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Highlights Trace amounts of long-chain gemini surfactants are essential for blade-coating of high-quality perovskite films, enabling a 17.05% efficient full printed carbon-based module (50 cm2 active area). Only when the surfactant chain is over a critical length will the gemini surfactant be effective for blade-coating of perovskite films. The surfactants increase the capillary number of perovskite precursor solution, reduce the local disturbance and combat inhomogeneous solidification during blade coating, thus allowing high-quality perovskite films to be formed.