Unsymmetrical squarylium dyes with π-extended heterocyclic components and their application to organic dye-sensitized solar cells

Novel π-extended squarylium dyes have been synthesized as sensitizers for use in dye-sensitized solar cells (DSSCs). Four quaternary salts with extended π-systems, quinolinium, benz[c,d]indolium, benzopyrylium, and benzo-1-thiopyrylium, were selected as electron-rich heterocyclic components and condensed with indole-based semisquarylium bearing a carboxyl group as an anchor for their immobilization on TiO2 to obtain four unsymmetrical squarylium dyes (SQ1–4). They exhibited intense absorption in the near-infrared region in solution and on the TiO2 surface (absorption edge: approximately 900 nm). Investigation of their electrochemical properties revealed that electron injection from the excited dyes to the TiO2 conduction band was thermodynamically permitted. A molecular-orbital calculation indicated that the electron distribution moved from the cyclobutene core to the heterocyclic components bearing the carboxyl group by photoexcitation of SQ1–4. The photovoltaic performances of DSSCs based on the squarylium dyes significantly depended on the structure of heterocyclic components. Among the series of squarylium dyes, SQ2–4 achieved comparatively high performance in metal-free NIR sensitizers (η = 1.1–1.6%), in accordance with the calculation results. These results suggest that the structure of heterocyclic components strongly affects not only the absorption properties of the dyes but also the photovoltaic performances of DSSCs based on these dyes.