Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer

Abstract

High efficiency inverted phosphorescence organic light-emitting diodes (PhOLEDs) based on ultrathin undoped and doped emitting layer (EML) have been developed. Compared to conventional device, the inverted PhOLED with 0.5nm undoped EML exhibits significantly larger external quantum efficiency (EQE), due to effective energy transfer from the excited host to the emitter. According to the atomic force microscopy image of EML, the 0.5nm emitter sandwiched by two hosts can be considered as the emitter doped in two hosts. The inverted device with intentionally doped ultrathin EML (1.5nm) exhibits the maximum EQE of 31.1%, which is attributed to optimized charge balance and preferred horizontal orientation of emitter. However, such inverted device has large efficiency roll-off at high brightness because of triplet–triplet annihilation process within the ultrathin EML. This can be improved by broadening the doped EML. The inverted device with 10.5nm doped EML has about EQE of 20 % at 10,000cd/m2. It is expected that our inverted PhOLED will promote development of high efficiency active-matrix organic light-emitting diodes based on the n-type Indium Gallium Zinc Oxide thin film transistor.