Abstract All-oxide solar cells are presently attracting extensive research interest due to their excellent stability, low-cost and non-toxicity. However, the band gap of metal oxides is lack of effective optimization and results in poor photovoltaic performance, thus hindering their practical applications. In this work, Co 3 O 4 was investigated for application as a photo-absorber in all-oxide solar cells, and its band gap was optimized by introducing Li dopant into the spinel structure. Li x Co 3−x O 4 nanoparticles, prepared via the hydrothermal method, were homogenously coated onto TiO 2 mesoporous films, which were then used to fabricate planar heterojunction TiO 2 /Li x Co 3−x O 4 solar cells (SCs). The effects of Li-doping on the heterojunction solar cell performance were further investigated. The findings revealed that the incorporation of Li ions into Co 3 O 4 led to a significant enhancement in short-circuit current density (J sc ). Remarkably, a high open-circuit voltage (V oc ) of 0.70 V was also achieved. Besides, reasons for the enhanced cell performance are the narrower band gap, reduced photogenerated carrier recombination and the more favorable energy band structure as compared with SCs assembled from pure Co 3 O 4 .