We demonstrate an InP heterojunction solar cell employing an ultrathin layer (∼10 nm) of amorphous TiO₂ deposited at 120 °C by atomic layer deposition as the transparent electron-selective contact. The TiO₂ film selectively extracts minority electrons from the conduction band of p-type InP while blocking the majority holes due to the large valence band offset, enabling a high maximum open-circuit voltage of 785 mV. A hydrogen plasma treatment of the InP surface drastically improves the long-wavelength response of the device, resulting in a high short-circuit current density of 30.5 mA/cm² and a high power conversion efficiency of 19.2%.

© 2015 American Chemical Society. The role of TiO2 thin films deposited by atomic layer deposition on p-InP photocathodes used for solar hydrogen generation was examined. It was found that, in addition to its previously reported corrosion protection role, the large valence band offset between TiO2 and InP creates an energy barrier for holes reaching the surface. Also, the conduction band of TiO2 is well-aligned with that of InP. The combination of these two effects creates an electronselective contact with low interface recombination. Under simulated solar illumination in HClO4 aqueous electrolyte, an onset potential of >800 mV vs RHE was achieved, which is the highest yet reported for an InP photocathode.