Dye-sensitized solar cells with shear-exfoliated graphene

Abstract

The efficiency of dye-sensitized solar cells (DSSC) has been limited by the inherent slow transport of charges in the TiO2 photoanode. The trap-limited diffusion of charges in the semiconductor prevented photoanodes from having thickness greater than 15 µm, thus limiting the potential of DSSC. This is especially with more recent research moving away from iodine-based electrolyte that requires a thicker photoanode. Thicker photoanodes means higher potential efficiency due to better light harvesting without resorting to scattering or reflecting layers to trap light as with thin photoanodes. Graphene had been added to TiO2 photoanodes in other research but in the form of rGO and suffered from increasing electron recombination at higher concentration. In this study, shear-exfoliated graphene was successfully incorporated into TiO2 nanofibers via electrospinning to produce fiber with diameter 52–73 nm in the photoanode with graphene in roll-up form inside the nanofibers eliminating edge effect of 2D graphene sheet that causes electron recombination. The nanofiber-based photoanode showed significantly longer electron diffusion length with little impact on recombination, primary due to higher electron diffusion coefficient. As a result, a thicker optimal photoanode, 23 µm, can be obtained with device efficiency measured at 8.9%, which is 22% higher than that without graphene with optimal thickness of 13 µm. The graphene also showed no interference in dye loading density of N719 dye. For the first time, we have demonstated that the electrospun TiO2/graphene nanofibers is a potential starting point to thicker, more efficient DSSCs.