Antimony sulfide (Sb 2 S 3 ) is emerging as a promising light harvesting material owing to its brilliant photoelectric property. However, the performance of Sb 2 S 3 -based solar cells is partly limited by serious back contact interface recombination and hole transportation resistance. High-efficiency Sb 2 S 3 devices typically use Spiro-OMeTAD and/or Au as back contact materials, but their stability and cost are a concern. In this sense, a surface modification scheme by lithium-doping is first introduced for Sb 2 S 3 via a facile molten salt method. The ions in the molten state have high mobility and activity, enabling doping reactions to complete within a short time. The lithium-doped Sb 2 S 3 thin film has a smooth and well-bonded surface, preferred (hk1) orientations, and an upshifted valence band maximum (VBM), which favors the hole extraction. Finally, a device using carbon as an electrode, which is more than a dozen times cheaper than gold, raises the short-circuit current density (J SC ) from 12.35 to 14.40 mA cm −2 , and the power conversion efficiency (PCE) from 4.47% to 6.16%. This is among the highest PCE reported for full-inorganic Sb 2 S 3 solar cells, which demonstrates a facile interface modification technique via molten alkali salt to improve the performance of Sb 2 S 3 solar cells.