Templated Assembly of Betanin Chromophore on TiO2: Aggregation-Enhanced Light-Harvesting and Efficient Electron Injection in a Natural Dye-Sensitized Solar Cell
journal contributionposted on 15.04.2016, 00:00 by Nicholas A. Treat, Fritz J. Knorr, Jeanne L. McHale
Enhanced light-harvesting and photoconversion efficiency of nanocrystalline TiO2 sensitized with the plant pigment betanin is observed in the presence of spectral signatures which reveal self-assembly of betanin on the surface. Though aggregation of sensitizing chromophores is generally considered detrimental to dye-sensitized solar energy conversion, solar cells constructed with aggregated betanin show a 2.5-fold increase in power conversion efficiency compared to those using mostly monomeric betanin. Dye aggregation results in a broadened absorbance spectrum with extended light harvesting at blue and red wavelengths. Variation in solution conditions and soaking times for film sensitization enable control of the relative amounts of adsorbed monomer and aggregate. Self-consistent modeling of the absorption spectra of betanin-sensitized TiO2 films as a function of dye loading suggests the templated formation of a betanin dimer on the TiO2 surface and associated splitting of the excited state. We show that dye aggregation increases the light-harvesting efficiency as well as the incident photon-to-current conversion efficiency (IPCE). Measurement of the absorbed photon-to-current conversion efficiency (APCE) reveals that electron injection and collection of the putative betanin dimer is more efficient than that of the monomer. Not only is this the first report of a dye-sensitized solar cell performance increase upon dye aggregation, but it also constitutes a record power conversion efficiency for a natural dye-based solar cell of 3.0%.