Time-resolved photoluminescence (TRPL) measurement has been performed on Cu(In,Ga)Se-2 (CIGS) absorbers. In this contribution, CIGS films on both rigid soda-lime glass and flexible stainless steel (SUS) substrates are fabricated by the so-called "multi-layer precursor method" consisting of Ga-Se/In-Se/Cu-Se stacks. The TRPL lifetime, demonstrating a positive relationship with PL intensity, exhibits a close correlation with all photovoltaic parameters. According to TRPL and sensitive capacitance measurements, the average band-gap energy (E-g) should be in a range of 1.25-1.30 eV, giving rise to sufficiently long TRPL lifetimes. Ultimately, CIGS absorber is fabricated with double graded E-g profile with a proper average E-g of 1.27 eV and back-surface field of 0.22 V/mu m, defined as a ratio of the change in E-g divided by the change in a depth range from 1 to 2 mu m from CIGS surface. This graded E-g profile results in the improvement of the efficiency of the CIGS solar cell on a flexible SUS substrate up to 16.22% without an anti-reflective layer. (C) 2014 Elsevier B.V. All rights reserved.