Giant Voc Boost of Low-Temperature Annealed Cu(In,Ga)Se2 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(O0.75S0.25)/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 degrees 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 Voc 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.