Silver-alloyed wide-gap CuGaSe2 solar cells

Crystal growth promotion for CuGaSe2 (CGSe) photovoltaic absorber by silver (Ag)-addition is demonstrated, enhancing device performance for wide-gap CGSe solar cells. By introducing Ag precursor, the grain size in the polycrystalline CGSe films deposited by the three-stage process is increased, and a residual Ga2Se3 phase during the growth is suppressed. Photoluminescence intensity is enhanced with Ag-addition, indicating reduced nonradiative recombination, which is related to the suppression of Ga2Se3 phase and reduction of undesirable deep defect levels. For the Ag-alloyed CGSe solar cells, Urbach energy defined from the sub-gap tail in exponential external quantum efficiency is monotonically reduced from 25.5 to 21.1 meV, suggesting the improvement of CGSe bulk quality by Ag-alloying. Open-circuit voltage is enhanced since carrier recombination in Agalloyed CGSe bulk is suppressed. Short-circuit current density is also enhanced as carrier collection efficiency is improved by widened depletion width due to the reduction of carrier concentration by Ag-alloying. On the other hand, excess Ag-addition leads to increased series resistance and deterioration of the built-in potential in the solar cells due to the reduced carrier concentration in the Ag-alloyed CGSe absorber. Finally, the conversion efficiency is maximized to 5.3% at [Ag] / ([Ag] + [Cu]) ratio of 0.07 for the Ag-alloyed CGSe solar cells. These findings will help the performance enhancement for the wide-gap CGSe solar cells.

Automated summary

Silver (Ag) addition promotes crystal growth of the CuGaSe2 (CGSe) photovoltaic absorber, enhancing device performance by reducing nonradiative recombination, improving carrier collection efficiency, and increasing open-circuit voltage. Excessive Ag addition, however, leads to increased series resistance and deterioration of built-in potential in the cells. Ultimately, the conversion efficiency is maximized at a specific Ag/Cu ratio.