Moreover, device WS7 using BDMAC-XT as Host 1 and BCz-TRZ as Host 2 is fabricated. It exhibits sharp decrease in EL performance, with a poor EQE of only 13.2%. Device WS8 adopting BDMAC-XT as Host 1 and TCP-BP-SFAC as Host 2 is also prepared, which gives an EQE of 17.6%, higher than that of WS7. The EQE enhancement in WS8 mainly results from the good electron-trapping of 4CzTPNBu in TCP-BP-SFAC, which reduces the exciton recombination in DBP and thus avoids exciton loss. However, the efficiency of WS8 is still much lower than that of W2-1. Based on these findings, it is considered that the higher lying HOMO of BDMAC-XT can form relatively large hole-injection barriers between BDMAC-XT and Host 2 in WS7 and WS8. In that case, more excitons tend to recombine in DBP. Moreover, device WS9 containing an EML of BDMAC-XT (9 nm)/1.5 wt% 4CzTPNBu: TCP-BP-SFAC (5 nm)/TCP-BP-SFAC (6 nm) is fabricated. WS11 has a PE of 116.7 lm W -1 and an EQE of 28.2%, which are lower than those of W1-6, further confirming that the excitons are partially recombined in BDMAC-XT. In addition, WS9 using BCz-TRZ and BDMAC-XT as Host 1 and Host 2, respectively, and device WS10 using BCz-TRZ and TCP-BP-SFAC as Host 1 and Host 2, respectively, are fabricated. WS9 exhibits a PE of 91.6 lm W -1 and an EQE of 27.5%. WS10 shows a PE of 96.2 lm W -1 and an EQE of 28.9%, which are slightly better than those of WS9. From the comparison between WS9 and WS10, it is concluded that TCP-BP-SFAC is an better host for 4CzTPNBu, due to its more suitable HOMO energy levels.