The role of MoS2 as an effective interfacial layer in graphene/silicon solar cells is systematically investigated by varying MoS2 film annealing temperature and thickness. It is found that the power conversion efficiency (PCE) is increased by similar to 100% from similar to 2.3% to similar to 4.4% with 80 degrees C annealed MoS2 film whereas it drops significantly to similar to 0.6% with 200 degrees C annealed MoS2 film. The results are well explained based on the device energy band diagram. That is, the incorporation of MoS2(80) films leads to the formation of type II structure, facilitating hole transport; while valence band mismatch is formed with MoS2(200) films due to the increase in the work function of MoS2. Besides, the PCE increases gradually with decreasing MoS2 film thickness, and ''saturates'' at about 2 nm. The PCE can be further enhanced to similar to 6.6% with the aid of silicon surface passivation. Our work demonstrates that MoS2 is an excellent interfacial layer to improve the PCE with low-temperature annealing (80 degrees C in air), which may be helpful in developing efficient and low-cost G/Si solar cells.