Comparison of Ag and Ga alloying in low bandgap CuInSe2-based solar cells

Solar cells based on CuInSe2 (CIS) with absorber bandgap 1.0 eV are excellent candidates for a bottom cell in a tandem solar cell. This work investigates the effect of alloys of Ag and small amounts of Ga as an approach to improve the efficiency of CIS-based solar cells with bandgap less than 1.1 eV. Ga and Ag influence the surface morphology of the absorber layer, and Ag alloyed solar cells also have an increased concentration of Ag relative to Cu at the surface. Despite these structural and compositional differences compared to CuInSe2, the device with the highest efficiency incorporates a mixture of Ga and Ag alloying to form (Ag,Cu)(In,Ga)Se-2, where Ga addition improves the open circuit voltage and Ag addition improves the short circuit current. Ag improves current collection from long wavelength light due to the larger space charge width of Ag alloyed solar cells. However, Ag alloyed devices demonstrate lower V-OC due to an interface recombination mechanism.