High performance red organic electroluminescent devices based on a trivalent iridium complex with stepwise energy levels

Abstract

In this work, electroluminescent (EL) devices with double light-emitting layers (EMLs) having stepwise energy levels were designed and fabricated to improve the EL performances of the red light-emitting trivalent iridium complex bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)₂(acac)]. To broaden the recombination zone and facilitate the balance of carriers on emitter molecules, the widely used p-type material 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TcTa) and bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) were chosen as host materials of EML1 and EML2, respectively due to their well matched energy levels. Interestingly, slight decomposition of Ir(MDQ)₂(acac) molecules was observed during the deposition of EML, which causes the rapidly decreased brightness at relatively high doping concentration. Finally, a high performance red EL device with maximum current efficiency of 44.76 cd A⁻¹, power efficiency of 40.19 lm W⁻¹, and external quantum efficiency (EQE) of 15.5% was obtained by optimizing the doping concentration of Ir(MDQ)₂(acac). Even at a high brightness of 1000 cd m⁻² (5.2 V), a current efficiency as high as 40.59 cd A⁻¹ (EQE = 14.4%) can still be retained by the same device.