We have systematically investigated the gallium grading in Cu(In,Ga)Se-2 (CIGS) solar cells grown on glass at low substrate temperature by a number of techniques including X-ray diffraction, scanning electron microscopy, energy X-ray dispersive spectrometry, and photoluminescence spectroscopy. The results reveal that the performance of the CIGS solar cells deposited at low substrate temperature strongly depends on the gallium grading and the micro-structures of the thin films. By modifying the conventional three-stage co-evaporation process from elemental sources, in particular in the first and third stage for CIGS growth to reduce Ga grading, it has been shown that fabrication of CIGS solar cells at low substrate temperature with efficiency comparable to that at high temperature is achievable. To optimize the device efficiency, the gallium grading in the (InGa)(2)Se-3 precursor grown in the first stage and the Ga content in the third stage are required to be carefully designed to match the employed substrate temperature. (C) 2013 Elsevier BY. All rights reserved.