Ultrafast Intermolecular Energy Transfer in OLED Materials: Excited-State Dynamics of a Blend of Poly(vinylcarbazole) and Oxadiazole Derivative in Solution and Film States

Abstract

Intermolecular energy transfer dynamics between layers of organic materials controls the efficiency of organic light-emitting diode (OLED) devices. Here, to understand the ultrafast intermolecular energy transfer dynamics occurring in widely used UV-OLED materials of poly(vinylcarbazole) (PVK, donor: hole-transporting material) and synthesized oxadiazole derivative (OXD, acceptor: electron-transporting material), their excited-state relaxation pathways are investigated in tetrahydrofuran (THF) and film. Upon addition of OXD, the nanosecond fluorescence lifetime studies of PVK exhibited efficient quenching of dynamics, supporting the occurrence of Förster resonance energy transfer from PVK to OXD with an efficiency of ∼90%. The femtosecond time-resolved absorption measurements of the mixture of PVK and OXD revealed the intermolecular resonance energy transfer with a time constant of ∼1.32 ps from all of the intermediates in the excited states of PVK to OXD. Interestingly, in the film state, the formation of exciplex beneficial for devices with a lifetime of ∼32.66 ns was observed at ∼516 nm upon excitation of PVK in a blend of OXD and PVK. Nanosecond transient absorption spectra confirmed the occurrence of energy transfer also from the triplet state of PVK. Understanding of these excited-state dynamics occurring in solution and film states is a prerequisite to design and improve the performance of OLED materials.