Ge-doped Hematite for an Unassisted Water Splitting System with Enhanced Efficiency

Abstract To boost the photoelectrochemical water oxidation performance of a hematite photoanode, high temperature annealing has been widely applied to enhance crystallinity and remove the physical interface between the hematite and the fluorine doped thin oxide (FTO) substrate. However, the high temperature also results in unintentional Sn-doping due to thermal diffusion from the bottom FTO substrate. Therefore, when using additional dopants and the subsequent high temperature annealing process to enhance performance, the procedure should more precisely be considered co-doping of the hematite photoanode. However, at present, the interaction between the unintentional Sn and intentional dopant is poorly understood. Here, using germanium (Ge), which has been proven a promising dopant in previously reported simulations, we investigated how Sn diffusion affects overall PEC performance in Sn:Ge co-doped systems. After revealing the negative interaction of Sn and Ge dopants, we developed a facile Ge-doping method which suppresses Sn diffusion from the FTO substrate, significantly improving hematite performance. The Sn:Ge-hematite photoanode showed a photocurrent density of 4.6 mA cm− 2 at 1.23 VRHE with an excellent low turn-on voltage. After combining with a perovskite solar cell, our tandem system achieved outstanding 4.8% solar-to-hydrogen conversion efficiency (3.9 mA cm− 2 in an unassisted water splitting system). Our work provides important insights on a promising diagnostic tool for future co-doping system design.