Monolayer and few-layer two-dimensional (2-D) transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) demonstrate excellent semiconducting and optical properties that have made them promising candidates for optoelectronic applications. However, fabricating high-quality and highly uniform MoS2 nano films on a large surface area remains a challenge. In this study, an effective synthesis method for large surface area of 2 cm by 2 cm, ultrathin MoS2 nanofilms using direct sulfurization of annealed molybdenum (Mo) foil was tested. Because of the unique band structure, the MoS2 layer not only serves as a carrier transport layer but also as an effective blocking layer for the diffusion of photo-generated holes. By optimizing the structural characteristics of MoS2 nanofilms, a dramatic increase in photovoltaic performance was measured as high as 11.2% in the novel graphene/MoS2/n-Si (G/MoS2/n-Si) Schottky junction solar cells. Our approach offers guidance to the synthesis of large surface area, ultrathin MoS2 nanofilms and furthers their appeal for use in highly efficient solar cell technologies.