Abstract
We report on the influence of the substrate temperature during the 2nd and 3rd stage of the Cu(In,Ga)Se2 3-stage co-evaporation process on the in-depth Ga and In concentrations and correlate these with the solar cell parameters and external quantum efficiency of soda-lime glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al devices. An increased homogenization of the [Ga]/[III] fraction ([III] refers to the total concentration of the group 3 elements Ga and In) with temperature is found. In the investigated temperature range, the highest efficiency was measured for the lowest temperature and the steepest Ga-profile. The tendency of the short-circuit current density matches well with the notch-deepness. Surprisingly, the open-circuit voltage decreases for higher substrate temperatures, even though the Ga-concentration in the space-charge region increases. We propose back-grading variations and reduced back-interface recombination to explain this observation. For the highest of the tested temperatures of 540 °C, a homogenization of the Ga and In concentrations close to the surface is found. We explain this by the appearance of a liquid Cu2-xSe phase at the end of stage 2 for this high temperature. Break-off experiments at this point are conducted and reveal morphological and compositional lateral inhomogeneities for Tsub < 540 °C.
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