The presence of localized trap states on the surface of CsPbCl3 perovskite nanocrystals (NCs) is one of the greatest challenges precluding the development of optoelectronic applications of these NCs. Passivation of these defect sites provides a promising pathway to remediating their electronic and optical properties, such as photoluminescence quantum yield (PLQY). Herein, we demonstrate a post-synthetic dual-surface treatment using trivalent metal ion salts, i.e., YCl3, as a new passivation approach that enhances the PLQY up to 60 % while preserving the NC size and crystal structure. Such remarkable enhancement of the PLQY along with prolongation of the average PL lifetimes of treated NC samples indicates effective passivation of the surface defects and subsequent suppression of the formation of surface non-radiative recombination centers. As a segue towards optoelectronic applications, we probed the photoelectric performance of the NCs using ultra-flexible devices; we found that YCl3-treated CsPbCl3 NC films exhibit an order of magnitude larger photocurrent compared to their non-treated counterparts. Our experimental and theoretical results provide an insightful understanding of the effective passivating roles of Y3+ and Cl- ions on the surface of CsPbCl3 NCs, as well as offers a new path to synthesize high-quality NCs for UV-light conversion applications.