Introducing MR-TADF Emitters into Light-Emitting Electrochemical Cells for Narrowband and Efficient Emission

Abstract

Organic semiconductors that emit by the process of multi-resonance thermally activated delayed fluorescence (MR-TADF) can deliver narrowband and efficient electroluminescence while being processable from solvents and metal free. This renders them attractive for use as the emitter in sustainable light-emitting electrochemical cells (LECs), but so far reports on narrowband and efficient MR-TADF emission from LEC devices are absent. Here, we address this issue through careful and systematic material selection and device development. Specifically, we show that the detrimental aggregation tendency of an archetypal rigid and planar carbazole-based MR-TADF emitter can be inhibited by its dispersion into a compatible carbazole-based blend host and an ionic-liquid electrolyte; and we further demonstrate that the tuning of this active material results in the desired achievement of balanced p- and n-type electrochemical doping, high solid-state photoluminescence quantum yield of 91%, singlet and triplet trapping on the MR-TADF guest emitter, and similarly sized electron and hole transport traps. The introduction of this designed metal-free active material into an MR-TADF LEC results in the attainment of bright blue electroluminescence of 500 cd/m2, which is delivered at a high external quantum efficiency of 3.8% and a narrow full-width-at-half-maximum of 31 nm.

Solution‐Processable Ultrapure‐Blue Light‐Emitting Electrochemical Cells

“…[32] The color purity of these results was better than that of recently reported LECs with MR-TADF emitters. [33,34] Therefore, we concluded that the realization of ultrapure blue EL emissions based on solution-processable LECs could contribute greatly to the creation of a sustainable society in the future.…”

“…The serious issue of low external quantum efficiency (<1% for both LECs, Figure S15, Supporting Information) and operational stability (Figure S16, Supporting Information) remains to be addressed; however, there are many recently proposed improvement methods that can solve this problem in the future. [34][35][36][37]…”