Abstract Tin oxide (SnO2)-based electron transport layers (ETLs) are used to make flexible perovskite solar cells (PSCs) because of their low-temperature properties. However, when pure SnO2 is used, the poor interface formed between the SnO2 ETL and the perovskite layer is detrimental to the efficiency of the PSCs. In this work, we report the synthesis and characterisation of a SnO2–NaCl composite ETL. Na+ and Cl− at the ETL/perovskite interface passivate the ETL/perovskite contact and suppress nonradiative recombination centres. Meanwhile, Na+ diffuses from the ETL to the perovskite film, passivating its grain boundaries. The outstanding capability of Na + to increase the grain size and consequently improve the charge dynamics and increase the power conversion efficiency was also verified. PSCs with a SnO2–NaCl ETL exhibited an open-circuit voltage of 1.11 V, which was higher than the reference (1.08 V) for lower band gap perovskite absorbers, and achieved a power conversion efficiency of 18.56% with negligible hysteresis.