Molybdenum (Mo) is the most commonly used material for rear contact applications in copper indium gallium selenide (CIGS) solar cells. Improving the functionality of the Mo rear electrode can help to further enhance the efficiency of CIGS solar cells. In this work, four types of rear contacts: (i) conventional bilayer Mo, (ii) and (iii) modified Mo rear contacts with a TiN or SiN barrier layer (TiN/Mo or SiN/Mo), and (iv) a modified Mo contact combined with a TiN barrier layer and a Mo:Na layer as an extrinsic sodium source deposited on soda-lime glass are utilised as substrates for solar cell fabrication. The influences of the rear contact modifications on the properties of the CIGS solar cells are investigated with respect to sodium diffusion from the glass substrate into the CIGS absorber formed by two-step process. We find that the introduction of a thin TiN layer into the stack helps to suppress Na diffusion from the glass substrate into the CIGS absorber, leading to improvements of the crystallinity and lateral uniformity of the CIGS absorber, as well as improved efficiency and long-term stability of the solar cells. A significant improvement of the solar cell efficiency from 8.6% using a conventional bilayer Mo rear contact to up to 11.1% using the modified Mo rear contact with a TiN barrier layer is achieved. This result indicates that the modified rear contact structure developed in this study could be beneficial for fabricating high efficiency CIGS solar cells.