Nitrogen and sulfur co-doped graphene counter electrodes with synergistically enhanced performance for dye-sensitized solar cells

A highly efficient nitrogen and sulfur co-doped graphene (NSG) nanosheet for dye-sensitized solar cells (DSSCs) was synthesized using a simple hydrothermal method, and its electrocatalytic activity towards the I-3(-)/I- redox reaction was investigated. The NSG materials showed a uniform distribution of nitrogen and sulfur heteroatoms throughout the graphene nanosheet. The doped nitrogen was present in the form of pyridinic, pyrrolic and graphitic states, and the doped sulfur was present in the C-S-C configuration. The DSSC with the NSG counter electrode exhibited a high conversion efficiency (7.42%), similar to that of the Pt counter electrode (7.56%) and much higher than that of the only N- or S-doped graphene electrodes. The high catalytic activity of the NSG electrode is attributed to the synergistic effect of the high charge polarization arising from the difference in electronegativity between nitrogen and carbon as well as the structural distortion caused by the bigger atomic size of the sulfur atom. To the best of our knowledge, the synergistic effect of co-doping of graphene on the counter electrode performance in DSSCs is demonstrated for the first time, and co-doping is proposed as a promising approach to enhance the photovoltaic performance of DSSCs.