Optoelectronic tunability of Hf-doped ZnO for photovoltaic applications

The need for a high optical spectrum throughput, high conductivity, and controlled energy levels of transparent conductive oxide used in solar cells stresses the development of novel materials that help reduce the existing dependency on indium-based oxides. ZnO is a promising material in this context, and in this work, we demonstrate how Hf doping of ZnO films allows engineering both electrical and optical properties to fit the requirements of different solar cell architectures and materials. We focus on the lightly doped domain where Hf substitution is believed to be the key for band gap tunability without negatively affecting the carrier transport behavior. We provide experimental analysis of controlled changes in the optical and electrical properties, including work function, and a detailed analysis of the structural behavior resulting from the deposition at elevated temperature. We finally present first-principles density functional theory simulations to elucidate the mechanisms responsible for the obtained electronic and electrical properties that predict a modification in the band structure of ZnO when Hf is substituted and/or embedded in the ZnO matrix as HfO₂ phases.