High‐Efficiency Solution‐Processed Organic Light‐Emitting Diodes Utilizing Multiple‐Resonance Emitters Incorporating Thiocarbazole Unit

Abstract Introducing heavy atoms to enhance spin‐orbit coupling represents an effective strategy for boosting the reverse intersystem crossing (RISC) rate constants of Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters. Nevertheless, the restricted diversity of molecular structures obstructs a thorough investigation of structure‐property relationships, thereby limiting the improvement of device performance, especially in the case of solution‐processed devices. Herein, an alkylthio‐substituted carbazole building block is designed and developed two new MR‐TADF emitters, namely BNCz‐2S and BNCz‐4S. Quantum simulations and photophysical studies have revealed that as the number of sulfur atoms increases, BNCz‐4S exhibits a higher photoluminescence quantum yield (PLQY), a smaller singlet‐triplet energy gap (ΔE_ST) and reorganization energy, along with a larger spin‐orbit coupling (SOC) constant and a higher reverse intersystem crossing rate (k_RISC) constants compared to BNCz‐2S. Consequently, solution‐processing devices based on BNCz‐4S exhibit a higher external quantum efficiency (EQE) of 24.06%, which is in the first tier of reported solution‐processed MR‐TADF organic light‐emitting diodes (OLED) to date.