With tunable bandgap and demonstrated high efficiency, the chalcopyrite CuInSe2 and its alloys have shown great potential as absorbers for single and multi-junction solar cells. However, the current deposition techniques mostly rely on expensive vacuum-based processing or involve complicated precursor solution preparation. These higher-cost absorber preparation processes make it difficult to commercialize this technology. In this work, CuInSe2−xSx (CIS) absorbers are deposited using a simple hydrazine-based solution process. Precursor solutions were prepared by dissolving the component metal chalcogenides and chalcogen in hydrazine, forming homogeneous solutions containing adjustable concentrations of desired elements mixed on a molecular level. These precursor solutions are then spin coated on substrates followed by a heat treatment in an inert environment to produce high quality CIS thin films. Significantly, no post deposition selenization process is required using this technique. Laboratory scale devices with conventional glass/Mo/CIS/CdS/i-ZnO/ITO structure have been fabricated using CIS absorbers deposited via this process. For the baseline low-bandgap CIS system with no Ga added (to compare with our previously reported results with Ga incorporated), AM1.5G conversion efficiency of as high as ∼9% has been achieved for devices with 0.45 cm2 effective area.