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.