Amphiphilic Indoline-Based Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells: Modulating the Dye-TiO₂/Electrolyte Interface for Nonaqueous and Aqueous Electrolytes

Abstract

A high molar extinction coefficient with sharp absorption properties from the range of visible to near-infrared regions makes squaraine dyes very attractive and potential chromophores for dye-sensitized solar cell (DSSC) applications. Here, we report a series of alkyl groups and glycolic chain [triethylene glycol (TEG)]-wrapped amphiphilic indoline-based unsymmetrical squaraine dyes, where the number of carbon atoms in the alkyl groups and TEG was systematically changed by incorporating the alkyl groups and TEG within the dye molecule for controlling the self-assembly of the dyes on the TiO2 surface and to improve the interfacial properties at the dye-TiO2/electrolyte interface. Due to the same skeletal characteristics, ASQ dyes showed similar photophysical and electrochemical properties in solution. Photovoltaic characterization of ASQ dyes was carried out in nonaqueous and aqueous electrolytes to evaluate the effect of alkyl groups and TEG on nonaqueous and aqueous DSSC device performances. V OC, J SC, and photovoltaic efficiencies were systematically enhanced by increasing the number of carbon atoms of alkyl groups into the dye molecules for nonaqueous DSSCs. Furthermore, addition of chenodeoxycholic acid (CDCA) decreased the charge recombination processes and resulted in enhanced efficiency, V OC, and J SC (enhanced incident photon-to-current conversion efficiency performance) compared to that without CDCA. The ASQ4 dye gave the highest nonaqueous DSSC efficiency of 6.39%, a V OC of 711 mV, and a J SC of 12.18 mA/cm2 with 2 equiv of CDCA in the ASQ dye series. Furthermore, increased carbon atoms in the alkyl groups showed a detrimental effect on the aqueous DSSC efficiency due to the mismatch in the polarity at the dye-TiO2/electrolyte interface, being the reason for the decreased device efficiencies from ASQ2 to ASQ4 dyes. The ASQ2 dye gave the highest aqueous DSSC efficiency of 2.36%, a V OC of 611 mV, and a J SC of 4.75 mA/cm2 without CDCA in the ASQ dye series.

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

“…Here, the SQ5 and SQS4 dyes formed well-packed controlled aggregated structures in the bad sensitizing solvent and showed better IPCE responses (more than 50%) compared to the SQ1 dye without CDCA; further addition of CDCA enhanced IPCE responses for all of the three dyes. Aggregated structures of D-A-D based squaraine dyes are the results of facile Ti periodic sites in TiO 2 (101 facet), which facilitate the formation of aggregates. ,,− Such types of monomeric, dimeric, aggregated, and highly aggregated structures have been also observed for D-A-D based squaraine dyes at the air–water interface, in aqueous solutions, in DMSO–water mixtures, in microheterogeneous media, and in Langmuir–Blodgett films. − Further, the lengths of the hydrophobic alkyl group in the squaraine dye showed relatively high V OC and J SC values in the bad sensitization solvent compared to that in the good solvent. Such high photovoltaic parameter values were observed due to the better surface passivation and controlled dye aggregation, whereas the low V OC and J SC values in the good sensitizing solvent are due to low dye loading for all of the dyes (Table S5).…”

D‐A‐D‐based Unsymmetrical Thiosquaraine Dye for the Dye‐Sensitized Solar Cells†

“…The device performance of SQ dyes were measured under standard simulator condition (AM 1.5 G, 100 mW cm −2 ), where the area of the black mask was 0.36 cm 2 . The solvents used for sensitization and electrolyte solutions were deoxygenated by purging with nitrogen gas (22).…”

“…Furthermore, modulation of various device components such as anode, cathode, electrolyte and light‐absorbing sensitizers make the dye‐sensitized solar cell (DSSC) (2–7) very attractive, besides other third‐generation photovoltaic technologies. Organic sensitizers offer different architectures to enhance the light‐harvesting properties of the dyes, such as D‐A (8–10), D‐π‐A (11–13), D‐A‐π‐A (14,15), D‐D‐π‐A (16,17), and D‐A‐D (18–23) types. Furthermore, dyes with a broad spectral response is vital besides the NIR‐light absorption and co‐sensitization for the enhanced photocurrent generation (24–29).…”

Ti3C2 MXene-Reduced Graphene Oxide Composite Polymer-Based Printable Electrolyte for Quasi-Solid-State Dye-Sensitized Solar Cells

“…In DSSCs, the electrolyte plays a very important role in both dye reduction regeneration and charge transfer. − The liquid electrolyte is the most widely used, but its leakage and sealing problems still limit the practical commercial application of DSSCs. , Currently, gel electrolytes can be utilized to replace the liquid electrolytes to overcome these limitations. Specifically, polymer gel electrolytes were reported to have great advantages, including their relatively high ionic conductivity, high permeability in porous titanium dioxide membranes, and thermal stability. − Table S1 lists the photovoltaic parameters that have been related to the development of quasi-solid-state electrolyte DSSCs in recent years. ,− So far, many polymers and copolymers, such as polypyrrole, polyethylene oxide (PEO) polyacrylonitrile, poly­(acrylic acid- co -ethylene glycol), and poly­(acrylic acid), have been successfully used to prepare gel electrolytes. − To further improve the efficiency of the cell, many studies have focused on the utilization of various nanomaterials, such as clay, graphene, carbon nanotubes, metal oxides, and so forth. − For example, Venkatesan et al reported a graphene oxide (GO) sponge as a printable electrolyte for nanofillers in a mixed polymer for quasi-solid-state DSSCs (QS-DSSCs), to achieve a PCE of 8.78% . In addition, Yu et al introduced three different metal oxide nanoparticles (SiO 2 , TiO 2 , and ZnO) into the electrolyte to improve ion dispersion and charge transportation, in which the DSSC containing 15% TiO 2 nanoparticles achieved the best performance of 8.32%.…”