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-(13Hdibenzo[b,i]phenothiazin-13-yl)butyl phosphonic acid (DB-PTZPA) as hole transport layer (HTL) for FA0.7MA0.3Sn0.4Pb0.6I3TLP solar cells. In contrast to popular PEDOT:PSS , DB-PTZPA shows more hydrophobic and favors the formation of a dense, void-free buried interface of high-quality FA0.7MA0.3Sn0.4Pb0.6I3 TLP film, reducing the defect state density from 2.19×1015 to 1.32×1015 cm-3. Depending on the expanded πconjugation, DB-PTZPA owns the optimal HOMO level closer to 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 record for the Sn0.4Pb0.6-based TLP branch, along with an enhanced light soaking stability.

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