ABSTRACT Solution phase deposition methods offer great potential for low‐cost photovoltaic device fabrication. We have previously developed a method for copper indium gallium disulfoselenide (CIGSSe) device fabrication based on drop‐casting copper indium gallium disulfide (CIGS) nanocrystals in a toluene or hexane‐based ink followed by chalcogen exchange in elemental selenium vapor at 500 °C. By starting with the chalcopyrite or sphaelerite phase of CIGS nanocrystals with controlled stoichiometry, superior composition uniformity can be achieved inherently. Here, we present a dramatic improvement in ink formulation using alkanethiol as the solvent, which enables the ability to create uniform nanocrystal coatings over large areas using a simple knife coating technique. In addition, we show a major improvement in device performance by a simple and low‐temperature method of incorporating sodium into the CIGSSe film based on soaking the films in aqueous NaCl solution. The addition of sodium plays an important role in improving the structural properties of the resulting CIGSSe films, where large and densely packed grain can be obtained. The improved film morphology significantly reduces recombination losses in the resulting device leading to a dramatically enhanced device performance. With the use of standard glass/Mo/CIGSSe/CdS/i‐ZnO/ITO device structure, photovoltaic devices yield total area power conversion efficiency as high as 12.0% under AM1.5 illumination without an anti‐reflection coating. Copyright © 2012 John Wiley & Sons, Ltd. A novel nanocrystal ink formulation and low‐temperature solution‐based method for the incorporation of Na for copper indium gallium disulfoselenide (CIGSSe) thin film have been developed. Simple ink formulation using alkanethiol as the solvent enables the deposition of uniform nanocrystal coatings over large area using knife coating. The addition of sodium plays an important role in improving the structural properties of the resulting CIGSSe films, which reduces recombination losses yielding devices with 12.0% total area efficiency.