A combined experimental and theoretical study of natural betalain pigments used in dye-sensitized solar cells

We report results of combined experimental and theoretical studies of betalain natural dyes used as sensitizers for TiO2 dye-sensitized solar cells (DSSCs). To analyze the compliance of the various forms of betalain dyes with the main criteria that should be met by a dye to be used as TiO2 sensitizer in a DSSCs, we performed density functional theory (DFT) calculations, which provided the optimized geometry, electronic structure and electronic spectrum of the dyes in fully protonated as well as partially deprotonated forms, in solution. We discuss the adsorption onto the substrate, the matching of the absorption spectrum of the dye with the solar spectrum, the energy level alignment with the semiconductor and the electrolyte, and the charge transfer to the substrate. By comparing the theoretical results with the experimental data we identify the betacyanins as the useful constituents of the betalain extracts and emphasize the role of extract purification.

► We study betalains as TiO2 sensitizers in dye-sensitized solar cells. ► All betalains favour adsorption to substrate through the same COO group. ► Betacyanins are better sensitizers than betaxanthines due to spectral matching. ► Betacyanins are better sensitizers than betaxanthines due to energy level alignment. ► Removal of betaxanthines increases short-circuit current and device efficiency.