Doping and Annealing Conditions Strongly Influence the Water Oxidation Performance of Hematite Photoanodes

Hematite (α-Fe₂O₃) is one of the most promising semiconductors for solar water splitting due to its high theoretical efficiency and low cost. However, its poor electronic properties strongly limit its performance. Furthermore, the impact of composition and processing conditions on such properties, and on the water splitting efficiency, is poorly understood. Here, we unravel the role of these contributions and provide guidelines for the fabrication of efficient hematite photoanodes. Aliovalent doping with tin and fluorine is found to improve the electrical conductivity of hematite, leading to higher performance. Annealing in an inert atmosphere, which is conventionally used to create oxygen vacancies, is found not to affect undoped hematite. However, a marked effect has been observed in doped hematite, and a model describing dopant activation rather than oxygen vacancy formation has been proposed. The synergy between the presence of both dopants and the annealing conditions provides optimal electrical properties, which enable the increase of the hematite thickness, leading to enhanced light absorption and limiting the detrimental charge recombination issues observed in undoped films or even in doped films processed in excess oxygen. Our work provides a deeper understanding of the interplay among all of these processing factors, resulting in hematite photoanodes with increased performance.