Suppression of benzophenone-induced triplet quenching for enhanced TADF performance

Efficient TADF relies on effective reverse intersystem crossing (rISC) from triplet to singlet manifold, and therefore the transition rate associated with this process must outcompete non-radiative decay rate of the triplets. Herein the issue of non-radiative triplet quenching in benzophenone-derived compounds frequently employed as blue TADF emitters in OLEDs is addressed and an effective way to suppress this quenching is proposed. TADF emitters designed for this study were based on phenone-derived electron-accepting units linked with multiple carbazolyl donors to deliver similar and large rISC rates (krISC ∼ 4 × 106 s−1), yet significantly different non-radiative triplet quenching rates (kTnr). We found that substitution of a loose phenyl moiety by a methoxy group in benzophenone suppresses kTnr by one order of magnitude (from 3 × 106 s−1 down to 0.3 × 106 s−1), while leaving rISC rate krISC almost unaffected. A 10 times enlarged ratio krISC/kTnr in 5tCzMeB as compared to that in 5tCzBP emitter suffices to retrieve the vast majority of triplet population via the rISC resulting in PL quantum yields of 0.99 and 0.58 in doped DPEPO and neat films, respectively. Sky-blue TADF OLEDs with doped and non-doped emissive layers based on 5tCzMeB demonstrated impressive maximum external quantum efficiency (EQE) values of 24.6% and 13.4%, respectively, whereas at 100 cd m−2 the devices exhibited 16.5% and 7.7% EQEs, respectively.