In recent years self-assembled monolayers (SAMs) have proven themselves to be suitable as hole-transporting materials (HTLs) for p-i-n perovskite solar cells, offering an alternative for the conventional HTLs that are based on metal oxides (e.g. NiOx) and organic polymers (e.g. PTAA). They offer multiple advantages such as manifold substrate compatibility, potential low cost, and easy processing through various techniques such as spin-coating, dip-coating, spray-coating, and evaporation, while only requiring a minimal amount of material in the process.

Additionally, proper energy level alignment can be ensured by finetuning the molecular structure of the SAM. The first SAMs that could compete with the conventional HTLs in perovskite solar cells were the carbazole-based phosphonic acids (2PACz ([2-(9H-carbazol-9-yl)ethyl]phosphonic acid) and MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid)), developed by Al-Ashouri et al. in 2019.[1] This sparked a surge in the research on incorporating SAMs as HTLs in p-i-n perovskite solar cells. Despite the vast quantity of research done on these self-assembled monolayers in recent years, most of the work focuses on SAMs with a similar molecular structure. More specifically, the organic core of the SAM generally consists of carbazole derivatives [2] or carbazole-like molecules such as phenothiazine.[3] While these carbazole-derived SAMs have proven their merit as HTLs in PSCs, they still cope with several disadvantages such as poor surface coverage leading to a low yield (percentage of working devices). Research into alternative organic cores for phosphonic acid SAMs to be used as an HTL in PSCs is limited. As a result, the influence of the organic core on the functioning of the phosphonic acid SAM as an HTL is poorly understood.

For this reason, we developed four novel SAMs based on four organic cores that are commonly employed in organic semiconductors. By incorporating them as interlayers in p-i-n perovskite solar cells we confirmed that it is necessary to have a proper energy level alignment between the SAM and active layer to obtain high-quality devices. On top of that one of our novel SAMs managed to clearly outperform the 2PACz reference both in terms of efficiency and yield, showcasing that further research in broadening the core structures of SAMs considered for hybrid perovskite solar cells is still useful for further device optimization.