Passivating Deep-Level Defects with Ag Alloying to Enhance the Efficiency of Cu(In,Ga)(S,Se)₂ Solar Cells

Chalcopyrite Cu(In,Ga)(S,Se)2 is one of the most promising photovoltaic candidates, presenting huge potential in future flexible and tandem solar cell applications. Ag alloying was recognized as an effective strategy to improve the efficiency of selenide Cu(In,Ga)Se2 solar cells, enabling record efficiencies. However, the fundamental mechanism underlying these remarkable performance enhancements remains elusive; especially, how Ag alloying alters the properties of point defects and the corresponding nonradiative recombination is still unclear. Here, we perform rigorous first-principles calculations to systematically investigate the native point defects in CuInSe2 and CuInS2 in comparison with those in AgInSe2 and AgInS2. We find that anion vacancies are critical deep-level defects that may act as nonradiative recombination centers, limiting the photovoltaic performance. We show that Ag alloying can effectively passivate these deep-level defects in both CuInSe2 and CuInS2, shallowing their charge–state transition levels, which is due to a band gap widening effect as a result of lattice expansion induced by Ag alloying; this beneficial effect is more pronounced in CuInS2 than in CuInSe2, which has been further verified by experiments on Cu(In,Ga)S2 solar cells with and without Ag alloying. Our insights will guide the optimization and design of Cu(In,Ga)(S,Se)2-based solar cells, pushing their performance toward the Shockley–Queisser limit.