Unsymmetrical squaraine dyes bearing pyrylium or thiopyrylium components were synthesized, and their light-absorbing properties and performance as sensitizer in dye-sensitized solar cells were investigated. Condensations of 2,6-di-tert-butyl-4-methylpyrylium and thiopyrylium salt with butyl squarate afforded corresponding semisquaryliums. The semisquaryliums were reacted with indolenium with carboxyl group to give target unsymmetrical dyes. They exhibited intense absorption in the far-red and near-infrared region in solution and on the TiO2 surface. Electrochemical measurements revealed that their oxidation potentials were moderately high although electron injection from the excited dyes to the TiO2 conduction band was thermodynamically permitted. Thus, pyrylium and thipyrylium components push absorbance into longer wavelength region as well as bring the highest occupied molecular orbitals (HOMOs) of these dyes in the upward level. A molecular-orbital calculation indicated that the electron distribution moved from the cyclobutene core to indolenine components bearing carboxy anchors upon photoexcitation. The dye-sensitized solar cells (DSSCs) based on these dyes exhibited moderate spectral response in the near-infrared region reflecting their absorption spectral features. However, their photovoltaic performance was far from state-of-the-art visible metal-free sensitizers owing to low open-circuit voltages and current density. These suppressed values might have originated from the high HOMO levels of these dyes, thus, pyrylium and thipyrylium components strongly affect not only the physical properties of the dyes but also the photovoltaic performances of DSSCs based on these dyes.

A symmetrical squaraine dye altered to possess dicyanovinylene functionality on a carbonyl group of the cyclobutene core (SQM1a) was synthesized to elucidate the effects of the conformationally locked-in cis-like form ensured by the steric repulsion of dicyanovinylenes on the performance of dye-sensitized solar cells (DSSCs) through a comparison with an unsymmetrical analogue bearing one carboxy group (SQM1b) and a standard squaraine dye (MSQ). The FT-IR spectrum of the TiO2 thin film immersed in the dye solution suggested that the linkages between the dye and TiO2 were formed by both carboxy groups symmetrically incorporated on the π-conjugation of dyes. The symmetrical and conformationally locked structure of SQM1a brought about a decrease in the non-radiative decay rate, which may have been induced by suppression of the possible photoisomerization of squaraines. In accordance with the finding of non-radiative decay, a DSSC fabricated with SQM1a exhibited a significantly enhanced short-circuit current density, which revealed an efficient electron injection derived from the double-anchored and conformationally locked structure of SQM1a. Consequently, the performance of DSSCs fabricated with SQM1a was higher than the performance of those fabricated with SQM1b and MSQ. This indicates that substantial advantages can be derived by restricting the conformation of symmetrical squaraine dyes.

Squarylium dyes with linearly π-extended structures involving three cyclobutene cores and four heterocyclic components were designed as metal-free sensitizing dyes that can enhance spectral responses in the range over 800 nm and successfully synthesized through the combination of Pd-catalyzed cross-coupling using stannylcyclobutenediones and condensation reactions. The dyes exhibited intense absorption in the NIR region with maxima at around 850 nm and onsets at around 950 nm. Although the HOMO levels remained at the same levels as the corresponding standard squaraine dye, the LUMO levels of the dyes were at lower energies owing to the lowering of the bandgaps of the dyes. Dye-sensitized solar cells (DSSCs) based on these dyes exhibiting significantly wide spectral response including the NIR region showed enhanced short-circuit current density. Co-sensitization of the NIR-absorbing dyes with a standard far-red absorbing squaraine dye produced a complementary effect in spectral response, resulting in the increase in short-circuit current density.

A novel class of near-infrared absorbing squarylium sensitizers with linearly extended π-conjugated structures, which were obtained by Pd-catalyzed cross-coupling reactions with stannylcyclobutenediones, has been developed for dye-sensitized solar cells. The cells based on these dyes exhibited a significant spectral response in the near-infrared region over 750 nm in addition to the visible region.

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.Highlights► Four unsymmetrical squarylium dyes with π-extended heterocycles were synthesized. ► π-Extended heterocyclic components lead to red-shifted absorption of the dyes. ► The squarylium dyes achieved acceptable performance in the NIR region (η < 1.6%). ► The photovoltaic performances depended on the structure of heterocyclic components.

A symmetrical squaraine dye altered to possess dicyanovinylene functionality on a carbonyl group of the cyclobutene core (SQM1a) was synthesized to elucidate the effects of the conformationally locked-in cis-like form ensured by the steric repulsion of dicyanovinylenes on the performance of dye-sensitized solar cells (DSSCs) through a comparison with an unsymmetrical analogue bearing one carboxy group (SQM1b) and a standard squaraine dye (MSQ). The FT-IR spectrum of the TiO2 thin film immersed in the dye solution suggested that the linkages between the dye and TiO2 were formed by both carboxy groups symmetrically incorporated on the π-conjugation of dyes. The symmetrical and conformationally locked structure of SQM1a brought about a decrease in the non-radiative decay rate, which may have been induced by suppression of the possible photoisomerization of squaraines. In accordance with the finding of non-radiative decay, a DSSC fabricated with SQM1a exhibited a significantly enhanced short-circuit current density, which revealed an efficient electron injection derived from the double-anchored and conformationally locked structure of SQM1a. Consequently, the performance of DSSCs fabricated with SQM1a was higher than the performance of those fabricated with SQM1b and MSQ. This indicates that substantial advantages can be derived by restricting the conformation of symmetrical squaraine dyes.