Photocatalytic water splitting for the sustainable production of hydrogen using a two-photon tandem device requires careful optimization of the semiconductors used as photon absorbers. In this work we show how the open-circuit voltage of photocathodes for the hydrogen evolution reaction based on p-GaP was increased considerably by sputtering of different n-type metal oxides on the surface and thereby forming an effective p–n heterojunction. Both n-TiO2 and n-Nb2O5 increased the VOC of the photocathodes, with the latter giving an ultimate VOC of 710 mV using Pt as the cocatalyst. This value is unprecedented for a p-GaP-based HER photocathode operating in an acidic electrolyte under simulated 1 Sun illumination. An additional, but highly significant benefit of a TiO2 layer is that it provides a remarkable operational stability of more than 24 h under constant operation. It was found that TiO2 and Nb2O5 overlayers, which were characterized by high donor density, caused a large built-in potential drop that is located almost exclusively in the p-type substrate. The large built-in potential drop in the GaP effectively separates charge carriers driving photogenerated electrons toward the surface of the electrode to perform the HER. According to this result, a further careful choice of materials having specific properties, such as optimal carrier concentration and band positions, could potentially increase VOC even more, paving the way for the realization of a non-assisted two-photon solar water splitting device.