Fused Fluorenylindolenine-Donor-Based Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells

A series of four unsymmetrical squaraine dyes, XSQ1-4, were synthesized using a fused fluorenylindolenine-based donor unit for dye-sensitized solar cells (DSSCs). The fused structure of fluorenylindolenine helped in moving the absorption toward the near-infrared (NIR) region, and the two sp³-C centers available on this donor were utilized to incorporate out-of-plane alkyl chains in opposite directions to control the dye-dye interactions on the TiO₂ surface. High extinction coefficient (ε ≥ 10⁵ M^-1 cm^-1) for absorbing NIR photons and suitable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels with respect to the conduction band of TiO₂ and electrolyte for charge injection and dye regeneration processes, respectively, make these dyes potential sensitizers for DSSCs. Introduction of branched alkyl groups in the π-framework helped in controlling dye aggregation to reduce exciton quenching and assisted in TiO₂ surface passivation to avoid the charge recombination process. Furthermore, having a naphthyl group on the indole part of the anchoring group containing segment helped to red-shift the absorption spectrum of dyes 15 nm toward the NIR region (XSQ3-4). Among all of the dyes under investigation, XSQ2 gave the best photovoltaic performance, having a short-circuit current density ( J_SC) of 13.99 mA cm^-2, open-circuit voltage ( V_OC) of 0.66 V, and a fill factor (ff) of 0.71, with a device performance (η) of 6.57%. Electrochemical impedance spectroscopy revealed higher electron lifetime on TiO₂ for XSQ2, which helps to avoid the charge recombination process.