Sensitized and Intrinsic Carrier Generation in Phenethylperylene/Tritolylamine Thin Film Structures

Abstract

The photoconductivity mechanism was investigated for vacuum evaporated phenethylperylene (PPEI) films deposited on a thin polycarbonate film doped with varying concentrations of tritolylamine (TTA) and subsequently exposed to methylene chloride vapors. Compared to structures without TTA in the polycarbonate layer, the presence of TTA leads to an increase of carrier generation efficiency and strong quenching of perylene fluorescence indicating a surface sensitized carrier generation process. Fluorescence quenching measurements on samples with and without TTA show a linear correlation between fluorescence quenching and carrier generation at high fields. In the presence of TTA, significant photoconductivity is observed much before the appearance of fluorescence quenching. A marked change of curvature (inflection point) in carrier generation accompanies the appearance of fluorescence quenching at fields in excess of 100 MV/m. These results demonstrate that in the samples containing TTA, two different carrier generation mechanisms are operating simultaneously. At low fields, carrier generation is dominated by the sensitized component. At high fields, although the sensitized component saturates, the intrinsic component causes a further increase in overall carrier generation. The change of slope in carrier generation at high fields is coincident with appearance of fluorescence quenching. The experimental results are consistent with the notion that the intrinsic photoconductivity component originates from direct dissociation of the fluorescent first excited singlet state into free carriers.