Band gap engineering of tandem structured CIGS compound absorption layer fabricated by sputtering and selenization

Band gap engineering was executed to fabricate a multi-junction stacked i.e. tandem Cu(In,Ga)Se-2 (CIGS) absorption layer. The CIGS absorption layers consist of multi-junction stacked CIS/CIGS/CGS thin films from bottom to top with increasing band gap. Tandem CIGS layers were fabricated by using three precursor of CuIn, In/CuGa/In, and CuGa onto the Mo coated soda-lime glass (SLG) by the sequential sputtering of CuIn, CuGa, and In targets. The CIG precursors were converted into CIGS absorption thin film by selenization process. From the X-ray diffraction (XRD) pattern of CIS/CIGS/CGS tandem layer, with the prominent peak shift for (112) reflections was attributed to the individual CIS, CIGS, and CGS phases at 26.76 degrees, 27.15 degrees, and 27.65 degrees diffraction angles, respectively. The morphologies and atomic (at%) composition uniformity onto the surface and along the depth were extensively analyzed with field effect scanning electron microscope (FESEM) attached energy dispersive spectroscopy (EDS) and secondary ion mass spectroscopy (SIMS). The optical properties such as transmittance, reflectance and absorbance were found to improve in the infrared region for all the tandem CIGS layers. Near the fundamental absorption edge, the absorption coefficient was approached to 10(5) cm (1) for CIS/CIGS/CGS tandem layer. The straight-line behavior indicates that the films have a direct band gap. The band gap was found to increase from 1.15 to 1.74 eV with the Ga-grading along the depth of individual CIS, CIGS, and CGS thin films. In the tandem CIGS layer, consist of CIS/CIGS, CIS/CGS, and CIS/CIGS/CGS thin films, the band gap was further increased from 1.42 to 2.06 eV, respectively. Such band gap engineering in CIGS tandem absorption layer will be a stepping stone to further improve the solar cell performance. (C) 2013 Elsevier B.V. All rights reserved.