Optical band-gap of reduced graphene oxide/TiO2 composite and performance of associated dye-sensitized solar cells

Abstract

The effect of reduced graphene oxide (rGO) on the performance of rGO/TiO2 composite-based dye-sensitized solar cells (DSSCs) is studied. rGO was mixed with TiO2 in an aqueous solution at different mass proportions, from 0% to 5%, to prepare the rGO/TiO2 composite films. The optical properties of the rGO/TiO2 films were correlated to the photovoltaic characteristics of associated DSSCs. The optical band-gap Eg of the rGO/TiO2 film decreases linearly, whereas the efficiency of the DSSC increases linearly with increasing rGO proportion. A model is proposed, in which the intercalated rGO nanosheets modulate the charge of the TiO2 film and redistribute the density of states (DOS). With increasing rGO proportion, the DOS distribution increases while shifting towards the gap, causing the conduction band (CB) edge to move to a critical level of a new mobility edge and restore the initial electron effective DOS. The DOS increase enhances the electron injection rate and, consequently, the efficiency of the DSSC. The hole-to-electron effective mass ratio mh/me is calculated and found to increase from values below unity near pure TiO2 to values above unity as the rGO proportion increases, in agreement with the literature.