values to the best HF devices employing these literature sulfur-or selenium-containing MR-TADF emitters (Table ), while, importantly, employing only a traditional device structure. This achievement can be attributed to the optimal regiochemistry of the 12H-benzo[4,5]thieno[2,3-a]carbazole (BT2) moiety, which maximizes the involvement of the sulfur orbitals in the low-lying excited states, thereby leading to larger SOC and faster kRISC compared to emitters utilizing other benzothienocarbazole isomers (Figure , Table ). Moreover, the rigidity of benzothienocarbazole suppresses molecular relaxation leading to narrowband emission, high ΦPL and fast kr in the solid state, which helps tCzBT2B achieve much larger 𝑘 1 # 𝐾 34 value compared to other heavy atom-containing emitters (Figure ).