Crystallinity-Dependent Donor Activation in RF-Sputtered SnO2 Electron-Transport Layers and the Interface-Character Origin of Their Predicted Device-Level Consequences

Abstract SnO2 is the electron-transport layer (ETL) in record n–i–p perovskite solar cells, grown by chemical bath deposition (CBD) without high-temperature crystallization; whether its crystallinity matters remains unresolved. Here, the same RF-sputtering process on fluorine-doped tin oxide (FTO), at room temperature or 300 °C, yields a predominantly amorphous or an FTO-templated polycrystalline rutile film. The measured film and interface properties anchor a calibrated TCAD model and a thirteen-parameter Sobol global sensitivity analysis (>33,000 simulations), which, unlike one-factor-at-a-time sweeps, resolves the interactions among parameters explicitly. At the film level, crystallinity governs donor activation: UPS, XPS, and STEM-EDS show that the amorphous film holds more fluorine yet has a Fermi level farther below the conduction band, so shallow donors attributed to substitutional fluorine exist in both films but are active only in the crystalline one. At the device level, however, the predicted gain arises from the crystalline film's interface rather than its doping. The Ag/SnO2 contact is rectifying on the amorphous film and ohmic on the crystalline one, through interface-state de-pinning rather than a band-offset shift. The Sobol analysis ranks the SnO2/perovskite interface trap density first (total-effect index ≈ 0.66). A degenerately doped film could screen such traps, but the amorphous film's rectification rules out that route; its deficit is interfacial. At its trap density (∼1013 cm–2), only deep acceptor-like states pin the Fermi level (acceptor-fraction index ≈ 0.83). The predicted crystallization gain is +5.6 pp (20.5 → 26.1%). FAPbI3 half-stack PL/TRPL supports this (1.35× longer lifetime on the crystalline ETL) and bounds that interface's trap density to ≲1012 cm–2. Crystalline growth is one route to the de-pinned interface that CBD ETLs reach chemically, provided that interface is preserved. The absolute efficiencies are calibrated predictions; the benefit's sign and the lever ranking are robust.