High efficiency thin-film silicon solar cells

The aim of this thesis project is to explore designs and technologies for achieving highefficiency thin-film silicon solar cells with potentially low cost.The work is based on the use of thin silicon films deposited on electronically inactive,heavily doped (1x1019 cm-3), p++-type, Czochralski-grown (CZ), low quality silicon substrates.The research was divided into three stages. In the first stage, a world record energyconversionefficiency of 15.6% has been achieved for a liquid-phase epitaxial (LPE) siliconthin-film solar cell using a passivated emitter solar cell (PESC) structure. In the second stage,a second world best efficiency of 16.4% has been achieved by creating a drift-field LPE thinfilmas a base layer and using simplified microgrooved PESC processing. In the third stage,another world best efficiency of 17.6% has been achieved with a multi-layer silicon thin-filmdeposited using chemical vapour deposition (CVD) and the inverted pyramid PESC structure.The multi-layer CVD cells exhibit an open-circuit voltage of 664.2 mV, a short-circuit currentdensity of 33.54 mNcm2 and a fill-factor of 82.5%, which are the highest parameters everachieved up to now for any thin-film silicon solar cell on an inert substrate.A series of novel designs and technologies, including the multi-layer cell structure, bufferlayer growth, drift-field base layer growth, substrate thinning, light-trapping, groove isolating,optimal double-layer anti-reflection coating, front surface and back surface passivation, postmetal anneal and so on, have been developed to enhance the opto-electronic characteristicsand achieve high cell performance. The highest short-wavelength spectral response, thelowest reflectance from the cell front surface, and a new efficiency level of 17.8% for a CVDmulti-layer thin-film silicon cell have been achieved using these techniques and the "inert"substrate mentioned above