Enhancing the electroluminescence efficiency by controlling the migration of excited states to quenching sites in a truxene-based oligomer

Abstract

Control of unwanted low-energy emission originating due to trap states is crucial for improving the emission efficiency and for maintaining the color purity in blue light-emitting diodes. Here, a truxene-based emitter (T-pyrene) that is a good candidate for the blue emitter is investigated using photophysical studies. Steady-state absorption of the emitter reveals the presence of a charge-transfer state, and the photoluminescence spectrum suggests an occurrence of a strong interchain/exciplex state. Time-resolved photoluminescence (TRPL) and fluence-dependent measurements of the material in the dilute solution show the presence of multiple excitonic processes contributing to the delayed fluorescence. Similar studies on the pristine film suggest that the delayed fluorescence is dominated by the presence of the traps. By blending the oligomer with a higher bandgap polymer matrix, poly(9-vinyl carbazole) (PVK), a type I heterojunction at the T-pyrene:PVK interface is obtained that confines the generated excitons in T-pyrene. With this architecture, device efficiencies with T-pyrene (20 wt. %):PVK show significantly enhanced efficiency than those of the pristine device. We elucidate the relevance of the matrix:T-pyrene system using TRPL and fluence-dependent studies on T-pyrene (20 wt. %):PVK. Time-resolved emission spectra on the T-pyrene (20 wt. %):PVK films do not show red-shifted emission with time delays, which suggests the passivation of the quenching sites due to the dispersion of T-pyrene in PVK to prevent the intermolecular interactions.