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

Abstract

Alkyl group wrapped donor−acceptor−donor (D-A-D) based unsymmetrical squaraine dyes SQ1, SQ5, and SQS4 were used to evaluate the effect of sensitizing solvents on dye-sensitized solar cell (DSSC) efficiency. A drastic change in DSSC efficiency was observed when the photo-anodes were sensitized in acetonitrile (bad solvent when considering dye solubility) and chloroform (good solvent) with an Iodolyte (I − /I 3 − ) electrolyte. The DSSC device sensitized with squaraine dyes in acetonitrile showed better photovoltaic performance with enhanced photocurrent generation and photovoltage compared to the device sensitized in chloroform. In a good sensitizing solvent, dyes with long hydrophobic alkyl chains are deleterious forming aggregates on the TiO 2 surface, which results in an incident photon-to-current conversion efficiency (IPCE) response mostly from monomeric and dimeric structures. Meanwhile, a bad sensitizing solvent facilitates the formation of well-packed self-assembled structures on the TiO 2 surface, which are responsible for a broad IPCE response and high device efficiencies. The photoanode sensitized in the bad sensitizing solvent showed enhanced V OC values of 642, 675, and 699 mV; J SC values of 6.38, 11.1, and 11.69 mA/cm 2 ; and DSSC device efficiencies of 3.0, 5.63, and 6.13% for the SQ1, SQ5, and SQS4 dyes in the absence of a coadsorbent (chenodeoxycholic acid (CDCA)), respectively, which were further enhanced by CDCA addition. Meanwhile, the photoanode sensitized in the good sensitizing solvent showed relatively low photovoltaic V OC values of 640, 652, and 650 mV; J SC values of 5.78, 6.79, and 6.24 mA/cm 2 ; and device efficiencies of 2.73, 3.35, and 3.20% for SQ1, SQ5, and SQS4 in the absence of CDCA, respectively, which were further varied with equivalents of CDCA. The best DSSC device efficiencies of 6.13 and 3.20% were obtained for SQS4 without CDCA, where the dye was sensitized in acetonitrile (bad) and chloroform (good) sensitizing solvents, respectively.

Visible-Light-Active Unsymmetrical Squaraine Dyes with Pyridyl Anchoring Groups for Dye-Sensitized Solar Cells

“…The dye SD1 exhibited a V OC of 0.706 V, a J SC of 3.20 mA/cm 2 , and a device efficiency (η) of 1.69%, whereas SD2 showed a device efficiency (η) of 0.17% ( V OC 0.606 V; J SC 0.45 mA/cm 2 ) (Table ). This is due to the good solvation of the alkyl groups around dyes on the TiO 2 in CHCl 3 , which leads to a lowered dye loading compared to the sensitization process in MeCN. , Further, with similar steric features, the carboxylic acid-containing dye SD3 showed the highest device efficiency (η) of 7.20% ( V OC 0.808 V; J SC 12.29 mA/cm 2 ) in MeCN (Table ) as compared to SD1 and SD2 . This is due to the combined effects of the anchoring group and long alkyl chains present in SD3 .…”

Effect of hydrophilicity-imparting substituents on exciton delocalization in squaraine dye aggregates covalently templated to DNA Holliday junctions

“…The spontaneous aggregation of dyes via non-covalent interaction is sensitive to the characteristics of the solvent, 18 ionic strength, 19 temperature, 20 and polarity. 21 Although many dyes self-assemble in solution under appropriate conditions, [18][19][20][21] the absence of a template makes it difficult to control the geometric arrangement and number of participant dyes in the aggregation.…”

“…The spontaneous aggregation of dyes via non-covalent interaction is sensitive to the characteristics of the solvent, 18 ionic strength, 19 temperature, 20 and polarity. 21 Although many dyes self-assemble in solution under appropriate conditions, [18][19][20][21] the absence of a template makes it difficult to control the geometric arrangement and number of participant dyes in the aggregation. In photosynthetic complexes of plants and bacteria, protein templates enable efficient electronic excitation energy transfer by controlling the geometry of the molecular aggregates and their exciton delocalization.…”

Steric and Electronic Effect in Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells

“…High efficient DSSC devices with the performances over 14–15.2% − by the cosensitization approach have been achieved under 1 Sun condition, and 37% has been achieved under ambient condition. Squaraines belongs to the polymethine dye family where the squaric acid unit is flanked between two aniline or indoline donor units, which possesses a high intense peak in the far-red region with the high molar extinction coefficient (>10 5 M –1 cm –1 ). − These dyes aggregate in solution as well as in bulk, and such properties have been widely utilized in developing metal-ion sensors and optoelectronic applications. , Though the aggregation of dyes on TiO 2 surface is considered to be a detrimental process for the device DSSC efficiency, it helps broadening the light harvesting efficiency (LHE), and hence, controlling the aggregation of dyes became a strategy to enhance the photocurrent generation from the aggregated structures. , Such types of controlled assembly of dyes on TiO 2 can be achieved by introducing a steric feature in the sensitizer by introducing alkyl groups, which resulted in the generation of photocurrent from aggregated structures, , and the hydrophobic alkyl groups passivate the anode and avoid the charge recombination process which improves the open-circuit potential ( V OC ) of the DSSC device. ,, Such aggregated structures can be modulated by the solvents in both OPV and DSSC research . Furthermore, the electronic properties of squaraine chromophores has been modulated significantly by extending the π-conjugation inside the polymethine framework, whereas only noticeable changes have been observed while extending the conjugation outside the polymethine framework. − Furthermore, fine-tuning in energy levels by introducing the various electron-rich and -deficient substituents known for squaraine chromophores that were utilized for field effect transistor, but such type of systematic study is required for the D–A–D type unsymmetrical squaraine dyes in DSSC.…”