Origin of significant visible-light absorption properties of Mn-doped TiO2 thin films

It is highly desirable to induce significant red-shift in the optical absorption edges of TiO2 phases so that this class of low-cost and environmentally friendly materials can be used as effective optical absorbing materials in novel photovoltaic cells with long-term sustainability or smart photo-catalysts beyond the ultraviolet range. This work focuses on studying the mechanisms of Mn-induced red-shift by combining theoretical modeling with advanced structural and spectroscopic characterization of doped thin films, aiming to provide fundamental guidance for effective doping through enhanced understanding of doping chemistry resulting from the interplay between doping atoms and defects. It is shown that Mn atoms doped into the Ti lattice sites are associated with oxidation valency higher than +3, resulting in maximized effectiveness in modifying the band structure to achieve remarkable optical red-shifting. The presence of oxygen vacancies reduces the Mn valency and its red-shifting effect, but their detrimental effect in bringing about localized defect levels is reduced owing to their association with Mn atoms, making Mn doping highly promising in activating various visible light functionalities of TiO2.