Wide-Range Near-Infrared Sensitizing 1 H-Benzo[ c, d]indol-2-ylidene-Based Squaraine Dyes for Dye-Sensitized Solar Cells

NIR absorbing squaraine dyes SQ1-SQ7 having 1 H-benzo[ c, d]indol-2-ylidene as a donor moiety were designed for application in DSSCs. Annulation of the benzene ring to an 3 H-indolium-based anchor moiety led to a red-shifted and broadened absorption band on TiO₂ film, which were reflected in the improved short-circuit current density of SQ2 (6.22 mA cm^-2) compared to the nonbenzene fused derivative SQ1 (4.39 mA cm^-2). Although the introduction of a butoxy (SQ4: 806 nm) or dialkylamino group (SQ5-SQ7: 815-820 nm) to the 1 H-benzo[ c, d]indol-2-ylidene-based donor moiety resulted in red-shifted absorption maxima in ethanol compared to the nonsubstituted derivative SQ2 (784 nm), the HOMO energy level of SQ4-SQ7 gave rise to an undesirable approximation to the redox potential of I^-/I₃^-. Thus, the butoxy (SQ4: 0.56) and dialkylamino (SQ5-SQ7: 0.25-0.30) derivatives had relatively lower conversion efficiencies. Since the 2-ethylhexyl derivative SQ3 exhibited red-shifted absorption (λ_max: 796 nm), suitable HOMO and LUMO energy levels, and relatively efficient restriction of charge recombination, this dye achieved the highest conversion efficiency (1.31%), along with a high IPCE response of over 20% over a wide range from 640 to 860 nm and an onset of IPCE at 1000 nm.