Understanding Phosphonic‐Acid Molecules Based Hole Transport Layers in Perovskite Solar Cells

ABSTRACT Self‐assembled monolayers (SAMs) have emerged as an important class of interfacial materials in perovskite solar cells (PSCs), enabling energy level tuning, efficient charge extraction, and improving device efficiency and stability. Among them, phosphonic‐acid‐based SAMs stand out due to their ability to form covalent bonds with transparent conductive oxides (TCOs), working as ultrathin, transparent, and tunable hole transport layers (HTLs). Despite the rapid progress in device development, several fundamental issues remain unclear, which may limit its broad application. In this perspective, we critically analyze the progress made by the community on several scientific questions. We first examine how SAMs are chemically bonded to TCOs, which is complicated by the difficulty in characterizing SAMs coverage on rough TCO substrates. We question the general perception of how charges are collected through SAMs layer to TCOs. And finally, the stability issues related to SAMs are discussed. Addressing these remaining challenges is necessary to bring the SAMs into commercial perovskite solar cell products.