Abstract: Copper indium gallium selenium (CIGS) solar cells on stainless steel substrate are widely used in the photovoltaics community due to their excellent photoelectric conversion efficiency and flexibility. However, the Fe element will diffuse into the CIGS thin film from the stainless-steel substrate during the process of the preparation of the CIGS thin films, which resulting deteriorated the performance of the devices. Therefore, it is necessary to insert a barrier layer between the stainless steel substrate and the Mo thin film to inhibit the diffusion of the Fe element. Mo(N,O) thin films were produced using reactive magnetron sputtering with varying O₂ flow rates. The influence of the O₂ flow rate on the crystal structure, microscopic morphology, and elemental composition of the Mo(N,O) thin films was analyzed using an XRD, SEM, and XPS. Base on secondary ion mass spectroscopy (SIMS) testing results, the addition of Mo(N,O) thin film to the fabrication process of CIGS solar cells results in a reduction of Fe element's relative intensity in the CIGS thin film from 30 counts to 2 counts. Moreover, current density-voltage (I-V) characteristic test demonstrated an increase in the photovoltaic conversion efficiency for the flexible CIGS solar cell on a stainless steel substrate, from 11.07% to 14.34%, upon the addition of the Mo(N,O) thin film.