Giant V${}_{oc}$ boost of low‐temperature annealed Cu(In,Ga)Se${}_2$ with sputtered Zn(O,S) buffers

Large‐scale industrial fabrication of Cu(In,Ga)Se${}_2$ (CIGS) photovoltaic panels would benefit significantly if the buffer layer chemical bath deposition could be replaced by a cadmium‐free dry vacuum process suitable for in‐line production. This Letter reports on the development of a Zn(O,S) buffer layer deposited by vacuum‐based magnetron sputtering from a single target onto commercial CIGS absorbers cut from a module‐size glass/Mo/CIGS stack. The buffer‐window stack consisting of Zn(O${}_{0.75}$S${}_{0.25}$)/i‐ZnO/ZnO:Al is optimized for layer thickness and optical and electronic properties, leading to an average device efficiency of 4.7%, which can be improved by annealing at 200 °C to a maximum of 10.5%, mainly due to a considerable increase in the open‐circuit voltage (V${}_{oc}$). Temperature‐dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed V${}_{oc}$ boost. Quantum efficiency shows improvements in the long and short wavelength region, setting in at different annealing temperatures, and photoemission depth profiling indicates interdiffusion of all atomic species at the CIGS/Zn(O,S) interface. Electrical device simulations explain the observed effects by a modification of the band offset at the interface and defects passivation. Both effects are attributed to the observed interdiffusion during annealing. (© 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)