Preparation and performance of prickly pear (Opuntia phaeacantha) and mulberry (Morus rubra) dye-sensitized solar cells

This study examined the use of natural dyes extracted from New Mexico prickly pear (Opuntia phaeacantha) and New Mexico mulberry (Morus rubra) in dye-sensitized solar cells (DSSC). The main pigments betalain (betanin plus betaxanthin molecules) in prickly pear and anthocyanin in mulberry were obtained in more concentrated form by column chromatographic separation of the extracts. UV–Vis spectrophotometry was used to determine pigment concentrations and the efficacy of extraction solvent. Solar cells were prepared using nano-crystalline TiO2 mesoporous films and the fruit extracts as sensitizers in both raw and separated form. Based on photovoltaic (PV) performance, aqueous methanol (90% methanol/10% water) was found to be the best extraction solvent for betalain from prickly pear, while 100% methanol was best for anthocyanin from mulberry. Modest separation of the betanin and betaxanthin from betalain was achieved, however no improvement in PV output was observed with these individual pigments. Removal of (non-pigment) organic components did improve adsorption and PV performance of the anthocyanin-sensitized cells. An overall maximum efficiency of 0.82% was achieved from a 1:1 cocktail blend of anthocyanin and betalain extracts, attributed in part to a broadened and heightened absorption spectrum. Moreover, cells sensitized with the anthocyanin and betalain blend showed improved photo-stability over those sensitized with the betalain alone, demonstrating a synergistic effect and additional advantage of natural dye combinations for DSSCs.