Abstract Posttreatment of titanium oxide (TiO 2 ) using lithium (Li) and cobalt (Co) precursors is widely adopted to modify the charge quenching property in perovskite solar cells (PSCs); however, the fundamental understanding of the effect of the modification layer on the material itself and, consequently, the photovoltaic performance stability is not complete. In this work, in situ X‐ray diffraction measurements show that the Li and Co ions can diffuse into TiO 2 and consequently accelerate the rutile phase transformation. X‐ray photoelectron spectroscopy results reveal the appearance of a Ti 3+ feature in both the Li‐ and Co‐treated samples, suggesting that the treatment ions are partially located at the subsurface/surface of the spin‐cast TiO 2 layer. The Li‐treated TiO 2 exhibits greatly upshifted conduction band edges, which benefits charge extraction properties and improves the average device parameters in a complete PSC. To complement the experiments, density functional theory calculations are performed. While Li treatment initially results in enhanced electronic properties, Li‐treated TiO 2 tends to have more surface vacancies over time and is more susceptible to adsorption and accumulation of iodide ions compared to the Co‐treated sample, which is experimentally supported by surface photovoltage spectroscopy and time‐resolved photoluminescence results.