Aggregation‐Free Organic Dyes Featuring Spiro[dibenzo[3,4:6,7]cyclohepta[1,2‐b]quinoxaline‐10,9′‐fluorene] (SDBQX) for Dye‐Sensitized Solar Cells

Abstract Three novel organic dyes coded as FHD4‐1, FHD4‐2, and FHD4‐3 featuring spiro[dibenzo[3,4:6,7]cyclohepta[1,2‐b]quinoxaline‐10,9′‐fluorene] (SDBQX) moieties are designed to inhibit dye aggregation to improve the performance of dye‐sensitized solar cells (DSSCs). The consistent absorption onsets of FHD4‐1, FHD4‐2, and FHD4‐3 in solutions and adsorbed on TiO2 films indicate that these dyes are aggregation‐free dyes. Therefore, coadsorption with chenodeoxycholic acid (CDCA) of these three dyes reduces the performance of DSSCs because no inhibition effect for dye aggregation is needed, but, on the contrary, the dye loading amount is reduced after addition of CDCA.

3a-3c and cyanoacetic acid to afford FHD4-1, FHD4-2, and FHD4-3, respectively. FHD4-1, FHD4-2, and FHD4-3 were characterized with 1 H NMR, 13 C NMR, and HRMS. Figure 1 shows the UV-vis absorption spectra of FHD4-1, FHD4-2, and FHD4-3 in CH 2 Cl 2 solutions (2 × 10 −5 m) and adsorbed on TiO 2 films. The relevant photophysical data are summarized in Table 1. In the solution absorption spectra, all of the three dyes exhibited two prominent peaks at around 410-430 and 490-550 nm. The intense absorption bands in short wavelength region could be assigned to the π-π* transition. [21] The absorption peaks in long wavelength region corresponding to the intramolecular charge transfer (ICT) from the electron donor to the electron acceptor were observed at 531, 545, and 490 nm for FHD4-1, FHD4-2, and FHD4-3, respectively. [21] Broader spectral coverages were observed for FHD4-1 and FHD4-2 compared with FHD4-3. Similar molar extinction coefficients (ε) were observed for these three dyes, the high ε values guaranteed their good light harvesting capabilities. [22] Based on the molecular exciton theory, dye aggregation would lead to a shift of absorption spectrum. [20] As for FHD4, an obvious bathochromic shift was observed for absorption spectrum on TiO 2 film compared with in solution, which suggested that the existence of J-aggregation for FHD4 molecules on the surface of TiO 2 . [21] The absorption onsets of FHD4-1, FHD4-2, and FHD4-3 in solutions and adsorbed on TiO 2 films had good consistency and no apparent shifts were observed. It demonstrates that dye aggregation could be suppressed efficiently by adopting donors with long alkyl chains, and hence FHD4-1, FHD4-2, and FHD4-3 are confirmed to be aggregation free dyes and their well performance in DSSC could be expected due to aggregation is a key adverse factor for DSSC performance.
Global Challenges 2019, 3,1900034  The redox potentials of FHD4-1, FHD4-2, and FHD4-3 were determined by cyclic voltammetry to evaluate the feasibilities of the electron injection and dye regeneration. The cyclic voltammograms are shown in Figure 2a and the corresponding data are summarized in Table 1. The first oxidation potentials (E ox ) of dyes FHD4-1 (0.86 V), FHD4-2 (0.97 V), and FHD4-3 (1.37 V) were sufficiently more positive than the I 3 − /I − redox potential (0.4 V vs normal hydrogen electrode, NHE), so efficient dye regeneration of the oxidized dyes by the I 3 − /I − electrolyte could be expected. [23] The energy band diagrams of the dyes were showed in Figure 2b. The optic bandgap energies (E 0-0 ) of FHD4-1 (1.76 V) and FHD4-2 (1.78 V) were much lower than that of FHD4-3 (2.04 V), resulting in broader adsorption spectra for FHD4 and FHD5, which is beneficial for their light harvesting capabilities. E red can be calculated from E ox -E 0-0 . The E red of dyes FHD4-1 (−0.90 V), FHD4-2 (−0.81 V), and FHD4-3 (−0.67 V) were negative than the conduction band (CB) edge of TiO 2 (−0.5 V vs NHE). The potential difference between these dyes and the CB edge of TiO 2 are getting smaller in the sequence of FHD4-1, FHD4-2, and FHD4-3. The latter one might not enough to guarantee the thermodynamic feasibility of charge injections from excited dye molecules to the CB of TiO 2 , which might restrict the photovoltaic performances for DSSCs. [24] The optimized geometrical structures and electron distributions of FHD4-1, FHD4-2, and FHD4-3 were simulated by density functional theory (DFT) calculations with the B3LYP exchange correlation functional under the 6-31G (d,p) basis set implemented in the Gaussian 09 program. The simulated electron distributions in highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the dyes are shown in Figure 3, while the isodensity surface values were fixed at 0.02. As shown in their molecular orbital profiles, the electrons mainly distributed over the triphenylamine moiety with a little contribution over the quinoxaline core for the HOMO of FHD4-1, and distributed over the whole molecules for the HOMOs of FHD4-2 and FHD4-3. For the LUMOs, the electrons located on the quinoxaline core and electron acceptors (furanylacrylic acid moiety) for all of these three dyes. The bandgaps between HOMOs and LUMOs according to DFT calculations (2.14, 2.42, and 2.63 V for FHD4-1, FHD4-2, and FHD4-3, respectively) exhibited consistent trend with the E 0-0 values obtained from onsets of the absorption spectra. Figure 4 shows the calculated dihedral angles between quinoxaline of SDBQX moieties and the aromatic rings connected with quinoxalines in the optimized structures of FHD4-1, FHD4-2, and FHD4-3. It is clear that the dihedral angles between quinoxaline moieties and electron donating groups are very similar (39.6°, 38.5°, and 41.2° for FHD4-1, FHD4-2, and FHD4-3, respectively), and the dihedral angles between quinoxaline moieties and furan rings are almost same (6.3°, 6.4°, and 6.8° for FHD4-1, FHD4-2, and FHD4-3, respectively). Based on the dihedral angle data, quinoxaline moieties showed an approximate coplanar geometry with the furan rings for FHD4-1, FHD4-2, and FHD4-3. The big dihedral angles between quinoxaline moieties and electron donating groups is beneficial to ICT, and hence enlarging the absorption spectra range, which could enhance the light harvesting capabilities of these dyes. [25,26] The photovoltaic performances of the DSSCs based on FHD4-1, FHD4-2, and FHD4-3 with or without chenodeoxycholic acid (CDCA) were evaluated under illumination simulated AM 1.5G irradiation (100 mW cm −2 ).    FHD4-1, FHD4-2, and FHD4-3.

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Global Challenges 2019, 3,1900034 further study the J sc of the DSSCs based on FHD4-1, FHD4-2, and FHD4-3. As shown in Figure 5b, the devices based on these three dyes exhibited broad spectral responses, which were well consistent with the absorption spectra on TiO 2 films (Figure 1b). Low response intensity for FHD4-1 and narrow response range for FHD4-3 resulting in there lower J sc values compared with FHD4-2.
Typically, the addition of CDCA would improve the power conversion efficiency because it could obstruct the dye aggregation on TiO 2 film. [27] Similar effects were observed for FHD4, FHD5, and FHD6 in our previous work. [21] However, after coadsorption with CDCA, lower efficiencies were observed for analogues FHD4-1, FHD4-2, and FHD4-3 (4.17%, 5.08%, and 4.34%, respectively) featuring with long alkyl chains mainly due to the reduction of J sc values. As we discussed above, FHD4-1, FHD4-2, and FHD4-3 are aggregation free dyes which means no auxiliary additive such as CDCA is necessary to prevent the dye aggregation. Moreover, the addition of CDCA could reduce the chance of dye adsorption, [28] so lower J sc values could be expected which is well coincident with our study. It could also explain the lower response intensity of IPCE for the DSSCs coadsorbed with CDCA. To examine this guess, dye loading amount (Γ) on TiO 2 of FHD4-1, FHD4-2, and FHD4-3 with or without CDCA were measured. As shown in Table 2, the Γ values of FHD4-1, FHD4-2, and FHD4-3 decreased from 1.41 × 10 −7 , 1.31 × 10 −7 , 1.79 × 10 −7 m cm −2 (no CDCA) to 1.13 × 10 −7 , 1.08 × 10 −7 , 1.32 × 10 −7 m cm −2 (with 3 × 10 −3 m CDCA), respectively. It is clear that dye loading amount reduced after coadsorption with CDCA for all of these three dyes as we expected above.

Supporting Information
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