Efficient blue phosphorescent organic light emitting diodes based on exciplex and ultrathin Firpic sandwiched layer

The high efficient blue phosphorescent organic light emitting diodes (PHOLEDs) employing ultrathin Firpic layer sandwiched between electron donor mCP and electron acceptor TmPyPB layers were fabricated, and the dependence of the electroluminescence (EL) performance on the ultrathin Firpic layer thickness were investigated. It was found that the Firpic layer thickness imposed vital influence on the OLEDs EL efficiency and the one with the 0.3 nm Firpic demonstrated the optimal power efficiency (PE) of 50 lm/w and the external quantum efficiency (EQE) close to 18%. The sandwiched ultrathin Firpic layer would not impede the formation of the exciplex between mCP and TmPyPB, and the balance between the exciplex formation and exciplex decaying showed strong dependence on Firpic layer thickness and imposed vital influence on the energy transfer, fluorescence and phosphorescence inside the emissive layer. There were three radiative decaying processes involved, i.e. the prompt and delayed fluorescence (PF and DF) of the exciplex as well as the phosphorescence (PH) of the Firpic, and the final EL relied on the competition of those processes. The optimal EL performance might be attributed to the combining benefits of the balanced PF, DF and PH decaying, alleviated Dexter-transfer-based triplet-triplet annihilation (TTA) and the triplets diffusion suppressed architecture.