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

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17 damage or stress in the silicon lattice in such a way that extended defects needed for trapping impurities are formed. These chemically reactive trapping sites are usually located at the wafer backside. Solar cells are minority-carrier devices and use nearly the entire bulk of the device. It is more attractive to apply external gettering techniques to clean up the bulk of the material. Phosphorous diffusion and Al alloying are some of the processes that have worked well for efficient gettering of solar cells. Because these processes are used extensively in solar-cell manufacturing for junction and contact formation, all Si solar cells experience a certain degree of gettering. 1.7 Passivation of Residual Impurities and Defects It should be noted that not all impurities can be completely gettered during solar-cell processing. Even impurities which are readily getterable remain in the solar cell at significant levels and introduce detrimental effects on solar-cell performance. In addition to the residual impurities, many crystallographic defects are stable at the processing temperatures. It is often observed that defect concentrations remain essentially unaltered by solar-cell processing. Therefore, it is important to identify methods of dealing with residual impurities and defects. Passivation has been applied to deal with residual impurities and defects. . It is known that the hydrogen passivation yields very good results in terms of passivation. One of the most promising methods is application of the hydrogenated amorphous silicon nitride layers (a-SiΝ :Η) deposited by plasma-enhanced chemical vapor deposition (PECVD). These layers are used as very effective antireflective coatings [37]. The major interest in these films, however, is attributed to the bulk and surface defect passivation of silicon. It has been shown that hydrogen released from
18 the SiNx:H layer during the thermal treatment of a solar cell can passivate silicon defects [38]. 1.8 Dissertation Outline Chapter 1 is an introduction to silicon solar cells, its history, status, and trends in the photovoltaic industry. The physics of solar cells, specifically Si solar cells and limiting factors of cell performance are discussed. Chapter 2 is an overview of silicon nitride layer as a multifunctional component in Si solar cells. PECVD hydrogen-rich silicon nitride (SiΝx :H) films, not only act as desirable AR coatings in PV industry, but are also capable of providing excellent passivation of surface defects as well as bulk passivation of impurities and defects. Chapter 3 gives a brief overview of numerical modeling of surface recombination velocity (SRV) based on the so-called extended SRH formalism, which calculates SRV at the SiN X:H-Si interface as a function of injection level. In order to overcome the discrepancy generated by the existing model, a modification is presented, which includes both the carrier recombination on the Si surface and the recombination across the space-charge-region (SCR) which is related to the charge and defects distribution in a damaged layer caused by ion bombardment during the PECVD process. A semi-quantitative model for H evolution mediated by SiΝ :H layer together with process-induced-damage is established based on the theory of H transport across SiN X:H medium and H trapping-detrapping mechanisms. The results of these models are used to establish properties of SiN X:H for the purpose of optimum passivation.
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 and 34: 14 Defects are generally categorize
Page 35: 16 copper, or nickel in concentrati
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
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
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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