Photogeneration of triplet states by polaron recombination in para-hexaphenyl

摘要

Dissociation of photogenerated excitons into charge carriers (polarons) is the working principle of organic photovoltaic cells, whereas recombination of field-injected charge carriers into luminescent singlet excitons and nonradiative triplet excitons governs the efficiency of organic light emitting devices (OLED). In this paper, we address these processes in a single layer OLED based on para-hexaphenyl (PHP) by conventional and electric field-assisted femtosecond pump-probe measurements. Due to its well defined conjugation length and its chemical structure, PHP exhibits well resolved photoinduced absorption (PA) features, that can be assigned to singlet, triplet and polaron absorption. This allows to monitor the dynamics of field-induced singlet dissociation and polaron recombination in a single layer OLED structure consisting of aluminum/PHP/ITO with 200 fs time resolution.