Tuning Self-Assembled Monolayers of Me-4PACz with Bisphosphonic Acid Groups to Improve Temperature and Cycling Stability of Perovskite Photovoltaics

Buried interface quality in p-i-n perovskite solar cells (PSCs) is shown to improve via the use of a mixed self-assembled monolayer (SAM) of [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) and (9H,9′H-[3,3′-bicarbazole]-9,9′-diylbis(butane-4,1-diyl))diphosphonic acid (Bi4PACz), eschewing the need for a NiOx layer. The bisphosphonic acid-based Bi4PACz minimizes SAM aggregation and enhances the wettability and uniformity. It also coordinates with the perovskite, passivating defects and suppressing PbI2 aggregation, thereby minimizing recombination losses. The improved buried interface quality leads to power conversion efficiencies (PCEs) of 24.48%, 25.65%, and 22.60% for triple cation PSCs with bandgaps of 1.61, 1.58, and 1.68 eV, respectively, demonstrating the wide applicability of this approach. Under temperature conditions, the PSCs retained 84% of initial PCE after 1200 h at 85 °C and 97% after 250 thermal cycles (−40 to +70 °C), indicating its potential for space-related applications. This mixed-SAM approach establishes a universal pathway toward efficient and temperature-stable perovskite photovoltaics.