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

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35 the passivation of dangling bonds at the SiN-silicon interface with the following possible reaction pathways: (i) A hydrogen radical in the plasma gas attaches to a Si dangling bond and forms a Si-H bond: Si° + .Η --> =Si—Η (ii) Silane radicals react with the surface: Si° + ·Si—Η2--> =Si—Si—Η2 (iii) A Si—H+ ion from the plasma attaches to a Si dangling bond by forming a weak bridging Si—H +—Si bond. One of the Si atoms then forms a Si—Si bond with a neighboring Si dangling bond at the silicon surface by hopping of an electron: Si—H+ + .Si 0 --> =Si—Η+ Si, Si—H+ —Si= + =Si° + / - =Si—Si= + =Sι—Η From the above model, it can be seen that .Η radical plays a fundamental role in the formation of complexes. 2.5.2 Hydrogen Passivation Effect Although the mechanism of bulk passivation of c-Si solar cells by SiΝ :Η films is not yet completely known, the passivation effect has been experimentally proved. Chen et. al. [71] studied PECVD of SiΝx films (-600 A) on top of PECVD-grown SiO2 (400 A) on both surfaces of samples followed by photo-assisted anneal at 350°C in forming gas ambient. The bulk and surface passivation effects were quantified and decoupled by a combination of internal quantum efficiency (IQE) measurements and computer modeling.
36 It was found that the bulk lifetime of PECVD passivated solar cells increased by 30% - 70% and the effective lifetime improved by a factor of 2.6 - 9.5 [71]. By using electron spin resonance (ESR) method, Fukui, et. al. [72] measured a decrease in unpaired electron-spin density from 4x 10 4 spins/cm3 to 2x 104 spins/cm3 as a result of deposition of PECVD SiN X films onto three different mc-Si substrates. Hence, they pointed out that PECVD SiN X film has bulk passivation effect and the effect is larger when the quality of substrate is lower [72]. According to Sopori and Zhang et. al. [42, 73], PECVD of SiN during solar cell fabrication serves as a step for H incorporation. The majority of the hydrogen atoms are trapped and "stored" in process-induced traps (PITs) in the surface damaged layer produced by the plasma process during the nitride deposition. It should be noted that there is a diffusion of H, but because of the traps, the H is primarily confined to the vicinity of the surface. In rapid thermal anneal (RTA) step, H is released from the surface and redistributed into the bulk region. Three steps are involved during the hydrogen diffusion: 1) release of hydrogen from the damaged layer, 2) bulk diffusion, and 3) if bulk trap level is high, the diffusion is stalled and controlled by trapping and detrapping in the vicinity of the diffusion front. The bulk Η may also be released from the chemical bonds in the SiN films, but is less important compared to the strong release of hydrogen from the surface. However, some researchers have questioned whether bulk hydrogenation from SiN occurs at all. Boehme and Lucovsky reported only 10-20 nm diffusion of deuterium (D) [67] into silicon from SiN. This degree of passivation is too small for improvement of solar cell emitter regions. They attribute the H loss during anneal to Η migration out of
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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 and 34: 14 Defects are generally categorize
Page 35 and 36: 16 copper, or nickel in concentrati
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: 34 2.5 Bulk Passivation of Si by Si
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 96 and 97: 77 5.2 Objective An electronic mode
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
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85 modeling. Wafers were selected f
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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
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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
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APPENDIX III COMPUTATIONAL METHOD F
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107 where, dscr is the width of the
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REFERENCES 1. E. Becquerel , C. R.
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44. L.L. Alt, S.W. Ing. Jr. and K.W
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90. B.L. Sopori, Y. Zhang, and N.M.
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