Thermally-enhanced minority carrier collection in hematite during photoelectrochemical water and sulfite oxidation†
Many metal-oxide light absorbing semiconductors, such as α-Fe2O3 (hematite), exhibit localized small polaron carrier conduction. The low electron/hole mobility hinders minority carrier transport, and is not readily modified through doping or nanostructuring. In this work, we demonstrate that thermal energy, which is available in moderately concentrated sunlight, enhances the minority carrier mobility and photoelectrochemical (PEC) water and sulfite oxidation of 30 nm-thick Ti-doped hematite thin-film photoanodes. In NaOH aqueous electrolyte, the instantaneous Tafel slope at 3 mA cm−2 decreases remarkably from 480 mV dec−1 (at 7 °C) to 240 mV dec−1 (at 72 °C) under 9 suns illumination, representing a substantial increase in the fill factor, which also depends on the doping level. Though the photovoltage decreases with temperature expectedly, we show that it can be mitigated by increasing the light intensity. In the presence of a Na2SO3 hole scavenger, the photocurrent at 1.23 V vs. reversible hydrogen electrode increased from 3.1 (at 7 °C) to 5.0 mA cm−2 (at 72 °C) under 8 suns illumination, and the onset potential was shown to depend weakly on the temperature. The strong increase in the photocurrent with temperature in the limit of fast reaction kinetics suggests that it arises from an improvement in the collection of minority carriers in the diffusion region of hematite. We show that room temperature and 1 sun illumination intensity is not the optimal reaction operating condition for hematite photoanodes. The thermally-enhanced minority carrier transport are likely generalizable to other small-polaron-type light absorbers for PEC and solar cells.