The influence of PCBM on the electronic energy levels of P3HT:PCBM photocathodes was investigated in a non-aqueous electrolyte using a benzoquinone redox couple. The presence of PCBM introduces a density of interface states at the semiconductor-electrolyte interface, consequently leading to the formation of an interfacial dipole layer and decrease in the built-in potential developed at the semiconductor-electrolyte interface. Photoelectrochemical measurements reveal a decrease in photovoltage from 0.26 V to 0.16 V and cathodic shift in onset potential from -0.27 V to -0.34 V (vs. Ag/Ag+) as PCBM content of the P3HT:PCBM photocathode is increased from 0 through 50 wt%. Additionally, depletion layer widths decreased from 90 nm to 50 nm. When the thickness of the active layer was controlled to coincide with the depletion layer thickness (e.g., 90 nm), photocurrent densities at pristine P3HT photocathodes were found to be higher than for P3HT:PCBM photocathodes. This suggests that the built-in electric field within the active layer is seemingly capable of effective exciton dissociation in the absence of a fullerene acceptor.