Ligand field states control photocatalytic efficiency of transition metal oxides

Efficient charge extraction in solar energy conversion devices requires materials that are capable of generating long-lived charge carriers. However, carrier lifetimes vary by orders of magnitude between photoabsorbers and there is no blueprint for the targeted design of materials with intrinsically long lifetimes. Here, we establish a fundamental link between the carrier lifetime and the electronic configuration of transition metal oxides (TMOs). By comparing their deactivation pathways across a range of electronic configurations, we identify a sub-ps relaxation mechanism via metal-centred ligand field (LF) states. These LF states open localised recombination channels that compromise charge and quantum yields in open d-shell TMOs (e.g., Fe2O3, Co3O4, Cr2O3, NiO), which is more reminiscent of molecular complexes than crystalline semiconductors. In contrast, in TMOs with d0 or d10 electronic configurations (e.g., TiO2, BiVO4), the absence of LF states enables larger yields of long-lived charges and thus more efficient photocatalysis. Notably, our results suggest that charge localisation in the form of polarons can mitigate rapid LF deactivation. These trends translate to other metal-containing semiconductors and open a new pathway to design absorbers with well-controlled non-radiative recombination channels for applications including photovoltaics, photocatalysis, and communication devices.