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-9 H -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 NiO x 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 PbI 2 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.