The thermally evaporated bottom-emitting devices with PhCz-O-DiKTa and PhCz-DiKTa were next fabricated using a device structure of ITO/HATCN (5 nm) /TAPC (40 nm)/TCTA (10 nm)/10 wt% emitter in mCP (20 nm)/TmPyPB (50 nm)/LiF (0.7 nm)/Al (100 nm) (Figure ), where ITO (indium tin oxide) serves as the transparent anode, HATCN (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile) acts as the hole-injection layer, TAPC (1,1-bis[(di-4-tolylamino)phenyl]cyclohexane) act as the hole-transporting layer, TCTA act as the electron blocking layer, mCP is the host, TmPyPB Although we have demonstrated that a high doping concentration leads to emission broadening (Figure ), the 10 wt% emitter doping concentration was chosen due to the recorded high F PL values. The EL data are summarized in Table . mitigate the efficiency roll-off due to the faster k RISC and shorter delayed lifetime, thus reducing triplet exciton populations at high current densities. However, most of the previously reported ketone-based MR-TADF OLEDs showed rather severe efficiency roll-off (> 45%); the origin of the large roll-off has not been explored in any detail. A summary of previously reported EL data related to ketone-based MR-TADF devices is presented in Figure . This report experimentally demonstrates that the T 2 state is the key to regulating k RISC (Figure ) and provides insight into improving the efficiency roll-off of devices, at least with ketone-containing MR-TADF derivatives.