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

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CHAPTER 4 MINORITY-CARRIER LIFETIME MEASUREMENTS 4.1 Quasi-Steady-State Photoconductance Decay One of the fundamental physical properties of a silicon solar cell is the minority-carrier lifetime. In practice, measurements on wafers yield an effective lifetime which can then be interpreted as a combination of bulk and surface recombination components. In order to do this correctly, the measurements of the effective lifetime must have a strong physical basis. Currently, there are three methods being deployed by the PV-Si industry for lifetime measurements: quasi-steady state photoconductance decay (QSSPCD), photoluminescence mapping and microwave reflection. In this thesis, QSSPCD was used to measure the minority-carrier lifetime of PV-Si. For long-lifetime wafers, the transient method (measurement after the light is extinguished) is preferred since it does not require knowledge of the photogeneration in the sample (reflection and absorption of photons in the sample) or the excitation wavelength. The QSS method has been found to be useful for lower-lifetime materials that are often used in the production of PV cells. The QSSPCD technique has been developed by Sinton [100]. This award-winning technique uses inductive coupling between a small coil that is placed under the sample platform and the sample under test. The frequency used is 10 MHz, and the light source is a very long duration (several milliseconds) flash lamp. The setup involves a zeroing procedure that accounts for the dark conductivity of the sample. The user also inputs the estimated carrier concentration. The sensor coil detects the photoconductivity produced by the flash lamp, and the interfaced computer processes the data. A highly calibrated onboard silicon cell, that is 65
66 interfaced with the measurement apparatus, monitors the instantaneous flash intensity. The quasi-steady state lifetime is monitored as function of the instantaneous photon density. The QSSPCD method uses the classic expression for steady state photoconductivity: Δn = GLτeff where, Δn is the phtogenerated electron/hole density, G L is the optical generation function, and τeff is the effective carrier lifetime. The physical quantity, which is measured by the inductive sensor system, is the photoconductivity, σ L : ΔσL = qΔn(μn + μ p )W Where μn, μp are the electron and hole mobilities respectively, Μ is the excess electron/hole density and W is the sample volume. The associated computer program calculates the carrier mobilities, based on the doping density, and excess carrier density. The reference cell measures the instantaneous generation rate, G, and therefore, the average lifetime is computed and displayed. This technique works at any injection level, and the display shows lifetime versus injection level as the intensity of the flash decays from the maximum value to zero. Figure 4.1 is a snap shot of a QSSPCD apparatus and user interface at NREL.
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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
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 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 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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