Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes

Abstract

In dye-sensitized solar cells co-photosensitized with an alkoxysilyl-anchor dye ADEKA-1 and a carboxy-anchor organic dye LEG4, LEG4 was revealed to work collaboratively by enhancing the electron injection from the light-excited dyes to the TiO₂ electrodes, and the cells exhibited a high conversion efficiency of over 14% under one sun illumination.

---

Dye-sensitized solar cells (DSSCs), which are composed of mesoporous nanocrystalline-TiO₂ thin layers modified with photosensitizing dyes as working electrodes, redox electrolytes and counter electrodes, have been actively investigated as photovoltaic devices in the next generation of alternatives to conventional silicon-based inorganic solar cells (Fig. S1, ESI†), because of their potentially low production costs, shorter energy and CO₂ payback times, low toxicity of the constituent elements and relatively high light-to-electric energy conversion efficiencies (η) especially under low-light intensities and scattered light conditions.^1–4 In DSSCs, η values of 11–13% under the simulated sunlight of one sun have been reported up to now through photosensitization using polypyridyl and porphyrin complexes of metals such as ruthenium or zinc, and a few metal-free organic dyes with carboxy-anchor moieties for binding to the surface of the TiO₂.^1–9

Organosilicon compounds such as silanols and alkoxysilanes have high metal-oxide surface bonding abilities by forming strong Si–O–metal bonds. While paying attention to the characteristics of silanols and alkoxysilanes, we have focused on the development of photosensitizing dyes for DSSCs possessing silyl-anchor moieties,^10–12 and recently we succeeded in achieving over 12% conversion efficiency in cells using a carbazole/alkyl-functionalized oligothiophene/alkoxysilyl-anchor moiety type compound, ADEKA-1 (Fig. 1a), as the photosensitizer.¹³ Besides the high photovoltaic performance, the TiO₂ photoelectrode sensitized with ADEKA-1 possesses much higher durability to solvents, e.g. nitrile, water and mixtures of them, and to surface modification using wet processes than those sensitized with carboxy-anchor dyes. The durability of the photoelectrode allows the co-adsorption of another sensitizing dye to the electrode for production of a co-sensitization effect, and actually we succeeded in improving the η to 12.8% by means of the co-sensitization of ADEKA-1 and a silyl-anchor coumarin dye SFD-5.¹⁴

Fig. 1 Molecular structures of photosensitizing dyes: (a) silyl-anchor dye ADEKA-1 and (b) carboxy-anchor dye LEG4.

For further improvement in the efficiency of ADEKA-1-sensitized DSSCs, we expanded the study of co-sensitizers for the cells to widely developed carboxy-anchor dyes, which have been demonstrated to possess high sensitizing properties as photosensitizers in DSSCs. In the investigation we found that the ADEKA-1-sensitized cells with the co-sensitizer LEG4¹⁵ (Fig. 1b) exhibited a considerably higher photovoltaic performance through collaborative sensitization by the dyes, and succeeded in achieving over 14% conversion efficiency in the cells. The efficiency shows the high potential of DSSCs to be practical light-to-electric energy conversion devices in the near future.

As co-sensitizers for the ADEKA-1-sensitized DSSCs, we selected carboxy-anchor organic sensitizing dyes which have been reported to have high sensitizing properties and an absorption band in the shorter wavelength region than ADEKA-1, i.e.LEG4, D35, L0 and D131 (Fig. S2–S6 and Table S1, ESI†). To check the potential of these dyes as co-sensitizers for ADEKA-1, we fabricated cells sensitized by ADEKA-1 and by ADEKA-1 with the dyes using an electrolyte solution containing an I₃⁻/I⁻ redox mediator (cell-A; the fabrication procedures of the cells are described in the ESI†). Among the cells, a significant and the largest improvement in the incident monochromatic photon-to-current conversion efficiency (IPCE) was observed in the cell photosensitized by ADEKA-1 with LEG4 (Fig. 2a and Fig. S7 in the ESI†). The cell exhibited much higher IPCE values of close to 90% compared to the cells sensitized only by ADEKA-1 and only by LEG4 in all of the visible region. The increase of the open-circuit photovoltage (V_oc) and the short-circuit photocurrent density (J_sc) compared to those of the cell sensitized only by ADEKA-1 resulted in the improvement of the η by a factor of 1.3 under simulated sunlight at one sun (AM-1.5G, 100 mW cm⁻²; Table S2, ESI†).

Fig. 2 (a) IPCE spectra of the cells photosensitized by ADEKA-1, by LEG4 and by ADEKA-1 with LEG4 (cell-A) and (b) a schematic drawing of the charge separation processes for the TiO₂ electrode sensitized collaboratively by ADEKA-1 and LEG4.

In the cell photosensitized by ADEKA-1 with LEG4, in which the relative amount of the dyes adsorbed on the TiO₂ electrode was estimated to be 1.0 : 0.25 for ADEKA-1 : LEG4, the improvement of the IPCE values compared to those of the ADEKA-1-sensitized cell was observed not only in the light-absorption wavelength region of LEG4 but also in the longer wavelength region where the light absorption by LEG4 was absent (Fig. S8, ESI†), which is different to the other co-sensitized cells. In order to clarify the origin of the peculiar and large improvement in the IPCE by the co-sensitization with LEG4, we examined MO calculations of the dyes (Fig. S9 and S10, and Tables S3 and S4 in the ESI†).

The light-to-electric energy conversion in DSSCs proceeds by light excitation of the sensitizing dye followed by charge separation produced through electron injection from the LUMO of the light-excited dye to the conduction band of TiO₂. In ADEKA-1 the alkoxysilyl-anchor moiety links to the chromophore (carbazole/alkyl-functionalized oligothiophene moiety) via the phenyl-amide moiety, and the LUMO has a small electron distribution around the silyl-anchor moiety. On the other hand, the LUMO of LEG4 has a large electron distribution around the carboxy-anchor moiety and thus LEG4 is expected to have a higher electron injection efficiency from the LUMO to the TiO₂ conduction band than ADEKA-1 (Fig. S9, ESI†). When comparing the energy levels of the LUMOs of the dyes, only LEG4 has a lower LUMO than ADEKA-1 which is different to the other dyes, D35, L0 and D131 (Fig. S6, ESI†), and emission analyses using an Al₂O₃ porous film modified by ADEKA-1 with LEG4 showed that the emission from ADEKA-1 was quenched almost completely by the existence of LEG4 as the co-adsorbent (Fig. S11, ESI†). From the MO properties and the results of the emission analyses, the large improvement of the photovoltaic performance in the cell photosensitized by ADEKA-1 with LEG4 is considered to be brought about by the collaborative sensitization by the dyes through an electron-injection enhancement effect due to the existence of the LEG4 molecules near the ADEKA-1 molecules on the TiO₂ electrode; the electron transfers from the light-excited ADEKA-1 to the co-adsorbent LEG4 and immediate electron injection occurs from LEG4 to the conduction band of the TiO₂ with much higher efficiency than the direct electron injection from the light-excited ADEKA-1 (Fig. 2b). Internal quantum efficiency (IQE) measurements revealed a considerably higher electron injection efficiency in the cell photosensitized by ADEKA-1 with LEG4, and the maximum IQE was evaluated to be 99 ± 2% (Fig. S12, ESI†). The increase of the V_oc (Table S2, ESI†), the decrease of the dark-current (Fig. S13, ESI†) and the elongation of the electron lifetime in the TiO₂ conduction band estimated from the transient open-circuit voltage decays (Fig. S14, ESI†) observed in the co-sensitization with LEG4 indicate that the adsorbed LEG4 on the TiO₂ electrode also works as a suppressor, preventing back electron transfer from the TiO₂ electrode to the electrolyte by covering the naked surface of the TiO₂ electrode with its plural alkyl-chain substituents.^15–18 By using an I₃⁻/I⁻ redox electrolyte solution with an experimentally optimized composition, the cell photosensitized collaboratively by ADEKA-1 and LEG4 (cell-B; the fabrication procedures of the cell are described in the ESI†) exhibited the η of 11.2% under AM-1.5G one sun illumination (entry 1 in Table 1).

Table 1 Photovoltaic parameters of the cells sensitized collaboratively by ADEKA-1 and LEG4 (cell-B) under the illuminations of the simulated sunlight (AM-1.5G)

Entry	Electrolyte:redox^a	Counter electrode	Light intensity (mW cm^−2)	J sc (mA cm^−2)	V oc (V)	FF	η (%)
a Electrolyte: (A) 0.07 M I2, 0.05 M LiI, 0.05 M NaI, 0.50 M DMPImI, 0.10 M EMImI, 0.05 M TBAI, 0.05 M THAI, 0.40 M TBP, 0.10 M MP, and 0.10 M GuSCN in MeCN/VN/THF (8 : 1 : 1 in volume); (F) 0.20 M [Co^2+(phen)3](PF6^−)2, 0.05 M [Co^3+(phen)3](PF6^−)3, 0.07 M LiClO4, 0.02 M NaClO4, 0.03 M TBAPF, 0.01 M TBPPF, 0.01 M HMImPF, 0.30 M TBP, 0.10 M TMSP, 0.10 M MP, 0.05 M CPrBP, 0.10 M CPeBP, and 0.05 M COcBP in MeCN. The data for the cells with other electrolytes (B–E) are listed in Table S6 (ESI). b The values are the averages of the results of the four cells which were prepared separately (Table S5, ESI).
1	A:I3^−/I^−	FTO/Pt	100	19.11	0.783	0.748	11.2
2	F:[Co(phen)3]^3+/2+	FTO/Pt	100	17.77	1.018	0.765	13.8
3^b	F:[Co(phen)3]^3+/2+	FTO/Au/GNP	100	18.27	1.014	0.771	14.3
4	F:[Co(phen)3]^3+/2+	FTO/Au/GNP	50	9.55	0.994	0.776	14.7

---

The maximum photovoltage (V_max) obtained in the DSSC is attributed to the energy gap between the quasi-Fermi level of the TiO₂ [approximately the energy level of the conduction-band edge (E_C.B.)] and the redox potential of the electrolyte, and the improvement of the efficiency of DSSCs is possible by the increase of the photovoltage through using an electrolyte having a more positive (lower) redox potential than I₃⁻/I⁻.^{1–4,7–9,11,13–15} The redox potential of the cobalt(III/II) tris(1,10-phenanthroline) complex ([Co(phen)₃]^3+/2+) is lower than that of I₃⁻/I⁻ by ca. 0.2 V,¹⁹ and the values of the HOMO levels of ADEKA-1 and LEG4 are still more positive than the redox potential of the cobalt(III/II) complex (Fig. S15, ESI†), which provides the thermodynamic driving force for the dye regeneration reaction by electron transfer from the Co²⁺-complex electrolyte to the oxidized dye.^13–15 Thus we employed [Co(phen)₃]^3+/2+ as the redox electrolyte for the ADEKA-1 and LEG4 co-sensitized cell for further improvement of the η of the cells.

In the fabrication of the cells using the cobalt(III/II) complex redox electrolytes (cell-B), the compositions of the electrolyte solutions, i.e. the Co²⁺/Co³⁺ ratio, the kind of cobalt(III/II) complex counter anion and the electrolyte additives, were optimized experimentally according to the literature^{7,13,14,20,21} using a platinum-deposited F-doped SnO₂ (FTO)-coated glass plate as the counter electrode. The cell using an electrolyte solution with the optimized composition exhibited a high V_oc of above 1 V and the η was improved to 13.8% under AM-1.5G one sun illumination (entry 2 in Table 1) as was expected from the more positive redox potential of [Co(phen)₃]^3+/2+. However, a decrease of the J_sc was also observed in the cell compared to the cell with the I₃⁻/I⁻ redox electrolyte solution. In order to recover the J_sc, we employed graphene nanoplatelets (GNPs) as the material for the counter electrode and prepared the counter electrode on a FTO-coated glass plate with a structure of FTO/Au/GNP, because the counter electrode has been reported to produce higher J_sc and fill factor (FF) in photocurrent–voltage properties than the standard platinum electrodes.^8,22,23Fig. 3 shows an example of the J–V curve under AM-1.5G one sun illumination (100 mW cm⁻²) and the IPCE spectrum of the cell co-sensitized by ADEKA-1 and LEG4. The photovoltaic parameters, assessed as the averaged values from the J–V curves of the four separately prepared cells, are listed in Table 1 as entry 3 (Table S5, ESI†). The J_sc was actually improved in the cell from 17.8 to 18.3 ± 0.1 mA cm⁻² by using the FTO/Au/GNP counter electrode and the maximum value in the IPCE spectrum reached 91%, resulting in the η of 14.3% with the V_oc above 1 V. The better photovoltaic performance in the lower light intensity is a characteristic of DSSCs. This is also observed in the present cell and the cell exhibited an η of close to 15% under simulated sunlight with a 50 mW cm⁻² intensity (entry 4 in Table 1, and Fig. S16 and S17 in the ESI†).

Fig. 3 A typical J–V curve of the cell photosensitized collaboratively by ADEKA-1 and LEG4 with an efficiency of over 14% (entry 3a in Table S5, ESI†) under the illumination of simulated sunlight (AM-1.5G, 100 mW cm⁻²). Inset shows the IPCE spectrum of the cell.

In conclusion, a carboxy-anchor organic dye LEG4 was revealed to work effectively as the collaborative sensitizer to the silyl-anchor dye ADEKA-1 in DSSCs, and we succeeded in obtaining a high IPCE of up to 91%, V_oc of above 1 V and 14.3% conversion efficiency in the cell with the optimized cobalt(III/II) complex redox electrolyte solution and the GNP counter electrode. The result is attributed basically to the strong adsorption properties of ADEKA-1 to the TiO₂ electrode and shows the validity of silyl-anchor dyes as photosensitizers for DSSCs. The observation of conversion efficiencies of over 14% in these DSSCs indicates the high potential of DSSCs as light-to-electric energy conversion devices. The collaborative sensitization by plural organic dyes including silyl-anchor dyes, which would bring a further improvement in the photovoltaic performance of DSSCs, is considered as a promising way to produce practical DSSCs.

This work was partly supported by the “Element Innovation” Project by the Ministry of Education, Culture, Sports, Science & Technology in Japan and by JSPS KAKENHI Grant Number 15H03848.

Notes and references

1. M. D. McGehee, Science, 2011, 334, 607.
2. M. Grätzel, Acc. Chem. Res., 2009, 42, 1788.
3. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Chem. Rev., 2010, 110, 6595.
4. Dye-sensitized Solar Cells, ed. K. Kalyanasundaram, EPFL Press, Lausanne, 2010.
5. M. A. Green, K. Emery, Y. Hishikawa, W. Warta and E. D. Dunlop, Prog. Photovoltaics, 2015, 23, 805.
6. M. K. Nazeeruddin, F. D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Grätzel, J. Am. Chem. Soc., 2005, 127, 16835.
7. 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.
8. S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazeeruddin and M. Grätzel, Nat. Chem., 2014, 6, 242.
9. M. Zhang, Y. Wang, M. Xu, W. Ma, R. Li and P. Wang, Energy Environ. Sci., 2013, 6, 2944.
10. K. Kakiage, M. Yamamura, E. Fujimura, T. Kyomen, M. Unno and M. Hanaya, Chem. Lett., 2010, 39, 260.
11. K. Kakiage, T. Tokutome, S. Iwamoto, T. Kyomen and M. Hanaya, Chem. Commun., 2013, 49, 179.
12. S. K. Matta, K. Kakiage, S. Makuta, A. Veamatahau, Y. Aoyama, T. Yano, M. Hanaya and Y. Tachibana, J. Phys. Chem. C, 2014, 118, 28425.
13. K. Kakiage, Y. Aoyama, T. Yano, T. Otsuka, T. Kyomen, M. Unno and M. Hanaya, Chem. Commun., 2014, 50, 6379.
14. K. Kakiage, Y. Aoyama, T. Yano, K. Oya, T. Kyomen and M. Hanaya, Chem. Commun., 2015, 51, 6315.
15. H. Ellis, S. K. Eriksson, S. M. Feldt, E. Gabrielsson, P. W. Lohse, R. Lindblad, L. Sun, H. Rensmo, G. Boschloo and A. Hagfeldt, J. Phys. Chem. C, 2013, 117, 21029.
16. H. Tian and L. Sun, J. Mater. Chem., 2011, 21, 10592.
17. S. M. Feldt, P. W. Lohse, F. Kessler, M. K. Nazeeruddin, M. Grätzel, G. Boschloo and A. Hagfeldt, Phys. Chem. Chem. Phys., 2013, 15, 7087.
18. M. Liang and J. Chen, Chem. Soc. Rev., 2013, 42, 3453.
19. S. M. Feldt, G. Wang, G. Boschloo and A. Hagfeldt, J. Phys. Chem. C, 2011, 115, 21500.
20. P. Salvatori, G. Marotta, A. Cinti, E. Mosconi, M. Panigrahi, L. Giribabu, M. K. Nazeeruddin and F. D. Angelis, Inorg. Chim. Acta, 2013, 406, 106.
21. T. M. Koh, H. Li, K. Nonomura, N. Mathews, A. Hagfeldt, M. Grätzel, S. G. Mhaisalkar and A. C. Grimsdale, Chem. Commun., 2013, 49, 9101.
22. L. Kavan, J.-H. Yum and M. Grätzel, Nano Lett., 2011, 11, 5501.
23. 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. M. Zakeeruddin, H. Tian and M. Grätzel, J. Am. Chem. Soc., 2014, 136, 5722.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cc06759f

---