Physical properties of Cu(In,Ga)Se2 film on flexible stainless steel substrate for solar cell application: A multi-layer precursor method

The process named “multi-layer precursor method”, consisting of Ga-Se/In-Se/Cu-Se stacked precursors, is proposed to prepare Cu(In,Ga)Se2 (CIGS) films on flexible stainless steel (SUS) substrates. In this work, precursor deposition temperature (TPRE), varying from about 250 to 430 °C, is first scrutinized. From Raman scattering measurement, it is suggested that the increase in the TPRE enhances Cu2−xSe secondary phase during film deposition, thus enlarging CIGS grain size, while decreases ordered vacancy compound (OVC) near surface of the finished film, caused by desorption of In2Se during the precursor deposition. Suitable TPRE is in a range of approximately 370-400 °C, resulting in long carrier lifetime. Next, according to the examinations of material compositions of CIGS absorbers, the optimized [Cu]/([In]+[Ga]) contributes to the absorbers with large grain size due to the help of Cu2−xSe phase during film deposition and OVC near their surfaces for pn homo-junction. The appropriate [Ga]/([In]+[Ga]) leads to optimized band-gap energy with high CIGS quality implied by long carrier lifetime. Ultimately, the effective [Cu]/([In]+[Ga]) and [Ga]/([In]+[Ga]) ratios for the flexible CIGS solar cell with high conversion efficiency under the so-called “multi-layer precursor method” are in ranges of about 0.74-0.84 and 0.28-0.35, respectively.