Delicate Balance in Design of High‐Efficiency and Heat‐Tolerant Deep Blue OLED Emitters with Barrierless High‐Lying Reverse Intersystem Crossing

The development of high‐efficiency deep blue organic light emitting diode (OLED) emitters is critical for display and solid‐state lighting applications. However, achieving optimal performance is challenging due to conflicts between color purity, photoluminescence intensity, and exciton utilization efficiency. In this study, two deep blue emitters based on pyrenyl phosphine oxides are synthesized by modifying a carbazole substituent, aiming to achieve a balance between moderate emission bathochromism, high photoluminescence quantum yield (PLQY), and favorable transition characteristics. Additional exciton harvesting via high‐lying reverse intersystem crossing (hRISC) markedly enhances the efficiency of OLED devices. The maximum external quantum efficiencies (EQEs) achieve 11.23% and 8.14%, with Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.07) and (0.15, 0.12), respectively. The presence of hRISC transitions also contributes to significant heat resistance to the devices, with a 22‐ and 71‐fold enhancement in electroluminescence (EL) in the deep blue and white OLEDs, respectively, as the temperature increases from room temperature to 500 and 450 K. This study demonstrates the strong potential for these emitters in energy‐saving OLED applications suitable for extreme environments.