Next, we turn to studies of photoinduced changes in charge distribution at the surface of our photocathodes using surface photovoltage (SPV) spectroscopy . In our SPV experiments, the contact potential difference (CPD), equivalent to the difference between the work functions of the sample and a gold reference probe, is measured under ambient conditions using the Kelvin probe technique. Surface photovoltage (SPV) is defined as the light-induced change of the CPD measured at the surface of a photoactive material. The sign of the SPV (i.e., CPD)