Achieving a Significantly Increased Efficiency in Nondoped Pure Blue Fluorescent OLED: A Quasi-Equivalent Hybridized Excited State

Excited state characters and components play a decisive role in photoluminescence (PL) and electroluminescence (EL) properties of organic light-emitting materials (OLEDS). Charge-transfer (CT) state is beneficial to enhance the singlet exciton utilizations in fluorescent OLEDs by an activated reverse intersystem crossing process, due to the minimized singlet and triplet energy splitting in CT excitons. However, the dominant CT component in the emissive state significantly reduces the PL efficiency in such materials. Here, the strategy is to carry out a fine excited state modulation, aiming to reach a golden combination of the high PL efficiency locally emissive (LE) component and the high exciton utilizing CT component in one excited state. As a result, a quasi-equivalent hybridization of LE and CT components is obtained in the emissive state upon the addition of only an extra phenyl ring in the newly synthesized material 4-[2-(4′-diphenylamino-biphenyl-4-yl)-phenanthro[9,10-d]imidazol-1-yl]-benzonitrile (TBPMCN), and the nondoped OLED of TBPMCN exhibited a record-setting performance: a pure blue emission with a Commission Internationale de L'Eclairage coordinate of (0.16, 0.16), a high external quantum efficiency of 7.8%, and a high yield of singlet exciton of 97% without delayed fluorescence phenomenon. The excited state modulation could be a practical way to design low-cost, high-efficiency fluorescent OLED materials.