Fabrication of TiO2/Dye-Sensitized Solar Cells (DSCs) Using Dye Extracts and Their Mixture as Photosensitizers

In this work we have reported an investigation on Hibiscus sabdariffa and Delonix regia dye extracts and their mixture as natural sensitizers for TiO2/DSCs. A shift in the absorption maximum toward the lower energy of the ultraviolet-visible spectrum was observed for the dye mixture and a shift in the absorption maximum towards the higher energy of the ultraviolet-visible spectrum was observed for the dye extracts. The optical band gaps obtained at the point where the absorption spectra showed strong cut offs range from 1.79eV to 2.40eV. Also, we have used TiO2 thin films of thickness 5.2μm and the Light Harvesting Efficiencies (LHE) of the dye extracts and the dye mixture adsorbed onto TiO2 surface were close to unity. The average diameter of the TiO2 films obtained from SEM is in the range of 25-40nm reflecting that the TiO2 films are transparent and suitable for DSC application. The XRD pattern revealed the TiO2 films to be of anatase form and the structure type is tetragonal with 3.53217Å as the d-spacing for the most prominent peak, 2θ=25.2139° (ICDD data file: 01-075-8897). Three (3) DSCs each of 0.52 cm2 active area were assembled and subjected to current-voltage characterization using a standard overhead Veeco viewpoint solar simulator equipped with AM 1.5 filter to give a solar radiation of 1000 W/m2 and coupled to Keithley source meter (model 4200SCS). The photoelectrochemical performance of the fabricated DSCs showed open-circuit voltages (Voc) varied from 0.42 to 0.53 V, the short-circuit current densities (Jsc) ranged from 0.10mAcm-2 to 0.90mAcm-2 and the fill factors (FF) varied from 12 to 38%. The best overall solar power conversion efficiency of 0.13% was obtained, under AM 1.5 irradiation and a maximum short circuit current density of 0.90mAcm-2. Nevertheless, pure Hibiscus sabdariffa and Delonix regia dye extracts proved to be rather poor sensitizers as can be seen by the low spectra absorption at lower energies with current densities of 0.17mAcm-2 and 0.10mAcm-2 respectively. The solar power conversion efficiencies for Hibiscus sabdariffa and Delonix regia dye extracts were 0.01% and 0.02% respectively. In our earlier studies, we highlighted an established fact that raw natural dye mixtures exhibit better performance than pure dye extracts. Thus, the power conversion efficiency of 0.13% observed for the dye mixture sensitized TiO2/DSC corresponds to an increment in the neighborhood of 85% to 92% over the pure dye extracts sensitized TiO2/DSCs.