Alkyl-Group-Wrapped Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells: Branched Alkyl Chains Modulate the Aggregation of Dyes and Charge Recombination Processes

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Electron transfer processes at the interfaces dictate the factors that improve the photovoltaic parameters, such as open-circuit voltage ( V oc ) and short-circuit current ( J sc ), of a dye-sensitized solar cell device, besides selection of a set of suitable anode, dye, electrolyte, and cathode materials. An inefficient charge injection process at the dye–TiO 2 interface and charge recombination at the TiO 2 –dye/electrolyte interface have detrimental effects on improving both J sc and V oc . Hence, tailoring the factors that govern the improvement of J sc and V oc will be an ideal approach to get the desired sensitizers with good device efficiencies. Squaraines are far-red-active zwitterionic dyes and have a high molar extinction coefficient along with unique aggregation properties due to the large dipole moment associated with them. Here, we report a series of unsymmetrical squaraine dyes, SQS1 to SQS6, with systematic variation of alkyl groups at the sp 3 -C and N-atoms of the indoline unit that is away from the anchoring group to control the dye–dye interactions on the TiO 2 surface. The branched alkyl groups help in modulating the self-assembly of sensitizers on the TiO 2 surface, besides passivating the surface that helps avoid the charge recombination processes. Light harvesting efficiency and cyclic voltammetry studies of dye-sensitized TiO 2 electrodes indicate that the aggregation and charge hopping process between the dye molecules can be modulated, respectively, by systematically increasing the number of carbon atoms in the alkyl groups. Such a variation in the branched alkyl group helps enhance V oc from 672 (SQS1) to 718 mV (SQS6) and J sc from 7.95 (SQS1) to 12.22 mA/cm 2 (SQS6), with the device efficiency ranging from 3.82% to 6.23% without any coadsorbent. Dye SQS4 achieves the highest efficiency of 7.1% ( V oc = 715 mV, J sc = 13.05 mA/cm 2 ) with coadsorbent chenodeoxycholic acid (CDCA) using an iodine ( I – / I 3 – ) electrolyte compared to its analogues. An analysis of the incident photon-to-current efficiency profiles indicates that the major contribution to photocurrent generation is from the aggregated squaraine dyes on TiO 2 .