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

Abstract

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 .

Squaraine Dyes for Photovoltaic and Biomedical Applications

“…Reproduced with permission. [ 5 ] Copyright 2020, American Chemical Society. d‐i) Absorption spectra of SQ4 and SQ22 in DMF; ii) IPCE spectra for DSCs sensitized with SQ4 and SQ22.…”

“…Reproduced with permission. [ 5 ] Copyright 2020, American Chemical Society. c) Summary of SQs for photovoltaic and biomedical applications.…”

“…[ 4 ] Condensed films of SQs are always polymorphic and thus show two limiting cases with respect to the overlap and mutual orientations of dyes, namely, the H‐aggregate with a “card pack” arrangement and a generally blue‐shifted absorption spectrum, and the J‐aggregate with a “slipped stack” arrangement and a red‐shifted absorption spectrum (Figure 1b). [ 5 ] Some research groups have found that the H‐ and J‐aggregates can be transformed with each other by adding small molecules or changing the environment, which have been shown to affect the performance of SQs considerably by changing the absorption of aggregates and/or causing fluorescence emission or quenching. [ 6 ]…”

Cause, Regulation and Utilization of Dye Aggregation in Dye-Sensitized Solar Cells

“…Aggregation of dyes on the TiO 2 surface depends on the length and the location of the flexible chains. A series of unsymmetrical squaraine dyes ( 3 – 8 ) with different alkyl chain lengths at the indoline unit were synthesized to modulate the aggregation of sensitizers on the TiO 2 surface, which resulted in an overall PCE ranging from 3.82% to 6.23% (see Table 1 ) [ 73 ]. In addition, the long chain not only reduces the aggregation, but also prevents the electrolyte from penetrating the dye [ 74 ].…”

“…For a series of squaraine dyes ( 3 – 8 ) (shown in Figure 3 ), although different alkyl chains were introduced to inhibit aggregation, the IPCE responses of the devices fabricated with six dyes were broad, which indicated that both the monomer and aggregated structures contributed to the charge injection process. Also, higher contributions to the IPCE response were observed from the aggregates rather than the monomeric form of the sensitizer [ 73 ].…”