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%.