We create and optimize a structurally well-defined electron donor-acceptor planar heterojunction interface in which electric-field and/or photon-induced charge transfer occurs. Electric-field-induced charge transfer in the dark and exciton dissociation at a pentacene/PCBM interface were probed by in situ thickness-dependent threshold voltage shift measurements in field-effect transistor devices during the formation of the interface. Electric-field-induced charge transfer at the interface in the dark is correlated with development of the pentacene accumulation layer close to PCBM, that is, including interface area, and dielectric relaxation time in PCBM. Further, we demonstrate an in situ test structure that allows probing of both exciton diffusion length and charge transport properties, crucial for optimizing optoelectronic devices. Competition between the optical absorption length and the exciton diffusion length in pentacene governs exciton dissociation at the interface. Charge transfer mechanisms in the dark and under illumination are detailed.