TiO₂/graphene nanocomposite layers for improving the performances of dye-sensitized solar cells using a cobalt redox shuttle

Progresses in dye-sensitized solar cells (DSSCs) have revealed limitations due to I⁻/I₃ ⁻ redox shuttle and the necessity of its replacement. Cobalt-complexe redox species are the most promising alternatives for the application. In the present work, we have optimized the preparation of solar cells based on a Co-complex redox shuttle and Z907 Ru-complex dye. The system has been studied using pristine TiO₂ and TiO₂_single-layered graphene composite mesoporous layers for the electron transport. Our results on TiO₂_graphene are promising since these cells, with the cobalt redox electrolyte, yielded to a power conversion efficiency close to that of a TiO₂ photoelectrode combined with the iodine electrolyte. Using the nanocomposite layers permitted a significant increase in the photoelectrode specific surface area, dye loading and then cell short circuit current (J_sc). Moreover, the use of [Co(phen)₃]^2+/3+ improved the cell V_oc due to a higher standard redox potential. However a limitation was found due to a shift of the TiO₂ conduction band towards lower energy. Impedance spectroscopy study of the systems revealed a higher recombination rate for the Cobalt electrolyte and that the photoelectrode conductivity was slightly increased by adding graphene. It is shown that, even if improved in the presence of graphene, the charge collection efficiency was lower for the cobalt electrolyte compared to the iodide one. Globaly graphene improved the DSSC performance mainly due to the enlargement of the photoelectrode specific surface area and then of the dye loading.