Here, we identified the optimal regiochemistry of a sulfur-containing benzothiophenocarbazole moiety in MR-TADF emitters for maximizing spin-orbit coupling and thus accelerating RISC. Our optimized MR-TADF emitter, tCzBT2B, possesses a high calculated SOCME value (𝐻 ,-. #/0/ = 0.60 cm -1 ) due to the significant involvement of sulfur atom in the low-lying excited states. In contrast, the regioisomer tCzBT1B shows a reduced 𝐻 ,-. #/0/ of 0.27 cm -2 . The large SOC in tCzBT2B translates into superior TADF properties, with fast kRISC rates of 2.7 × 10 5 s -1 and a relatively large figure of merit 𝑘 1 # 𝐾 34 of 1.9 × 10 5 s -1 as 4 wt% doped films in PhCzBCz, both values being over two times faster compared to those of tCzBT1B. As a result, the devices with tCzBT2B do not require either a TADF host or assistant dopant to efficiently harvest triplet excitons, achieving an impressive EQEmax of 34.3% and mild efficiency roll-off (EQE1000: 22.4%). By contrast, the device with tCzBT1B showed similarly high EQEmax of 34.9% but the efficiency roll-off was more several with an EQE1000 of 18.0%, attributed to the slower kRISC of the emitter. Overall, these findings underscore the importance of judicious heteroatom placement within the design of high-performance MR-TADF emitters and devices.