Nanostructured heterojunction solar cells of CdS/Cu2S were grown by simple chemical ion exchange reaction by immersing the CdS thin films in Cu ion solution at room temperature. The as-grown nanostructured thin films were annealed at 250 degrees C in air for improving the interface and crystallinity of the heterojunction. These as grown and annealed nanostructured heterojunction were characterized for photovoltaic and other optoelectronic characteristics. Increase in conversion efficiency is obtained from annealed (eta = 0.24%) than the as grown heterojunction (eta = 0.09%), on illuminating to 100 mW/cm(2) light source. The X-ray diffraction (XRD) pattern confirms improvement in crystallinity with increase in crystallite size from 29 to 32 nm on annealing. The optical absorbance strength observed to be increased along with red shift in energy band gap value from E-g = 2.02-2.20 eV after annealing. (C) 2011 Elsevier Ltd. All rights reserved.