Expanding the hole delocalization range in excited molecules for stable organic light-emitting diodes employing thermally activated delayed fluorescence

Metal-free, thermally activated delayed fluorescent (TADF) emitters have emerged as a promising new generation of organic light-emitting diode (OLED) materials. Donor-acceptor (D-A) structures are widely used in TADF molecular design to ensure a small energy splitting between the singlet and triplet excitons. Here, a series of efficient bluish-green TADF emitters are constructed using one or two phenyltriazine acceptors and one tercarbazole, bicarbazole or indolo[2,3- b ]carbazole donor through an ortho -linkage. The impact of the D/A ratio on the photoluminescence and electroluminescence stability of these emitters in doped films is thoroughly investigated. According to the two-exciton dynamics and the degradation products, device degradation is deduced to be a result of electrophilic substitution between two charge-transfer excitons. Within a limited molecular weight range, increasing the number of acceptor moieties leads to a decrease in the hole delocalization range in the excited state, which facilitates the substitution reaction. Based on an optimized device structure, the device containing an emitter with bulk a tercarbazole donor achieves a long half-life of 1512 hours with an initial luminescence of 1000 cd m −2 . Our findings reveal a possible mechanism for exciton-exciton and exciton-polaron annihilation-induced device degradation and provide new approaches for achieving stable OLEDs employing TADF. The degradation in TADF OLEDs is found to be governed by the radical electrophilic substitutions between two charge-transfer (CT) excitons. Expanding the mean localization distance (RLOL) of hole in the CT state can improve device stability.