The even distribution of the electron density of the HOMO across multiple donors also is in operation in TmCzTRZ (∆EST: 0.07 eV; FPL: 99% in toluene), which also possesses a smaller DEST and a higher FPL than DCzmCzTRZ (∆EST: 0.20 eV; FPL: 84% in toluene), which has two different carbazole-based donors. OLEDs with these three emitters achieved comparably high efficiencies, with EQEmax of 25% and CIE coordinates of (0.18, 0.33) for the device with TCzTRZ and an EQEmax of 25.5% and CIE coordinates of (0.25, 0.50) for the device with TmCzTRZ; the red-shift in the EL of the latter is due to the use of a stronger dimethylcarbazole donor. The EQEmax decreased to 21.3%, with CIE coordinates of (0.23, 0.46), for the device with DCzmCzTRZ, reflecting to the lower FPL for the emitter. permission. Lee et al. later probed the effect of the regiochemistry of the three carbazole donors on the photophysical properties of the emitters. A cross-comparison of the photophysical properties of compounds 234CzTRZ (∆EST: 0.07 eV; FPL: 90% in DPEPO; τd: 4.1 μs), 235CzTRZ (∆EST: 0.14 eV; FPL: 100% in DPEPO ; τd: 8.4 μs) and 245CzTRZ (∆EST: 0.17 eV; FPL: 98.4% in DPEPO ; τd: 9.7 μs) revealed that the substitution pattern of the three carbazole donors affects both ∆EST and τd; all three compounds possess a shorter delayed fluorescence lifetime than that of TCzTRZ (τd: 13.5 μs). As shown in the Figure , the ortho-substituted carbazoles to the diphenyltriazine acceptor adopt a large dihedral angle, which leads to a shortened delayed fluorescence lifetime, compound 234CzTRZ, with three carbazoles packed closely to each other, showed the shortest delayed lifetime of 4.1 μs. The EQEmax of the OLEDs using 234CzTRZ (λEL: 497 nm; CIE = (0.20, 0.44)), 235CzTRZ (λEL: 495 nm; CIE = (0.20, 0.45)) and 245CzTRZ (λEL: 490 nm; CIE = (0.17, 0.39)) are all quite similar at 20.4%, 18.7%, and 22.0%, respectively. At a luminance of 1000 cd m -2 , the device with 234CzTRZ can still maintain 18.4% while the EQE1000 of the OLEDs with 235CzTRZ and 245CzTRZ drop to 14.7% and 13.8%, respectively. However, even though these emitters exhibited shorter tDF and smaller ∆EST than those of TCzTRZ, their devices performance are inferior to the OLED based on TCzTRZ. Operational lifetimes of the TADF and hyperfluorescent OLEDs. Reproduced with permission. Copyright © 2020, Nature Publishing Group.