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

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15 micro-volume defects. Growth of PV multicrystalline (mc) Si materials usually requires a high pulling rate, so they contain a higher level of vacancies than interstitials [33]. Dislocations are an example of line defects [(b) above]. Dislocations represent boundaries between slipped and unslipped regions of a crystal. The formation of dislocation lowers the total free energy to relieve the tension caused by the temperature gradient during crystal growth and cooling. Dislocations in silicon may be dissociated into glide and be involved in the deformation behavior of silicon [34]. As a result of elastic distortions associated with a dislocation, band bending occurs in its vicinity. Dangling bonds are also created along the core of the dislocation and introduce energy levels in the bandgap. Stacking faults [(e) and (h) above], grain boundaries and twin planes are examples of area defects. Stacking faults arise from excess silicon self-interstitials generated during oxidation. Grain boundaries are formed during crystal growth. In polycrystalline silicon material grown by casting process, a large amount of grain boundaries can be induced. Grain boundaries may be treated as an assemblage of dislocations whose properties depend on the crystallography of the boundary; and, their electrical activities are connected with the set of dislocations and constitute a boundary. Precipitates [(d) above] and impurity clusters are examples of volume defects. During processing, contamination by metallic impurities is also present. Unlike the intentional doping of shallow level impurities, metallic impurities may be incorporated without notice due their high solubilities in silicon. Transition atoms, such as Fe, Co, Cr, Ni, Cu, in the silicon lattice are believed to introduce energy levels in the bandgap [35]. Multicrystalline silicon (mc-Si) solar cells can tolerate iron,
16 copper, or nickel in concentrations up to 10 14-10 15 cm 3 [36] because metals in mc-Si are often found in less electrically active inclusions or precipitates at structural defects (e.g., grain boundaries) rather than being atomically dissolved. Within the crystal, impurities can act in isolation as recombination centers or can be precipitated at crystallographic defects, with the combined defect acting as an efficient recombination site. Once precipitated, it is generally considered more difficult to remove the impurity. In order to achieve high device efficiency, cell fabrication processing must include steps that can remove as-grown impurities and defects as much as possible and passivate the remaining impurities and defects. However, to maintain cost effectiveness, these processes must be included as a part of a typical cell-fabrication sequence without increasing the number of process steps. 1.6 Impurity Gettering in PV-Si The performance of solar cells would be quite poor if the concentration of impurities in the device is as high as in the as-grown PV-Si. Some of the impurities are removed during device processing. This mechanism, called gettering, has been used in microelectronic devices to trap impurities away from the active region of the device by oxygen precipitates. The general mechanism of gettering can be described by the following steps: 1) the impurities are released into solid solution from whatever precipitate they are in; 2) they undergo diffusion through the silicon; 3) they are trapped by defects such as dislocations or precipitates in an area away from device regions. There are two general classifications of gettering, namely, extrinsic, and intrinsic. Extrinsic gettering refers to gettering that employs external means to create
Page 1 and 2: Copyright Warning & Restrictions Th
Page 3 and 4: ABSTRACT SURFACE AND BULK PASSIVATI
Page 5 and 6: SURFACE AND BULK PASSIVATION OF MUL
Page 7 and 8: APPROVAL PAGE SURFACE AND BULK PASS
Page 9 and 10: Chuan Li, B.L. Sopori, P. Rupnowski
Page 11 and 12: ACKNOWLEDGEMENT The work presented
Page 13 and 14: TABLE OF CONTENTS (Continued) Chapt
Page 15 and 16: LIST OF TABLES Table Page 2.1 Posit
Page 17 and 18: LIST OF FIGURES (Continued) Figure
Page 19 and 20: LIST OF FIGURES (Continued) Figure
Page 21 and 22: 2 percent; however, soon, more adva
Page 23 and 24: 4 Figure 1.1 World solar module pro
Page 25 and 26: 6 bond is called a hole. It too can
Page 27 and 28: 8 Figure 1.4 The I-V characteristic
Page 29 and 30: 10 First generation cells consist o
Page 31 and 32: 12 Basically, materials for manufac
Page 33: 14 Defects are generally categorize
Page 37 and 38: 18 the SiNx:H layer during the ther
Page 39 and 40: CHAPTER 2 SILICON NITRIDE LAYER FOR
Page 41 and 42: 22 reflectance of polished Si can b
Page 43 and 44: 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
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67 Figure 4.1 Α photograph of QSSP
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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
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75 resistivities and lifetime) do n
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77 5.2 Objective An electronic mode
Page 98 and 99:
79 Figure 5.2 is a photograph of a
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81 impurity-gettering methods which
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83 distribution of local currents a
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85 modeling. Wafers were selected f
Page 106 and 107:
87 Figure 5.5 A comparison of (a) d
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89 alloying results in metallizatio
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91 (i) Defect clusters are the prim
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93 SiNX induced charge density on t
Page 114 and 115:
APPENDIX I PROGRAMS TO CALCULATE SR
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97 phin = -ΕΙ - 1 / beta * log(nd
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99 ίter3 = 0 for xi=1 to nmax/2-1
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101 input "output file name {XXXXXX
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103 F (i) = (exp (beta * (phip - ph
Page 124 and 125:
APPENDIX III COMPUTATIONAL METHOD F
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107 where, dscr is the width of the
Page 128 and 129:
REFERENCES 1. E. Becquerel , C. R.
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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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