Substrate temperature optimization for Cu(In, Ga)Se2 solar cells on flexible stainless steels

Cu(In, Ga)Se-2 (CIGS) thin films are deposited on flexible stainless steel (SS) substrates using the so called 3-stage co-evaporation process at different substrate temperatures ranging from 440 degrees C to 640 degrees C during the 2nd stage and the 3rd stage (T-S2). The effects of T-S2 on the properties of CIGS thin films are systematically investigated. It is found by secondary ion mass spectrometry measurement that CIGS thin films deposited at different T-S2 show different Ga/(Ga + In) ratio (GGI) profiles along the growth direction. High T-S2 facilitates the grain growth and leads to larger grain size. However, high T-S2 worsens the spectral response of CIGS solar cells in the long wavelength range, which is partly attributed to the too much iron atom diffusion from the SS substrates into the CIGS thin films. All CIGS thin films show (112) preferred orientations with a shift to higher angle due to variation of compositions. A shoulder-like two-peak structure of (112) and (220/204) peaks appears for CIGS thin films deposited at lower T-S2. Conversion efficiency of 11.3% is obtained for CIGS thin film solar cells deposited at the T-S2 of 500 degrees C. (C) 2016 Elsevier B.V. All rights reserved.