Photoelectrochemical cells (PECs) have attracted enormous attention for solar hydrogen generation. An overwhelming majority of the work has focused on room-temperature PECs, as it is believed that the efficiency of photovoltaic devices decreases with increasing temperature. While that the photovoltage generally decreases with temperature due to the rising intrinsic carrier concentration, electrocatalytic activity and minority/majority carrier transport properties in many materials actually improve with temperature. Hematite, in particular, is a promising material for elevated-temperature PECs (achieved through moderate concentration) because both the minority and majority carrier mobilities increase exponentially with temperature as a result of improved electron hopping dynamics. Therefore, room temperature is not likely the optimal temperature to operate hematite-based PECs.
In this work, we used various Ti-doped hematite thin films grown by pulsed-laser deposition as a model system to study the effect of temperature and light intensity on the photoelectrochemical properties. To eliminate possible microstructural effects across multiple samples, all of the photoanodes were dense and smooth (roughness < 1nm). These hematite-based photoanodes were characterized in a temperature-controlled photoelectrochemical cell (5 - 80 C) using a concentrated solar simulator (up to 10 suns). The photovoltage decreased as expected with increasing temperature. However, at the same time, we observed a significant enhancement in the photocurrent under the synergetic effect of temperature and light intensity. Tuning both temperature and light intensity opens up the opportunity to further enhance PEC efficiency, and to understand the semiconductor/liquid interface.