Open circuit voltage recovery in GaAsSbN-based solar cells: Role of deep N-related radiative states

In this work we investigate the effect of rapid thermal annealing (RTA) on the performance of solar cells consisting of different GaAsSbN-based structures and correlate the device results with modifications of the optical and structural properties of the alloy. In particular, bulk layers grown at different growth rates and type-II GaAsSb/GaAsN superlattices with different period thickness are analyzed. We find evidences of material quality improvement after the annealing process such as a reduction of N-related radiative defects and Sb clusters. These RTA-induced changes lead to a notable enhancement of the open circuit voltage (V-OC), which results in values of the bandgap-voltage offset (W-OC = E-G/q-V-OC) comparable to that of a non-optimized reference GaAs solar cell with the same device structure (W-OC similar to 0.63 eV). The decrease in W-OC after annealing shows a correlation with the reduced radiative recombination at low energy N-related sub-bandgap states. These results suggest that radiative recombination in a broad band of deep defect states is a source of V-OC degradation in GaAsSbN solar cells.