Betalain pigments for dye-sensitized solar cells

A dye-sensitized solar cell based on natural betalain pigments from red beet roots is described. Reddish-purple betanin is an easily oxidized, water-soluble pigment with strong visible light absorption, having a maximum molar absorptivity of about 65,000 M−1 cm−1at 535 nm. A photoanode was fabricated from nanocrystalline TiO2 on transparent conductive glass, treated with ethanolic HCl solution, then sensitized with raw beet extract or a betanin solution from which yellow betaxanthin pigments were removed by column chromatography. The betanin-sensitized film when employed in a dye-sensitized solar cell gave a maximum photocurrent of 2.42 mA/cm2 and open-circuit photovoltage of 0.44 V in the presence of methoxypropionitrile containing I−/I3− redox mediator. Photocurrents and photovoltages are also reported for the DSSC sensitized with yellow betaxanthin and a brown betalain-derived oxidation product. Wavelength-dependent incident photon-to-electron conversion efficiencies (IPCEs) show maximum values of 14% and 8% for betaxanthin- and betanin-based solar cells, respectively.