Low-Temperature Plasma-Enhanced Atomic Layer Deposition of ZnMgO for Efficient CZTS Solar Cells

Abstract

Cu2ZnSnS4 (CZTS) solar cells are an emerging photovoltaic technology owing to their earth abundance, all-dry processability, and environmental friendliness. Further power conversion efficiency enhancement of the Cd-free CZTS device necessitates the substitution of traditionally used intrinsic ZnO by an alternative wide-band-gap window layer. Here, we demonstrate deposition of a ZnMgO window layer of controlled thickness, composition, and electro-optical properties by atomic layer deposition (ALD). The amorphous ZnMgO deposited at low temperature down to 100 °C using plasma-enhanced ALD showed smoothness superior to that of high-temperature plasma-enhanced ALD and doping density comparable to that of high-temperature thermal ALD but with a much lower electron affinity. The overall charge carrier recombination at the CZTS/ZnSnO/ZnMgO region was reduced due to the optimized ZnMgO conduction band minimum, thus reducing the VOC value and fill factor loss for a CZTS solar cell. In addition, the thinner- and larger-band-gap ZnMgO was believed to reduce the parasitic absorption, improving the JSC value and boosting the efficiency to 10%.