Having identified the 5 wt% doped film in mCP is optimal, we next proceeded to obtain the steady-state PL spectra and time-resolved PL decays. This film emits at lPL of 501 nm and has a FWHM of 41 nm/0.20 eV (Figure ). The DEST value is slightly smaller at 0.13 eV compared to that in 2-MeTHF (Figure ). The time-resolved PL decay at 300 K under vacuum reveals a prompt fluorescence lifetime, τp, of 7.6 ns and a long delayed lifetime, τd, of 1.2 ms (Table ). The increased intensity of delayed emission with increasing of temperature from 77 to 300 K confirms that there is TADF (Figure ). The much longer τd of 2,7-tBuCzNB compared to that of Cz-SCz (τd of 425 µs) may be due to the much lower SOCME values (0.026, 0.050 cm -1 for S1-T1, S1-T2 of 2,7-tBuCzNB compared to 0.150, 0.653 cm -1 for S1-T1, S1-T2 of Cz-SCz) (Figure ), which result in a lower RISC rate constant (kRISC = 5.4 × 10 2 s -1 , Table ). The device structure and performance are summarized in Figure and Table