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In this paper, we analyze the theoretical impact of the Molybdenum (Mo) back contact and the MoSe 2 interfacial layer on the performances of a Cu 2 ZnSnSe 4 (CZTSe)-based solar cell. MoSe 2 layers are formed spontaneously in the Mo/CZTSe interface during the annealing of the absorber, but disagreeing interpretations about their actual role in affecting the device figures of merit (V O C , J S C , FF, and η) have been proposed in the literature. In our approach, we have simulated three structures presenting different conditions at the back contact: ideal-contact/CZTSe (flat-band), Mo/CZTSe, and Mo/MoSe 2 /CZTSe. For these three layers, an accurate explanation of the selection of critical material parameters is given. The numerical simulations, performed with SCAPS 3.2.01, show that the low values of Mo work function (≤ 4.95 eV) would have a strong detrimental effect on the V O C and FF of the cell if no interfacial layers were present at the Mo/CZTSe interface. On the other hand, a beneficial effect of the MoSe 2 layer on the V O C of the device is demonstrated when this layer is included in the structure. This trend is confirmed by experimental measurements. The expected band diagram of the full ZnO/CdS/CZTSe/MoSe 2 /Mo structure is provided.