Surface Fluorination of ALD Tin Oxide by SF ₆ Plasma for Operationally Stable Perovskite Solar Cells

ABSTRACT

Colloidal tin oxide (c‐SnO _x ) is the dominant electron transport layer (ETL) used in efficient n–i–p perovskite solar cells (PSCs), yet intrinsic oxygen‐related vacancies undermine operational stability. Here, we report a fluorinated SnO _x electron transport layer (ETL) fabricated by SF ₆ plasma‐assisted atomic layer deposition (ALD), in which plasma‐generated fluorine species chemisorb preferentially at oxygen vacancy (O _v ) sites. Density functional theory calculations show that F and F ⁻ bind at a bridging O _v with adsorption energies of −5.66 and −4.95 eV, respectively, whereas neutral SF ₆ , SF _5, and SF ₄ species bind weakly (−0.04 to −0.45 eV). Bader charge analysis confirms electron transfer from the SnO _x surface to the adsorbed fluorine. Experimentally, the fluorination induces a surface dipole that shifts the vacuum level and elevates the work function without altering the intrinsic valence band onset of the F‐containing SnO _x . This dipole‐induced vacuum level shift influences the interfacial energetics governing directional charge extraction and suppresses interfacial charge accumulation, which enhances device performance and operational stability. PSCs with SF ₆ ‐plasma ALD SnO _x achieve a power conversion efficiency of 21.0%, retaining 17.29% after 1400 h without encapsulation and sustaining stable power outputs for nearly 7000 min under continuous tracking. These results demonstrate superior performance and operational stability compared to thermal ALD‐grown SnO _x without fluorination. The results establish plasma‐enabled interfacial fluorination as a strategy to engineer surface electrostatics and enhance the long‐term stability in perovskite photovoltaics.