Electron-beam evaporated polycrystalline silicon thin-film solar cells: paths to better performance

Photovoltaics is likely to become one of the world major energy sources in the future providing it is able to meet demands of a significantly lower production cost. It requires cheap materials, equipment and processes on the one hand, and high energy conversion efficiency on the other hand. The focus of this thesis is on polycrystalline silicon thin-film solar cells on glass prepared by electron-beam evaporation and solid-phase crystallisation – an emerging technology with a potential to meet the requirements. It combines the advantages of the mature crystalline silicon technology with low material usage and large- area monolithic construction typical for the thin-film approach. Importantly, it replaces an expensive and complicated Si film fabrication technique, plasma enhanced chemical vapour deposition, with a much simpler and cheaper electron beam evaporation. This thesis presents the author’s research results on evaporated polycrystalline silicon cells. It covers a wide range of important topics: silicon material preparation, cell fabrication, glass substrate texturing and back surface reflectors. It can be categorised into two parts. The first part introduces the process of fabricating a metallised photovoltaic device from silicon feedstock. Important issues are addressed, such as doping control, comparison of p-type cells and n-type cells, sample wrinkling, contamination protection, metallisation of planar cells and textured cells, etc. The second part is a systematic study on light trapping, a very critical issue for thin-film silicon solar cells. It is found that glass texturing is hardly compatible with evaporated cells due to the defects associated with the micro-columnar structures introduced by the texture topography. In the case when no suitable texturing can be used, an efficient back surface reflector is extremely important as the only means to enhance the light-trapping. The best back surface reflector found by the author is a combined structure: nanoparticles/magnesium fluoride/white paint, ...