For comparison with film I (1.5 wt% 4CzTPNBu: TCP-BP-SFAC (15 nm)), films V and VI are constructed with the same configurations of 1.5 wt% 4CzTPNBu: BCz-TRZ (15 nm) and 1.5 wt% 4CzTPNBu: BDMAC-XT (15 nm), respectively. As shown in Fig. , the energy levels of both S 1 and T1 states of 4CzTPNBu are lowered progressively by doping in BDMAC-XT host to TCP-BP-SFAC host and then to BCz-TRZ host. The ∆EST values are 0.05, 0.05 and 0.11 eV for 4CzTPNBu in films I, V and VI, respectively, estimated from the fluorescence and phosphorescence spectra at 77 K (Supplementary Fig. ), indicating TCP-BP-SFAC and BCz-TRZ are more suitable hosts than BDMAC-XT to promote RISC process and thus delayed fluorescence of 4CzTPNBu. The different ∆EST values can rationally influence the EL efficiencies of 4CzTPNBu in different hosts and consequently alter the performances of WOLEDs. Besides, the theoretical calculation is conducted for comprehensive investigation. According to the previous study, the host-tuning effect on TADF materials is virtually associated with the different polarities and the valence shell transition types of S1 and T1 states. The natural transition orbital analysis discloses that the S1 and T1 states of 4CzTPNBu are dominated by π-π* transition (Supplementary Fig. ), indicative of the similar sensitivity towards the polarity of matrix. The calculated molecular polarity index (MPI) values of S1 and T1 states of TCP-BP-SFAC, BCz-TRZ and BDMAC-XT are listed in Supplementary Table . It can be observed that BCz-TRZ has the largest MPI values, and can lead to the lowest energy levels of S1 and T1 states of 4CzTPNBu, which is in good agreement with the experimental results. Such kind of host-tuning effect should be a powerful method to modulate the PL and EL behaviors of luminescent materials.