Influence of Surface Charge from Gold Nanoparticles on Titanium Dioxide-Graphene Based Dye Sensitized Solar Cell

Abstract

Dye-sensitive solar cells (DSSCs) have attracted interest as a suitable candidate for photovoltaic (PV) technology to plug the widening global energy demand and supply gap. Their full potential scalability has been hindered by low cell stabilities and low photo-electric conversion efficiency. This study has investigated inclusion of monolayer graphene as transparent conducting material and gold nanoparticles (AuNPs) on Titanium dioxide (TiO2) thin films as novel materials for dye-sensitive solar cells application. Titanium dioxide thin films on Fluorine doped tin oxide (FTO) were prepared using the sol-gel technique and heat treated at different annealing rates. Optical properties recorded using a UV-Vis spectrophotometer revealed that film annealed at 1 oC/min registering 85.2% of transmittance with corresponding bandgap energy of 3.162 eV. FTIR analysis revealed that film annealed at 1 oC/min exhibited high adhesion properties with a bonding energy of 4.16 eV, consistent with bonding transition temperature and maximized crystallinity between interface layers. AuNPs were synthesized using a reduction process and analyzed using UV-Vis spectroscopy. 0.0508 M of HAuCl4.3H2O was reduced by trisodium citrate (Na3Ctr). The molarity of trisodium citrate was varied from 0.015 M to 0.035 M, which yielded AuNP of sizes ranging from 26.55 nm to 17.04 nm. The AuNPs were used to prepare AuNP/TiO2 on graphene composite films and their electrical properties namely; work function, surface contact potential, hall coefficient, film resistivity and carrier mobilities were examined. The pristine graphene had a work function of 4.60 eV while TiO2/graphene recorded 4.56 eV. Introduction of AuNPs in the mesoporous TiO2 lowered the work function to 4.51 eV. Pristine graphene exhibited a uniform contact potential of +450 mV. The addition of TiO2 and AuNPs reduced the potential by 16.77%. The resistivity of the films increased with the addition of AuNPs, where TiO2/graphene film had 5.70×10-2 Ωcm while AuNP/TiO2 on graphene film had 8.29×10-2 Ωcm. Five nanocomposite photoanodes with different AuNP sizes were prepared and used to fabricate platinum-based DSSCs in order to elucidate the effect of surface charge on the performance of DSSC. The cell with 17.04 nm AuNPs recorded the highest PCE due to the establishment of the scottky barrier, high electron transport through the various layers in the cell, more photon absorption due to increased dye loading, an increased spectrum range for photon absorption and inhibited charge recombinations. The plasmonic phenomena had the effect of light amplification within the internal structure of DSSC which effectively increased the absorption range. The presence of graphene in this cell greatly improved the heat dissipation and therefore be projected to slow recombination mechanisms of photo generated electrons hence increasing the Voc of the cell.