Self-Assembly of Cis-Configured Squaraine Dyes at the TiO₂–Dye Interface: Far-Red Active Dyes for Dye-Sensitized Solar Cells

To synergize both steric and electronic factors in designing the dyes for dye-sensitized solar cells, a series of cis-configured unsymmetrical squaraine dyes <b>P11–P15</b> with suitably functionalized alkyl groups and squaric acid units containing the electron-withdrawing groups were synthesized, respectively. These dyes capture the importance of (i) the effect and position of branched alkyl groups, (ii) mono- and di-anchoring groups containing dyes, and (iii) further appending the alkyl groups through the cyanoester vinyl unit on the central squaric acid units of D–A–D-based cis-configured squaraine dyes. All the above factors govern the controlled self-assembly of the dyes on the TiO₂ surface which helps to broaden the absorption profile of the dyes with an increased energy-harvesting process. With respect to the position of the branched alkyl groups, dye <b>P11</b> with the sp³-C and N-alkyl groups away from the TiO₂ surface showed a better device efficiency of 5.98% (<i>J</i>_sc of 14.46 mA cm^–2, <i>V</i>_oc of 0.576 V, and ff of 71.8%) than its positional isomer <b>P12</b> with 3.45% (<i>J</i>_sc of 8.78 mA cm^–2, <i>V</i>_oc of 0.554 V, and ff of 70.9%). However, with respect to the dyes containing mono- and di-anchoring groups, <b>P13</b> with two anchoring units exhibited a superior device performance of 7.58% (<i>J</i>_sc of 17.12 mA cm^–2, <i>V</i>_oc of 0.618 V, and ff of 71.7%) in the presence of optically transparent co-adsorbent <b>CDCA</b> (3α,7α-dihydroxy-5β-cholanic acid) than dyes <b>P11</b> and <b>P12