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Great efforts have been made to improve the composition and structure of perovskite light‐emitting diodes (PeLEDs) through methods such as dimensional reduction or halide engineering, thereby reducing non‐radiative recombination. However, deep‐blue PeLEDs still face a deep valence band issue. The mismatched energy level alignment between the perovskite and the hole transport layer (HTL) leads to charge accumulation, resulting in imbalanced carrier transport and injection. Herein, to address the issues of imbalanced carrier injection and defect states in PeLEDs, a deep‐blue perovskite emitter using [4‐(3,6‐Dimethyl‐9H‐carbazol‐9‐yl)butyl]phosphonic acid (Me‐4PACz) as the material to promote hole transport and passivate defects is presented. The stepwise energy level structure design can effectively reduce the hole injection barrier and improve the carrier injection efficiency. Additionally, the electron‐rich P═O bond can effectively passivate the unsaturated Pb2+ in perovskite, reducing non‐radiative recombination caused by defects. Ultimately, stable deep‐blue PeLEDs (≈458 nm) are successfully fabricated with an external quantum efficiency (EQE) of 4.33%. This study provides new insights into the application of self‐assembled monolayers (SAMs) in PeLEDs. [ABSTRACT FROM AUTHOR]

• The appropriate functional groups are determined when selecting molecules for the passivation of multiple types of defects. • The concept of comprehensive passivation is proposed into perovskite photovoltaics. • The research content aims at the buried interface, solving the problem of current immature buried interface modification technology. • This topic gives new consideration for future molecular design of passivation candidates in perovskite photovoltaics. Defect passivation emerges as a potent approach to enhance the performance of perovskite solar cells (PSCs). In particular, the buried interface shows a higher defect density than the upper one. Despite recent growth in buried interface passivation studies, it remains less mature than upper interface or precursor doping passivation, possibly hindering imminent efficiency breakthroughs. In addition, most of the recent buried passivation work passivates only one or two kinds of defects, the remaining defects at the buried interface will still damage the performance of PSCs. Herein, a chlorine-rich ammonium salt named chloroacetamidine hydrochloride (CAHC) is selected to achieve comprehensive passivation of the buried interface. CAHC effectively addresses various defects in tin oxide (SnO 2) and perovskite at the buried interface, significantly boosting the performance of PSCs. The power conversion efficiency (PCE) of the device based on CAHC modification reaches 24.78%. This work showcases promising advancements in buried interface passivation, offering a pathway for future breakthroughs in the performance of PSCs. [ABSTRACT FROM AUTHOR]