Rational Design and Visualization of Multifunctional Phenothiazine‐Based Self‐Assembled Monolayers for Better Interface Contact in High‐Efficiency and Stable Perovskite Solar Cells

Abstract Interfacial modification using self‐assembled monolayers (SAMs) is crucial for defect passivation and energy level alignment in perovskite solar cells (PSCs), yet scaling SAMs remains a challenge. Organic SAMs are often too thin for large‐area homogeneous layers through spin‐coating and their hydrophobic nature complicates solution‐based perovskite fabrication, hindering uniform film formation. This study introduces SAM based on phenothiazine core that involves synergistic co‐adsorption of a hydrophilic phosphonic acid with phenothiazine core unit for use as a hole transport layer in p‐i‐n PSCs. The PTZ‐PA SAM improves film formation, energy alignment, and hole extraction, achieving a power conversion efficiency above 23.2%. It also maintains stable performance for over 500 h under continuous illumination, indicating its potential for durable PSCs. PTZ‐PA increases surface energy, overcoming non‐wetting issues and enabling the formation of high‐quality perovskite films with improved morphology and crystallinity. The phosphonic acid group coordinates with lead iodide in the perovskite, enhancing electronic charge transfer and mechanical absorption, which facilitates effective p‐type charge‐selective contacts.