Cu(ln,Ga)Se-2 (CIGS) absorbers with several Ga/III, Ga/(In + Ga), profiles were fabricated by the so-called "multi layer precursor method" under the control of Ga flux rate during film's deposition. Open-circuit voltage (V-oc) of CIGS solar cell is principally dependent on average band-gap energy (E-g) in space-change region (SCR) of below 1.3 eV. This average E-g in SCR, principally controlled by average Ga/III ratio in SCR, is estimated by the peak position of (220/204)-oriented CIGS films on soda-lime grass (SLG) substrates investigated by grazing incidence X-ray diffraction (GIXRD) with 0.1 degrees incident angle, whereas the average E-g in SCR is predicted by Raman or photoluminescence (PL) peak positions of CIGS absorbers on both SLG and stainless steel (SUS) substrates. Ultimately, with the average E-g in SCR of below 1.3 eV, the (220/204) peak position (GIXRD) can be well used as a predictor of the V-oc on rigid SLG substrates. On the other hand, Raman peak and PL peak positions can be well utilized as indicators of the V-oc on not only rigid SLG but also flexible SUS substrates before solar cell fabrication. These are fast and non-destructive methods to evaluate the Ga content, E-g near CIGS surface, and corresponding V-oc for high cell performance. (C) 2014 Elsevier Ltd. All rights reserved.