The Photoconductivity of Poly(N-vinylcarbazole). III. Photoconductivity in a Sandwich-type Cell.

Photoconductivity in a sandwich-type cell of a PVCz film was investigated in a high vacuum and in the air. A large photociirrent was observed in the π–π* absorption region and a small photocurrent in the visible region. The former showed the following interesting points in its behavior. In low applied fields, no difference was found between the photoconductivity with positive-electrode illumination and that with negative-electrode illumination. In moderate or high fields, however, the photoconductivity with positive-electrode illumination was significantly different from that with negative-electrode illumination. The former showed a superlinear dependence on the applied field (consequently, iph+>>iph−) and a spectral response coinciding with the absorption spectrum. This photoconductive behavior was explained by assuming the mechanism of carrier generation, namely, the fieldassisted thermal dissociation of exciplexes and the detrapping of trapped carriers by singlet excitons. Carriers are supposed to migrate through the overlap of π-electrons of neighboring carbazyl rings in the same polymer chain.