Separation of voltage loss mechanisms in polycrystalline solar cells

The operating voltage of polycrystalline solar cells, even in the best CuInSe₂ and CdTe, is 150-250 mV lower than that of single-crystal cells normalized to an equivalent bandgap. Reasons are examined for the size of the difference between open-circuit voltage and bandgap, and ways in which it might be narrowed are suggested. The major loss mechanisms are found to be low built-in potential due to heavy compensation, excess forward current attributed to additional recombination paths, modification to forward current with illumination, and higher than necessary series resistance. It is found that these mechanisms can be separated experimentally and that the common cause of the first three is extraneous bandgap states, mostly likely associated with crystallite surfaces.