  A deep understanding of defects is essential for the optimisation of
materials for solar energy conversion. This is particularly true for metal
oxide photo(electro)catalysts, which typically feature high concentrations of
charged point defects that are electronically active. In photovoltaic
materials, except for selected dopants, defects are considered detrimental and
should be eliminated to minimise charge recombination. However, photocatalysis
is a more complex process where defects can play an active role, for example,
by stabilising charge separation and mediating rate-limiting catalytic steps.
Here, we review the behaviour of electronic defects in metal oxides, paying
special attention to the principles underpinning the formation and function of
trapped charges in the form of polarons. We focus on how defects alter the
electronic structure, statically or transiently upon illumination, and discuss
the implications of such changes in light-driven catalytic reactions. Finally,
we consider the applicability of lessons learned from oxide defect chemistry to
new photocatalysts based on carbon nitrides, polymers and metal halide
perovskites.
