We investigate the effect of a post heat treatment of the absorber layer in air for antimony sulfide (Sb₂S₃) sensitized solar cells. Phenomenologically, exposing the Sb₂S₃ surface of sensitised solar cells to air at elevated temperatures is known to improve device performance. Here, we have investigated the detailed origins of this improvement. To this end, samples were annealed in air for different time periods and the build-up of an antimony oxide layer was monitored by XPS. A very short heat treatment resulted in an increase in power conversion efficiency from η = 1.4% to η = 2.4%, while longer annealing decreased the device performance. This improvement was linked to a reduction in charge carrier recombination at the interface of Sb₂S₃ with the organic hole conductor, arising from the oxide barrier layer, as demonstrated by intensity modulated photovoltage spectroscopy (IMVS).