The work functions of solid layers of various thicknesses of dithioketopyrrolopyrrole (DTPP), evaporated onSnO2:F,were measured by the Kelvin probe technique. Surface photovoltage (SPV) measurements confirm thep-conductivity type. By front and back side illumination of the thick DTPP layers, two space-charge layers are distinguished, allocated to the air/DTPP and DTPP/SnO2:Finterface, respectively. The SPV of thinner layers is a superposition of the front side signal and back side signal, modulated by a filter effect through the organic layer. This permitted the construction of the energy diagram of the DTPP/SnO2:Fjunction. ©1997 American Institute of Physics.