The photophysical results validate efficient FET from 4CzTPNBu to DBP through the out-of-phase sensitization configuration, finally producing impressive EL performance of W2-1~W2-3. To gain in-depth insights into the device mechanism, the impact from host-dopant interaction is also studied. Two typical TADF materials with close emission colors, 3,6-bis(9,9-dimethylacridin-10-yl)-xanthen-9-one (BDMAC-XT) and 9-(4-(4,6-diphenyl-1,3,5-triazin-2yl)phenyl)-9'-phenyl-9H,9'H-3,3'-bicarbazole (BCz-TRZ), are selected as control hosts for comparison with TCP-BP-SFAC. The single carrier devices of BDMAC-XT and BCz-TRZ neat films and doped films with 4CzTPNBu dopant are fabricated and studied (Supplementary Fig. ). The electron-trapping effect from 4CzTPNBu is found to be still significant in BDMAC-XT, but is less effective in BCz-TRZ, probably due to its lower lying LUMO. Three-color white EMLs with the configuration of 1 wt% DBP: Host 1 (9 nm)/1.5 wt% 4CzTPNBu: Host 2 (5 nm)/TCP-BP-SFAC (6 nm) are applied for investigation in detail (Fig. ). Device WS5 using BDMAC-XT as both Host 1 and Host 2 shows a low EQE of 23.2%, much inferior to that of W2-1 (Supplementary Fig. and Supplementary Table ). Similarly, device WS6 adopting BCz-TRZ as both Host 1 and Host 2 has a low EQE of 23.9% neither. Since the electron-trapping effect in BDMAC-XT is comparable to that in TCP-BP-SFAC, the low EQEs of WS5 and WS6 should be attributed to the efficiency decrease of 4CzTPNBu in BDMAC-XT host. As for WS6, the inefficient electron-trapping by 4CzTPNBu causes inadequate blocking of electrons, and thus exciton recombination can occur in DBP. In consequence, there is severe triplet exciton loss, resulting in a low EQE.