Performance of selected natural dyes in dye sensitized solar cells

Abstract

Organic dyes have the potential for use as sensitizers in dye sensitized solar cells (DSSC). To this end, the performance of anthocyanins in DSSC has been investigated. Anthocyanins were extracted from fresh flowers of Acanthus Pubescens and leaf stocks of Manihot Esculenta Crantz, and used independently as sensitizers. The influence of concentration and pH of anthocyanins on solar conversion efficiency in DSSC was then investigated. Anode materials; titanium dioxide , titanium dioxide-zirconium dioxide and titanium-graphene were screen printed on pre- cleaned fluorine-doped tin oxide (FTO) glass substrates. The zirconium dioxide shell on top of the anode was introduced to suppress electron-hole recombination at the titanium-dye interface. To improve electron mobility in the anode, graphene nano particles were introduced into the titanium anode. Cathodes were made by applying plastisol on pre-cleaned FTO glass substrates using an artistic brush and later annealed at 450 °C for 20 minutes to activate platinum. Photovoltaic performance was measured using a computer controlled digital source meter under solar simulation of 1 sun. Electron transport was studied using electrochemical impedance spectroscopy (EIS). Results showed improvement in solar conversion efficiency as concentration of anthocyanins increased, and dependence of solar conversion efficiency on different molecular forms of anthocyanins, owing to variation in pH of the sensitizing anthocyanins. The improvement in solar conversion efficiency was attributed to increase in absorption of light and photo excitation of electrons into the conduction band of titanium. EIS studies linked the improvement in solar conversion efficiency to increase in electron density in the conduction band of titanium. The use of zirconium dioxide shell on top the titanium anode resulted into increase in recombination rate constant, and decrease in electron lifetime. The increase in recombination rate constant and decrease in electron lifetime indicated that, the shell did not suppress electron-hole recombination at the anode-dye interface. The titanium-graphene hetero-structures showed improvement in solar cell performance at low concentrations of graphene content. The highest efficiency of was observed at 0.02% graphene content. The higher efficiencies were attributed to improved charge mobility, owing to the superior conductivity of graphene.