Next, we investigated the electroluminescence properties of these three emitters. Multilayer OLEDs were fabricated using these materials as dopants within the emissive layer, with the following device structures: ITO/HATCN (5 nm)/TAPC (40 nm)/TCTA (10 nm)/mCP (10 nm)/PPT: emitter (10 wt%) (30 nm)/PPT (10 nm)/TmPyPB (40 nm)/LiF (0.8 nm) Al (100 nm). Here, HATCN acts as the hole injection layer, TAPC and TCTA are used as hole transporting layers, and mCP is the exciton blocking layer. PPT was selected as the host material due to the good carrier transport ability and the high ΦPL values of the three emitters in 10 wt% doped films, and TmPyPB is the electron-transporting material. The performance of the devices is summarized in Figure . The devices based on the three emitters exhibit turn on voltage (Von) at 3.0 V, and the maximum luminance (Lummax) exceeded 10 4 cd m -2 . The OLEDs based on 4CzPyz exhibit sky blue emission with λEL of 486 nm, and CIE coordinates of (0.20,0.36). Thanks to the high ΦPL and TADF character of 4CzPyz in PPT host (Figures and), a maximum EQE (EQEmax) of 24.1% is achieved at 3 cd m -2 , which is much higher than our previously reported pyrazine-based TADF emitter DTCz-Pz (EQEmax: 11.6% at 2 cd m -2 ). The EQE reduced to 7.0% and 2.6% at 100 cd m -2 and 1000 cd m -2 , respectively.