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Molecular Design of Organic Dye toward Retardation of Charge Recombination at Semiconductor/Dye/Electrolyte Interface: Introduction of Twisted π-Linker Jun-ichi Nishida,† Tatsuya Masuko,† Yan Cui,‡ Kohjiro Hara,‡ Hideshi Shibuya,§ Manabu Ihara,§ Tomohiro Hosoyama,| Ryota Goto,| Shogo Mori,*,| and Yoshiro Yamashita*,† Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, National Institute of AdVanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, Research Center for Carbon Recycling and Energy, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan, and DiVision of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu UniVersity, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: August 9, 2010
A triphenylamine (TPA) dye with two hexyl groups at the opposable 3,3′-positions of a bithiophene linker (1a) was prepared and compared to a dye with a regio regular 4,3′-dihexylbithiophene unit (1b). The absorption spectrum of 1a showed a blue shift in comparison to 1b, suggesting the structure of 1a was twisted in comparison to 1b. The twisted structure agreed with the structure optimized by DFT calculations. By replacing one thiophene unit of 1a and 1b with a pyridine ring (2a and 2b, respectively), a further blue shift was observed. Dye-sensitized solar cells (DSSCs) were prepared from these dyes and a conventional Ru dye (N719). Under one sun conditions, DSSCs/2a showed comparable or higher open-circuit voltage (Voc) than did DSSCs/N719. The high Voc was attributed solely to long electron lifetime in the DSSCs/2a. A previous study has suggested that TPA dyes with long π-conjugation unit suffer from larger dispersion forces between the dyes and acceptors, I3- and/or I2, causing short electron lifetime and thus low Voc. The present study shows that this problem can be overcome by increasing steric hindrance by attaching obstacle units to the π-linker without a significant increase of polarizability. The obstacle unit is to increase the intermolecular distance between the π-linker and acceptor species in electrolytes. Twisted structure is suggested to be one strategy to add such an effect. Introduction Dye-sensitized solar cells (DSSC) using nanocrystalline TiO2 electrodes have recently attracted considerable attention because of the high potential providing electricity with low-cost and high efficiency.1 The highest conversion efficiency (η) from solar light to electric power for DSSCs is over 11%, which has been obtained from porous TiO2 electrode, Ru polypyridine complexes (named N719 and black dye) as sensitizers, and an organic electrolyte containing I-/I3- as a hole transport media.2 Besides the Ru complex dyes, huge efforts have been made to design new dyes, especially for metal-free organic dyes.3 However, they have shown lower η than those of Ru complexes. The lower values are mostly due to lower open circuit voltages (Voc) caused by the enhanced charge recombination by adsorbed dyes on the TiO2 surface. Such facilitation has been observed not only for metal-free organic dyes4 but also for metal complexes including Ru ones.5 It has been shown that the increased recombination was caused not due to recombination with dye cation4 but by the increased concentrations of acceptors, that is, I3- and/or I2, at the vicinity of TiO2 surface, and such increases could be induced by dispersion force,4 electrostatic force,4 and/or complex formation.5 The first case is related to * Corresponding author. E-mail:
[email protected] (S.M.),
[email protected] (Y.Y.). † Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology. ‡ National Institute of Advanced Industrial Science and Technology. § Research Center for Carbon Recycling and Energy, Tokyo Institute of Technology. | Shinshu University.
polarizability of sensitizers, and thus to molecular size and shape. The latter two are related to the location of partial charges, and the last one is related to its electron donor ability. Charge recombination is also affected by the packing density of adsorbed dyes and thickness of the dye layer, where the dye layer acts as a blocking layer to prevent I3-/I2 from approaching to the TiO2 surface.4 In this view, we can design dye molecules to block the approach of I3-/I2. Along the strategy, alkyl chains were added to the dye framework, and the electron lifetime in the DSSCs with the organic dyes is nearly the same as those with Ru complex dyes.6 However, further improvement is still desired, and thus it is essential to design dye molecules to reduce the acceptors’ concentration at the TiO2 surface by reducing molecular interactions between the dyes and acceptors. Many of the recent reported dyes have a donor part based on aryl amines, a π-linker composed from thienyl or vinyl units, and a cyanoacrylic acid part as electron acceptor and anchoring groups.7,8 In the case of these dyes without π-linker (e.g., dye named 1b7a and L08a), Voc is high, and they do not seem to enhance recombination.8a However, with an increase of π-conjugation unit to widen absorption spectrum, charge recombination was facilitated.8a A similar trend has been also observed for Ru complex dyes.9 If this trend is related to polarizability, faster recombination for wide spectrum absorbing dyes is the intrinsic property. To avoid this dilemma, an increase of π-conjugation resulting in the enhancement of recombination, we propose a design of introducing a twisted π-linker between the donor and acceptor moiety. In this Article, dyes 1a and 2a with two hexyl groups at the opposable 3,3′-positions of a bithiophene spacer were
10.1021/jp912047u 2010 American Chemical Society Published on Web 09/29/2010
Molecular Design of Organic Dye
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17921
CHART 1: Structures of Sensitizers
Figure 1. Absorption spectra of dyes in 2-MeTHF. Thin line, 1b; bold, 1a; thin broken line, 2b; bold broken, 2a.
prepared (Chart 1) and compared to dyes 1b and 2b with a regio regular 4,3′-dihexylbithiophene unit. Along this design guide, we demonstrate that longer electron lifetimes than those in DSSCs using Ru complex dye, N719, are obtained. Between dyes 1 and 2, one thiophene unit was replaced with a pyridine ring to check if the pyridine enhances recombination by acting as a donor to form complex with I2. Experiments Organic dyes were synthesized as described in the Supporting Information. Nanocrystalline TiO2 photoelectrodes were prepared by a screen printing technique reported in a previous paper.10 Commercially available TiO2 nanoparticles, NanoxideT20 (Solaronix), were used. We also added a scattering layer, which consisted of large TiO2 particles (av. 100 nm). The TiO2 paste was printed on a glass substrate coated with transparent conducting oxide (TCO, F-doped SnO2) and subsequently sintered at 500 °C in air for 1 h. For the other set of TiO2 particles (18NR, Catalysis & Chemicals Inc. Co. Ltd.), PEG (40 wt %) was added to the water solution containing the TiO2. The 18NR paste or Nanoxide-T (Solaronix) was applied on TCO by a doctor blade technique, and sintered at 450 or 550 °C for 30 min, respectively. The thickness of the TiO2 thin films, measured with an Alpha-Step 300 profiler (TENCOR INSTRUMENTS), was ca. 2-15 µm. The dyes were dissolved at a concentration of 0.3 mM in a mixed solvent of 50:50 (vol. %) tert-butanol (Kanto Chemical) and acetonitrile (Kanto, dehydrated for organic synthesis). The TiO2 films were immersed in the dye solutions and then kept at 25 °C for more than 12 h to allow the dye to adsorb to the TiO2 surface. Photovoltaic cells were prepared from the dye-sensitized TiO2 electrode, a Pt counter electrode sputtered (ca. 200 nm) onto a TCO-coated glass plate, a film spacer (30 µm thickness), and an organic electrolyte. The photovoltaic performance of the solar cells was measured with a source meter (ADVANTEST, R6243). We employed an AM 1.5 G solar simulator (WACOM, WXS-80C-3, or YAMASHITA DENSO Co., YSS-100A) as the light source. The incident light intensity was calibrated by using a standard solar cell composed of a crystalline silicon solar cell and an IR-cut off filter (SCHOTT, KG-5), giving the photoresponse range of amorphous silicon solar cell (produced and calibrated by Japan Quality Assurance Organization). For the thickest samples, we used an aperture mask (0.2354 cm2) attached onto the top of the cells in the photovoltaic measurements. Action spectra of the monochromatic incident photon-to-current conversion efficiency (IPCE) of the solar cell were measured with a CEP-99W system (BUNKOH-KEIKI Co., Ltd.).
Electron lifetimes in the DSSCs were obtained by stepped light-induced transient measurements of photovoltage. For the measurements, TiO2 electrodes were prepared from Solaronix-T by sintering at 550 °C for 30 min in air. The electrolyte was 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI) + 0.1 M LiI + 0.05 M I2 + 0.5 M 4-tert-butylpyridine (TBP) in AN. The experimental procedure of the measurements is described in detail elsewhere.11 In short, DSSCs were irradiated by a blue LED, and the decay of open circuit voltage, caused by stepwise decrease of a small fraction of the LED intensity, was measured. The measurement was repeated with various initial LED intensities, giving different electron density in the DSSCs. Electron lifetime (τ) was obtained by fitting an exponential function, exp(-t/τ), to the voltage decay. The electron density was estimated by the charge extraction method introduced by Peter and co-workers.12 Our experimental setup is described elsewhere.13 In short, under LED irradiation, bias potential was applied to the cell to obtain open circuit conditions. Next, the LED was turned off simultaneously with changing the bias potential to zero respect to the counter electrode. Resulting current transients were recorded, and the electron density was obtained by numerical integration of the currents. Computations Calculations were carried out using Chem3D Pro 7.0,14 Gaussian 03,15 and Gaussian 09.16 The structures of organic dyes were first optimized by the MM2 level of theory17 implemented in Chem3D. The constants used for MM2 are shown in Table S1 (Supporting Information). From the structure, the groundstate geometries were optimized in the gas phase by DFT using the B3LYP functional18,19 and 6-31 g(d) basis set with Gaussian 03. With the geometries, TDDFT calculations were carried out with Gaussian 09 using CAM-B3LYP20 in the gas phase and in THF using the C-PCM model.21 Polarizabilities were also calculated with Gaussian 09 at the same level of theory used for TDDFT. We did not take the solvent effects into the groundstate geometry optimization because not a significant effect on the geometry is expected.22 The calculations were performed for free dye molecules without taking into account of the effect of adsorption onto the TiO2 surface. Experimental Results Figure 1 shows the absorption spectra of these dyes in 2-MeTHF. Although the number of π-electron is the same, the absorption maxima of these dyes are dependent on the positions of the hexyl groups. The absorption maximum of dye 1b containing a regio regular 4,3′-dihexylbithiophene unit is observed at 464 nm. Introduction of opposable two hexyl groups at the 3,3′-position of bithiophene induced a blue-shift of the
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TABLE 1: I-V Characteristics of DSSCs with Various Dyes under One Sun Conditions dye
TiO2a
wb/µm
Jsc/mA cm-2
Voc/V
FF
η/%
1a 1b 2a 2b N719 2a N719 1a 1b 2a 2b
Sol-T Sol-T Sol-T Sol-T Sol-T 18NR 18NR T20 T20 T20 T20
2.4 2.4 2.4 2.4 2.4 3.4 3.3 15c 15c 15c 15c
5.6 7.3 5.1 7.0 5.7 5.8 8.0 10.1 11.9 8.4 11.1
0.76 0.70 0.75 0.72 0.75 0.79 0.75 0.75 0.72 0.76 0.74
0.69 0.68 0.66 0.66 0.69 0.70 0.72 0.75 0.73 0.75 0.71
2.9 3.5 2.5 3.3 3.0 3.3 4.3 5.7 6.3 4.8 5.8
a TiO2 electrode was prepared from Nanoxide-T (Solaronix), 18NR (Catalysis & Chemicals Inc. Co. Ltd.), or Nanoxide-T20 (Solanonix) nanoparticles. b Thickness of TiO2 layer. c TiO2 electrode consists of ca. 10 µm thick nanoporous layer and ca. 5 µm scattering layer prepared from homemade TiO2 particles, with TiCl4 treatment. Electrolyte was 0.1 M LiI, 0.6 M DMPImI, 0.05 M I2, and 0.5 M tBP in AN.
absorption maximum to 436 nm in dye 1a. This result indicates that the C-C bond around the bithiophene axis takes a twist conformation. A larger energy is necessary to undergo an intramolecular charge transfer when the π-spacer unit takes a twist conformation. Replacement of a thiophene in dye 1a to a pyridine ring affords a remarkable blue-shift to 376 nm (dye 2a). On the other hand, emission peaks of these dyes in 2-MeTHF appeared at almost similar wavelengths ranging from 631 to 655 nm, implying that the excited states of these dyes might adopt similar conformations such as planar quinoid structures. I-V characteristics of the DSSCs with these dyes and N719 under AM 1.5 G illumination are listed in Table 1. Figure 2
shows absorption spectra on TiO2 film and the incident photon to current conversion efficiency (IPCE) of the DSSCs. The onset wavelength became longer in the order 2a < 1a < 2b < 1b. For the DSSCs prepared from Nanoxide-T, the values of Voc for 2a and 1a dyes were comparable to that with N719. The difference of Voc between 2a and N719 became larger when different TiO2 particles (18NR) were employed. Figure 3 shows the electron lifetime (τ) and Voc as a function of electron density (n). The longest lifetime was observed with 2a. Little difference of Voc at the same electron density (Figure 3b) suggests negligible shift of TiO2 conduction band edge potentials (Ecb) by the adsorption of the dyes. The slope for double logarithmic plot of τ versus n in Figure 3a is different between the organic dyes and N719. Thus, with the increase of light intensity, τ with 2b and 1b probably becomes shorter than that of N719, resulting in lower values of Voc for these dyes than N719. For 2a and 1a, because DSSCs/Nanoxide-T showed comparable Voc with N719 under one sun conditions, while they have similar Ecb, the τ value is expected to be comparable to N719 under one sun conditions. For the case of the cells from 18NR, the slopes of the double logarithmic plot of τ versus n are closer between 2a and N719 (Figure S4a in Supporting Information). Based on the Voc under one sun conditions, and similar Ecb from Voc versus n plot (Figure S4b), the τ in the DSSCs/2a is probably longer than with N719 even under one sun conditions. In view of charge recombination, the adsorbed dye layer on the TiO2 can act as blocking layer and/or facilitation layer, in comparison to bare TiO2. The layer of N719 acts as a blocking layer.4 For 2a to 1b, the observed electron lifetime could be due to a good blocking effect, but they could still have the same nature with other organic dyes, that is, to interact with I3-/I2. Because these dyes did not change the Ecb, such nature may be distinguished simply by I-V measurements at dark. In comparison to the cell with bare TiO2, the onset voltages of DSSCs/
Figure 2. Absorption spectra of dyes on TiO2 films (left) and IPCE of DSSCs (right). Thin red line, 1b; bold red, 1a; thin broken blue, 2b; bold broken blue, 2a. Thickness of TiO2 used for IPCE measurement was 2.4 µm.
Figure 3. Electron lifetime (a) and Voc (b) of DSSCs as a function of electron density in the DSSCs. The cells were the same as shown in Table 1 for 2.4 µm thick TiO2 (Nanoxide-T). The highest LED intensity used for these measurements gave ca. 0.8 mA cm-2 at short circuit conditions for these cells.
Molecular Design of Organic Dye
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17923 TABLE 2: Experimental and Calculated Spectral Properties of Dyes in THF Solution at CAM-B3LYP/6-31G(d) Level of Theory exp. λmax/nm 1a 1b 2a 2b L0
a
434 465a 376a 448a 386d
fc
calc. λmaxb/nm 320, 314, 317, 307, 261,
349, 345, 335, 330, 277,
437 455 399 432 384
0.66, 0.39, 0.71, 0.30, 0.19,
a In 2-MeTHF. b By TDDFT using C-PCM model. oscillator strengths. d In ethanol, ref 7a.
Figure 4. Dark I-V curves of the solar cells prepared with dye 2a, 1b, N719, and without dye (bare TiO2). Amount of dye was fully loaded or less by reducing immersion time into dye solution.
Figure 5. Electron lifetime in DSSCs prepared from fully and partially loaded 2a using TiO2 film prepared from 18 NR particles by annealing at 450 °C for 30 min.
2a, /1b, where currents start flowing, were higher, and when the amount of dyes was reduced, the values became similar to that for the bare TiO2 cell (Figure 4). At the low dye load condition where little blocking effect is expected, similar onset voltage of the DSSCs/2a, /1b with the case of bare TiO2 cell suggests little facilitation of recombination by these dyes. For the case of DSSCs/18NR/2a, the electron lifetime was also measured with full and partial dye load conditions, and the decrease of electron lifetime with the reduction of the dye amount was confirmed (Figure 5). Note that we excluded the possibility of recombination to these dye cations based on the dark I-V measurements, where there were no dye cations during the measurements. Computational Results and Comparison with Experiments DFT calculations were performed for the organic dyes examined here. The B3LYP functional was chosen for groundstate geometries according to a previous report studying TPA dyes.22,23 For selected dyes, ground-state optimization and polarizability calculations were performed with various basis sets. Because the calculated polarizabilities were close among the basis sets (Table S4, Supporting Information), we selected 6-31G(d) for the rest of dyes. TDDFT calculations at B3LYP/ 6-31G(d) level of theory were applied for all the dyes, and the results are shown in Table S5-1 (Supporting Information). As it has been pointed out, B3LYP cannot predict the absorption spectrum well,22,23 while tendencies of the absorption wavelength and oscillator strength agreed with those of experimental values. On the other hand, the long-range corrected version of B3LYP using the coulomb-attenuated method (CAM-B3LYP) has demonstrated good agreements of excitation energies with experimental results.23 Table 2 shows the results of our
0.13, 0.10, 0.09, 0.02, 0.02, c
1.73 1.81 1.79 1.88 1.12
Calculated
calculations including the L0 dye (structure shown in Figure S6 in Supporting Information). The good agreement of the calculated and experimental values of absorption maxima supports the validation of the calculated optimized geometries. From the absorption spectrum, it was indicated that the introduction of the opposable two hexyl groups induced twisted confirmation. This agreed with the calculated geometry showing the dihedral angle at C-C bond around bithiophene was 39° for 1b and 59° for 1a. For 2b and 2a, the tendency was the same, but smaller values than 1b and 1a were obtained, that is, 31° for 2b to 57° for 2a. Table 3 summarizes the calculated polarizabilities of the dyes including other TPA dyes published in a previous paper (structures shown in Figure S6, Supporting Information).8a The new dyes examined here showed comparable values, and the values were much larger than that of the L0 dye. The values calculated in THF solution were larger than that calculated in vacuum (Table S5-2, Supporting Information), while the tendencies were the same among the theories employed here. Discussion For the case of TPA dyes, dispersion force, that is, induced dipole-induced dipole interaction, may dominate the intermolecular forces between the dyes and I3-/I2.8a The force is proportional to the polarizability (R′) of molecules and r-6, where r is a distance between two molecules. In a previous study, the electron lifetime in DSSCs employing TPA dyes (L0-L3) was decreased with the increase of π conjugation. This can be interpreted with the increase of the interaction between the dyes and I3- due to larger values of R′.8a Table 3 summarizes the values of R′, and onset wavelengths (λ) of IPCE, and electron lifetime of DSSCs examined here and in the previous study. Note that the lifetime in DSSCs/L0 is comparable to that in the DSSCs/N719. When alkyl chains are attached to dyes, they can act as obstacles for I3-/I2 to get closer to the mainframe of the dye. Thus, the alkyl chains could be used not only to increase the blocking effect by covering the TiO2 surface4 but also to reduce the dispersion force between the dye and I3-/I2. These effects can explain the long electron lifetime observed with 2b and 1b, in comparison to L2 and L3, which have similar λ and R′. By introducing a twisted π-linker into the TPA dyes, the R′ could be decreased, causing lesser interaction. However, the calculated value of R′ of 2a (121 Å3) is just slightly lower than that of 2b (125 Å3). Thus, it cannot explain the difference in τ. Instead, the increased τ for the cases of 2a and 1a in comparison to 2b and 1b, respectively, seems to be caused by an additional steric hindrance effect between the π-linker and I3-/I2 due to the dense positioning of two nonplanarly alkyl chains. We mean that the twisted structure itself may not be essential, but direction and local density of the alkyl chains would be more important,
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TABLE 3: Calculated Polarizability, and Measured IPCE Onset Wavelength and Electron Lifetime
dye 1a 1b 2a 2b L0 L2 L3 N719 a
polarizability/Å3 (CAM-B3LYP/6-31G(d) in THF solution)
polarizability/Å3 (B3LYP/6-31G(d) in vacuum)
125 128 121 125 54
110 114 110 113 44 76
polarizability/Å3 (B3LYP/LANL2DZ in vacuum)
onset wavelength for IPCE/nm
electron lifetimeb/ms
46a 80a 106a 106c
610 650 600 640 520a 600a 630a 710
220 80 300 120 100a 7a
