Atomic-Scale Interface Modification Improves the Performance of Cu(In1\u2013xGax)Se2/Zn(O,S) Heterojunction Solar Cells

Cadmium-free buffer layers deposited by a dry vacuum process are mandatory for low-cost and environmentally friendly Cu(In1\u2013xGax)Se2 (CIGS) photovoltaic in-line production. Zn(O,S) has been identified as an alternative to the chemical bath deposited CdS buffer layer, providing comparable power conversion efficiencies. Recently, a significant efficiency enhancement has been reported for sputtered Zn(O,S) buffers after an annealing treatment of the complete solar cell stack; the enhancement was attributed to interdiffusion at the CIGS/Zn(O,S) interface, resulting in wide-gap ZnSO4 islands formation and reduced interface defects. Here, we exclude interdiffusion or island formation at the absorber/buffer interface after annealing up to 200 \ub0C using high-resolution scanning transmission electron microscopy (HR-STEM) and energy-dispersive X-ray spectroscopy (EDX). Interestingly, HR-STEM imaging reveals an epitaxial relationship between a part of the Zn(O,S) buffer layer grains and the CIGS grains induced by annealing at such a low temperature. This alteration of the CIGS/buffer interface is expected to lead to a lower density of interface defects, and could explain the efficiency enhancement observed upon annealing the solar cell stack, although other causes cannot be excluded

Giant Voc Boost of Low\u2010Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers

Large-scale industrial fabrication of Cu(In,Ga)Se (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(OS)/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). Temperature-dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed V 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.This work was partially supported by the European Union's Horizon 2020 research and innovation program under grant agreement No. 641004 (project Sharc25). The authors acknowledge additional support by the Spanish project AIC‐B‐2011‐0806, and by the “Micro‐concentrator thin film solar cells (MiconCell)” project (028922) and the “Correlated Analysis of Inorganic Solar Cells in and outside an Electron Microscope (CASOLEM)” project (028917), both co‐funded by FCT and ERDF through COMPETE2020. DC acknowledges the European Commission for funding the NanoTrainforGrowth II project No. 713640 through the Marie Curie Cofund programme