Developing red thermally activated delayed fluorescence (TADF) emitters,a ttainable for both high-efficient red organic light-emitting diodes (OLEDs) and non-doped deep red/near-infrared (NIR) OLEDs,ischallenging. Now,two red emitters,B PPZ-PXZ and mDPBPZ-PXZ, with twisted donor-acceptor structures were designed and synthesized to study molecular design strategies of high-efficiency red TADF emitters.B PPZ-PXZ employs the strictest molecular restrictions to suppress energy loss and realizes red emission with aphotoluminescence quantum yield (F PL )of100 AE 0.8 %and external quantum efficiency (EQE) of 25.2 %i nadoped OLED.Its non-doped OLED has an EQE of 2.5 %owingto unavoidable intermolecular p-p interactions.mDPBPZ-PXZ releases two pyridine substituents from its fused acceptor moiety.A lthough mDPBPZ-PXZ realizes al ower EQE of 21.7 %i nt he doped OLED,i ts non-doped device shows asuperior EQE of 5.2 %with adeep red/NIR emission at peak of 680 nm.

The conformational distribution and mutual interconversion of thermally activated delayed fluorescence (TADF) emitters significantly affect the exciton utilization. However,t heir influence on the photophysics in amorphous film states is still not knownd ue to the lacko fasuitable quantitative analysis method. Herein, we used temperaturedependent time-resolved photoluminescence spectroscopyt o quantitatively measure the relative populations of the conformations of aT ADF emitter for the first time.W ef urther propose an ew concept of "self-doping" for realizing highefficiency nondoped OLEDs.Interestingly,this "compositionally" pure film actually behaves as afilm with adopant (quasiequatorial form) in am atrix (quasi-axial form). The concentration-induced quenching that may occur at high concentrations is thus expected to be effectively relieved. The "selfdoping" OLED prepared with the newly developed TADF emitter TP2P-PXZ as an eat emitting layer realizes ah igh maximum external quantum efficiency of 25.4 %a nd neglectable efficiency roll-off.