Orange photoluminescent N-doped graphene quantum dots as an effective co-sensitizer for dye-sensitized solar cells

In this research, nanocoral (NC) film of TiO2 was decorated with nitrogen-doped graphene quantum dots (NGQDs, average size of similar to 9 nm) and photovoltaic properties were investigated for dye-sensitized solar cell application. TiO2 NCs and NGQDs synthesized separately using the hydrothermal method and TiO2 NCs were decorated with NGQD solution by spin coating. FESEM images prove that TiO2 NCs are composed from nanorods, with an average diameter of about 60 nm. Optical characterization shows that the NGQDs are highly orange-luminescent (emission at 590 nm) and absorb UV and visible light photons. Using NGQDs together with N719 dye as co-sensitizers led to an improvement in efficiency of DSSC, as investigated by photoelectrical measurements. The experimental analysis reveals that this improvement arises from enhancement of charge separation and collection due to the cascaded energy levels because of presence of NGQDs. By addition of NGQDs into TiO2 NC photoanode, we were able to increase the short-circuit current density and efficiency by 40% (from 12.61 to 17.65 mA/cm(2)) and 31% (from 5.72 to 7.49%), respectively.