kRISC is largely dependent on the magnitude of the SOC (V #%$ ), which is proportional to the atomic nuclear charge 𝑍 * + of the atoms involved in the emissive transition. Previous works have shown that introducing heavy atoms such as bromine, iodine, sulfur, and selenium can induce a large SOC in MR-TADF molecules, thereby accelerating kRISC to greater than 10 5 s -1 , and leading to an improve exciton utilization in the device. The benzothienocarbazole (BTC) regioisomeric moieties, with their moderately heavy S atom (ZN =16), rigid skeleton, and moderately strong electron-donating strength, have been used in D-A TADF emitter design. Both Cai et al. and Shin et al. have shown that the location of the sulfur atom affects the SOC between low-lying triplet and singlet excited states, impacting excitonic kinetics. The emitters BTCZPZ2 and BTCZPZ1, containing different regioisomeric 12H-benzo [4,5]thieno [2,3a]carbazol (BT2) and 5H-benzo [4,5]thieno [3,2-c]carbazole (BT1) donors (Figure ), respectively,