Interface modification of hole transport layers in tin-based halide perovskite solar cells†

PEDOT:PSS is one of the most widely used hole transport materials in tin-based perovskite solar cells. However, the acid residues in PEDOT:PSS cause chemical/physical reactions with the ITO electrode. In this study, we utilized [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) and [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl] phosphonic acid (Me-4PACz) as buffer layers between PEDOT:PSS and ITO. The charge carrier transport ability of the SAM/PEDOT:PSS layer was significantly improved compared to that of the pure PEDOT:PSS layer. With SAM/PEDOT:PSS as the bottom layer, the tin-based perovskite films showed an improved morphology, suppressed Sn²⁺/Sn⁴⁺ oxidation, and a prolonged carrier lifetime; meanwhile, the perovskite film showed a reduced defect density, especially deep-level defect densities. Consequently, the champion tin-based devices achieved an increase in PCE from 5.7% (control device) to 7.3% (Me-4PACz/PEDOT:PSS device), while also retaining 80% of the initial PCE after 750 hours of storage in a N₂ environment.