Metal-centred states control carrier lifetimes in transition metal oxide photocatalysts
Michael Sachs, Liam Harnett-Caulfield, Ernest Pastor, Bernadette Davies, Daniel J. C. Sowood, Benjamin Moss, Andreas Kafizas, Jenny Nelson, Aron Walsh, James R. Durrant
Abstract
Abstract Efficient sunlight-to-energy conversion requires materials that can generate long-lived charge carriers upon illumination. However, the targeted design of semiconductors possessing intrinsically long lifetimes remains a key challenge. Here using a series of transition metal oxides, we establish a link between carrier lifetime and electronic configuration in transition metal-based semiconductors. We identify a subpicosecond relaxation mechanism via metal-centred ligand field states that compromise quantum yields in open d -shell transition metal oxides (for example, Fe 2 O 3 , Co 3 O 4 , Cr 2 O 3 and NiO), which is more reminiscent of molecular complexes than crystalline semiconductors. We found that materials with spin-forbidden ligand field transitions could partially mitigate this relaxation pathway, explaining why Fe 2 O 3 achieves higher photoelectrochemical activity than other visible light-absorbing transition metal oxides. However, achieving high yields of long-lived charges requires transition metal oxides with d 0 or d 10 electronic configurations (for example, TiO 2 and BiVO 4 ), where ligand field states are absent. These trends translate to transition metal-containing semiconductors beyond oxides, enabling the design of photoabsorbers with better-controlled recombination channels in photovoltaics, photocatalysis and communication devices.