Synergistic Effects of Cu Doping and Spin‐Coating Engineering on the Properties of SnO 2 Thin Films for Organic Solar Cells

High‐performing organic solar cells (OSCs) require electron transport layers (ETLs) that combine high transparency, low roughness, and controlled thickness for efficient charge extraction and reduced optical losses. Here, sol–gel SnO 2 and Cu‐doped SnO 2 thin films (3 mol% Cu) were deposited by spin coating, and the coupled influence of spin speed (2500–8000 rpm), layer number (1–3), precursor concentration (0.4–0.8 mol L − 1 ), annealing temperature (200°C–550°C), substrate type (glass, ITO, FTO, Si), and cleaning protocol was assessed by stylus profilometry, spectroscopic ellipsometry, UV–vis spectroscopy, and atomic force microscopy (AFM). Thickness decreased monotonically with spin speed, from 213.8 nm at 2500 rpm to 20.6 nm at 8000 rpm; single‐layer undoped films spun at 8000 rpm were uniform (20–25 nm), smooth (RMS < 40 nm), and highly transparent. Cu incorporation increased roughness by 40%–60% and reduced transmittance by 9%–20%, consistent with stronger scattering and additional sub‐bandgap absorption, and produced systematically thicker films, reflecting higher solute loading and the more viscous character of Cu‐containing sols. Moderate annealing improved uniformity, whereas higher temperatures promoted anisotropic growth and microcracking. Rigorous substrate cleaning most effectively minimized contamination‐driven roughness, especially on ITO, defining a practical processing window for smooth, transparent SnO 2 ‐based ETLs suitable for OSC integration across different substrate platforms.