Efficiency of Blue Organic Light-emitting Diodes Enhanced by Employing an Exciton Feedback Layer**Supported by the National Natural Science Foundation of China under Grant No 60906022, the Natural Science Foundation of Tianjin under Grant No 10JCYBJC01100, the Scientific Developing Foundation of Tianjin Education Commission under Grant No 2011ZD02, and the Key Science and Technology Support Program of Tianjin under Grant No

Abstract

We report that a novel exciton feedback effect is observed by introducing the bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (BAlq) inserted between the emitting layer (EML) and the electron transporting layer in blue organic light emitting diodes. As an exciton feedback layer (EFL), the BAlq does not act as a traditional hole blocking effect. The design of this kind of device structure can greatly reduce excitons' quenching due to accumulated space charge at the exciton formation interface. Meanwhile, the non-radiative energy transfer from EFL to the EML can also be utilized to enhance the excitons' formation, which is confirmed by the test of photolumimescent transient lifetime decay and electroluminescence enhancement of these devices. Accordingly, the optimal device presents the improved performances with the maximum current efficiency of 4.2 cd/A and the luminance of 24600 cd/m2, which are about 1.45 times and 1.75 times higher than those of device A (control device) without the EFL, respectively. Simultaneously, the device shows an excellent color stability with a tiny offset of the CIE coordinates (Δx = ±0.003, Δy = ±0.004) and a relatively lower efficiency roll-off of 26.2% under the driving voltage varying from 3 V to 10 V.