Indenoquinaldine‐Based Unsymmetrical Squaraine Dyes for Near‐Infrared Absorption: Investigating the Steric and Electronic Effects in Dye‐Sensitized Solar Cells

Abstract

A series of near-infrared (NIR)-responsive unsymmetrical squaraine dyes (ISQ1-3) incorporating a fused indenoquinaldine-based donor have been designed and synthesized. C alkyl chains were incorporated at the sp -hybridized carbon center of the indene unit of the indenoquinaldine in an out-of-plane orientation to control dye aggregation on the surface of titanium dioxide, and indole (ISQ1), benzo[e]indole (ISQ2), and quinoline (ISQ3) moieties were included as the donor component bearing the anchoring carboxy group to extend the absorption in the NIR region and to systematically study the effect of the electronic modification on the performance of dye-sensitized solar cells (DSSC). All the dyes exhibit intense absorption (ϵ≥10 m cm ) in the NIR region, and the dye-adsorbed TiO films exhibit broad panchromatic absorption. The incident photon-to-current efficiency (IPCE) spectrum of the ISQ3-based DSSC device displays a panchromatic IPCE response up to 880 nm. Additionally, the ISQ3-sensitized device provides the best efficiency of 4.15 % with a short circuit current density (J ) of 10.02 mA cm , open-circuit voltage (V ) of 0.58 V, and fill factor (ff) of 72 % in the presence of 10 equivalents of 3α,7α-dihydroxy-5β-cholanic acid (CDCA). Electrochemical impedance spectroscopy analysis showed attenuated charge recombination in the ISQ3-sensitized DSSC, which contributes to its higher value of V compared with the other dyes.

Dye-sensitized solar cells strike back

“…Squaraine-based dyes have shown some of the deepest NIR photon use in DSC devices known. 309 Squaraines typically absorb intensely in the NIR region often between 600–900 nm with molar absorptivities often above 100 000 M −1 cm −1 ; however, absorption is typically weak in the higher energy spectral region. The literature surrounding this class of materials is expanding dramatically since high performing NIR absorbing chromophores are urgently needed to improve DSC devices.…”

“…Interestingly, a symmetric core bis-quinoline squaraine dye (ISQ3) shows appreciable light harvesting efficiency on TiO 2 reaching 1000 nm, but an IPCE onset near 850 nm. 309 This suggests significant influence of the electrolyte on the dye absorbance energy with quinoline-squaraine based materials.…”

Redox Shuttle-Based Electrolytes for Dye-Sensitized Solar Cells: Comprehensive Guidance, Recent Progress, and Future Perspective

“…An outstanding PCE of over 14% has been achieved by using the [Co(bpy) 3 ] 3+ /[Co(bpy) 3 ] 2+ redox shuttle, however, slow mass transport and large reorganization energy in the counter electrode still limit the V oc value to 1 V. Recently, using a Cu complex, a [Cu(tmby) 2 ] 2+ /[Cu(tmby) 2 ] + redox shuttle, the PCE of DSSC under 1-sun illumination has reported to be 15.2% with a V oc of over 1 V. Although the V oc value was improved significantly by using the [Cu(tmby) 2 ] 2+ /[Cu(tmby) 2 ] + redox shuttle, further improvement of J sc value using this redox shuttle is difficult, due to the competition of the visible light absorption of Cu complexes and the requirement of using a dye with a higher positive HOMO level, which limits the scope of the entire range of visible light utilization. Due to the higher positive redox potential of [Cu(tmby)] 2+ /[Cu(tmby)] + complexes (∼0.87 V vs NHE), most of the developed high-efficiency porphyrin dyes, such as SM315 ( E HOMO ≈ 0.89 V), 105 SGT-021 ( E HOMO ≈ 0.82 V), 4 and XW17 ( E HOMO ≈ 0.68 V), 178 some organic dyes, such as SGT-149 ( E HOMO ≈ 0.84), 9 and near-infrared (NIR) absorbing organic dyes, such as ISQ1 ( E HOMO ≈ 0.62 V) 179 and AP3 ( E HOMO ≈ 0.74 V), 178 cannot be explored with this Cu redox shuttle because of the insufficient driving force for dye regeneration. Therefore, research is needed to find a suitable redox shuttle with a redox potential of 0.45–0.65 V or replacement of a ligand to tune the redox potential of the Cu complexes, so that most of the porphyrin and organic dyes can be utilized.…”

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

“…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. Therefore, to understand the effect of various substitutes in squaraine dye on the DSSC device performance, a series of alkyl-group-wrapped unsymmetrical squaraine dyes with both electron-donating and electron-withdrawing substituents at the nonanchoring indoline donor moiety have been designed, synthesized, and characterized (Figure ).…”