Abstract In the past, considerable effort are focused on advancing blue organic light‐emitting diodes (OLEDs) based on 1,3‐bis(N‐carbazolyl)benzene (mCP). To enhance and stabilize the device performance in vacuum‐processed OLEDs, it is essential to optimize the thermally stable organic materials constituting the emitting layer. The proposed novel carbazole derivative, Ad‐mCP , is designed and synthesized as a host material for blue thermally activated delayed fluorescence OLEDs, which are ideal for vacuum processing. Ad‐mCP exhibits a high glass transition temperature and triplet energy (T 1 ), making it highly compatible with the blue dopant 2,11,14‐tri‐ tert ‐butyl‐N,N,5‐tris(4‐( tert ‐butyl)phenyl)−5 H ‐5,8b‐diaza‐15b‐borabenzo[ a ]naphtho[1,2,3‐ hi ]aceanthrylen‐7‐amine emitter. The incorporation of a rigid adamantane unit into the carbazole group modifies the molecular structure of mCP, preserving crucial photophysical properties, such as T 1, while enhancing the morphological and thermal stability of the host material in its film state. Moreover, the spatially hindered molecular structure of Ad‐mCP improves the photoluminescent quantum yield of the emissive layer and enhances charge balance, crucially contributing to the overall efficiency of the OLEDs. As a result, the exceptional performance of Ad‐mCP as a host material for OLEDs resulted in a remarkably high maximum external quantum efficiency of 29.9%, demonstrating superior stability in efficiency even after thermal annealing compared to mCP devices.