  Metal halide perovskite solar cells have gained widespread attention due to
their high efficiency and high defect tolerance. The absorbing perovskite layer
is as a mixed electron-ion conductor that supports high rates of ion and charge
transport at room temperature, but the migration of mobile defects can lead to
degradation pathways. We combine experimental observations and drift-diffusion
modelling to demonstrate a new framework to interpret surface photovoltage
(SPV) measurements in perovskite systems and mixed electronic ionic conductors
more generally. We conclude that the SPV in mixed electronic ionic conductors
can be understood in terms of the change in electric potential at the surface
associated with changes in the net charge within the semiconductor system. We
show that by modifying the interfaces of perovskite bilayers, we may control
defect migration behaviour throughout the perovskite bulk. Our new framework
for SPV has broad implications for developing strategies to improve the
stability of perovskite devices by controlling defect accumulation at
interfaces. More generally, in mixed electronic conductors our framework
provides new insights into the behaviour of mobile defects and their
interaction with photoinduced charges, which are foundational to physical
mechanisms in memristivity, logic, impedance, sensors and energy storage.
