Interactions in Dye-sensitized Solar Cells

• Main Author: Greijer Agrell, Helena
• Format: Doctoral Thesis
• Language: English
• Published: Uppsala universitet, Fysikalisk-kemiska institutionen 2003
• Subjects: Physical chemistry; solar cells; photovoltaics; dye-sensitized; mesoporous; nanostructured; Raman scattering; Fysikalisk kemi
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3752; http://nbn-resolving.de/urn:isbn:91-554-5786-X

The interactions between the molecular constituents in dye-sensitized solar cells were studied with UV-VIS and IR spectroscopy, Raman scattering, conductivity and electron accumulation measurements. From stability studies of the dye, bis(tetrabutylammonium)cis-bis(thiocyanato) bis(2,2’-bipyridine-4-carboxylic acid, 4’-carboxylate) ruthenium(II), in the complete solar cell, the thiocyanate ion ligand was found to be lost from the dye. A method was developed to study mechanisms in a sealed dye-sensitized solar cell using resonance Raman scattering (RRS). RRS studies of a complete dye-sensitized solar cell including iodine and lithium iodide in the electrolyte indicate that triiodide exchange the SCN- ligand of the dye. It was proposed that an ion pair Li+…I3- formation occurred, which, by a reduced electrostatic repulsion between I3- and SCN- facilitated the exchange of these anions at Ru(II) of the dye. The additive 1-methylbenzimidazole suppressed the SCN-/I3- ligand exchange by forming a complex with Li+. In order to study charge transport in nanostructured TiO2 films permeated with electrolyte, a technique was developed for determining activation energies of conduction, electron accumulation and effective mobility. Two regions were distinguished from the relation between conductivity and electron concentration. In the first region (~1-20 electrons per TiO2 particle), which resembles best the region where the nanostructured dye-sensitized solar cell operates, the results can be fitted to some extent with a trapping/detrapping or a hopping model for charge transport, but not with a conduction band model. For the second region (> 20 electrons per TiO2 particle), charge transport by electrons in the conduction band seems to be the most applicable model. Through this work many effects from the interplay between the solar cell components were observed. These observations emphasize the importance of well-balanced interactions in dye-sensitized solar cells.