Bulk and surface trap-states in the Sb_2S_3 films are considered one of the crucial energy loss mechanisms for achieving high photovoltaic performance in planar Sb_2S_3 solar cells. Because ionic liquid additives offer interesting physicochemical properties to control the synthesis of inorganic material, in this work we propose the addition of 1-Butyl-3-methylimidazolium hydrogen sulfate (BMIMHS) into a Sb_2S_3 hydrothermal precursor solution as a facile way to fabricate low-defect Sb_2S_3 solar cells. Lower presence of small particles on the surface, as well as higher crystallinity are demonstrated in the BMIMHS-assisted Sb_2S_3 films. Moreover, analyses of dark current density-voltage J–V curves, surface photovoltage transient and intensitymodulated photocurrent spectroscopy have suggested that adding BMIMHS results in highquality Sb_2S_3 films and a successful defect passivation. Consequently, the best-performing BMIMHS-assisted device exhibits a 15.4 power conversion efficiency enhancement compared to that of control device. These findings show that ionic liquid BMIMHS can effectively be used to obtain high-quality Sb_2S_3 films with low-defects and improved optoelectronic properties.
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