Influences of Fe and absorber thickness on photovoltaic performances of flexible Cu(In,Ga)Se^sub 2^ solar cell on stainless steel substrate

Cu(In,Ga)Se2 (CIGS) films on flexible stainless steel (SUS) substrates were deposited by so-called “multi-layer precursor method” with varying Fe concentration from 2.3 × 1017 to 2.1 × 1018 atom/cm3. It has been revealed that the Fe leads to the reduction of the carrier lifetime of the CIGS films, thereby having the detrimental impact on their photovoltaic performances. Conversion efficiency (η) of the flexible CIGS solar cell on SUS substrate is increased to 16.9% when Fe concentration is minimized to 2.3 × 1017 atom/cm3. Next, the thickness of the CIGS films with Fe concentration of about 2.3 × 1017 atom/cm3 was decreased from 2.50 to 0.86 µm. It is disclosed that the double-grading GGI ([Ga]/([Ga]+[In])) profiles are realized with increasing GGI from 0.24 to 0.30, when decreasing the thickness from 2.50 to 1.15 µm, respectively. Consequently, η values of the resulting flexible CIGS solar cells are 17.3, 16.4, and 15.9% for the CIGS absorber thicknesses of 2.50, 1.56, and 1.15 µm, respectively, where open-circuit voltage and fill factor are not varied very much while shot-circuit current density is clearly reduced. Nevertheless, all photovoltaic parameters are decreased with further decrease in CIGS thickness to 0.86 µm since GGI content is relatively high at 0.38 and the double-grading GGI profile is not obtained, thus reducing the η to 11.9%.