Electric-field-induced quenching of photoluminescence in photoconductive organic thin film structures based on Eu3+ complexes

A large electric field effect on photoluminescence (PL) from electroluminescent emitters sandwiched between two high-work-function electrodes is reported and a model of the effect formulated. We examine the PL behavior of Eu3+ complex-based organic thin films subjected to increasing electric field applied to either two high-work-function [indium tin oxide (ITO) and Au] or two low-work-function (Al∕Al) electrodes. The progressive drop (up to 40%) of the PL for the first structure is observed while the current increases by about two orders of magnitude under illumination within the first absorption band of diamine derivative (TPD) molecules acting as an antenna harvesting exciting photons within the common matrix bonded with polycarbonate. This behavior is interpreted in terms of a field-dependent branching between ligand occupation by excited triplets and by electrons injected from negatively biased ITO or Au electrodes by TPD excitons. In contrast, the PL quenching in the Al/organic film/Al structure is due to exciton dissociation solely. This allows to extract the quenching component for the ITO/organic film/Au structure, associated with the injected charge only. The recombination of thermally injected holes with exciton-injected electrons makes the charge-induced quenching efficiency component to nonmonotonically evolve with a decrease down to negative values (PL enhancement) at high electric fields. The quenching of the emission from host (TPD) is attributed to singlet-exciton–charge-carrier interactions.