Solvent-Dependent Functional Aggregates of Unsymmetrical Squaraine Dyes on TiO₂ Surface for Dye-Sensitized Solar Cells

Abstract

Alkyl group wrapped donor-acceptor-donor (D-A-D) based unsymmetrical squaraine dyes <b>SQ1</b>, <b>SQ5</b>, and <b>SQS4</b> were used to evaluate the effect of sensitizing solvents on dye-sensitized solar cell (DSSC) efficiency. A drastic change in DSSC efficiency was observed when the photo-anodes were sensitized in acetonitrile (bad solvent when considering dye solubility) and chloroform (good solvent) with an Iodolyte (I^-/I₃^-) electrolyte. The DSSC device sensitized with squaraine dyes in acetonitrile showed better photovoltaic performance with enhanced photocurrent generation and photovoltage compared to the device sensitized in chloroform. In a good sensitizing solvent, dyes with long hydrophobic alkyl chains are deleterious forming aggregates on the TiO₂ surface, which results in an incident photon-to-current conversion efficiency (IPCE) response mostly from monomeric and dimeric structures. Meanwhile, a bad sensitizing solvent facilitates the formation of well-packed self-assembled structures on the TiO₂ surface, which are responsible for a broad IPCE response and high device efficiencies. The photoanode sensitized in the bad sensitizing solvent showed enhanced <i>V</i>_OC values of 642, 675, and 699 mV; <i>J</i>_SC values of 6.38, 11.1, and 11.69 mA/cm²; and DSSC device efficiencies of 3.0, 5.63, and 6.13% for the <b>SQ1</b>, <b>SQ5</b>, and <b>SQS4</b> dyes in the absence of a coadsorbent (chenodeoxycholic acid (CDCA)), respectively, which were further enhanced by CDCA addition. Meanwhile, the photoanode sensitized in the good sensitizing solvent showed relatively low photovoltaic <i>V</i>_OC values of 640, 652, and 650 mV; <i>J</i>_SC values of 5.78, 6.79, and 6.24 mA/cm²; and device efficiencies of 2.73, 3.35, and 3.20% for <b>SQ1</b>, <b>SQ5</b>, and <b>SQS4</b> in the absence of CDCA, respectively, which were further varied with equivalents of CDCA. The best DSSC device efficiencies of 6.13 and 3.20% were obtained for <b>SQS4</b> without CDCA, where the dye was sensitized in acetonitrile (bad) and chloroform (good) sensitizing solvents, respectively.