Field Dependence of Exciton‐Induced, Contact‐Limited Photocurrents in Molecular Crystals

The current–field dependence of contact‐limited photocurrents resulting from dissociation of Frenkel excitons at the interface between molecular crystals and a metal or a fatty acid layer is investigated. In the field range 3 × 10 3 Vcm −1 < F < 10 5 Vcm −1 both photocurrent and dark current follow a j = j 0 (1 + bF ) relationship with no indication of a field‐independent limiting current. An explanation is given in terms of the Onsager model for thermally activated diffusional escape of a carrier inside the crystal from the attractive Coulombic potential of its countercharge. At higher fields the Frenkel‐Poole effect becomes operative. It is concluded that exciton dissociation initially produces a thermalized carrier inside the crystal.