High-injection conditions at dislocations in silicon: A mechanism for dependence of lifetime on photogeneration rate

Experimental observations are presented concerning the dependence of the spectral response of silicon photovoltaic cells upon the optical illumination intensity. These effects are significant only in silicon materials with many extended defects such as dislocations or grain boundaries. In this case, the short-circuit current response to a chopped monochromatic optical excitation increases monotonically as a background optical (white light) bias is increased to approximately AM1 intensity. These results may be explained in terms of SRH recombination through defect states at dislocations which under these conditions enter a high-injection regime at the defect sites, as a result of their associated space-charge regions. The mechanism helps to understand the observation that the photocurrents in solar cells made from cast polycrystalline silicon are not appreciably lower than for crystalline silicon.