The Effect of Sodium on a Molybdenum Layer by Varying the Growth Pressure on CIGS Solar Absorp-tion Layer

Flexible Cu (In, Ga) Se2 (CIGS) solar cells on stainless-steel (STS) substrates face the problem of efficiency deterioration when iron impurities diffuse into the absorber layer. Iron, the main component of stainless steel, can diffuse through the back contact into the CIGS absorber, where Fe impurities are known to reduce the solar cell performance. In this work, the sodium-doped Molybdenum (Mo-Na) layer was made as a diffusion barrier on STS substrate for various growth pressures. The formed Mo-Na diffusion barrier layer was analyzed by Scanning Electron Microscopy, X-ray diffractometer and UV-Vis Spectrophotometer. X-ray diffraction showed that the films grown on STS substrates had a pure chalcopyrite phase with a preferred (112) orientation Mo back contact and the CIGS layer was subsequently deposited by co-sputtering technique and selenization process, to investigate the Na diffusion through the diffusion barrier into the CIGS absorption layer. The concentrations of Na and Fe diffused in the CIGS layer were also measured by secondary ion mass spectroscopy (SIMS). The SIMS depth profile and optical measurement results demonstrated that the diffusion of Na into the CIGS absorber layer was controlled by varying the working pressure of the Mo-Na layer.