A Near-Infrared cis-Configured Squaraine Co-Sensitizer for High-Efficiency Dye-Sensitized Solar Cells

A cis-configured squaraine dye (HSQ1), synthesized by incorporation of a strongly electron-withdrawing dicyanovinyl group into the central squaric acid moiety, is employed in dye-sensitized solar cells (DSCs). In solution, HSQ1 displays an intense absorption in the near-infrared region with a maximum at 686 nm and when the dye is adsorbed on a TiO2 surface, the absorption spectrum broadens in both the blue and the near-infrared regions, which is favorable for efficient light harvesting over a broad wavelength range. A solar cell sensitized with HSQ1 shows a broader incident photon-to-current conversion efficiency (IPCE) spectrum (from 400 to 800 nm) and a higher IPCE in the long-wavelength region (71% at 700 nm) than a cell sensitized with squaraine dye SQ1. Furthermore, a solar cell co-sensitized with HSQ1 and N3 dye shows remarkably improved short-circuit current density and open-circuit voltage compared to those of a DSC based on N3 alone and fabricated under the same conditions. The energy-conversion efficiency of the co-sensitized DSC is 8.14%, which is the highest reported efficiency for a squaraine dye-based co-sensitized DSC without using Al2O3 layer.