Progress in epitaxial deposition on low-cost substrates for thin-film crystalline silicon solar cells at IMEC

In recent years, research on epitaxial growth for photovoltaics became more important due to the increasing interest in thin-film silicon solar cells. Two significant challenges need to be resolved before this technique can become a competitive industrial alternative to the current dominating technology of bulk silicon solar cells: (i) the availability of a high-throughput and cost-effective epitaxial CVD reactor and (ii) efficiencies approaching those of bulk Si solar cells.In this paper, two CVD systems are studied: an AP-CVD commercial reactor, as a reference system, and an experimental LP-CVD system for optimization of a low-cost semi-industrial process. For low growth rates, an LP-CVD process is realized with a defect density around 5×103 defects/cm2, comparable with the layers grown in the commercial reactor with a growth rate of 3.9 μm/min. First solar cells, grown in the LP-CVD reactor show an efficiency of 8.2% on mono-crystalline samples. Cells on various low-cost substrates, grown in the reference reactor, show efficiencies between 12% and 13% with IMEC's industrial screen-printing process.The short-circuit current of epitaxial cells is limited to 28 mA/cm2 (typically 5 mA/cm2 less than for bulk Si cells). Therefore, the thin epitaxial cell concept requires optimal light trapping, increasing the optical path length. Experiments show that a porous silicon (PS) intermediate layer as an internal reflector can fulfill this role adequately, giving an internal reflectance up to 80%. However, the effectiveness of this reflector depends on its influence on the quality of the epi-layer. Measurements show a lower-quality epi-layer for samples with a PS intermediate layer, but indicate that further optimization of the pre-deposition bake could lead to a compromise between current gain by internal reflectance and losses caused by the increased defect density.