Spiro-Linked Bisphosphonate Self-Assembled Monolayers for Efficient and Stable Perovskite Solar Cells

Bisphosphonate-anchored self-assembled monolayers (SAMs) play a pivotal function in the development of high-performance inverted perovskite solar cells (PSCs). However, the strong aggregation tendency of SAM molecules on substrates significantly limits the power conversion efficiency (PCE) of the devices. In this study, a spirocyclic linkage strategy is introduced for the first time into the molecular design of bisphosphonate-based SAM hole-transporting materials, and three target compounds, SCF-4PADCB, SF-4PADCB, and SFT-4PADCB, are successfully synthesized. The twisted spiro-conformation effectively suppresses intermolecular π-π stacking aggregation. In addition, SCF-4PADCB hole-transporting layer optimizes the interfacial energy level alignment, and passivates the defects of perovskite. Consequently, the inverted PSCs based on SCF-4PADCB achieve a champion PCE of 26.58%. Moreover, the unencapsulated device retains 91.26% of its initial PCE after 1200 h of maximum power point tracking and exhibits outstanding ultraviolet stability. Impressively, SCF-4PADCB also demonstrates excellent versatility, delivering a PCE of 23.46% in 1.68 eV wide-bandgap PSCs and a superior indoor PCE of 43.65% under 1000 lux LED illumination. This work provides a promising molecular design strategy for developing novel SAMs toward highly efficient and stable PSCs.