Photocurrent Enhancement by Spontaneous Formation of a p-n Junction in Calcium-Doped Bismuth Vanadate Photoelectrodes.

The application of bismuth vanadate (BiVO4 ) photoelectrodes for solar water splitting is hindered by the poor carrier transport. To overcome this, multiple donor-doping strategies (e.g. dual doping, gradient doping) have been explored. Here, we show for the first time the successful introduction of calcium (Ca) as an acceptor-type dopant into BiVO4 photoelectrodes. Interestingly, instead of generating cathodic photocurrents, the Ca-doped BiVO4 photoelectrodes show anodic photocurrents with an enhanced carrier separation efficiency. Hard X-ray photoelectron spectroscopy (HAXPES) shows that this enhancement is caused by out-diffusion of Ca during the deposition process, which spontaneously creates a p-n junction within the BiVO4 layer. Overall, a significant two-fold improvement of the AM1.5 photocurrent is obtained upon Ca-doping. This study highlights the importance of controlled doping beyond simply modifying carrier concentration and may enable new device architectures in photoelectrode materials.