Development of D–π–A dyes with a pyrazine ring as an electron-withdrawing anchoring group for dye-sensitized solar cells

Abstract

D–π–A and (D–π–)₂A fluorescent dyes (OUK-1 and OUK-2) with a pyrazine ring as an electron-withdrawing anchoring group capable of forming a hydrogen bond or a pyrazinium ion at the Brønsted acid sites on TiO₂ surface have been developed as a new type of D–π–A dye sensitizer for dye-sensitized solar cells.

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Among several photovoltaic systems utilizing the sun as a free and inexhaustible source of energy, TiO₂-based dye-sensitized solar cells (DSSCs) are one of the most promising photovoltaic cells.^1–6 To further improve the photovoltaic performance of DSSCs, several kinds of donor–acceptor π-conjugated (D–π–A) dyes have been developed as dye sensitizers during the last decade because the D–π–A dye sensitizers with both electron-donating (D) and electron-accepting (A) groups linked by π-conjugated bridges, which possess broad and intense absorption spectral features, would be expected to be one of the most promising classes of organic dye sensitizers.^4–8 Most of the D–π–A dye sensitizers have carboxylic acid, cyanoacrylic acid or rhodanine-3-acetic acid moieties, which act as electron acceptors as well as anchoring groups for attachment on a TiO₂ surface. Thus, the conventional D–π–A dye sensitizers were adsorbed on the TiO₂ film through bidentate bridging linkage between a carboxyl group of the dye and the Brønsted acid sites (surface-bound hydroxyl groups, Ti–OH) on TiO₂ surface. On the other hand, a new-type of D–π–A dye sensitizers with a pyridyl group as an electron-withdrawing anchoring group have been recently designed and developed by some research groups.^9,10 We previously reported that a new type of D–π–A dye sensitizers were predominantly adsorbed on the TiO₂ by coordinate bonding between the pyridyl group of the dye and the Lewis acid site (exposed Tiⁿ⁺ cations) on the TiO₂ surface.⁹ It was demonstrated that a new-type of D–π–A dye sensitizers can inject electrons efficiently from the pyridyl group to the conduction band (CB) of the TiO₂ electrode by coordinate bonding rather than the bidentate bridging linkages of the conventional D–π–A dye sensitizers with carboxyl groups. Recently, DSSCs based on D–π–A dye with a pyridyl group reached solar energy-to-electricity conversion yield (η) of 4.02%.^10e Moreover, Goutsolelos reported an η value of 6.12% for DSSC based on porphyrin dye with four pyridyl groups.^10b

In this work, to direct molecular design toward creating efficient D–π–A dye sensitizers with an azine ring as electron-withdrawing anchoring group, we have designed and synthesized D–π–A and (D–π–)₂A fluorescent dyes (OUK-1 and OUK-2) with a pyrazine ring as an electron-withdrawing anchoring group and (diphenylamino)carbazole as a D–π moiety (Scheme 1; see Scheme S1 in ESI† for synthetic procedures). It was found that the dyes OUK-1 and OUK-2 are adsorbed onto the TiO₂ surface by the formation of pyrazinium ion or by hydrogen bonding at the Brønsted acid sites. Here, we reveal the effects of interaction between the pyrazyl group of the dye and TiO₂ surface on the photovoltaic performances of OUK-1 and OUK-2.

Scheme 1 Chemical structures of D–π–A dye sensitizer OUK-1 and (D–π–)₂A dye sensitizer OUK-2.

The absorption and fluorescence spectra of OUK-1 and OUK-2 in 1,4-dioxane are shown in Fig. 1a, and their spectral data are summarized in Table 1. The dyes show an absorption maximum (λ^abs_max) at around 400 nm, which is attributed to the intramolecular charge-transfer (ICT) excitation from electron donor moiety (diphenylamino group) to electron acceptor moiety (pyrazine group). The ICT band for OUK-2 is broadened because of a shoulder at around 420 nm. Molar extinction coefficient (ε) for the ICT band of OUK-2 is 65 600 M⁻¹ cm⁻¹, which is higher than that (ε = 45 400 M⁻¹ cm⁻¹) of OUK-1. This result shows that the (D–π–)₂A dyes with two (diphenylamino)carbazoles can lead to the broadening of ICT band and the enhancement of ε, resulting in the improvement of light-harvesting efficiency (LHE). The fluorescence maxima (λ^fl_max) for OUK-1 and OUK-2 occur at 478 and 486 nm, respectively. The fluorescence quantum yields (Φ_fl) of OUK-1 and OUK-2 are 0.46 and 0.47, respectively. The density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level indicate that for the dyes the HOMOs were mostly localized on the diphenylamine-carbazole moiety containing a thiophene ring, and the LUMOs were mostly localized on the thienylpyrazine moiety (Fig. S1 in ESI†). Accordingly, the DFT calculations revealed that dye excitations upon light irradiation induced a strong ICT from the diphenylamine-carbazole moiety to the pyrazine ring.

Fig. 1 (a) Absorption (–) and fluorescence (⋯) spectra of OUK-1 and OUK-2 in 1,4-dioxane and (b) absorption spectra of OUK-1 and OUK-2 adsorbed on TiO₂ film (9 μm).

Table 1 Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance parameters of OUK-1 and OUK-2

Dye	λ^absmax/nm (ε^a/M^−1 cm^−1)	λ^flmax/nm (Φf)^b	E^ox1/2^c/V	HOMO^d/V	LUMO^d/V	Molecules^e cm^−2	Jsc^f/mA cm^−2	Voc^f/mV	FF^f	η^f (%)
a In 1,4-dioxane.b In 1,4-dioxane. Fluorescence quantum yields (Φf) were determined using a calibrated integrating sphere system (λex = 402 and 397 nm for OUK-1 and OUK-2, respectively).c Half-wave potentials for oxidation (E^ox1/2) vs. Fc/Fc^+ were recorded in DMF/Bu4NClO4 (0.1 M) solution.d vs. Normal hydrogen electrode (NHE).e The dye-coated film was immersed in a mixed solvent of THF–DMSO–NaOH aq. 1 M (5 : 4 : 1), which was used to determine the amount of dye molecules adsorbed onto the film by measuring the absorbance. Adsorption amount per unit area of TiO2 film was controlled by concentration of dye solution in THF.f Under a simulated solar light (AM 1.5, 100 mW cm^−2).g Under the adsorption condition of 0.1 mM dye solution in THF.h Under the adsorption condition of 1.0 mM dye solution in THF.
OUK-1	402 (45 400)	478 (0.46)	0.39	1.11	−1.68	0.8 × 10^16^g	0.84^g	416^g	0.62^g	0.22^g
						3.0 × 10^16^h	2.99^h	448^h	0.67^h	0.89^h
OUK-2	397 (65 600)	486 (0.47)	0.39	1.11	−1.60	1.2 × 10^16^g	3.49^g	508^g	0.57^g	1.01^g
						3.3 × 10^16^h	4.14^h	524^h	0.58^h	1.26^h

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The absorption band of OUK-2 adsorbed on the TiO₂ film is broadened because of a shoulder at around 450 nm (Fig. 1b) compared with that of OUK-1. However, the absorption peak wavelengths of OUK-1 and OUK-2 adsorbed on TiO₂ are similar to those in 1,4-dioxane. Thus, this result indicates that the dyes form weak π-stacked aggregates on TiO₂ surface.

To elucidate the adsorption states of the dyes on TiO₂ nanoparticles, we measured the FTIR spectra of the dye powders and the dyes adsorbed on TiO₂ nanoparticles (Fig. 2). For the powders of the two dyes the C=N stretching band of pyrazine ring was clearly observed at around 1490 cm⁻¹. When the two dyes were adsorbed on the TiO₂ surface, where the amount of the dye adsorbed on TiO₂ electrode is low (0.8 × 10¹⁶ molecules per cm² for OUK-1 and 1.2 × 10¹⁶ molecules per cm² for OUK-2), the band at 1490 cm⁻¹ disappeared completely and a new band appeared at around 1650 cm⁻¹, indicating the formation of a pyrazinium ion¹¹ with Brønsted acid sites on TiO₂ surface (see Fig. S3 in ESI† for FTIR spectra of 2,6-dimethylpyrazine, 2,6-dimethylpyrazium chloride and 2,6-dimethylpyrazine adsorbed on TiO₂ nanoparticles).¹¹ More interestingly, when the amount of the dye adsorbed on TiO₂ electrode is high (3.0 × 10¹⁶ molecules per cm² for OUK-1 and 3.3 × 10¹⁶ molecules per cm² for OUK-2), the C=N stretching band at 1490 cm⁻¹ is shifted by 6 and 2 cm⁻¹ for OUK-1 and OUK-2, respectively, to a higher wavenumber, i.e., the band can be assigned to the hydrogen-bonded pyrazyl group to the Brønsted acid sites on the TiO₂ surface. These observations indicate that at the initial stages of dye adsorption both the dyes OUK-1 and OUK-2 are predominantly adsorbed on TiO₂ surface by the formation of a pyrazinium ion with Brønsted acid sites. However, both the dyes are adsorbed on the TiO₂ surface through hydrogen bonding at Brønsted acid sites as the dye adsorption progresses; however, dye molecules adsorbed on the TiO₂ surface through the formation of pyrazinium ion with Brønsted acid sites still exist.

Fig. 2 FTIR spectra of the powders and the dyes (0.8 × 10¹⁶ and 3.0 × 10¹⁶ molecules per cm² for OUK-1 and 1.2 × 10¹⁶ and 3.3 × 10¹⁶ molecules per cm² for OUK-2) adsorbed on TiO₂ nanoparticles for (a) OUK-1 and (b) OUK-2.

The electrochemical properties of OUK-1 and OUK-2 were determined by cyclic voltammetry (Fig. S2 and Table S1 in ESI†). The reversible oxidation waves for the two dyes were observed at 0.42 V vs. ferrocene/ferrocenium (Fc/Fc⁺). The corresponding reduction waves for the two dyes appeared at 0.35 V, showing that the oxidized states of the two dyes were stable. The HOMO energy level was evaluated from the half-wave potential for oxidation (E^ox_1/2 = 0.39 V for the two dyes). The HOMO energy level was 1.11 V vs. normal hydrogen electrode (NHE), thus indicating that the HOMO energy level is more positive than the I₃⁻/I⁻ redox potential (0.4 V). This assures the efficient regeneration of the oxidized dyes by electron transfer from the I₃⁻/I⁻ in the electrolyte. The LUMO energy level was estimated from the E^ox_1/2 and an intersection of absorption and fluorescence spectra (445 nm; 2.79 eV for OUK-1 and 458 nm; 2.71 eV for OUK-2). The LUMO energy levels for OUK-1 and OUK-2 were −1.68 and −1.60 V, respectively, which are higher than the energy level of the CB of TiO₂ (−0.5 V), thus the two dyes can efficiently inject electrons to the TiO₂ electrode.

The DSSC was prepared using the dye-adsorbed TiO₂ electrode (9 μm), Pt-coated glass as a counter electrode, and an acetonitrile solution with iodine (0.05 M), lithium iodide (0.1 M), and 1,2-dimethyl-3-propylimidazolium iodide (0.6 M) as an electrolyte. The photocurrent–voltage (I–V) characteristics were measured under simulated solar light (AM 1.5, 100 mW cm⁻²). When the amount of the dye adsorbed on TiO₂ electrode is low (0.8 × 10¹⁶ molecules per cm² for OUK-1 and 1.2 × 10¹⁶ molecules per cm² for OUK-2), the I–V curves show that the short-circuit photocurrent density (J_sc) and η of OUK-2 (3.49 mA cm⁻² and 1.01%) are larger than that of OUK-1 (0.84 mA cm⁻² and 0.22%) (Fig. 3a). At low adsorption of the dye, the binding mode of the dye on TiO₂ surface is the formation of pyrazinium ion with Brønsted acid sites. The incident photon-to-current conversion efficiency (IPCE) spectrum of OUK-2 is broadened when compared with that of OUK-1 (Fig. 3b), which is in good agreement with the absorption spectra of the dyes adsorbed on TiO₂ film. The maximum IPCE value of OUK-2 is 46% at 420 nm, which is higher than that (11% at 425 nm) of OUK-1. Consequently, the (D–π–)₂A dye OUK-2 possessing a high LHE because of broad and intense absorption spectral features can lead to a relatively high IPCE and J_sc values when compared with those of D–π–A dyes OUK-1. On the other hand, when the adsorption amount of the dye adsorbed on TiO₂ electrode is high (3.0 × 10¹⁶ molecules per cm² for OUK-1 and 3.3 × 10¹⁶ molecules per cm² for OUK-2), the J_sc (4.14 mA cm⁻²), η values (1.26%) and maximum IPCE value (50% at 420 nm) for OUK-2 are moderately increased compared with those of low adsorption amount of the dye. At this high adsorption amount of the dye, the binding mode of dye on TiO₂ surface is the formation of hydrogen bonds with Brønsted acid sites. For OUK-1 the J_sc (2.99 mA cm⁻²), η values (0.89%) and maximum IPCE value (38% at 420 nm) are ca. 4 times as high as those at the low adsorption amount of the dye because of the enhancement of LHE with increasing dye loading on TiO₂ electrode. Thus, this result reveals that the formation of a pyrazinium ion between the pyrazyl group of the dye and the Brønsted acid sites exhibit relatively efficient electron injection from the dye to the CB of TiO₂ when compared with the formation of hydrogen bonds between the pyrazyl group of dye and the Brønsted acid sites. Moreover, it is worth mentioning that the open-circuit photovoltage (V_oc) values of OUK-2 (524 mV) are higher than that of OUK-1 (448 mV). Thus, electrochemical impedance spectroscopy (EIS) analysis was performed to study the electron recombination process in DSSCs based on these two dyes, where the adsorption amount of the dyes adsorbed on TiO₂ electrode is ca. 3.0 × 10¹⁶ molecules per cm² for the two dyes. The Nyquist plots (Fig. 4a) show that the resistance value for the large semicircle of OUK-2 (36 Ω) is larger than that of OUK-1 (26 Ω), indicating that the electron recombination resistance of OUK-2 is higher than that of OUK-1. The electron recombination lifetimes (τ_e) expressing the electron recombination between the injected electrons in TiO₂ and I₃⁻ ions in the electrolyte, which was extracted from the angular frequency (ω_rec) at the midfrequency peak in the Bode phase plot (Fig. 4b) using τ_e = 1/ω_rec, are 10 ms for OUK-1 and 13 ms for OUK-2, respectively, which is consistent with the V_oc values in the DSSCs. Consequently, the dye OUK-2 with sterically-hindered (D–π–)₂A structure can efficiently suppress the charge recombination between the injected electrons in the CB of TiO₂ and I₃⁻ ions in the electrolyte, arising because of the approach of I₃⁻ ions to the TiO₂ surface, which is responsible for the higher V_oc value of OUK-2.⁴

Fig. 3 (a) I–V curves and (b) IPCE spectra and of DSSCs based on OUK-1 and OUK-2. The adsorption amount of the dye adsorbed on TiO₂ electrode is 0.8 × 10¹⁶ and 3.0 × 10¹⁶ molecules per cm² for OUK-1 and 1.2 × 10¹⁶ and 3.3 × 10¹⁶ molecules per cm² for OUK-2, respectively.

Fig. 4 (a) Nyquist plots and (b) Bode phase plots of DSSCs based on OUK-1 and OUK-2. The adsorption amount of the dye adsorbed on TiO₂ electrode is ca. 3.0 × 10¹⁶ molecules per cm².

In conclusion, it was found that the D–π–A dye OUK-1 and the (D–π–)₂A dye OUK-2 with a pyrazine ring as electron-withdrawing anchoring group are adsorbed on the TiO₂ surface by the formation of pyrazinium ion or hydrogen bonds at the Brønsted acid sites of TiO₂. Therefore, this work revealed that D–π–A dye sensitizers with azine rings can adsorb onto the TiO₂ surface through the formation of hydrogen bonds, pyrazinium ions or coordinate bonding. This work also indicates that the formation of a pyrazinium ion between the pyrazyl group of dye and the Brønsted acid sites on the TiO₂ surface exhibit relatively efficient electron injection from the dye to the CB of TiO₂ when compared with the formation of hydrogen bonds between the pyrazyl group of the dye and the Brønsted acid sites on the TiO₂ surface.

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) (24102005 and 24550225) and by Takahashi Industrial and Economic Research Foundation.

Notes and references

1. B. O'Regan and M. Grätzel, Nature, 1991, 353, 737.
2. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem. Rev., 2010, 110, 6595.
3. (a) T. Bessho, S. M. Zakeeruddin, C.-Y. Yeh, E. W.-G. Diau and M. Grätzel, Angew. Chem., Int. Ed., 2010, 49, 6646; (b) A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin and M. Grätzel, Science, 2011, 334, 629; (c) L.-L. Li and E. W.-G. Diau, Chem. Soc. Rev., 2013, 42, 291; (d) S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, Md. K. Nazeeruddin and M. Grätzel, Nat. Chem., 2014, 6, 242; (e) A. Yella, C.-L. Mai, S. M. Zakeeruddin, S.-N. Chang, C.-H. Hsieh, C.-Y. Yeh and M. Grätzel, Angew. Chem., Int. Ed., 2014, 53, 2973.
4. (a) Z. Ning and H. Tian, Chem. Commun., 2009, 5483; (b) Z. Ning, Y. Fu and H. Tian, Energy Environ. Sci., 2010, 3, 1170.
5. A. Mishra, M. K. R. Fischer and P. Bäuerle, Angew. Chem., Int. Ed., 2009, 48, 2474.
6. (a) Y. Ooyama and Y. Harima, Eur. J. Org. Chem., 2009, 18, 2903; (b) Y. Ooyama and Y. Harima, ChemPhysChem, 2012, 13, 4032.
7. K. Ladomenou, T. N. Kitsopoulos, G. D. Sharma and A. G. Coutsolelos, RSC Adv., 2014, 4, 21379.
8. (a) X. Wang, J. Yang, H. Yu, F. Li, L. Fan, W. Sun, Y. Liu, Z. Y. Koh, J. Pan, W.-L. Yim, L. yan and Q. Wang, Chem. Commun., 2014, 50, 3965; (b) S.-G. Li, K.-J. Jiang, J.-H. Huang, L.-M. Yang and Y.-L. Song, Chem. Commun., 2014, 50, 4309; (c) D. K. Panda, F. S. Goodson, S. Ray and S. Saha, Chem. Commun., 2014, 50, 5358; (d) K. Kakiage, Y. Aoyama, T. Yano, T. Otsuka, T. Kyomen, M. Unno and M. Hanaya, Chem. Commun., 2014, 50, 6379; (e) A. Amacher, C. Yi, J. yang, M. P. Bircher, Y. Fu, M. cascella, M. Grätzel, S. Decurtins and S.-X. Liu, Chem. Commun., 2014, 50, 6540; (f) J. Yang, P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi, P. Comte, K. Pei, T. W. Holcombe, M. K. Nazeeruddin, J. Hua, S. K. Zakeeruddin, H. Tian and M. Grätzel, J. Am. Chem. Soc., 2014, 136, 5772.
9. (a) Y. Ooyama, S. Inoue, T. Nagano, K. Kushimoto, J. Ohshita, I. Imae, K. Komaguchi and Y. Harima, Angew. Chem., Int. Ed., 2011, 50, 7429; (b) Y. Ooyama, T. Nagano, S. Inoue, I. Imae, K. Komaguchi, J. Ohshita and Y. Harima, Chem.–Eur. J., 2011, 17, 14837; (c) Y. Ooyama, N. Yamaguchi, I. Imae, K. Komaguchi, J. Ohshita and Y. Harima, Chem. Commun., 2013, 49, 2548; (d) Y. Ooyama, Y. Hagiwara, T. Mizumo, Y. Harima and J. Ohshita, New J. Chem., 2013, 37, 2479; (e) Y. Ooyama, T. Sato, Y. Harima and J. Ohshita, J. Mater. Chem. A, 2014, 2, 3293.
10. (a) M.-D. Zhang, H.-X. Xie, X.-H. Ju, L. Qin, Q.-X. Yang, H.-G. Zheng and X.-F. Zhou, Phys. Chem. Chem. Phys., 2013, 15, 634; (b) D. Daphnomili, G. Landrou, P. Singh, A. Thomas, K. Yesudas, B. K. G. D. Sharma and A. G. Goutsolelos, RSC Adv., 2012, 2, 12899; (c) L. Wang, X. yang, S. Li, M. Cheng and L. Sun, RSC Adv., 2014, 4, 13677; (d) T. Sakurada, Y. Arai and H. Segawa, RSC Adv., 2014, 4, 13201; (e) J. Mao, D. Wang, S.-H. Liu, Y. Hang, Y. Xu, Q. Zhang, W. Wu, P.-T. Chou and J. Hua, Asian J. Org. Chem., 2014, 3, 153.
11. (a) T. J. Dines, L. D. MacGregor and C. H. Rochester, Phys. Chem. Chem. Phys., 2001, 3, 2676; (b) M. I. Zaki, M. A. Hasan, F. A. Al-Sagheer and L. Pasupulety, Colloids Surf., A, 2001, 190, 261; (c) O. Kasende and T. Zeegers-Huyskens, J. Phys. Chem., 1984, 88, 2132; (d) H. Takahashi, K. Mamola and E. K. Plyler, J. Mol. Spectrosc., 1966, 21, 217; (e) M. A. Montańez, I. L. Tocón, J. C. Otero and J. I. Marcos, J. Mol. Struct., 1999, 482–483, 201.

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental procedures, synthesis and characterization of compound. See DOI: 10.1039/c4ra03999h

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