Conjugated Group Tuning of Self‐Assembled Monolayer for Efficient Hole‐Transport Layer in Organic Solar Cells

P‐type carbazole‐derived self‐assembled monolayers (SAMs) have garnered significant attention as promising hole transport layers (HTLs) in the development of highly efficient organic solar cells (OSCs). However, it still lacks the effective navigation to modulate the terminal functional groups of SAMs to achieve a compromise between the highest occupied molecular orbital (HOMO) energy levels and self‐aggregation behavior. Herein, the terminal functional groups are adjusted and three SAMs are synthesized, namely, t‐Bu‐3PACz, Ph‐3PACz, and Bz‐3PACz to comprehensively investigate their intrinsic properties and influence on photovoltaic performance. Among them, Ph‐3PACz featuring an exceptionally suitable conjugated region and steric hindrance exhibits the best compatibility with the active layer, superior electrical conductivity, HOMO energy level aligning with polymer donor, and ordered film packing. As a result, the photovoltaic devices based on Ph‐3PACz exhibit an open‐circuit voltage ( V OC ) of 0.850 V, a short‐circuit current density ( J SC ) of 28.7 mA cm −2, and a fill factor (FF) of 78.5%, thus resulting in a remarkable power conversion efficiency (PCE) of 19.2%. This work provides an effective and easily navigable method to modulate the molecular packing and energy levels of SAMs, thereby achieving highly efficient OSCs.