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 TiO 2 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 Al 2 O 3 layer.