Most phosphorescent devices suffer from severe triplet−triplet annihilation (TTA), and the efficiency recording at high luminance is much lower than that at low luminance, making the practicality worse than expected. In this study, a series of donor−acceptor (D−A) molecules consisting of dicyano-imidazole and phenylcarbazole were synthesized and applied to the host materials. For electroluminescence applications, imM-m-Cz-based green-emitting organic light-emitting diodes show a maximum luminance of 1.68 × 10 5 cd m −2 at 11.2 V, which is 20% higher than that of the benchmark host 4,4′-bis(N-carbazolyl)-1,1′-biphenyl at 13.4 V; in addition, the turn-on voltage (V on ) is only 2.3 V. In terms of hole and electron mobility, imM-m-Cz shows one of the most balanced carrier mobilities in the reported literature (electron and hole mobilities of 3.64 × 10 −5 and 4.23 × 10 −5 cm 2 V −1 s −1 , respectively). The balanced carrier mobility can help expand the recombination region and thus reduce the formation of TTA. Furthermore, the high maximum luminance of about 2.80 × 10 5 cd m −2 obtained in an imM-m-Cz-based tandem device demonstrates a sufficiently high current density/luminance, and the peak efficiency achieves an even higher efficiency of 40.6% (1.51 × 10 5 cd A −1 and 98.6 lm W −1 ), which is among the highest reported properties based on imidazole-based host materials. Combining the balanced carrier mobility with the proper device design in this study, the efficiency roll-off under high luminance can be greatly reduced, and the practicality of the resulting phosphorescent device can be significantly improved.

A series of dicyano-imidazole-based molecules with thermally activated delayed fluorescence (TADF) properties were synthesized to obtain pure blue-emitting organic lightemitting diodes (OLEDs). The targeted molecules used dicyano-imidazole with a short-conjugated system as the electron acceptor to strong intermolecular π-π interactions, and provide a relatively shallow energy level of the lowest unoccupied molecular orbital (LUMO). The cyano group was selected to improve imidazole as an electron acceptor due to its prominent electron-transporting characteristics. Four different electron donors, that is, 9,9-dimethyl-9,10-dihydroacridine (DMAC), 10H-spiro(acridine-9,9'-fluoren) (SPAC), and 9,9diphenyl-9,10-dihydroacridine (DPAC), were used to alternate the highest occupied molecular orbital (HOMO) energy level to tune the emission color further. The crowded molecular structure in space makes the electron donor and acceptor almost orthogonal, reducing the energy gap (ΔE ST ) between the first excited singlet (S 1 ) and the triplet (T 1 ) states and introducing significant TADF property. The efficiencies of the blue-emissive devices with imM-SPAC and imM-DMAC obtained in this work are the highest among the reported imidazole-based TADF-OLEDs, which are 13.8 % and 13.4 %, respectively. Both of Commission Internationale de l'Eclairage (CIE) coordinates are close to the saturated blue region at (0.17, 0.18) and (0.16, 0.19), respectively. Combining these tailor-made TADF compounds with specific device architectures, electroluminescent (EL) emission from sky-blue to deep-blue could be achieved, proving their great potential in EL applications.

Small-molecule hole-transporting materials (HTMs) have gained much attention due to their structural flexibility, material properties, and stabilities, allowing for enhanced operational durability in perovskite photovoltaics. We synthesized and investigated a...

Pure blue‐emitting organic light‐emitting diodes (OLEDs) based on dicyano‐imidazole with thermally activated delayed fluorescence (TADF) have been studied at Providence University (of which a building is shown). The efficiencies obtained in this work are the highest among the reported imidazole‐based TADF‐OLEDs. By combining these tailor‐made TADF compounds with specific device architectures, electroluminescent emission from sky blue to deep blue could be achieved. More information can be found in the Full Paper by C.‐W. Lu et al. (DOI: 10.1002/chem.202101807).

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