An antibonding valence band maximum enables defect-tolerant and stable GeSe photovoltaics

Abstract In lead–halide perovskites, antibonding states at the valence band maximum (VBM)—the result of Pb 6 s -I 5 p coupling—enable defect-tolerant properties; however, questions surrounding stability, and a reliance on lead, remain challenges for perovskite solar cells. Here, we report that binary GeSe has a perovskite-like antibonding VBM arising from Ge 4 s -Se 4 p coupling; and that it exhibits similarly shallow bulk defects combined with high stability. We find that the deep defect density in bulk GeSe is ~10 12 cm −3 . We devise therefore a surface passivation strategy, and find that the resulting GeSe solar cells achieve a certified power conversion efficiency of 5.2%, 3.7 times higher than the best previously-reported GeSe photovoltaics. Unencapsulated devices show no efficiency loss after 12 months of storage in ambient conditions; 1100 hours under maximum power point tracking; a total ultraviolet irradiation dosage of 15 kWh m −2 ; and 60 thermal cycles from −40 to 85 °C.