Design, Synthesis, and Photophysical Characterization of Multifunctional Far‐Red Squaraine Dyes for Dye‐Sensitized Solar Cells

Abstract

Novel far‐red sensitive symmetrical squaraine (SQ) dyes aiming toward multifunctional properties such as control of dye aggregation, promotion of molecular self‐assembly, and introduction of electrolyte function by alkyl chain terminal modification are designed. Newly designed dyes without terminal modification as reference (SQ‐5) along with iodine (SQ‐77) and imidazole (SQ‐79) alkyl terminal‐modified dyes are successfully synthesized, characterized, and subjected to detailed photophysical investigations. Iodine terminal modification (SQ‐77) leads to enhanced molar extinction coefficient, dye aggregation, dye loading, and binding strength on the TiO2 surface, which is found by just the opposite after the imidazole (SQ‐79) terminal modification. Demonstration of nearly similar photovoltaic performance by SQ‐77 and SQ‐79 in the absence of iodine and chenodeoxycholic acid validates their multifunctional role as electrolyte function and dye aggregation prevention, respectively, in addition to their main role as photosensitizer. Dye‐sensitized solar cells fabricated with SQ‐5 as a sensitizer show efficient far‐red to near‐infrared photo sensitization and photon harvesting with short‐circuit photocurrent density, open‐circuit voltage, and fill factor of 11.98 mA cm−2, 0.61 V, and 0.57, respectively, leading to a photoconversion efficiency of 4.2% under simulated solar irradiation.