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J. Phys. Chem. C 2009, 113, 2618–2623
Efficient Electron Transfer Ruthenium Sensitizers for Dye-Sensitized Solar Cells Zhengzhe Jin,† Hideki Masuda,*,† Noriyo Yamanaka,‡ Masaki Minami,‡ Tsutomu Nakamura,‡ and Yoshinori Nishikitani‡ Department of Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan, and Central Technical Research Laboratory, Nippon Oil Corporation, Yokohama 231-0815, Japan ReceiVed: September 21, 2008; ReVised Manuscript ReceiVed: NoVember 7, 2008
Triarylamine-functionalized ruthenium dyes J13 and J16 were synthesized as novel efficient dye-sensitized solar cells (DSSCs). Under standard global AM 1.5 solar conditions, the J13- and J16-sensitized solar cells demonstrate short circuit photocurrent densities of 15.65 and 15.71 mA cm-2, open circuit voltages of 703 and 702 mV, fill factors of 0.712 and 0.701, and overall conversion efficiencies of 7.83% and 7.73%, respectively. The N719 dye under identical measurement conditions gives 7.91%. DFT/TDDFT calculations were performed on the two sensitizers to gain insights into their structural, electronic, and optical properties. These results indicate that the triarylamine ligand acts as an electron donor in a manner similar to thiocyanato ligands and Ru metal, and the TDDFT calculations predict that the absorption in the visible region originates from electron transfer from the electron donor groups to the anchoring ligand dcbpy, through which excited electrons are directly transferred to TiO2. It is suggested that the triarylamine-functionalized ruthenium dyes are highly promising for DSSCs. Introduction Dye-sensitized solar cells (DSSCs) based on mesoporous nanocrystalline TiO2 films have attracted intensive interest for scientific and industrial applications due to their high phototo-electricity conversion efficiency and low production cost.1,2 One of the essential strategies for improving the performance of DSSCs is provided by modification of the dye. Although numerous sensitizers such as metal-free organic dyes3-6 and nonruthenium metal dyes7-10 have been employed, the best energy conversion efficiency of up to 11% was achieved only by using ruthenium dyes, such as N3, N719, and black dye,11-13 in standard global air mass 1.5 sunlight. To further increase the efficiency of these cells, much effort has been directed toward the development of highly efficient solar sells based on ruthenium dyes.14 There are several essential design requirements for an efficient sensitizer. The LUMO of the dye must be sufficiently high in energy to promote efficient charge injection into the TiO2 film and the HOMO should be sufficiently low in energy for efficient regeneration of the oxidized dye by the hole-transport material (HTM).15 The increase in the open-circuit photovoltage (Voc) can be explained by two different mechanisms: (i) retardation of the recombination between injected electrons and oxidized species in the electrolyte or (ii) a band edge movement with respect to the redox potential.16-19 Otherwise the short circuit current density (Jsc) is related to the interaction between TiO2 and the sensitizer as well as the absorption region and molar extinction coefficient of the sensitizer. A recent strategy for identification of new ruthenium dyes is to replace one of the 4,4′-dicarboxylic acid2,2′-bipyridine (dcbpy) ligands of N3 with a highly conjugated ancillary ligand. However, this strategy has two main disadvantages: (i) a relatively low adsorption of dye molecules onto * Corresponding author. E-mail:
[email protected]. † Nagoya Institute of Technology. ‡ Nippon Oil Corporation.
the TiO2 particles and (ii) the electron excited on the highly conjugated ancillary ligand is not injected into the electrode because it is not directly connected to the TiO2 particles.20,21 For these ruthenium dyes, the LUMO is localized on the anchoring ligand and the excited electrons can be directly transferred to the TiO2 band, but the LUMO+1 is localized on the ancillary ligand and the excited electrons cannot be directly transferred to the TiO2 band. These effects result in a lower conversion efficiency ( 0.01 and their compositions larger than 25% are noted. M: the mixed Ru-NCS. L1: dcbpy. L2: triarylamine.
J ) 8.8 Hz, 1H), 6.69 (d, J ) 8.7 Hz, 1H), 6.51 (m, 1H), 4.09 (t, J ) 6.6 Hz, 2H), 1.83 (m, 2H), 1.59-1.31 (m, 10H), 0.90 (t, J ) 7.2 Hz, 3H). FT-IR 2099 (-NCS), 1724 (-COOH). ESI-MS m/z 417.4 [M - 2H] 2-. Anal. Calcd for C38H37N7O5RuS2 · H2O · MeOH: C, 52.81; H, 4.89; N, 11.05. Found: C, 52.82; H, 4.64; N, 11.01. Theoretical Calculations. All of the calculations were performed with the Gaussian 03 program package,31 using the Fujitsu PRIMEPOWER HP2500 instrument at the Information Technology Center of Nagoya University. Becke’s threeparameter hybrid functional32 with the LYP correlation functional (B3LYP)33 was used together with the Los Alamos effective core potential LanL2DZ.34 Geometry optimizations were performed in vacuo without any symmetry constraints. Vibrational frequency calculations were performed to confirm the stability of the optimized geometries. The electronic structures were determined in ethanol solution on the geometries optimized in vacuo. Solvent effects were included by means of the nonequilibrium implementation35 of the polarization conductor calculation model (CPCM).36,37 The vertical excitation energies and oscillator strengths were calculated in ethanol solution by using TD-DFT with the LanL2DZ basis set, along with the corresponding pseudopotential for the metal atom. The absorption spectra of these ruthenium dyes were calculated with use of the SWizard program.38
Results and Discussion Scheme 1 shows step-by-step details for the syntheses of the J13 and J16 dyes. The syntheses of these two triarylamine ligands (L) were carried out by using Ullmann condensation methods, where copper(II) sulfate and KOH are used as catalysts. The key intermediate [RuCl(L)(p-cymene)]Cl was synthesized from the triarylamine ligand and [Ru(Cl)2(pcymene)]2 in EtOH in a 4 h reaction. The ruthenium complexes J13 and J16 were then prepared in a one-pot synthesis, and confirmed with use of NMR, ESI-TOF mass, and FT-IR spectroscopies and elemental analysis. The absorption spectra of J13 and J16 are shown in Figure 2. The two triarylamine ruthenium complexes exhibit two broad absorption bands in the visible region. The broad bands at 507 nm originate from metal-to-ligand charge transfer (MLCT) transitions from Ru(NCS) to the dcbpy ligand and the broad bands at 380 nm originate from the mixture of MLCT transitions from Ru(NCS) to dcbpy and triarylamine ligands. In the UV region, both complexes have an intense band at 302 nm, which originates from the π-π* transitions of the dcbpy and triarylamine ligands. The absorption spectra of J13 and J16 adsorbed on TiO2 electrodes are similar to the corresponding solution spectrum, although a red shift is observed due to the interaction of the anchoring groups with the surface titanium ions. In addition, the dye coverage was 1.27 × 10-7 and 1.55 × 10-7 mol cm-2 for J13 and J16, respectively, as estimated from the absorbance change. This result indicates that there is only a small change in the amount of adsorbed dyes on the film according to the change of the alkoxy substituent. The ionization potentials of J13 and J16 on the TiO2 electrode were measured on a Riken Keiki AC-2 instrument. The ionization potential is -5.1 eV for J13 dye, which is 0.3 eV more negative than that of the iodide/triiodide redox couple in the electrolyte. This provides a large thermodynamic driving force for sensitizer regeneration by iodide. The emission maximum of J13 appeared at 716 nm. The excited state oxidation potential derived from the ionization potential and the zero-zero excitation energy is -3.4 eV, which is 0.5 eV more positive than the TiO2 conduction band. This provides an efficient electron transfer from the excited dye to the TiO2 conduction band. A similar ionization potential (-5.1 eV) and excited state oxidation potential (-3.4 eV) were also measured for the J16 dye. To gain insight into the geometrical, electronic, and optical properties of the triarylamine-functionalized ruthenium dyes, DFT and TDDFT calculations were intensively investigated.
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Jin et al.
TABLE 2: Photovoltaic Performance of Dye-Sensitized Solar Cells Based on J13, J16, and N719 Dyes under AM 1.5 Illumination (100 mW cm-2) dye
Voc [V]
Jsc [mA cm-2]
FF
η [%]
J13 J16 N719
0.703 0.702 0.690
15.65 15.71 15.70
0.712 0.701 0.730
7.83 7.73 7.91
Optimized geometrical structures of J13 and J16 are shown in Figure 3, and the molecular orbital energies calculated for J13 are shown with isodensity plots of the HOMO-N (N ) 3, 2, 1, 0) and LUMO+M (M ) 0, 1, 2, 3) in Figure 4. The molecular orbital energy and isodensity plots for J16 (not shown) are qualitatively similar to those of J13. The three highest occupied molecular orbitals (HOMO, HOMO-1, and HOMO-2) have predominately ruthenium t2g character with a significant contribution originating from the NCS ligand orbitals, and we note that, according to the introduction of the triarylamine derivative ligand, a small contribution originates from the N atom of the triarylamine ligand in the HOMO and HOMO-1 orbitals. The HOMO-3 indicates a contribution from a pure SCN π orbital. The three lowest unoccupied molecular orbitals (LUMO, LUMO+1, and LUMO+2) are localized homogeneously on the dcbpy anchoring ligand. The excited electrons are directly transferred to TiO2 through these orbitals. Such similar locations of LUMO, LUMO+1, and LUMO+2 are also found in N3, N719, and black dye.39,40 However, for the other Ru complexes, the three lowest unoccupied molecular orbitals are localized on the two different ligands.41-43 LUMO+3, which is found 1.31 eV above the LUMO, is localized on two pyridinyl rings of triarylamine. The calculated absorption spectra for J13 and J16 in EtOH are shown in the inset of Figure 2, and the most representative calculated optical transitions for J13 are listed in Table 1. Surprisingly, the calculated and experimentally determined absorption positions and relative intensities are in good agreement. In the visible region, the first broad bands at 505 nm originate from MLCT transitions from Ru(NCS) to the dcbpy ligand, and the second broad bands at 377 nm originate from the mixture of MLCT transitions from Ru(NCS) to dcbpy and triarylamine ligands. In the UV region, the intense band at 298 nm originates from the π-π* transitions of the dcbpy and triarylamine ligands. It is interesting to note that the absorptions in the visible region from 400 to 800 nm originate from the MLCT transitions from Ru(NCS) to the anchoring ligand dcbpy, through which the excited electrons are directly transferred to TiO2. Similar results are also found for the J16 dye. The findings obtained from DFT-TDDFT calculations indicate that the triarylamine-functionalized ruthenium dyes should be more efficient for electron transfer to occur from the excited dye to the TiO2 conduction band. Figure 5 (left) shows the incident monochromatic photo-tocurrent conversion efficiency (IPCE) for J13 and J16. The IPCE data exceed 60% in a spectral range from 420 to 610 nm and from 410 to 620 nm for J13 and J16, respectively. The IPCE data of J13 and J16 plotted as a function of excitation wavelength exhibit a strikingly high efficiency of 79.3%. It is quite notable that the IPCE data extend the spectral coverage to about 850 nm. This indicates that addition of triarylamine with an alkoxy substituent at the phenyl ring of arylamine promotes extension of the spectral coverage. The IV characteristics are shown in Figure 5 (right), and the detailed device parameters are listed in Table 2. Under standard global AM 1.5 solar conditions, the J13-sensitized solar cell
produces a Jsc value of 15.65 mA cm-2, a Voc value of 703 mV, and a fill factor of 0.712, corresponding to an overall conversion efficiency (η) of 7.83%, and the J16-sensitized solar cell produces a Jsc value of 15.71 mA cm-2, a Voc value of 702 mV, and a fill factor of 0.701, corresponding to an overall conversion efficiency (η) of 7.73%. These values are similar to those of the N719-sensitized cell (η ) 7.91%) under identical measurement conditions. Conclusion In conclusion, novel efficient sensitizers as solar cells were developed by replacing one of the dcbpy ligands of N3 with a highly conjugated triarylamine derivative as an ancillary ligand. The introduction of triarylamine derivatives with an alkoxysubstituted phenyl group provides directionality to the efficient electron transfer from the excited dye to the TiO2 conduction band. These observations are also interpreted by TD-DFT theoretical calculations. The efficient photo-to-electricity conversion efficiency demonstrates that molecular engineering of the ancillary ligand of the ruthenium complexes to achieve higher performance could be expanded by altering the substituent functional group. Further optimizations of the cell parameters and stability experiments under long-term irradiation are now in progress in order to take full advantage of these new sensitizers. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research from the Ministry of Education, Science, Sports and Culture, to which our thanks are due. Supporting Information Available: Absorption, emission spectra, isodensity surface plots, MO energies, excitation energies, and oscillation strengths for J16, and photovoltaic date of N719. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Gra¨tzel, M. Nature 2001, 414, 338. (3) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (4) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (5) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarstrolm, L. J. Phys. Chem. B 2005, 109, 19403. (6) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Pe´chy, P.; Takata, M.; Miura, H.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2006, 18, 1202. (7) Kuciauskas, D.; Freund, M. S.; Gray, H. B.; Winkler, J. R.; Lewis, N. S. J. Phys. Chem. B 2001, 105, 392. (8) Islam, A.; Sugihara, H.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida, M.; Takahashi, Y.; Murata, S.; Arakawa, H. Inorg. Chem. 2001, 40, 5371. (9) Wang, Q.; Campbell, W. M.; Bonfantani, E. E.; Jolley, K. W.; Officer, D. L.; Walsh, P. J.; Gordon, K.; Humphry-Baker, R.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 15397. (10) Geary, E. A. M.; Yellowlees, L. J.; Jack, L. A.; Oswald, I. D. H.; Parsons, S.; Hirata, N.; Durrant, J. R.; Robertson, N. Inorg. Chem. 2005, 44, 242. (11) Nazzruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (12) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.; Vlachopoulos, N.; Shklover, V.; Fischer, C.-H.; Gra¨tzel, M. Inorg. Chem. 1999, 38, 6298. (13) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (14) Gra¨tzel, M. Bull. Jpn. Soc. Coord. Chem. 2008, 51, 3. (15) Robertson, N. Angew. Chem., Int. Ed. 2006, 45, 2338.
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JP8083748
