Large‐scale industrial fabrication of Cu(In,Ga)Se2 (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 °C to a maximum of 10.5%, mainly due to a considerable increase in the open‐circuit voltage (Voc). 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.
An open circuit voltage boost from 0.33 to 0.50 V is reported by M. Zutter et al. (article no. http://doi.wiley.com/10.1002/pssr.201900145) for solar cells comprising industrial‐grade Cu(In,Ga)Se2 absorbers and sputtered Zn(O,S) buffer layers. The research shows that thermal annealing up to 200 °C shifts the dominant recombination pathway from interface to bulk. Concurrently, ultraviolet‐photoelectron spectroscopy reveals an increase of the work function of Zn(O,S) films deposited on Cu(In,Ga)Se2. The change of conduction band offset and the observed wavelength‐dependent improvement of charge‐carrier collection are consistent with solar cell capacitance simulations. The study identifies future steps needed to achieve efficient cells based on all‐vacuum processed components.
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