shows excellent PE and EQE of 126.8 lm W -1 and 30.6%, respectively, but the EL spectrum of WS3 is barely changed in comparison with that of W1-6, in which the characteristic peak of DBP cannot be observed. The primary cause for the failed sensitization is the large span between the recombination zones of 4CzTPNBu and DBP, due to severe electron trapping of 4CzTPNBu as illustrated in Supplementary Fig. . Another supplementary device WS4 with an EML consisting of a single doped film of 1 wt% DBP: TCP-BP-SFAC (20 nm) is fabricated (Supplementary Table ). WS4 has an EQE of only 4.9%, close to the EQE limit of the conventional fluorescence devices, indicating TCP-BP-SFAC is not a suitable sensitizer for DBP and causes severe exciton loss, namely DBP cannot be sensitized by TCP-BP-SFAC host.