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Interfacial engineering is an efficient strategy for enhancing the performance of perovskite photovoltaic cells. Here, we present a novel method to modify the surface of TiO2 with thiourea. X-ray photoelectron spectroscopy indicates that thiourea treatment inactivates the TiO2 surface groups. Kelvin probe force microscopy and ultraviolet photoelectron spectra demonstrate an energy level downshift of the modified TiO2. Photoluminescence spectra reveal the reduced interface charge recombination and an improved charge transportation/extraction. Tafel and Nyquist plots elucidate a decrease of trap-state density and an increase of conductivity of the modified TiO2. Hence, the planar perovskite solar cell based on the TiO2 electron transport layer (ETL) modified with 0.1 M thiourea yields an efficiency of 19.18%, which is 7.5% larger than that of the device based on the pristine TiO2 ETL. The simple procedure and the significant performance improvement render this method promising.