Abstract: In recent years, solar cells on transparent substrates have attracted attention in a wide range of emerging applications, such as Building-Integrated PhotoVoltaics（BIPV）, owing to their bifacial transmittance and high potential for power generation. In particular, Cu（In, Ga）Se₂（CIGS） thin film solar cells, with advantages of high absorption coefficients and flexibility, are considered as one of ideal candidates for these applications. However, it is hard to obtain an acceptable ohmic contact between the transparent substrate and CIGS due to the mismatch of band structures at the back contact and the generation of GaO_X during high-temperature processes. To solve this problem, the molybdenum layers with different thickness as an interlayer between the FTO transparent substrate and the CIGS are introduced, and their effects on the solar cell performance are explored. The results show that the introduction of Mo layer results in a more sufficient elemental diffusion and improves crystalline quality of the CIGS film. With the increase of Mo thickness, the defect density and barrier height are diminished, and recombination at the back interface is reduced, but the transmittance is degraded. The Mo layer of 40 nm thickness can significantly improve the cell performance while the transmittance of the substrate is not deteriorated. The result provides a useful reference for further optimization of the design of CIGS solar cells with transparent substrate.