Advanced intermediate reflector layers for thin film silicon tandem solar cells
Tandem solar cells based on thin film silicon benefit from an intermediate reflector layer between the top and bottom cells since it enhances the absorption in the top cell. The top cell can thus be manufactured thinner and less prone to light induced degradation. Made from a thin layer of nanocrystalline silicon oxide, the interlayer provides a second functionality since it aids in the spatial separation of local shunts occurring in both sub-cells. Recently, the reflector morphology received attention since it can provide a third function; here, a substantial difference exists between the commonly used configurations, i.e. superstrate or substrate. In the former, the thin layer of nanocrystalline silicon oxide reproduces the morphology of the underlying top cell. Its surface may thus be too rough for the growth of the bottom cell. In the latter configuration, reflectors made from a thick layer of ZnO can yield an adequate texture for the top cell, but conductive ZnO loses the effect of shunt quenching. We present our recent progress with improved intermediate reflector layers in both cell types. For silicon oxide based interlayers, we introduce a smoothing lacquer layer with self-organized openings that allow current transport. For ZnO based interlayers, we demonstrate that a treatment in oxygen plasma is capable of tuning the in-plane resistivity.