Ga]/([Ga]+[In]) profile controlled through Ga flux for performance improvement of Cu(In,Ga)Se2 solar cells on flexible stainless steel substrates

Abstract

We present flexible Cu(In,Ga)Se2 (CIGSe) solar cells on stainless steel substrates with enhanced photovoltaic performance. CIGSe absorbers with thicknesses of 1.4 and 1.0 µm were deposited by the multilayer precursor method using precursor depositions of Ga–Se/In–Se/Cu–Se and then annealed under In–Se/Ga–Se/In–Se evaporations. The pronounced double-graded [Ga]/([Ga] + [In]) (GGI) profiles of the 1.4-µm-thick and 1.0-µm-thick CIGSe films were obtained using a relatively high Ga flux (up to 5.4 × 10−4 Pa), which shortened the annealing time. The pronounced double-graded GGI profiles of the 1.4-µm-thick and 1.0-µm-thick CIGSe films and CuInSe2 phase separation remarkably enhanced photovoltaic performances because a high back slope in the double-graded GGI profile induces back surface field, and the pronounced double-graded GGI profile with CuInSe2 phase separation yields the minimum GGI values, increasing light absorptionat longer wavelengths and in turn, conversion efficiency (η). Ultimately, η values of the flexible CIGSe solar cells increased to 17.3%, 17.2%, and 14.3% for CIGSe films with thicknesses of 2.5, 1.4, and 1.0 µm, respectively.