A recent study has reported a power-conversion efficiency of 5.1% for solar cells employing mesoporous TiO2 films sensitized with quantum dots of stibnite (Sb2S3). Here, a first-principles atomic-scale investigation of the interface between TiO2 and Sb2S3 is presented. The proposed atomistic interface model is free of defects, and the calculated energy-level alignment at the interface indicates that the ideal open-circuit voltage is as high as 1.6 V. Films sensitized with the isostructural compounds bismuthinite (Bi2S3) and antimonselite (Sb2Se3), which exhibit band gaps closer to the ideal Shockley–Queisser value are also examined. In the case of Bi2S3 the calculations indicate that the lowest unoccupied molecular orbital is too low in energy to inject electrons into TiO2, in agreement with experimental data. For antimonselite (Sb2Se3) the calculations predict a type-II heterojunction with TiO2, and suggest that Sb2Se3 sensitization may lead to higher power conversion efficiencies than found in the TiO2/Sb2S3 system.