Extremely simple two-color all-TADF WOLEDs with a single-emissive-layer singe-host/dopant system provide a bright future for high-efficiency low-cost vacuum-deposited WOLEDs.

An organometallic Ir complex exhibited enhanced photovoltaic performance compared to an organic ligand.

Electron acceptors based on the electron-deficient carboline of CbPyCN exhibited better electroluminescence performance than CzPyCN in exciplex or non-exciplex mixed cohosts and exciplex-emitters.

Since the pioneering work of Tang et al., organic light-emitting diodes (OLEDs) unveil a bright future for the applications in new-generation solid-state lightings, full-color flat-panel displays, and flexi ble displays due to their low power consumption, brilliant colors, fast response time, and self-emission. [1,2] According to the emission mechanism, the phosphorescent OLEDs (PHOLEDs) or thermally activated delayed fluorescence (TADF) OLEDs, which can simultaneously harvest both singlet and triplet excitons through intersystem crossing (ISC) or reverse intersystem crossing (RISC), to reach maximum internal quantum efficiency (IQE) of 100%, are expected to achieve higher efficiencies than the traditional fluorescent devices with theoretical IQE of only 25%. [2] However, in order to suppress the concentration quenching and triplettriplet annihilation in phosphorescent or TADF OLEDs, the phosphorescent or pure organic TADF emitter has to be dispersed into a suitable organic host matrix to achieve high device efficiency. [3] Thus, the exploration of host materials is essential to acquire ideal phosphorescent/TADF OLEDs. [3,4] In comparison to the traditional single-carrier-transport host materials, bipolar hosts comprising of both hole-and electron-transport moieties have been widely reported to broaden exciton recombination zones, extend the lifetime of excitons, improve charge carrier injection and transport balance, and thus to increase the device performance and reduce efficiency roll-off. [3][4][5] It is well proved that the strategy of using bipolar hosts with extremely low singlet and triplet bandgap (ΔE ST ) is an effective way to lower driving voltage and drastically increase the power efficiency of the devices owing to better matched frontier energy levels of host materials with those of the adjacent hole transporting materials and electron transporting materials. [6] This is due to the fact that the high first triplet energy level (T 1 ) can significantly prevent reverse energy transfer from the phosphorescent dopant to host and the relatively low first singlet energy level (S 1 ) is able to remarkably improve and balance carrier injection and transport in the emissive layers, 1,3,5-tris(2-(pyridin-2-yl)-1H-benzo[d]imidazol-1-yl)benzene (i TPyBIB), 1,1′,1′′-(pyridine-2,4,6-triyl)tris(2-phenyl-1H-benzo[d]imidazole) (i TPBIPy), and 1,1′,1′′-(pyridine-2,4,6-triyl)tris(2-(pyridin-2-yl)-1H-benzo[d]imidazole) (i TPyBIPy) are designed and synthesized. Compared to the commercial electron-transport 2,2,2-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TPBI) with the C-atom in benzoimidazole linked to the central phenyl ring, the introduction of isomeric N-linkage to the central phenyl or pyridine ring in three compounds remarkably simplifies the synthesis to a one-step CN coupling reaction. iTPyBIB, iTPBIPy, and iTPyBIPy exhibit comparable highest occupied molecular orbital (≈6.0 eV) and lowest unoccupied molecular orbital (≈2.5 eV) energy levels, similar triplet energy levels of ≈2.65 eV with TPBI, whe...