Highly efficient exciplex-based OLEDs incorporating a novel electron donor

Abstract

A novel triphenylamine (TPA) derivative molecule 4,4,8,8,12,12-hexaphenyl-8,12-dihydro-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine (DEX) is presented as an electron donor to form an effective exciplex system with an electron acceptor molecule ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T) for fabricating high performance exciplex-based organic light-emitting diodes (OLEDs). If the DEX:PO-T2T exciplex itself is used as an emitting layer without any doped luminescent emitter in an OLED, it can exhibit green emission with a maximum power efficiency (PE_max), current efficiency (CE_max), and external quantum efficiency (EQE_max) of 44.6 lm W⁻¹, 36.0 cd A⁻¹, and 11.2%, respectively. The high device performance indicates that the green exciplex would have balanced charge transport properties, a broad recombination zone, and a suitable triplet energy level (E_T), which is therefore expected to serve as an ideal host for a red phosphorescent emitter. Indeed, due to the desirable thermal and electrical properties of donor DEX, an optimized exciplex-based red phosphorescent OLED (PhOLED), doped with iridium(III) bis(2-methyldibenzo[f,h]quinoxaline)acetylacetonate (Ir(MDQ)₂(acac)), achieved an impressive PE_max, CE_max, and EQE_max of 46.1 lm W⁻¹, 36.0 cd A⁻¹, and 24.5% as well as a low operating voltage (2.4 V at 0.2 mA cm⁻²). Remarkably, both an excellent EQE over 22% and low operating voltage of 3.4 V remained even at a high brightness of 1000 cd m⁻². The high EQE and low efficiency roll-off reveal that the new exciplex system using novel donor DEX enables improved OLED performance, which paves the way to practical applications of OLEDs.