Hybrid dye sensitized solar cell based on single layer graphene quantum dots

Abstract

Single layer graphene quantum dots (SLGQDs, average size of ∼ 9 nm) were added into N719/TiO2 nanoparticles photoanode (prepared using a doctor blade method) as cosensitizer and photovoltaic properties were investigated for dye sensitized solar cell (DSSC) application. Low-cost and high-yield SLGQDs solution was synthesized with only glucose and DI water as precursor using a hydrothermal method. This method allows the tuning of optical properties and energy states by using appropriate precursors and synthesis conditions. Optical characterization reveals that the SLGQDs solution absorbs UV and visible light photons with wavelengths up to ∼ 700 nm. For engineering a suitable energy state, and then to obtain an efficient DSSC, lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the SLGQDs were determined against vacuum energy level by utilizing a cyclic voltammetry measurement. Electrical measurements indicated an improvement in power conversion efficiency for the DSSC fabricated based on SLGQDs and N719 as co-sensitizers. The experimental analysis shows that this improvement arises from enhancement of charge collection and separation due to the cascaded alignment of the energy levels between N719/SLGQDs/TiO2 interfaces. By addition of SLGQDs into N719/TiO2 nanoparticles photoanode, we were able to increase the short circuit current density and efficiency by 39% (from 14.47 to 20.03 mA/cm2) and 35% (from 6.57% to 8.92%), respectively.