Contribution of silicon substrates to the efficiencies of silicon thin layer solar cells

Although silicon solar cells based on layers less than 50 μm thick have become very popular, little attention has been paid to the role of the underlying silicon substrate. This treatment uses the device simulation program PC-1D and the ray tracing program SUNRAYS to examine the role of the substrate in contributing to the current and efficiency of textured and non-textured thin layer solar cells. For the case of a heavily doped silicon substrate, substrate contributions can be significant for cells with sufficiently thin base layers. For example, for the case of a silicon thin layer cell with a base layer thickness of 20 μm and a substrate doping of 6 × 10¹⁸ cm⁻³, the substrate contributes no more than 4% of the total short-circuit current. However, decreasing the base width to 5 μm results in an increase in this substrate contribution to 20%. Light trapping tends to alleviate the substrate contribution by increasing the effective path length in the base. Examination of the current components under forward bias reveals that for a thin layer cell with a high quality base and good front surface passivation, back diffusion of electrons into the substrate limits cell performance.