Perovskite solar cells (PSCs) have emerged as one of the most intensively studied photovoltaic technologies in the past decade, reaching certified power conversion efficiencies (PCEs) of 27%. Their rapid rise in performance underscores the importance of developing device components that are not only highly efficient but also stable and compatible with scalable processing routes. A critical element in PSC architecture is the hole transporting layer (HTL), which governs charge selectivity and strongly impacts both efficiency and device lifetime. Conventional HTLs, while effective, often rely on dopants and exhausting multi-step fabrication processes that may compromise reproducibility and hinder large-scale implementation.

Self-assembled monolayers (SAMs) have recently gained prominence as promising alternatives due to their ability to form ultrathin, homogeneous, and strongly bound interfacial films on transparent conducting oxides such as indium tin oxide (ITO). Their unique structural characteristics minimize the thickness of the HTL, reduce the need for additives, and enable processing through simple deposition methods, thereby aligning with the requirements of scalable device manufacturing. Despite these clear advantages, a comprehensive understanding of how structural modifications – particularly anchoring group variation – influence interfacial properties and photovoltaic performance remains limited. ^[1-2]

In this work, we introduce a new family of enamine-based SAMs incorporating diverse anchoring functionalities, including carboxylic, acetic, cyanoacrylic, phosphonic, methylene phosphonic, and cyanovinyl phosphonic acid groups. These molecules were synthesized through one- to three-step procedures. To probe surface characteristics of the SAMs on ITO substrates, contact angle measurements were carried out using water droplets. Carboxyl-functionalized SAMs produced more hydrophilic surfaces, exhibiting wettability values comparable to the benchmark material MeO-2PACz, whereas phosphonic acid derivatives formed more hydrophobic layers. Such variations in wettability are indicative of differences in molecular packing and surface energy, which are known to affect film formation and subsequent charge extraction.

The photovoltaic properties of the new SAMs were evaluated in inverted (p-i-n) PSC configurations. Devices incorporating carboxyl-anchored SAMs consistently yielded the best performance, achieving efficiencies around 24%. Phosphonic acid-based monolayers also showed competitive efficiencies reaching even above 24%, underscoring their suitability as effective HTLs. Moreover, SAMs with cyanoacrylic and cyanovinyl phosphonic groups exhibited promising interfacial behavior, suggesting potential for further optimization of charge extraction and energy level alignment.

These results demonstrate that rational molecular design – particularly the selection of anchoring groups – plays a decisive role in tailoring the surface and interfacial properties of SAM-based HTLs. By providing insights into the structure-function relationships of enamine-based monolayers, this study contributes to the broader understanding of interfacial engineering strategies for PSCs. The findings support the continued exploration of SAMs as versatile, scalable, and efficient interlayers, offering new opportunities for the development of stable and commercially viable perovskite photovoltaics.