Photogeneration processes in poly (N-vinylcarbazole)

Abstract

The photogeneration efficiency of holes in films of highly purified poly (N-vinylcarbazole) (PVCA) and of PVCA containing 0.1% by weight trichloroacetic acid (TCAA) has been studied as a function of applied field (5×104−2×106 V/cm) and excitation energy (3.4–5.2 eV), and the influence of extrinsic effects such as aging, ambient and photodecomposition upon the carrier generation efficiency, fluorescence, and absorption has been examined. It is observed that light and aging degrades the film. Possible photo-oxidation processes are discussed. Oxygen is found to reduce the yields for carrier generation and fluorescence. The carrier generation in PVCA becomes less field dependent but more efficient with increasing excitation energy. The addition of TCAA to PVCA reduces the field dependence of the carrier generation, sensitizes it at low fields, but quenches it at high fields. A kinetic model is developed which assumes that the field dependence of the generation yield can be described in terms of the Onsager model for geminate recombination. The essential parameters from this analysis are φ0∼0.14 and ∼0.11 for the primary quantum yield for pure and acid doped PVCA, respectively. For PVCA the separation of the photoexcited electron-hole pair increases from r0∼22.5 Å for excitation into the first singlet state (1S1) to r0∼30 Å for excitation into the third singlet state (1S3). For the acid doped film, we find r0∼30 Å for 1S1 excitation. The kinetic model enables us to relate φ0 to molecular parameters, to account for the reduction of φ0 on doping with acids, and to predict the influence of extrinsic effects upon the spectral dependence of the carrier generation.