Enhanced reliability of molybdenum by Al doping after damp-heat exposure for CIGS solar cells

The effect of Al doping into Mo, used as the back electrodes of CuIn_1-XGa_XSe₂-based solar cells, was systematically investigated. The transitions of Mo_1-XAl_X films, where X represents 14at.%, 21at.%, 28at.% 34at.%, and 39at.%, from the Al solid solution in Mo films to the mixture of Al solid solution in Mo films and the amorphous Mo-Al regions were fabricated by using co-sputtering from the Al and Mo target. When the Al content below 14at.%, the Al solid solution with Mo polycrystalline films was observed. In the content of Al from 14at.% to 39at.%, the analysis of XRD and SEM revealed that Mo-Al films become the microstructures of Al solid solution with polycrystalline Mo matrix along with the amorphous Mo-Al regions. To verify the reliability, all samples were exposed in a damp-heat (DH) chamber with 85°C and 85% relativity humidity for 168 hours. The electrical and optical properties of Mo-Al films after DH exposures were better than those of pure Mo films. According to the XPS analysis, it could be explained that the effect of Al dopant enhanced the reliability of Mo films by the passive oxide films of Al₂O₃ formed on the surface. In addition, the formations of MoSe₂ manipulated by the content of Al were also demonstrated.