Effect of Cyclization and Alkyl Group Wrapping in Visible-Light-Active Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells

A series of alkyl group-appended indoline- and carboxylic acid-functionalized indolenine-based visible-light-active unsymmetrical squaraine dyes, SQA7–10, were designed, synthesized, and utilized for the dye-sensitized solar cells device fabrication. The number of alkyl group has been increased systematically on the indoline moiety to control the self-assembly of dyes on TiO2 and to passivate the photoanode that helps in improving the open-circuit voltage (VOC) by avoiding the charge recombination process. SQA7–10 dyes showed an absorption between λmax 536–540 nm, with molar extinction coefficients of 1.62–2.13 × 105 M–1 cm–1 in CH3CN. Further, ultraviolet–visible (UV–vis) studies on TiO2 indicated the formation of H-aggregated dyes (505–510 nm), which can be controlled by appending the alkyl groups. The energy levels of the highest occupied molecular orbital (HOMO) of these dyes are well aligned with the Nernst potentials of I–/I3–electrolyte and [Cu(tmby)2]+/2+ electrolytes with sufficient overpotentials required for the dye regeneration process. DSSC devices made with these dyes showed systematic enhancements of open-circuit voltage (VOC) and device efficiency with respect to alkyl group incorporation for SQA7–10 with I–/I3– electrolytes. All of the SQA dyes exhibited the device VOC of more than 805 mV, where the introduction of alkyl groups systematically enhanced the VOC in the order of SQA7 (805 mV) < SQA8 (829 mV) < SQA9 (843 mV) < SQA10 (862 mV) without any addition of CDCA. Within the SQA dye series, SQA10 dye has achieved the highest DSSC device efficiency of 7.52% (Jsc of 11.16 mA/cm2, Voc of 862 mV, and ff of 78%) with I–/I3– electrolyte, whereas use of [Cu(tmby)2]+/2+ redox shuttle with SQA10 showed enhanced Voc (1080 mV) and device efficiency (8.35%). The IPCE profile for the device fabricated with SQA dyes showed good response at 480 and 560 nm, which indicates the photocurrent generation from the aggregated structures.