Comment on “Enhanced and Passivated Co-Doping Effect of Organic Molecule and Bromine on Graphene/HfO₂/Silicon Metal-Insulator-Semiconductor (MIS) Schottky Junction Solar Cells”

Graphene/Silicon Schottky junction was found to have a high potential for photovoltaic applications. In ACS Appl. Energy Mater. 2022, 5, 9, 10509–10517, Kadam and co-workers proposed high-performance solar cells based on graphene/HfO2/silicon MIS structure with certain doping and passivation. The thin HfO2 layer passivates the surface silicon dangling bonds, reduces the trapping of photogenerated carriers, and enhances the power conversion efficiency and long-term stability of the solar cells. However, we note some inaccuracies in applying the Schottky emission model and extracting the Schottky barrier heights from the current–voltage characteristics. Here we propose that the current–voltage behavior of the Gr/HfO2/Si MIS structure is better explained by direct tunneling and the Fowler–Nordheim tunneling mechanisms. Instead of being governed by the Schottky barrier only, we suggest that the current–voltage characteristics should be mainly governed by the barrier height between the Si/HfO2 and Gr/HfO2 interface as well as the insulator thickness. With the exact proposed mechanisms, we consistently explain some unexplained phenomena, such as the current enhancement at the large forward voltage and the effects of covering the devices with a thick insulating layer in their devices. These mechanisms suggest a different way for further device performance improvement. The thickness of the dielectric film as well as its band offset with Si are important parameters for process optimization.