Locking excitons in two-dimensional emitting layers for efficient monochrome and white organic light-emitting diodes

Simplified organic light-emitting diodes (OLEDs) with high efficiency are key for successful products. Here, we demonstrate simplified OLEDs by combining ultra-thin non-doped emitting layers (UN-EMLs) with charge-transfer (CT)-type mixed donor-acceptor recombination layers. The CT recombination layers provide a bipolar recombination zone and improved long-range coupled Förster energy transfer. The UN-EMLs not only remain similar in optical properties compared to the doped-EMLs including photoluminescence quantum yield and emitter orientation but also mitigate charge trapping by the emitters in the CT-host systems despite the large energy level differences. The results of time-resolved photoluminescence decay reveal that although a five times smaller volume emitter is used in the UN-EMLs, the intensity of triplet-triplet annihilation stays comparably low by restricting the exciton migration to the two-dimensional plane. Making use of the excellent optoelectronic properties of the proposed system, green OLEDs with extremely simplified structures reach an external quantum efficiency (EQE) of 23.2%, nicely matching with the optical simulation results. Furthermore, white OLEDs with the same structure achieve a maximum EQE of 18.7% with considerably low efficiency roll-off. The combined system demonstrated here provides a novel approach for significantly simplified and tunable device structures of OLEDs, while maintaining high performance.