In order to understand the impressive EL performance, comprehensive researches on the working mechanism of these devices are conducted. Device W1-0 can be regarded as a dual emissive layer device, whose EL efficiencies are even lower than those of the reported monochromatic OLEDs based on 4CzTPNBu, implying there is energy loss in W1-0. The single carrier devices of the neat film of TCP-BP-SFAC and doped film of 1.5 wt% 4CzTPNBu: TCP-BP-SFAC disclose that 4CzTPNBu has neglectable hole-trapping effect but significant electron-trapping effect in TCP-BP-SFAC host (Supplementary Fig. ). In that case, the exciton recombination zone in W1-0 will be greatly constrained in a narrow zone in orange EML (Supplementary Fig. ), which induces unnecessary energy loss due to triplet-triplet annihilation and triplet-singlet annihilation. By increasing the thickness of TCP-BP-SFAC layer adjacent to PPF layer, the excitons can partly recombine during electron shifting to 4CzTPNBu. Thereby, the excitons can be distributed in a dispersed manner in a wide zone, which alleviates exciton quenching caused by accumulation as depicted in Fig. . The appropriate frontier molecular orbitals of TCP-BP-SFAC also bring about low turn-on voltages comparable with those of WOLEDs based on exciplex hosts. The EL efficiencies tend to decline by continuously increasing the thickness of TCP-BP-SFAC layer (x = 7 and 8), because the orange EML gets too far away to harvest sufficient excitons and many excitons are recombined in TCP-BP-SFAC layer as depicted in the EL spectra (Supplementary Fig. ). The EL efficiency of TCP-BP-SFAC in neat film is lower than that of 4CzTPNBu in doped film, which is probably responsible for the efficiency decline in devices W1-7 and W1-8.