Expanding the conjugated benzene rings of phenothiazine phosphonic acid for efficient tin–lead mixed perovskite solar cells with a Sn/Pb ratio of 0.4 : 0.6

Interfacial defect states, redox reactions, and energy-level mismatch limit the progress of promising bandgap tunable tin-lead mixed perovskite (TLP) solar cells. To address these issues, in this work, we designed and synthesized a novel small molecule, 4-(13<i>H</i>-dibenzo[<i>b</i>,<i>i</i>]phenothiazin-13-yl)butyl phosphonic acid (DB-PTZPA), as a hole transport layer (HTL) for FA_0.7MA_0.3Sn_0.4Pb_0.6I₃ TLP solar cells. In contrast to the popular PEDOT:PSS, DB-PTZPA is more hydrophobic and favors the formation of a dense, void-free buried interface with high-quality FA_0.7MA_0.3Sn_0.4Pb_0.6I₃ TLP film, reducing the defect state density from 2.19 × 10¹⁵ to 1.32 × 10¹⁵ cm^-3. Depending on the expanded π-conjugation, DB-PTZPA has an optimal HOMO level closer to that of our TLP film with the energy offset decreasing from 0.19 to 0.08 eV, compared with PTZPA. Both attributes contribute to a champion device PCE of 22.73%, creating a new record for Sn_0.4Pb_0.6 TLP solar cells, along with an enhanced light soaking stability.