Photoelectrochemical properties of p-type tandem $CuBi_2O_4$|CuO thin films by a simple solution process

Abstract

Metal oxide semiconductors are drawing much attention for low cost photoelectrochemical (PEC) water splitting cells because of material abundance and facile synthetic methods. $CuBi_2O_4$ is a p-type semiconductor with band gap of 1.5-1.8 eV and is promising for a top layer in double junction tandem PEC devices. However, $CuBi_2O_4$ (CBO) is suffering from low hydrogen evolution reaction (HER) photocurrent density from poor charge transport in $CuBi_2O_4$ and charge injection kinetics at the CBO/electrolyte interface. Here, we demonstrate $CuBi_2O_4$ and CuO bilayer thin films prepared by simple solution process for improved PEC HER. A CuO layer is introduced to enhance light absorption for its smaller band gap (~ 1.2 eV). For the tandem electrodes prepared by simple drop casting followed by annealing, PEC ORR performance was higher when CuO layer was on top of $CuBi_2O_4$ under AM 1.5G illumination. We suggest that conduction band edge alignment of CuO and $CuBi_2O_4$ improves charge separation between the FTO|$CuBi_2O_4$|CuO and electrolyte interface. For the further improvement of tandem electrode for PEC HER, a spin coating method is introduced utilizing Polyvinylpyrrolidone (PVP) as helping agent. CBO film from spin coating method was highly dense, and the role of PVP-metal interaction toward film formation mechanism is explored. This CBO film was subjected to photoelectrochemical oxygen reduction reaction (PEC ORR) and found that most dense film shows the best ORR performance.