Redox-Active Flavonoid Interlayers Enable Strain-Relieved and Efficient Sn–Pb Perovskite Solar Cells

Tin–lead (Sn–Pb) alloyed perovskites offer narrower bandgaps than Pb-based analogues, enabling enhanced photovoltaic performance, but their practical application is hindered by Sn 2+ oxidation and poor buried-interface control. Here, we report plasma-driven oxidation of catechin, a flavonoid polyphenol, to regulate the buried interface in Sn–Pb perovskite solar cells. Oxygen plasma treatment converts catechin deposited on PEDOT:PSS/FTO into a quinone-rich form, termed catechin-Q, which induces favorable upward band bending, improves energy-level alignment, and suppresses interfacial nonradiative recombination. In addition, coordination between semiquinone/quinone moieties and Sn/Pb centers relieves tensile strain at the buried interface. As a result, catechin-Q-modified perovskite solar cells achieve a power conversion efficiency of 23.48% with a high open-circuit voltage of 873 mV. The modified devices also exhibit excellent operational durability, retaining more than 95% of the initial efficiency after 4,800 h under the ISOS-D-1I protocol.