CuInSe₂ (CIS) absorber layers for thin film solar cells were fabricated via a non-vacuum route using amorphous Cu–In–Se nanoparticle precursors prepared by a low temperature colloidal process within one minute of reaction without any external heating. In particular, we intentionally added a chelating agent to the nanoparticle colloid in order to increase the density of the final films by enhancing the viscous flow of precursor materials during high temperature selenization. This is based on the decreased reactivity of precursor particles due to the formation of chelate complexes at particle surfaces. While the CIS films formed from the amorphous Cu–In–Se particles without surface modification were found to have large voids, those formed from surface modified particles showed flat and dense morphologies. In accordance with the improvements in the film morphology and density, efficiencies of the devices were also significantly increased from 0% (complete short circuit in the case without surface modification) to 4.41% (with surface modification).