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

摘要： Electron transfer processes at the interfaces dictate the factors to improve the photovoltaic parameters such as open-circuit voltage (Voc) and short-circuit current (Jsc) of a dye-sensitized solar cell (DSSC) device besides selecting a set of suitable anode, dye, electrolyte and cathode materials. Inefficient charge injection process at dye-TiO2 interface and charge recombination at the TiO2-dye/electrolyte interface have detrimental effects in improving both Jsc and Voc. Hence tailoring the factors that governs to improve the Jsc and Voc will be an ideal approach to get the desired sensitizers with good device efficiencies. Squaraines are far-red active zwitterionic dyes, has high molar extinction coefficient along with unique aggregation properties due to the larger dipole moment associated with it. Here we report a series of unsymmetrical squaraine dyes, SQS1 to SQS6, with systematic variation of alkyl groups at sp3-C and N-atoms of indoline unit that was away from the anchoring group to control the dye-dye interactions on the TiO2 surface. The branched alkyl groups help to modulate the self-assembly of sensitizers on the TiO2 surface besides passivating the surface that helps to avoid the charge recombination processes. Light harvesting efficiency (LHE) and cyclic voltammetry studies of dye-sensitized TiO2 electrode indicated 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 variation in the branched alkyl group helps to enhance the Voc from 672 (SQS1) to 718 mV (SQS6), Jsc from 7.95 (SQS1) to 12.22 mA/cm2 (SQS6), with the device efficiency ranging from 3.82% to 6.23% without any coadsorbent. Dye SQS4 has achieved highest efficiency of 7.1% (Voc = 715 mV, Jsc = 13.05 mA/cm2) with coadsorbent chenodeoxycholic acid (CDCA) using iodine (I-/I3 -) electrolyte compared to its analogs. Analysis of IPCE profile indicates that the major contribution of photocurrent generation is from the aggregated squaraine dyes on TiO2.