Among the limiting factors that affect the performance of Graphene (Gr)/ Silicon (Si) Schottky barrier solar cells (SBSC) - are the high recombination at Gr/Si interface and the growth of undesirable native oxide that resulted in instability issue. In this work, hafnium oxide (HfO2) grown by atomic layer deposition (ALD) has been investigated as an interfacial layer for Gr/Si Schottky junction solar cells. Engineering the interface with HfO2 contributed to enhancing the power conversion efficiency remarkably from 3.9% to 9.1%. The obtained efficiency with HfO2 interfacial layers grown by ALD is considered among the highest reported for Gr/Si SBSC using pristine graphene.