Recently, a TADF emitter 5Cz-TRZ (∆EST: 0.06; FPL: 99% (15wt% mCBP); τd: 0.8 μs in DMF) was reported (Figure ) that contains five carbazole donors. Compound 5Cz-TRZ possesses a very high kRISC of 1.5 × 10 7 s -1 , which is in part due to the large density of excited states and the small DEST. The authors contended that the fast RISC in 5Cz-TRZ would contribute to a smaller efficiency roll-off in the device as efficiency roll-off is typically caused by the accumulation of long-lived triplet excitons. The resulting device showed an outstanding performance with an EQEmax of 29.3%, which decreased a mere 2.3% at 1,000 cd m -2 , and a device lifetime, T90, of ~600 h from an initial brightness of 1,000 cd m -2 . All the photophysics and electrochemical characteristics of the aforementioned materials are summarized in Table . Representative device performance is summarized in Table . PIC-TRZ (λPL: 466 nm; τd: 120 μs; ΔEST: 0.11 eV; ΦPL: 39%; 6 wt% in mCP) was the first reported purely organic TADF emitter for use in OLEDs (Figure ). Unlike the most commonly used design strategy for TADF emitters, which is to separate electron-donor and electronacceptor groups by a conjugated phenylene bridge, PIC-TRZ contains two carbazole derivatives that are directly connected to a triazine core. A relatively small DEST (0.11 eV) was obtained due to the large twist angle between the carbazole-based donors and the triazine acceptor. Even though the reported EQEmax could only reach 5.3% for the sky-blue OLED, this nevertheless indicated that up-conversion of triplet excitons to singlet excitons was occurring as the theoretical EQEmax of the device was limited to 2% based on its relatively low FPL of 39% and assuming no triplet exciton harvesting.