Perovskite solar cells (PSCs) have emerged as one of the most promising photovoltaic technologies over the past decade, achieving power conversion efficiencies (PCE) of 27%, as certified by the National Renewable Energy Laboratory (NREL). A critical component in PSC architecture is the hole transporting layer (HTL), which facilitates effective hole extraction at the perovskite/HTL interface. Recently, self-assembled monolayers (SAMs) have garnered increasing attention as alternative HTLs due to their ability to form highly ordered, compact films on indium tin oxide (ITO) substrates via robust interactions between anchoring groups and the oxide surface. These materials enable the minimization of HTL thickness, eliminate the need for dopants, and present a viable route toward scalable and stable PSC fabrication through simple deposition techniques.^[1-2]

Despite these advantages, the influence of different anchoring functionalities within SAM structures on key device parameters remains insufficiently understood. In this study, a novel series of enamine-based SAMs bearing various anchoring groups – namely carboxylic acid, acetic acid, cyanoacrylic acid, phosphonic acid, methylene phosphonic acid, and cyanovinyl phosphonic acid – were synthesized via one-, two-, or three-step procedures. The surface properties of the resulting SAMs on ITO substrates were characterized by water contact angle measurements. Films incorporating carboxyl-functionalized SAMs exhibited enhanced hydrophilicity comparable to that of the benchmark material MeO-2PACz, whereas phosphonic acid-based SAMs displayed more hydrophobic characteristics.

To evaluate their functionality in photovoltaic applications, inverted (p-i-n) PSCs were fabricated incorporating the synthesized SAMs as hole-selective interfacial layers. Preliminary device measurements revealed that SAMs containing carboxyl groups yielded the highest performance, with PCE values reaching around 24%. Devices based on phosphonic acid-functionalized monolayers demonstrated comparable efficiencies, highlighting their potential as effective HTLs.

These findings indicate that enamine-based SAMs, particularly those with cyanoacrylic acid and cyanovinyl phosphonic acid anchoring groups, can provide favorable interfacial properties for efficient hole extraction, contributing to high device performance. This work contributes to a deeper understanding of molecular design strategies for SAM-based HTLs and offers valuable insights for the future development of stable, scalable, and high-efficiency PSCs.