figure
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LIST OF FIGURES (Continued) Figure
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2 percent; however, soon, more adva
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4 Figure 1.1 World solar module pro
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6 bond is called a hole. It too can
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8 Figure 1.4 The I-V characteristic
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10 First generation cells consist o
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12 Basically, materials for manufac
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14 Defects are generally categorize
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16 copper, or nickel in concentrati
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18 the SiNx:H layer during the ther
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CHAPTER 2 SILICON NITRIDE LAYER FOR
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22 reflectance of polished Si can b
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24 information is application-orien
Page 45 and 46: 26 film fed growth (EFG) ribbon sil
Page 47 and 48: 28 Figure 2.5 Deposition of SiΝ :
Page 49 and 50: 30 Figure 2.6 shows the dependence
Page 51 and 52: 32 atoms, the interface states are
Page 53 and 54: 34 2.5 Bulk Passivation of Si by Si
Page 55 and 56: 36 It was found that the bulk lifet
Page 57 and 58: CHAPTER 3 MODELING OF SURFACE RECOM
Page 59 and 60: 40 Figure 3.2 Schematic diagram of
Page 61 and 62: 42 σ and σp are the capture cross
Page 63 and 64: 44 Qsi — charge density induced i
Page 66 and 67: 47 Figure 3.5 The calculated depend
Page 68 and 69: 49 * 10 Λ m; m is in a range from
Page 70 and 71: 51 Na, sigma_n, sigma_p: enter x.xx
Page 72 and 73: 53 Figure 3.7 Measured Seff(Δn) de
Page 74 and 75: 55 curves converge to a single valu
Page 76 and 77: 57 seen that, initially Ss decrease
Page 78 and 79: 59 carrier recombination within the
Page 80 and 81: 61 recombination in the SCR influen
Page 82 and 83: 63 Figure 3.13 shows that: 1) after
Page 84 and 85: CHAPTER 4 MINORITY-CARRIER LIFETIME
Page 86 and 87: 67 Figure 4.1 Α photograph of QSSP
Page 88 and 89: 69 work. The most convenient is 1 m
Page 90 and 91: 7Ι dependence of the minority carr
Page 92 and 93: 73 It was tempting to assume that l
Page 94 and 95: 75 resistivities and lifetime) do n
Page 98 and 99: 79 Figure 5.2 is a photograph of a
Page 100 and 101: 81 impurity-gettering methods which
Page 102 and 103: 83 distribution of local currents a
Page 104 and 105: 85 modeling. Wafers were selected f
Page 106 and 107: 87 Figure 5.5 A comparison of (a) d
Page 108 and 109: 89 alloying results in metallizatio
Page 110 and 111: 91 (i) Defect clusters are the prim
Page 112 and 113: 93 SiNX induced charge density on t
Page 114 and 115: APPENDIX I PROGRAMS TO CALCULATE SR
Page 116 and 117: 97 phin = -ΕΙ - 1 / beta * log(nd
Page 118 and 119: 99 ίter3 = 0 for xi=1 to nmax/2-1
Page 120 and 121: 101 input "output file name {XXXXXX
Page 122 and 123: 103 F (i) = (exp (beta * (phip - ph
Page 124 and 125: APPENDIX III COMPUTATIONAL METHOD F
Page 126 and 127: 107 where, dscr is the width of the
Page 128 and 129: REFERENCES 1. E. Becquerel , C. R.
Page 130 and 131: 44. L.L. Alt, S.W. Ing. Jr. and K.W
Page 132 and 133: 90. B.L. Sopori, Y. Zhang, and N.M.
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