Beyond The RISC Rate: A Figure of Merit for MR-TADF Materials and the Path to High Efficiency OLEDs Having Minimal Efficiency Roll-Off and High Stability

The relatively slow reverse intersystem cross rate constant ( k RISC ) of multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters has been recognized as perhaps the most critical factor governing their rather significant efficiency roll-off in organic light-emitting diodes (OLEDs). A recent study identified a figure of merit for TADF materials, which captures the relevant exciton dynamics processes and correlates these with efficiency roll-off in the resulting device. Two MR-TADF emitters tBTCzBN and tSeCzBN, containing, respectively, dibenzothiophenocarbazole and dibenzoselenophenocarbazoles were designed. Their rigid p-and n-doped nanographene structures, incorporating heavy chalcogen atoms at optimal locations, result in fast 𝑘 γ S s of 1.7×10 8 and 7.7 ×10 7 s -1 , k RISC values of 2.6 ×10 5 and 3.9 ×10 6 s -1 , and FOMs of 1.8 ×10 5 and 3.9 ×10 5 s -1 for tBTCzBN and tSeCzBN, respectively. The sensitizer-free OLEDs with tBTCzBN and tSeCzBN exhibited green electroluminescence [CIE coordinates of (0.14, 0.61), and (0.18, 0.61)]. Notably, the device with tSeCzBN delivered superior high maximum external quantum efficiencies (EQE max ) of 37.1%, with EQE 1000 of 31.7%, which is one of the highest reported EQE1000 values, and is matched by a remarkably low efficiency roll-off of 14%, especially compared to other MR-TADF OLEDs. Moreover, the incorporated chalcogen atoms modulate the frontier molecular orbital levels of the emitter, resulting in much-improved operational stability as a result of suppressed hole trapping. This work highlights the distinct benefits and the importance of judicious incorporation of heavy atoms within MR-TADF emitter structures to facilitate efficient exciton harvesting and enhance device performance.