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

Visible-light-active alkyl group-wrapped unsymmetrical squaraine dyes SD1-SD3 were synthesized, featuring an indoline donor and pyridine and carboxylic acid anchoring groups. Their photophysical, electrochemical, and photovoltaic characteristics were examined by fabricating a dye-sensitized solar cell (DSSC) device. Both carboxylic acid and pyridine anchoring groups containing squaraine dyes SD3 and SD2 possess similar photophysical and electrochemical characteristics. However, their photovoltaic performances were completely different. The SD3 dye with the carboxylic acid anchoring group displayed a DSSC device efficiency of 7.20% (VOC 0.81 V; JSC 12.29 mA/cm2) using iodolyte (I-/I3-) electrolyte, compared to SD1 (VOC 0.659 V; JSC 4.97 mA/cm2; and η - 2.34%) and SD2 (VOC 0.629 V; JSC 1.68 mA/cm2; and η - 0.84%), which were featured with pyridyl anchoring groups. These results were attributed to dye loading on the Lewis and Brønsted acidic sites of TiO2 and the importance of aggregated structures for photocurrent generation. In the incident photon-to-current efficiency (IPCE) analysis, SD1 dye-sensitized devices exhibited photocurrent generation from both monomeric and aggregated dyes on the TiO2 surface. In contrast, SD2 showed photocurrent generation solely from aggregated states. Despite the introduction of long alkyl chains to reduce dye aggregation and charge recombination, the results indicated preferential charge injection from only the aggregated SD2 dye on TiO2. Fluorescence-quenching experiments indicated an efficient charge transfer from the aggregated SD2 dye to TiO2 compared to that of the monomeric dye. Cosensitization, a method to enhance the light-harvesting efficiency and photocurrent generation in DSSCs, was explored by simultaneously cosensitizing pyridyl-based dyes (SD1 and SD2) with a blue-colored carboxylic acid-based squaraine dye SD4. IPCE analysis demonstrated that both SD1 and SD4 contributed to generating a photocurrent of 9.11 mA/cm2. The sequential cosensitization of SD1 and SD4 with the coadsorbent CDCA showed the highest performance, with a VOC of 0.663 V, a JSC of 11.43 mA/cm2, and an efficiency (η) of 5.20%.

Synthesis, photophysical and electrochemical properties of pyridine, pyrazine and triazine-based (D-π-)2A fluorescent dyes

The donor-acceptor-π-conjugated (D-π-)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 with two (diphenylamino)carbazole thiophene units as D (electron-donating group)-π (π-conjugated bridge) moiety and a pyridine, pyrazine or triazine ring as electron-withdrawing group (electron-accepting group, A) have been designed and synthesized. The photophysical and electrochemical properties of the three dyes were investigated by photoabsorption and fluorescence spectroscopy, Lippert-Mataga plots, cyclic voltammetry and density functional theory calculations. The photoabsorption maximum (λ_max,abs) and the fluorescence maximum (λ_max,fl) for the intramolecular charge-transfer characteristic band of the (D-π-)₂A fluorescent dyes show bathochromic shifts in the order of OUY-2 < OUK-2 < OUJ-2. Moreover, the photoabsorption bands of the (D-π-)₂A fluorescent dyes are nearly independent of solvent polarity, while the fluorescence bands showed bathochromic shifts with increasing solvent polarity (i.e., positive fluorescence solvatochromism). The Lippert-Mataga plots for OUY-2, OUK-2 and OUJ-2 indicate that the Δμ (= μ_e - μ_g) value, which is the difference in the dipole moment of the dye between the excited (μ_e) and the ground (μ_g) states, increases in the order of OUY-2 < OUK-2 < OUJ-2. Therefore, the fact explains our findings that OUJ-2 shows large bathochromic shifts of the fluorescence maxima in polar solvents, as well as the Stokes shift values of OUJ-2 in polar solvents are much larger than those in nonpolar solvents. The cyclic voltammetry of OUY-2, OUK-2 and OUJ-2 demonstrated that there is little difference in the HOMO energy level among the three dyes, but the LUMO energy levels decrease in the order of OUY-2 > OUK-2 > OUJ-2. Consequently, this work reveals that for the (D-π-)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 the bathochromic shifts of λ_max,abs and λ_max,fl and the lowering of the LUMO energy level are dependent on the electron-withdrawing ability of the azine ring, which increases in the order of OUY-2 < OUK-2 < OUJ-2.

Bi-anchoring Organic Dyes that Contain Benzimidazole Branches for Dye-Sensitized Solar Cells: Effects of π Spacer and Peripheral Donor Groups

Benzimidazole-branched bi-anchoring organic dyes that contained triphenylamine/phenothiazine donors, 2-cyanoacrylic acid acceptors, and various π linkers were synthesized and examined as sensitizers for dye-sensitized solar cells. The structure-activity relationships in these dyes were systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The wavelength of the absorption peak was more-heavily influenced by the nature of the π linker than by the nature of the donor. For a given donor, the absorption maximum (λmax ) was red-shifted on changing the π linker from phenyl to 2,2'-bithiophene, whilst the dyes that contained triphenylamine units displayed higher molar extinction coefficients (ϵ) than their analogous phenothiazine-based triphenylamine dyes, which led to good light-harvesting properties in the triphenylamine-based dyes. Electrochemical data for the dyes indicated that the triphenylamine-based dyes possessed relatively low-lying HOMOs, which could be beneficial for suppressing back electron transfer from the conduction band of TiO2 to the oxidized dyes, owing to facile regeneration of the oxidized dye by the electrolyte. The best performance in the DSSCs was observed for a dye that possessed a triphenylamine donor and 2,2'-bithiophene π linkers. Electron impedance spectroscopy (EIS) studies revealed that the use of triphenylamine as the donor and phenyl or 2,2'-bithiophene as the π linkers was beneficial for disrupting the dark current and charge-recombination kinetics, which led to a long electron lifetime of the injected electrons in the conduction band of TiO2 .

Development of a D–π–A pyrazinium photosensitizer possessing singlet oxygen generation

(D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) bearing a counter anion (X⁻ = Br⁻ or I⁻) have been newly developed as a photosensitizer possessing singlet oxygen (¹O₂) generation.

Development of a functionally separated D–π-A fluorescent dye with a pyrazyl group as an electron-accepting group for dye-sensitized solar cells

A functionally separated D–π-A dye OUK-3 with a pyrazyl group as an electron-accepting group and a carboxyl group as an anchoring group has been newly developed as a photosensitizer for dye-sensitized solar cells.

Synthesis, structure, physical properties and OLED application of pyrazine–triphenylamine fused conjugated compounds

Several donor–acceptor compounds based on pyrazine–triphenylamine fused conjugated molecules have been fabricated in OLED devices. And the maximum EQE of 7.37% have been obtained.

Photoinduced electron injection from an organic dye having a pyridyl anchor to Lewis acid site of TiO2 surface

Adsorbability of dye onto TiO₂ is not a principal factor for a high electron injection probability from dye to TiO₂.

Synthesis, optical, electrochemical and photovoltaic properties of a D–π–A fluorescent dye with triazine ring as electron-withdrawing anchoring group for dye-sensitized solar cells

D–π–A fluorescent dye OUJ-1 with 1,3,5-triazine ring as electron-withdrawing anchoring group has been newly developed and its optical and electrochemical properties, and photovoltaic performance in a dye-sensitized solar cell were investigated.