Investigation of Cu(In,Ga)Se2 absorber by time-resolved photoluminescence for improvement of its photovoltaic performance

Time-resolved photoluminescence (TRPL) measurement has been performed on Cu(In,Ga)Se2 (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 (Eg) 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 Eg profile with a proper average Eg of 1.27 eV and back-surface field of 0.22 V/μm, defined as a ratio of the change in Eg divided by the change in a depth range from 1 to 2 μm from CIGS surface. This graded Eg 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.