Abstract The continuous search for low-cost and environment-friendly materials in photovoltaic applications has become a priority, as well as the understanding of the various strategies to boost the photovoltaic performance. In this work, we investigate the effect of TiCl 4 treatment on a compact TiO 2 layer used as an electron transport material (ETM) in Sb 2 S 3 planar solar cells. After TiCl 4 treatment, TiO 2 exhibits higher crystallinity, lower density of hydroxyl groups acting as traps, and better surface coverage of the FTO substrate. Although no major structural changes are observed in Sb 2 S 3 films grown on pristine or TiCl 4 treated TiO 2 films, there are differences in preferential growth of Sb 2 S 3 (hk1) planes, sulfur-enrichment of the chalcogenide film, and superior substrate coverage after the TiCl 4 treatment, leading to the decrease of interfacial trap states. The driving force for electron injection in the TiO 2 /Sb 2 S 3 heterojunction is also favored by the shift on the VB and CB positions of TiCl 4 treated TiO 2 . These findings are in agreement with the improved power conversion efficiency of the planar solar cell FTO/TiO 2-Treated /Sb 2 S 3 /SbCl 3 /spiro-OMeTAD/Au.