Photovoltaic Performance of Platinum-Graphene Based Counter Electrode for Dye Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) have emerged as new class of low cost energy conversion devices. DSSCs were fabricated using graphene on platinum-based counter electrodes, electrolyte and titanium dioxide thin films deposited on fluorine-doped tin oxide (FTO) using doctor blade technique. TiO2 thin films as working electrode (WE’s) were annealed at different rates; 2 OC/min, 1 OC/min, one step annealed and as deposited. Surface roughness and optical properties of TiO2 thin films were examined using optical microscopy and UV-VIS spectroscopy respectively. Thin films annealed at low rate (1 OC/min) were found to have high transmittance, lower band gap energy and improved surface roughness for greater surface area for dye adsorption that resulted in better conversion efficiency of a solar cell. The prepared counter electrodes were characterized using UV-VIS spectrophotometry and four-point probe for optical transmittance and sheet resistance respectively. Transmittance of single layer, double graphene on platinum was found to be high (above 70 %) at visible wavelengths. It was found that each layer increase in graphene corresponds to decrease of 2.4 % in the optical transmittance of these films. Sheet resistance (Rs) was found to reduce with increase in number of graphene layers with 1100, 620 and 180 Ω/sq for single-layer (SLG), double layer (DLG) and multi-layer graphene (MLG) on FTO respectively. DSSCs were then characterized by analyzing the photocurrent-voltage characteristics. The results showed incorporation of SLG and DLG increase the short circuit current density and photoelectric conversion efficiency (η). However, incorporation of MLG led to reduction of η. Pt on SLG, Pt on DLG and Pt on MLG CE’s had a conversion efficiency of 3.30, 3.41 and 2.16 % respectively. Pt on double Layer-Gr based CEs showed the highest conversion efficiency and improvement of 5.01 % on ƞ as compared to that of reference platinum-based CEs.