Controlled back slope of Ga/(In+Ga) profile in Cu(In,Ga)Se2 absorber fabricated by multi layer precursor method for improvement of its photovoltaic performance

Cu(In,Ga)Se2 (CIGS) absorbers with several Ga/III, Ga/(In+Ga) ratio, profiles were deposited by the so-called “multi layer precursor method” consisting of Ga-Se/In-Se/Cu-Se stacks. The effects of average Ga/III ratios and back slopes of the Ga/III profiles on the photovoltaic performance were investigated. The optimum average Ga/III ratio of approximately 0.36 leads to the appropriate average band-gap energy as well as sufficiently long carrier lifetime in CIGS absorber, thus enhancing open-circuit voltage. In addition, the back slope of 0.50 μm−1, defined as the ratio of the change in Ga/III ratio to the change in the depth range from 1 to 2 μm from CIGS surface, induces back surface field of 0.34 V/μm, thereby increasing short-circuit current density. Ultimately, the CIGS absorber on flexible stainless steel substrate with the double graded Ga/III profile (the average Ga/III of 0.36 and the back slope of 0.50 μm−1) was fabricated by the multi layer precursor method. This results in the 14.36%-efficient CIGS solar cell on the flexible substrate without an anti-reflective layer.