Charge‐selective contacts critically influence carrier dynamics and overall performance in halide perovskite solar cells (PSCs). Self‐assembled monolayers (SAMs) have emerged as a powerful strategy for precise interfacial engineering, enabling tailored energy level alignment and interfacial interactions to enhance charge extraction. Despite their promise, clear structure–function relationships for SAMs—particularly as electron‐selective contacts (ESCs)—remain poorly developed. Here, a series of naphthalimide (NI)‐based SAMs functionalized with cyano, bromo, or methoxy groups and varying alkyl linker lengths are systematically evaluated as ESCs in n‐i‐p PSCs. Devices incorporating these SAMs exhibit power conversion efficiencies (PCEs) ranging from 5.8% to 20.6%, depending on molecular structure. The highest PCE is achieved using a SAM with a strongly electron‐withdrawing cyano group and a short ethyl linker, attributed to deep LUMO alignment and efficient charge transport at the interface. In contrast, SAMs with longer linkers or higher energy levels yield inferior performance. These results reveal critical design principles for high‐performance SAM‐based ESCs and establish a new PCE benchmark for PSCs employing standalone SAMs, without auxiliary metal oxide layers. Overall, this work underscores the potential of molecularly engineered SAMs to enable scalable, efficient, and commercially viable perovskite photovoltaics through optimized interfacial control.