Effects of Electric Field on the Performance of Graphene-Based Counter Electrodes for Dye-Sensitized Solar Cells: A Theoretical Study

We have investigated the influence of an external electric field on the performance of graphene-based counter electrode (CE) materials such as pristine, boron-doped graphene (BC5), and nitrogen-doped graphene (NC5) for dye-sensitized solar cells (DSSCs) using density functional theory calculations. The stable I-2 adsorption geometries, electronic properties, and charge transfer between I-2 and different graphene nanosheets in the presence and absence of an external electric field are calculated. We find that the NC5 graphene sheet exhibits high affinity for the I-2 molecule and greater sensitivity to the external electric field. Our calculations indicate that the I-2 dissociation on the NC5 graphene sheet is thermodynamically and kinetically favorable even in the absence of an external electric field; however, the I* atomic desorption energy is relatively large (0.70 eV), and it is the rate-determining step in the I-3(-) reduction reaction. We demonstrate that a negative external electric field decreases the interaction between the I* atom and the NC5 graphene sheet and eases the I* desorption. The excellent catalytic property of the NC5 graphene sheet toward I-2 dissociation and I* atom desorption makes it as an alternative nonmetallic CE for DSSCs.