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 (JSC) of 13.99 mA cm–2, open-circuit voltage (VOC) 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.