1984 Volume 57 Issue 6 Pages 1626-1630
The carrier-photogeneration efficiency of holes in poly(N-vinylcarbazole) (PVCz) films was investigated by means of the time-of-flight method as a function of applied electric field, temperature, and excitation band. The extrinsic effects on the efficiency were examined; these included photooxidation of film surface and doping with a weak electron-acceptor, dimethyl terephthalate (DMTP). The experimental results were analyzed according to the Onsager theory of geminate recombination of a bound electron-hole pair with an interionic separation of ro. The primary yield of the electron-hole pair, φo, increased from 0.02 to 0.059 with decreasing excitation wavelength. By photooxidation, it increased up to 0.09 for all the excitation bands. The ro was independent of the excitation band, and was identical between fresh and photooxidized films. In DMTP(1.8 mol%)-doped films, both φo and ro were independent of the excitation band, although their values were different from those of undoped films. These results indicate that the extrinsic carrier-photogeneration via nonrelaxed exciplexes formed between singlet excitons and acceptor-like impurities or photooxidation products is predominant in PVCz films for all the excitation bands.
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