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

摘要

Thin- film poly-Si materials on glass substrate are promising candidates for low costphotovoltaics, since the stability and reliability of crystalline Si material has provedto be most decisive advantage. Despite their close resemblance to mono-crystallinesilicon in terms of structural and optical properties, their electronic properties areespecially influenced by relatively shallow subgap states, resulting in markedlydifferent temperature sensitivities of the thin- film solar cell open circuit voltage andshort 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 hasbeen studied, including both n-type and p-type base layers prepared by eitherPECVD or e-beam. The result points towards a fundamental difference in the carrierrecombination process between poly-Si thin- film on glass and bulk single-crystal Simaterials. Additionally, the light generated current JL increases significantly withtemperature in the poly-Si cells at temperatures when diffusion lengths are shorterthan the base thickness, which indicates that there are relatively shallow subgapstates at some energy |Eext - ET| from the closest band edge acting either as fastminority carrier traps or recombination centres. This is further supported by theobservations of photoluminescence at energies below the silicon bandgap in poly-Sithin- film materials, which is direct evidence of carriers trapped in subgap statesundergoing radiative recombination.TEM analysis shows that dislocations are broadly present in poly-Si thin- filmdeposited on glass substrate, with these mainly generated during the SPC process.The RTA treatment contributes to the reduction of these dislocations, but the densityremains at a relatively high level. The dislocation density results show uniformdistribution in different grains and relatively small variation across samples withrelatively large variation in the Voc. The dislocation density is determined by thequantitative analysis of weak-beam dark field (WBDF) TEM images, giving valuesin the range of 0.955×1010 (±0.18) to 1.68×1010 (±0.11) cm-2 for the investigatedsamples. Such result is 4~5 orders of magnitude higher than the good epitaxiallygrown silicon layers.