Conjugated Bisphosphonic Acid Self-Assembled Monolayers for Efficient and Stable Inverted Perovskite Solar Cells

Chemically modifiable self-assembled monolayer (SAM)-based hole transport layers are crucial for achieving high-efficiency inverted perovskite solar cells (PSCs). However, designing molecular structures that simultaneously ensure strong binding affinity, interfacial stability, and optimized energy level alignment remains challenging. Here, we introduce TPA2P ((2-(4-(diphenylamino)phenyl)-1-phosphonovinyl)phosphonic acid), a novel SAM material featuring a conjugated bisphosphonic acid anchoring group. This dual phosphonic acid configuration enhances substrate binding on indium tin oxide (ITO), improves SAM uniformity, and increases interfacial stability. Furthermore, the ethylene bridge facilitates efficient intramolecular charge transfer (ICT) from the electron-donating triphenylamine unit to the electron-accepting bisphosphonic acid group. This ICT induces significant charge redistribution in TPA2P, resulting in a deep HOMO level at -5.47 eV. This optimized energy alignment reduces interfacial energy losses and significantly enhances hole extraction efficiency. As a result, TPA2P-based inverted PSCs achieve a high power conversion efficiency of 26.11%, an exceptional fill factor of 85.03%, and outstanding operational stability under continuous illumination. These findings provide an effective molecular design strategy for advancing high-performance and stable perovskite photovoltaics.