Efficient Solid-State triplet-triplet annihilation up-conversion electroluminescence device by incorporating intermolecular intersystem-crossing dark sensitizer

Abstract

The efficient conversion of electron-hole (e-h) pairs into triplet excitons is a challenge for blue organic light-emitting diodes (OLEDs) based on triplet–triplet annihilation upconversion (TTAUC). The 25% fraction of e-h pairs that create singlet excitons represents a significant loss channel that can lead to parasitic red-shifted emission. In this study, an intermolecular intersystem crossing that relies on a “dark sensitizer” (DS) layer consisting of tris-(8-hydroxyquinoline)aluminum (Alq3) doped with tris[2-phenylpyridinato-C2,N]Iridium(III) (Ir(ppy)3) is demonstrated to enhance the TTAUC process. Carriers recombination and excitons generation are formed on Alq3 molecules. Alq3 singlet excitons are then quenched by the Ir(ppy)3 triplet state, followed by energy transfer to the Alq3 triplet state. The non-emissive, long-lived Alq3 triplets migrate from the sensitizer layer to an emitter layer where they undergo TTAUC to give blue fluorescence emission. This DS-TTAUC process promises no green emission from the Alq3. The efficiency of a blue OLED utilizing DS-TTAUC was improved by 34.2% compared to a standard TTAUC OLED. In addition, the device exhibited CIE coordinates of (0.15, 0.09). Furthermore, a high photoluminescence quantum yield fluorescence emitter is incorporated to enhance the singlet exciton emission in the emitter layer. A maximum external quantum efficiency of 7.54% can be achieved with recorded-high quantum yield of the TTAUC (ΦTTAUC) = 37.6% in solid state.