Simultaneous measurements of thermoluminescence and thermally stimulated currents in poly(N-vinylcarbazole)/polycarbonate blends

Abstract

Photoconducting model systems: poly(N-vinylcarbazole)/polycarbonate (PVK/PC) blends were investigated by simultaneous measurements of thermally stimulated currents (TSC) and thermoluminescence (TL). It was found that with increasing applied electric field, the TSC maximum increases and shifts towards lower temperatures, but the TL maximum decreases and its position on temperature scale remains unchanged. Moreover, the TSC maximum shifts also towards lower temperatures with increasing PVK content and with decreasing sample thickness and its position depends on the applied electric field polarity in a case when the sample is excited at low temperatures with a strongly absorbed light. The position of the TL maximum is unaffected by these factors. It is concluded that the field dependent TSC maximum cannot be interpreted in terms of the Poole-Frenkel effect, but it is the so-called ‘‘transport peak,’’ determined by transport and trapping phenomena, and can be regarded as thermally stimulated time-of-flight signal. Spectral analysis of TL indicates that it is characteristic of PVK phosphorescence. It results from recombination of charge carriers thermally liberated from neutral traps under the influence of a charged recombination center, and it can be described by a model based on the Onsager theory of geminate recombination. The detrapping is probably stimulated by the so called ‘‘wet dog’’ effect, related to molecular γ-relaxation in PVK.