Conjugated Oligoelectrolyte-Driven Self-Assembled Monolayer for Bidirectional Interfacial Engineering in Sn–Pb Perovskite Solar Cells

Tin-lead (Sn-Pb) mixed perovskites exhibit ideal bandgaps (1.21-1.25 eV) for high-efficiency single-junction and tandem solar cells, yet they suffer from interfacial instability arising from Sn vacancies, Sn oxidation, and poor film morphology. While self-assembled monolayers (SAMs) have emerged as promising hole-selective interlayers, conventional monophosphonate-based SAMs show weak interfacial binding and poor wettability, challenges that become more pronounced in scalable Sn-Pb perovskite solar cells (PSCs). Herein, a rationally designed SAM, 6,6'-(2,7-bis(9-(4-phosphonobutyl)-9H-carbazol-2-yl)-9H-fluorene-9,9-diyl)bis(<i>N,N,N</i>-trimethylhexan-1-ammonium bromide) (4PACz-TMABr), is developed, based on conjugated oligoelectrolytes featuring both phosphonic acid groups and ionic moieties. The dual phosphonic acid groups improve interfacial coverage on indium tin oxide, while the quaternary ammonium bromide ionic moieties suppress interfacial perovskite defects and Sn²⁺ oxidation. These dual interactions promote the orderly alignment of the SAM and facilitate its function as a bidirectional interfacial linker. The formation of uniform, high-crystallinity Sn-Pb perovskite films is further supported by density functional theory calculations. Consequently, 4PACz-TMABr-based Sn-Pb PSCs achieve a champion power conversion efficiency of 22.67% in small-area devices and 17.61% in 1 cm² devices, along with improved thermal stability. This work highlights a strategic molecular approach to SAM design, offering a pathway toward scalable, stable, and efficient Sn-Pb perovskite PSCs.