Mechanism of extrinsic carrier photogeneration in poly-N-vinylcarbazole. II. Quenching of exciplex fluorescence by electric field

Abstract

Based upon the observation that the exciplex fluorescence of poly‐N‐vinylcarbazole film doped with a weak electron acceptor is partly quenched by an electric field, a model is proposed for the extrinsic carrier photogeneration mechanism involving the field‐assisted thermal dissociation into free carriers of an ion pair produced from a ’’nonrelaxed’’ exciplex state. Interaction between a migrating singlet exciton and an electron acceptor produces an electron–hole pair having an interionic separation of 22 Å through the nonrelaxed exciplex state and subsequent thermalization of its excess energy. The ion pair produced dissociates into free carriers with assistance from the applied field according to the Onsager probability of escaping geminate recombination. Recombination leads to the ordinary relaxed fluorescent exciplex state. The initial separation r₀ of the ion pair was determined by comparing experimental values of the field‐induced exciplex fluorescence decrement and the theoretical prediction. The temperature dependence of the field‐induced exciplex fluorescence quenching provides further proof for this mechanism. As the electron affinity of a series of added acceptors is increased, r₀ becomes larger as expected.