Efficient green fluorescent organic light-emitting diodes with extended lifetimes by exploiting an iridium complex as a sensitizer

Abstract

Conventional fluorescent materials are considered potential candidates for constructing low-cost and stable organic light-emitting diodes (OLEDs). However, their applications are impeded by low efficiency to some extent. Herein, a feasible method of realizing efficient and stable green fluorescent OLEDs was demonstrated by exploiting an iridium complex to sensitize a conventional fluorescent emitter. To reveal the mechanism of obtained sensitized devices, the charge mobility of the host materials, and the transient electroluminescent (EL) spectra and carrier distribution within the light-emitting layer were investigated. The research results indicated that the thermally activated delayed fluorescence (TADF) type host material possesses superior charge mobility and that the triplet excitons in the sensitized devices can be utilized via sufficient energy transfer. In addition, the formation of triplet excitons on fluorescent emitter molecules following direct charge trapping was restrained by the high-lying energy levels of the TADF type host material and iridium complex. Compared with the reference device, the sensitized devices exhibited modified EL performances. Consequently, a sensitized EL device with a maximum external quantum efficiency (η_{ext, max}), power efficiency (η_{p, max}) and external quantum efficiency (η_{ext, 1000}) at an initial brightness of 1000 cd m⁻² up to 25.1% and 94.95 lm W⁻¹ and 24.4%, respectively, was achieved. Furthermore, the optimal device realized a lifetime LT₈₀ (period over which brightness decreases to 80% of its initial value) of 2080 h.