Photovoltaic Materials and Devices Based on the Alloyed Kesterite Absorber (Ag x Cu 1– x ) 2 ZnSnSe 4

The photovoltaic absorber Cu 2 ZnSn(S x Se 1– x ) 4 (CZTSSe) has attracted interest in recent years due to the earth‐abundance of its constituents and the realization of high performance (12.6% efficiency). The open‐circuit voltage in CZTSSe devices is believed to be limited by absorber band tailing caused by the exceptionally high density of Cu/Zn antisites. By replacing Cu in CZTSSe with Ag, whose covalent radius is ≈15% larger than that of Cu and Zn, the density of I–II antisite defects is predicted to drop. The fundamental properties of the mixed Ag‐Cu kesterite compound are reported as a function of the Ag/(Ag + Cu) ratio. The extent of band tailing is shown to decrease with increasing Ag. This is verified by comparing the optical band gap extrapolated from transmission data with the position of the room‐temperature photoluminescence peak; these values converge for the pure‐Ag compo…