Different RISC rates in benzoylpyridine-based TADF compounds and their implications for solution-processed OLEDs

Abstract Reverse intersystem crossing (RISC) rate is among the key parameters of TADF compounds affecting triplet upconversion and annihilation processes, and thus determining efficiency roll-off in TADF-OLEDs. Herein, control of RISC rate (kRISC) in a series of benzoylpyridine-based TADF compounds via carbazole donor linking positions is demonstrated. Regardless of the similar singlet-triplet energy splitting found in the compounds with meta- or para-carbazole linking topologies, meta-Cz compounds exhibited stronger singlet-triplet state coupling resulting in 10-fold larger kRISC (up to ∼2 × 106 s−1) as that in para-Cz counterparts. Different kRISC was shown to have important implications for the solution-processed TADF-OLEDs based on the benzoylpyridines. Explicitly, in the non-doped devices based on neat meta-Cz emissive layers, larger kRISC was crucial for the reduced roll-off. Conversely, in 1.2 wt% rubrene-doped hyperfluorescent devices employing benzoylpyridines as TADF hosts, insignificant roll-off was observed irrespective of kRISC, provided it is above 105 s−1. The obtained results give insight into the possibilities of controlling kRISC in benzoylpyridines and demonstrate its importance for efficiency roll-off of TADF and hyperfluorescent OLEDs.