Electrical and microstructural characterisation of thin-film polycrystalline silicon solar cells on glass

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Copyright: Huang, Jialiang
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Abstract
Thin- film poly-Si materials on glass substrate are promising candidates for low cost photovoltaics, since the stability and reliability of crystalline Si material has proved to be most decisive advantage. Despite their close resemblance to mono-crystalline silicon in terms of structural and optical properties, their electronic properties are especially influenced by relatively shallow subgap states, resulting in markedly different temperature sensitivities of the thin- film solar cell open circuit voltage and short circuit currents compared to mono-crystalline (c-Si) counterparts. In this thesis, the consistency of the value of an effective bandgap discrepancy, Ego - Eact1 (0.15-0.18eV), across several types of poly-Si thin- film solar cells on glass has been studied, including both n-type and p-type base layers prepared by either PECVD or e-beam. The result points towards a fundamental difference in the carrier recombination process between poly-Si thin- film on glass and bulk single-crystal Si materials. Additionally, the light generated current JL increases significantly with temperature in the poly-Si cells at temperatures when diffusion lengths are shorter than the base thickness, which indicates that there are relatively shallow subgap states at some energy |Eext - ET| from the closest band edge acting either as fast minority carrier traps or recombination centres. This is further supported by the observations of photoluminescence at energies below the silicon bandgap in poly-Si thin- film materials, which is direct evidence of carriers trapped in subgap states undergoing radiative recombination. TEM analysis shows that dislocations are broadly present in poly-Si thin- film deposited on glass substrate, with these mainly generated during the SPC process. The RTA treatment contributes to the reduction of these dislocations, but the density remains at a relatively high level. The dislocation density results show uniform distribution in different grains and relatively small variation across samples with relatively large variation in the Voc. The dislocation density is determined by the quantitative analysis of weak-beam dark field (WBDF) TEM images, giving values in the range of 0.955×1010 (±0.18) to 1.68×1010 (±0.11) cm-2 for the investigated samples. Such result is 4~5 orders of magnitude higher than the good epitaxially grown silicon layers.
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Huang, Jialiang
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Publication Year
2011
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Thesis
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PhD Doctorate
UNSW Faculty
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