In this paper, the cyclic voltammetric studies were used to realize the element's reduction potential and chemical reaction mechanism for presuming the formation routes of quaternary Cu(In,Al)Se-2 crystals. Thereafter, the prior adjustment of deposited potential from -0.6 V to -1.0 V can be identified a suitable potential as co-electrodeposition. The material characteristics of Cu(In,Al)Se-2 films are dominated by the percentage of aluminum content. Thus, the influence of aluminum and indium concentrations in solutions on the percentage composition, surface morphology, structural and crystal properties, and optical energy band gap of Cu(In,Al)Se-2 films were investigated. Energy dispersive X-ray spectroscopy (EDS) indicated that the ratio of Al to (Al + In) in Cu(In,Al)Se-2 films varied from 0.21 to 0.42 when adjusting aluminum and indium concentrations in solutions. Scanning electron microscopy (SEM) shows that the surface morphology changed from round-like structures into cauliflower-like structures and became rough when the aluminum concentration increased and indium concentration decreased in solutions. X-ray diffraction (XRD) patterns revealed three preferred growth orientations along the (1 1 2), (2 0 4/2 2 0), and (1 1 6/3 1 2) planes for all species. The (alpha h upsilon)(2) versus h upsilon plots (UV-Visible) shows that the optical energy band gap of the Cu(In,Al)Se-2 films can be successfully controlled from 1.17 eV to 1.48 eV by adjusting the aluminum and indium concentrations. Furthermore, the shift of the (1 1 2) peak in the XRD patterns and variation of optical band gap are evidence that the incorporation of aluminum atoms into the crystallitic CuInSe2 forms Cu(In,Al)Se-2 crystals. (c) 2013 Elsevier B.V. All rights reserved.