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Effect of dye coverage on photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO2 films
Chemical Physics Letters 489 (2010) 202–206
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Effect of dye coverage on photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO2 films Ryuzi Katoh a,*, Nobuhiro Fuke b, Akihiro Furube a, Naoki Koide b a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan SHARP Corporation, 282-1 Hajikami, Katsuragi, Nara 639-2198, Japan
a r t i c l e
i n f o
Article history: Received 22 October 2009 In final form 28 February 2010 Available online 3 March 2010
a b s t r a c t The effect of dye coverage on the photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO2 films was studied by transient absorption spectroscopy. We evaluated the electron injection efficiency of films with several dye concentrations on the surface and found no significant difference between them. We also studied the suppression of the efficiency by the addition of tBP (4-tert-butylpyridine) and found that the effect of tBP was more pronounced for films with lower dye loading. This suggests that sensitizer dyes adsorbed densely onto the surface suppress the adsorption of tBP. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Since highly efficient dye-sensitized solar cells (DSSCs) were first reported [1], substantial research has been carried out to improve their performance. N3 dye [cis-di(thiocyanato)-bis(2,20 bipyridyl-4,40 -dicarboxylate)ruthenium(II); Ru(dcbpy)2(NCS)2] is one of the most commonly used sensitizing dyes for high-performance DSSCs. Nazeeruddin et al. studied modifications of this dye and found that one of its derivatives, namely N719 (Fig. 1), in which two protons are replaced by tetrabutylammonium cations (TBA), shows high solar-energy-to-electricity-conversion efficiency (g > 11%) [2]. Despite much effort toward the development of high-performance solar cells, their efficiency has not been improved dramatically. Thus, many studies of the primary processes in such cells have been conducted in order to determine the factors controlling their performance. One of the most important primary processes is electron injection from excited dyes to semiconductor films. Thus, the photochemical process of dye-sensitized nanocrystalline films, which serve as active electrodes in DSSCs, has been studied by means of transient absorption (TA) spectroscopy in several research groups [3–16]. Using femtosecond TA spectroscopy, nonexponential ultrafast electron injection was observed in the 100fs to 100-ps time range for N3 dyes adsorbed on nanocrystalline TiO2 films (N3/TiO2) [3–7] and for N719/TiO2 [6,7]. Since recombination between injected electrons and parent cations occurs slowly in the millisecond time range [5,8], the efficiency of electron injection (Uinj) can be evaluated through microsecond TA measurements. We estimated the absolute values of Uinj to be almost unity for N3/TiO2 [10,11] and N719/TiO2 [14]. To clarify the factors * Corresponding author. Fax: +81 29 861 5301. E-mail address: [email protected] (R. Katoh). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.02.076
controlling electron injection efficiency and for further development of high-performance solar cell devices, it is very important to understand the primary processes in DSSCs. In this context, we have studied Uinj under different experimental conditions, such as various excitation wavelengths [12], free energy changes [13], particle sizes [14], and additives [15,16]. The coverage of sensitizer dyes on nanocrystalline semiconductor films is a factor of considerable importance for controlling the performance of DSSCs because performance reduction for highly loaded films has frequently been reported [17–20]. To clarify the origin of the lowered performance, TA measurements were examined as a function of dye concentration. Hirata found that electron injection efficiency decreases with increasing concentration of dye on the surface. They explained that this is due to multi-layer adsorption of dyes on the surface [20]. Moser and co-workers reported that the electron injection rate was reduced by the aggregation of N3/TiO2 films by comparing them with N719/TiO2 films, in which aggregation is expected to be reduced by bulky TBA groups [6]. Recently, Pellnor et al. carefully studied the difference in electron injection dynamics between N3/TiO2 and N719/TiO2 by varying experimental conditions such as the observed wavelength and the density of dye on the surface [7]. Although they pointed out that the injection dynamics difference between N3/TiO2 and N719/TiO2 was mainly due to the difference in temporal spectral change, they observed that the injection dynamics were affected by the density of dye on the surface for N3/TiO2 films. Hence, more careful studies are needed to clarify the effect of dye coverage on the electron injection process. The coverage of sensitizer dyes on nanocrystalline semiconductor films is also a factor of importance for controlling the electron loss process, i.e., the reaction between the injected electrons and in an electrolyte. This is because such reactions triiodide ions I 3 occur at the bare surface of dye-sensitized films, which is
R. Katoh et al. / Chemical Physics Letters 489 (2010) 202–206
amplified with an AC-coupled pre-amplifier (NF Electronic Instruments, SA-230F5) and a voltage amplifier (NF Electronic Instruments, 5305). Signals were processed with a digital oscilloscope (Tektronix, TDS380) and analyzed with a computer. Using the pre-amplifier, the DC offset of the photocurrent from the detector could be subtracted, and therefore a small absorbance change (<105) could be detected. The time resolution of the system was about 500 ns. To calculate the absorbance change, it was necessary to measure the intensity of the probe light without laser excitation by modulating the probe light intensity with an optical chopper placed in front of the detector. All measurements were carried out at room temperature.
2.5 N719 / TiO2 2
COOR
36 h (538 nm)
HOOC
N NCS
N Ru
Absorbance
N
1.5
HOOC
2 h (537 nm)
1
0.5
203
NCS N
COOR R = TBA (tetrabutylammonium)
0.5 h (534 nm)
3. Results and discussion 3.1. Dye coverage
0 400
450
500
550 600 650 700 Wavelength / nm
750
800
Fig. 1. Ground state absorption spectra of N719 dye adsorbed on TiO2 nanocrystalline films prepared with different immersion times ti = 0.5, 2, and 36 h. The wavelength values in parentheses indicate the peak positions. The molecular structure of N719 is also shown.
confirmed by the observation that the lifetime of the injected electron decreases with decreasing dye coverage [21]. To reduce the effect, small additive molecules, such as of 4-tert-butylpyridine (tBP), have been used as an adsorbate to cover the vacancies on the surface. Another function of tBP is to increase the energy level of the conduction band edge ECB. In fact, tBP improves the open-circuit voltage (VOC) and fill factor (FF) of DSSC devices [21–26]. However, short-circuit current (JSC) is often suppressed. This suggests that Uinj is reduced by the addition of tBP, and we observed such an effect in a black dye-sensitized system [15,16]. The effect of tBP is controlled by the density of tBP adsorbed on the surface and therefore the effect of tBP on Uinj may be affected by dye coverage on the surface, namely, bare surface area is required to realize the function of tBP molecules. Here we evaluate Uinj quantitatively by using a microsecond time-resolved TA technique to clarify the relationship between dye coverage and the tBP effect on Uinj in N719/TiO2 films.
2. Experimental N719 was purchased from Solaronix SA. TiO2 nanocrystalline films were prepared from a commercially available TiO2 paste (Solaronix SA, T/SP series). The paste was painted onto a glass plate substrate with a screen printer. Nanocrystalline films were prepared by calcination of the painted substrate for 1 h at 450 °C. The films obtained had an area of 10 10 mm2 and a thickness of 13 lm. The N719 dyes were dissolved at a concentration of 0.3 mM, which is close to the solubility limit in a solvent consisting of a 1:1 mixture of tert-butylalcohol and acetonitrile. The nanocrystalline TiO2 films were immersed into the dye solution, so that the dye could adsorb onto the semiconductor surface. The films were rinsed in ethanol, and then dried in air. For the TA measurement, a Nd3+:YAG laser (HOYA Continuum, Surelite II) was used for the pumping light source. The repetition rate of the laser was 10 Hz. The second-harmonic pulse (532 nm) was used for the excitation. A halogen lamp (100 W) was used as the probe light source. The light transmitted through the sample specimen was detected with a Si photodiode (Hamamatsu, S1722) after being dispersed with a monochromator (Acton Research, SpectraPro-150). The photocurrent from the detector was
Fig. 1 shows the ground-state absorption spectra of N719 dye adsorbed on TiO2 nanocrystalline films prepared with different immersion times ti. The distinct peaks at around 535 nm are due to the metal-to-ligand charge transfer (MLCT) absorption band characteristic of N719 dye. The peak positions of the MLCT band (Fig. 1) shift slightly toward longer wavelengths with increasing concentration of N719 dye. This suggests that the peak position is affected by the intermolecular interaction between dye molecules. The number of dye molecules Ddye per unit area of the film was estimated assuming that the absorption coefficient of N719 is the same as that of N3 dye (e = 14 000 dm3 mol1 cm1 at the peak). The number of particles Dparticle per unit area on the film can be estimated to be Dparticle = 1.5 1014 cm2 by using the following values: radius of a TiO2 particle rp = 10 nm, thickness of the film dfilm = 13 lm, and porosity of the film P = 0.5. Accordingly, for the film prepared with ti = 0.5 h, 100 dye molecules adsorb on one particle and the area Sd occupied by an N719 dye molecule is estimated to be 13 nm2. For the films prepared with ti = 2 and 36 h, Sd is estimated to be 5.1 and 2.7 nm2, respectively. Although we assumed homogeneous adsorption of N719 dyes in nanocrystalline films in depth direction, there has been argued heterogeneous adsorption [27]. In the present study, we confirmed that local density of dye increases with ti from the results of the peak shift of ground-state absorption and transient absorption spectra. For detail discussion, direct evaluation of depth profile of dye concentration is required. The absorbances of the films prepared with ti > 12 h were almost the same indicating that the adsorption of dye molecules was saturated. Even under the saturated condition, Sd (2.7 nm2 at ti = 36 h) was larger than the size of an N3 dye molecule, which was estimated to be 1 nm2 [28,29]. There are two possible reasons for this. One is that adsorption of dye is not saturated under this condition; in other words, a higher concentration of dye solution is required. In the present study, this scenario is impossible because the concentration of dye solution is close to the solubility limit. Another possibility is that the model used to estimate the surface area is too simple. Necking between particles should be considered. Thus, the estimated values give an upper limit of the surface area of the film. 3.2. Effect of dye concentration on the efficiency of electron injection Fig. 2 shows TA spectra of N719/TiO2 films immersed in acetonitrile recorded 20–40 ls after excitation normalized by the number of absorbed photons, which is represented by (1–10A), where A is the absorbance at the excitation wavelength (532 nm). Peaks around 800 nm were observed, which can be assigned to the cation
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0.0006
1.2 0.5 h 2h 36 h
in MeCN
1
0.5 h tBP 0M
0.8
tBP 1M
0.6
0.0004
0.4 0.2
0.0003
0 0.0002 0.0001 0 650
700
750 800 850 Wavelength / nm
900
950
Fig. 2. Transient absorption (TA) spectra of N719/TiO2 films prepared with different immersion times ti = 0.5, 2, and 36 h in acetonitrile recorded 20–40 ls after excitation. The spectra are normalized by the number of absorbed photons.
Normalized absorbance
Absorbance / (1-10-A)
0.0005
1.2 1
2h
tBP 0M tBP 1M
0.8 0.6 0.4 0.2 0
1.2 state of N719 dye [8]. The position of the peak due to the N719 cations seems to shift slightly toward longer wavelengths with increasing dye concentration (Fig. 2). Because the ground-state absorption spectra (Fig. 1) were not sensitive to the dye concentration, this shift is difficult to explain by a change in the contribution of ground-state bleaching in the TA spectra. Thus, a likely explanation of the spectral shift is that it arises from a change in electronic structure due to intermolecular interaction. Although TA spectra due to N719 cations change with the concentration (Fig. 2), the absorbances at the peak wavelengths are similar to each other when normalized by the number of absorbed photons. This suggests that the electron injection efficiency is not sensitive to the dye concentration on the surface. Hirata reported that a reduction of electron injection efficiency occurs for highly loaded films in N719/TiO2 through TA measurements [20]. This was explained as being due to multi-layer dye adsorption in highly loaded films. In fact, the Sd in these highly loaded samples was estimated to be smaller than 1 nm2, which is the size of the dye molecule. Accordingly, the situation of Hirata’s study is different from the present study; namely, we conducted our studies under less than full coverage density conditions.
3.3. Effect of tBP on the efficiency of electron injection To understand the effect of tBP on the electronic structure of N719 dye on a TiO2 surface, the TA spectra of N719/TiO2 films in a 1 M tBP solution of acetonitrile were measured (Fig. 3). These spectra were normalized at the peak. No spectral shape change was observed following the addition of tBP, suggesting that it did not change the electronic structure. Note that the relative efficiency of the electron injection as a function of tBP concentration can be evaluated by observing the TA signal at a particular wavelength. The inset in Fig. 4 shows the time profile of the TA signal for ti = 0.5 h film recorded at 800 nm. The absorbance decreases in the presence of tBP, indicating suppression of the electron injection efficiency. Decay of these signals reflects charge recombination and the effect of tBP on the recombination kinetics has been discussed previously [16]. The absolute value of the efficiency of the electron injection Uinj can be estimated by using the values of the absorbance change due to the generation of the oxidized form of the sensitizer dye (DAox) and its molar absorption coefficients eox:
1
36 h
tBP 0M tBP 1M
0.8 0.6 0.4 0.2 0 650
700
750 800 850 900 Wavelength / nm
950 1000
Fig. 3. Transient absorption (TA) spectra, after normalization at the peaks, of N719/ TiO2 films prepared with different immersion times ti = 0.5, 2, and 36 h in acetonitrile and in 1 M tBP solution recorded 20–40 ls after excitation.
Uinj ¼
DAox
eox ð1 TÞN0
ð1Þ
where T and N0 represent the transmittance of excitation light and the number of incident excitation photons per unit area, respectively. Earlier we estimated the value of eox for N3 dye at the peak to be 6000 mol1 dm3 cm1 [10]. Assuming that N719 has the same value, which is reasonable because the molecular structure of N3 dye is very similar to that of N719, we estimate the electron injection efficiency to be about 0.8. Based on our previous experiments, this is a reasonable value [10,14]. This high efficiency also indicates that there is no efficient fast recombination between an injected electron and a parent dye-cation. By comparing the absorbance in tBP solution at time = 0 ls with that in neat acetonitrile, the relative electron injection efficiency Urel inj ¼ Uinj ðtBPÞ=Uinj ð0Þ as a function of tBP concentration can be obtained (Fig. 4). Urel inj decreases with increasing tBP concentration and tends to saturate at higher tBP concentration. For the film with ti = 0.5 h, the electron injection efficiency decreased by 33% in a 1 M tBP solution. As discussed above, the electronic structure of N719 dye is not affected by the presence of tBP. This suggests that the suppression of the efficiency of electron injection is due to the increase in ECB by the adsorption of tBP onto the TiO2 surface. Note that the tBP effect is more pronounced for films with lower dye loading. This clearly indicates that the adsorption of tBP onto the surface is protected by densely adsorbed N719 dye on the surface. To discuss the effect of dye coverage in detail, all data points of Urel inj shown in Fig. 4 are plotted as a function of surface density
R. Katoh et al. / Chemical Physics Letters 489 (2010) 202–206
completely protect the association of tBP on the TiO2 surface at the given concentration. The Urel inj obtained by extrapolating the data to ds = 0 gives the efficiency without protection by dye adsorption on the surface at the given concentration of tBP. It seems that the change in the efficiency is proportional to the logarithm of the concentration of tBP. It is known that ECB of TiO2 is proportional to the logarithm of the concentration of protons, i.e., pH [30]. If the effect of tBP occurs in the same manner, this suggests Uinj is proportional to ECB. The change in ECB by the addition of 1 M tBP can be estimated to be about 0.2 eV from the observed VOC change [21–26]. Thus, a linear relationship between Uinj and log [tBP] is not surprising because we observed that Uinj changes linearly within a very small energy region of free energy change (DG) for various dye-sensitized nanocrystalline ZnO films [13]. Notably, the electron injection efficiency was still affected by the tBP in the higher concentration region tested here (0.3–1 M). This concentration at 1 M corresponded to a 1:19 M ratio between tBP and MeCN. These results indicate that the adsorption of tBP on the surface of TiO2 was not effective.
36 h
2h 0.8 0.5 h 0.6 0.5 h
in MeCN (0M tBP)
Absorbance
Relative efficiency of electron injection
1
0.4
205
in 1M tBP
0.2 0
0 -0.2
0
0.2
50 Time / μs
0.4
0.6
4. Conclusion
100
0.8
1
1.2
[tBP] / M Fig. 4. Relative electron injection efficiency Urel inj ¼ Uinj ðtBPÞ=Uinj ð0Þ as a function of tBP concentration of the films prepared with different immersion times ti = 0.5, 2, and 36 h. Inset: A typical transient absorption (TA) signal of the film prepared by immersion time ti = 0.5 h in acetonitrile and in 1 M tBP solution observed at 800 nm.
The effect of tBP on the photo-induced electron injection process in N719-sensitized nanocrystalline TiO2 films was studied by TA spectroscopy. We evaluated the electron injection efficiency as a function of dye concentration and tBP. We found that the electron injection efficiency is suppressed by the addition of tBP. This tBP effect is suppressed in films with higher dye loading. This suggests that sensitizer dye densely adsorbed onto the surface can protect the surface from association with tBP. These results imply that the effect of dye coverage should be optimized to obtain high performance in DSSCs. Acknowledgment
1
We gratefully acknowledge financial support from the New Energy and Industrial Technology Development Organization (NEDO).
0.9 0.1 M
Φinj
References 0.8 0.3 M
0.7 1M
0.5 h 2h 36 h
0.6
0
0.1
0.2
0.3
0.4
0.5
ds / nm-2 . Fig. 5. Relative electron injection efficiency Urel inj ¼ Uinj ðtBPÞ Uinj ð0Þ as a function of density of dye adsorbed on the surface.
ds = 1/Sd of dye on the surface (Fig. 5). At ds = 0.45 nm2 obtained by the extrapolation of the lines to Urel inj ¼ 1, no change of the efficiency of electron injection is expected. This density corresponds to Sd = 2.2 nm2, which is larger than the estimated size of an N3 dye molecule (1 nm2 [28,29]). Although direct comparison between these values is not easy as mentioned above, N719 dyes
[1] M.K. Nazeeruddin et al., J. Am. Chem. Soc. 115 (1993) 6382. [2] M.K. Nazeeruddin et al., J. Am. Chem. Soc. 127 (2005) 16835. [3] J.B. Asbury, E. Hao, Y.Q. Wang, H.N. Ghosh, T.Q. Lian, J. Phys. Chem. B 105 (2001) 4545. [4] R. Katoh, A. Furube, A.V. Barzykin, H. Arakawa, M. Tachiya, Coord. Chem. Rev. 248 (2004) 1195. [5] J.R. Durrant, S.A. Haque, E. Palomares, Chem. Commun. (2006) 3279. [6] B. Wenger, M. Grätzel, J.E. Moser, J. Am. Chem. Soc. 127 (2005) 12150. [7] M. Pellnor, P. Myllyperkiö, J. Korppi-Tommola, A. Yartsev, V. Sundström, Chem. Phys. Lett. 462 (2008) 205. [8] R. Katoh, M. Kasuya, A. Furube, N. Fuke, N. Koide, L.Y. Han, Chem. Phys. Lett. 471 (2009) 280. [9] C.A. Kelly, G.J. Meyer, Coord. Chem. Rev. 211 (2001) 295. [10] T. Yoshihara et al., J. Phys. Chem. B 108 (2004) 2643. [11] R. Katoh et al., J. Phys. Chem. B 108 (2004) 4818. [12] R. Katoh, A. Furube, M. Murai, Y. Tamaki, K. Hara, M. Tachiya, C. R. Chimie 9 (2006) 639. [13] R. Katoh, A. Furube, K. Hara, S. Murata, H. Sugihara, H. Arakawa, M. Tachiya, J. Phys. Chem. B 106 (2002) 12957. [14] R. Katoh, A. Huijser, K. Hara, T.J. Savenije, L.D.A. Siebbeles, J. Phys. Chem. C 111 (2007) 10741. [15] R. Katoh, A. Furube, M. Kasuya, N. Fuke, N. Koide, L. Han, J. Mater. Chem. 17 (2007) 3190. [16] R. Katoh, M. Kasuya, S. Kodate, A. Furube, N. Fuke, N. Koide, J. Phys. Chem. C 113 (2009) 20738. [17] A.C. Khazraji, S. Hatchandani, S. Das, P. Kamat, J. Phys. Chem. B 103 (1999) 4693. [18] S. Kambe, K. Murakoshi, T. Kitamura, Y. Wada, S. Yanagida, H. Kominami, Y. Kera, Sol. Energy Mater. Sol. Cells 61 (2000) 427. [19] K. Keis, J. Lindgren, S.E. Lindquist, A. Hagfeldt, Langmuir 16 (2000) 4688. [20] N. Hirata, Ph.D. Thesis, Imperial College, London, UK, 2005.
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[21] S. Nakade, T. Kanzaki, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, J. Phys. Chem. B 109 (2005) 3480. [22] S.A. Haque, E. Palomares, B.M. Cho, A.N.M. Green, N. Hirata, D.R. Klug, J.R. Durrant, J. Am. Chem. Soc. 127 (2005) 3456. [23] M.K. Nazeeruddin et al., J. Am. Chem. Soc. 115 (1993) 6382. [24] S.Y. Huang, G. Schlichthörl, A.J. Nozik, M. Grätzel, A.J. Frank, J. Phys. Chem. B 101 (1997) 2576. [25] H. Kusama, Y. Konishi, H. Sugihara, H. Arakawa, Sol. Energy Mater. Sol. Cells 80 (2003) 167.
[26] G. Boschloo, L. Haggman, A. Hagfeldt, J. Phys. Chem. B 110 (2006) 13144. [27] M. Dürr, A. Schmid, M. Obermaier, A. Yasuda, G. Nelles, J. Phys. Chem. A 109 (2005) 3967. [28] V. Shklover, Y.E. Ovchinnikov, L.S. Braginsky, S.M. Zakeeruddin, M. Grätzel, Chem. Matter. 10 (1998) 2533. [29] A. Sasahara, C.L. Pang, H. Onishi, J. Phys. Chem. B 110 (2006) 4751. [30] T. Watanabe, A. Fujishima, O. Tatsuoki, K. Honda, Bull. Chem. Soc. Jpn. 49 (1976) 8.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Effect of dye coverage on photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO2 films Ryuzi Katoh a,*, Nobuhiro Fuke b, Akihiro Furube a, Naoki Koide b a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan SHARP Corporation, 282-1 Hajikami, Katsuragi, Nara 639-2198, Japan
a r t i c l e
i n f o
Article history: Received 22 October 2009 In final form 28 February 2010 Available online 3 March 2010
a b s t r a c t The effect of dye coverage on the photo-induced electron injection efficiency in N719-sensitized nanocrystalline TiO2 films was studied by transient absorption spectroscopy. We evaluated the electron injection efficiency of films with several dye concentrations on the surface and found no significant difference between them. We also studied the suppression of the efficiency by the addition of tBP (4-tert-butylpyridine) and found that the effect of tBP was more pronounced for films with lower dye loading. This suggests that sensitizer dyes adsorbed densely onto the surface suppress the adsorption of tBP. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Since highly efficient dye-sensitized solar cells (DSSCs) were first reported [1], substantial research has been carried out to improve their performance. N3 dye [cis-di(thiocyanato)-bis(2,20 bipyridyl-4,40 -dicarboxylate)ruthenium(II); Ru(dcbpy)2(NCS)2] is one of the most commonly used sensitizing dyes for high-performance DSSCs. Nazeeruddin et al. studied modifications of this dye and found that one of its derivatives, namely N719 (Fig. 1), in which two protons are replaced by tetrabutylammonium cations (TBA), shows high solar-energy-to-electricity-conversion efficiency (g > 11%) [2]. Despite much effort toward the development of high-performance solar cells, their efficiency has not been improved dramatically. Thus, many studies of the primary processes in such cells have been conducted in order to determine the factors controlling their performance. One of the most important primary processes is electron injection from excited dyes to semiconductor films. Thus, the photochemical process of dye-sensitized nanocrystalline films, which serve as active electrodes in DSSCs, has been studied by means of transient absorption (TA) spectroscopy in several research groups [3–16]. Using femtosecond TA spectroscopy, nonexponential ultrafast electron injection was observed in the 100fs to 100-ps time range for N3 dyes adsorbed on nanocrystalline TiO2 films (N3/TiO2) [3–7] and for N719/TiO2 [6,7]. Since recombination between injected electrons and parent cations occurs slowly in the millisecond time range [5,8], the efficiency of electron injection (Uinj) can be evaluated through microsecond TA measurements. We estimated the absolute values of Uinj to be almost unity for N3/TiO2 [10,11] and N719/TiO2 [14]. To clarify the factors * Corresponding author. Fax: +81 29 861 5301. E-mail address: [email protected] (R. Katoh). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.02.076
controlling electron injection efficiency and for further development of high-performance solar cell devices, it is very important to understand the primary processes in DSSCs. In this context, we have studied Uinj under different experimental conditions, such as various excitation wavelengths [12], free energy changes [13], particle sizes [14], and additives [15,16]. The coverage of sensitizer dyes on nanocrystalline semiconductor films is a factor of considerable importance for controlling the performance of DSSCs because performance reduction for highly loaded films has frequently been reported [17–20]. To clarify the origin of the lowered performance, TA measurements were examined as a function of dye concentration. Hirata found that electron injection efficiency decreases with increasing concentration of dye on the surface. They explained that this is due to multi-layer adsorption of dyes on the surface [20]. Moser and co-workers reported that the electron injection rate was reduced by the aggregation of N3/TiO2 films by comparing them with N719/TiO2 films, in which aggregation is expected to be reduced by bulky TBA groups [6]. Recently, Pellnor et al. carefully studied the difference in electron injection dynamics between N3/TiO2 and N719/TiO2 by varying experimental conditions such as the observed wavelength and the density of dye on the surface [7]. Although they pointed out that the injection dynamics difference between N3/TiO2 and N719/TiO2 was mainly due to the difference in temporal spectral change, they observed that the injection dynamics were affected by the density of dye on the surface for N3/TiO2 films. Hence, more careful studies are needed to clarify the effect of dye coverage on the electron injection process. The coverage of sensitizer dyes on nanocrystalline semiconductor films is also a factor of importance for controlling the electron loss process, i.e., the reaction between the injected electrons and in an electrolyte. This is because such reactions triiodide ions I 3 occur at the bare surface of dye-sensitized films, which is
R. Katoh et al. / Chemical Physics Letters 489 (2010) 202–206
amplified with an AC-coupled pre-amplifier (NF Electronic Instruments, SA-230F5) and a voltage amplifier (NF Electronic Instruments, 5305). Signals were processed with a digital oscilloscope (Tektronix, TDS380) and analyzed with a computer. Using the pre-amplifier, the DC offset of the photocurrent from the detector could be subtracted, and therefore a small absorbance change (<105) could be detected. The time resolution of the system was about 500 ns. To calculate the absorbance change, it was necessary to measure the intensity of the probe light without laser excitation by modulating the probe light intensity with an optical chopper placed in front of the detector. All measurements were carried out at room temperature.
2.5 N719 / TiO2 2
COOR
36 h (538 nm)
HOOC
N NCS
N Ru
Absorbance
N
1.5
HOOC
2 h (537 nm)
1
0.5
203
NCS N
COOR R = TBA (tetrabutylammonium)
0.5 h (534 nm)
3. Results and discussion 3.1. Dye coverage
0 400
450
500
550 600 650 700 Wavelength / nm
750
800
Fig. 1. Ground state absorption spectra of N719 dye adsorbed on TiO2 nanocrystalline films prepared with different immersion times ti = 0.5, 2, and 36 h. The wavelength values in parentheses indicate the peak positions. The molecular structure of N719 is also shown.
confirmed by the observation that the lifetime of the injected electron decreases with decreasing dye coverage [21]. To reduce the effect, small additive molecules, such as of 4-tert-butylpyridine (tBP), have been used as an adsorbate to cover the vacancies on the surface. Another function of tBP is to increase the energy level of the conduction band edge ECB. In fact, tBP improves the open-circuit voltage (VOC) and fill factor (FF) of DSSC devices [21–26]. However, short-circuit current (JSC) is often suppressed. This suggests that Uinj is reduced by the addition of tBP, and we observed such an effect in a black dye-sensitized system [15,16]. The effect of tBP is controlled by the density of tBP adsorbed on the surface and therefore the effect of tBP on Uinj may be affected by dye coverage on the surface, namely, bare surface area is required to realize the function of tBP molecules. Here we evaluate Uinj quantitatively by using a microsecond time-resolved TA technique to clarify the relationship between dye coverage and the tBP effect on Uinj in N719/TiO2 films.
2. Experimental N719 was purchased from Solaronix SA. TiO2 nanocrystalline films were prepared from a commercially available TiO2 paste (Solaronix SA, T/SP series). The paste was painted onto a glass plate substrate with a screen printer. Nanocrystalline films were prepared by calcination of the painted substrate for 1 h at 450 °C. The films obtained had an area of 10 10 mm2 and a thickness of 13 lm. The N719 dyes were dissolved at a concentration of 0.3 mM, which is close to the solubility limit in a solvent consisting of a 1:1 mixture of tert-butylalcohol and acetonitrile. The nanocrystalline TiO2 films were immersed into the dye solution, so that the dye could adsorb onto the semiconductor surface. The films were rinsed in ethanol, and then dried in air. For the TA measurement, a Nd3+:YAG laser (HOYA Continuum, Surelite II) was used for the pumping light source. The repetition rate of the laser was 10 Hz. The second-harmonic pulse (532 nm) was used for the excitation. A halogen lamp (100 W) was used as the probe light source. The light transmitted through the sample specimen was detected with a Si photodiode (Hamamatsu, S1722) after being dispersed with a monochromator (Acton Research, SpectraPro-150). The photocurrent from the detector was
Fig. 1 shows the ground-state absorption spectra of N719 dye adsorbed on TiO2 nanocrystalline films prepared with different immersion times ti. The distinct peaks at around 535 nm are due to the metal-to-ligand charge transfer (MLCT) absorption band characteristic of N719 dye. The peak positions of the MLCT band (Fig. 1) shift slightly toward longer wavelengths with increasing concentration of N719 dye. This suggests that the peak position is affected by the intermolecular interaction between dye molecules. The number of dye molecules Ddye per unit area of the film was estimated assuming that the absorption coefficient of N719 is the same as that of N3 dye (e = 14 000 dm3 mol1 cm1 at the peak). The number of particles Dparticle per unit area on the film can be estimated to be Dparticle = 1.5 1014 cm2 by using the following values: radius of a TiO2 particle rp = 10 nm, thickness of the film dfilm = 13 lm, and porosity of the film P = 0.5. Accordingly, for the film prepared with ti = 0.5 h, 100 dye molecules adsorb on one particle and the area Sd occupied by an N719 dye molecule is estimated to be 13 nm2. For the films prepared with ti = 2 and 36 h, Sd is estimated to be 5.1 and 2.7 nm2, respectively. Although we assumed homogeneous adsorption of N719 dyes in nanocrystalline films in depth direction, there has been argued heterogeneous adsorption [27]. In the present study, we confirmed that local density of dye increases with ti from the results of the peak shift of ground-state absorption and transient absorption spectra. For detail discussion, direct evaluation of depth profile of dye concentration is required. The absorbances of the films prepared with ti > 12 h were almost the same indicating that the adsorption of dye molecules was saturated. Even under the saturated condition, Sd (2.7 nm2 at ti = 36 h) was larger than the size of an N3 dye molecule, which was estimated to be 1 nm2 [28,29]. There are two possible reasons for this. One is that adsorption of dye is not saturated under this condition; in other words, a higher concentration of dye solution is required. In the present study, this scenario is impossible because the concentration of dye solution is close to the solubility limit. Another possibility is that the model used to estimate the surface area is too simple. Necking between particles should be considered. Thus, the estimated values give an upper limit of the surface area of the film. 3.2. Effect of dye concentration on the efficiency of electron injection Fig. 2 shows TA spectra of N719/TiO2 films immersed in acetonitrile recorded 20–40 ls after excitation normalized by the number of absorbed photons, which is represented by (1–10A), where A is the absorbance at the excitation wavelength (532 nm). Peaks around 800 nm were observed, which can be assigned to the cation
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0.0006
1.2 0.5 h 2h 36 h
in MeCN
1
0.5 h tBP 0M
0.8
tBP 1M
0.6
0.0004
0.4 0.2
0.0003
0 0.0002 0.0001 0 650
700
750 800 850 Wavelength / nm
900
950
Fig. 2. Transient absorption (TA) spectra of N719/TiO2 films prepared with different immersion times ti = 0.5, 2, and 36 h in acetonitrile recorded 20–40 ls after excitation. The spectra are normalized by the number of absorbed photons.
Normalized absorbance
Absorbance / (1-10-A)
0.0005
1.2 1
2h
tBP 0M tBP 1M
0.8 0.6 0.4 0.2 0
1.2 state of N719 dye [8]. The position of the peak due to the N719 cations seems to shift slightly toward longer wavelengths with increasing dye concentration (Fig. 2). Because the ground-state absorption spectra (Fig. 1) were not sensitive to the dye concentration, this shift is difficult to explain by a change in the contribution of ground-state bleaching in the TA spectra. Thus, a likely explanation of the spectral shift is that it arises from a change in electronic structure due to intermolecular interaction. Although TA spectra due to N719 cations change with the concentration (Fig. 2), the absorbances at the peak wavelengths are similar to each other when normalized by the number of absorbed photons. This suggests that the electron injection efficiency is not sensitive to the dye concentration on the surface. Hirata reported that a reduction of electron injection efficiency occurs for highly loaded films in N719/TiO2 through TA measurements [20]. This was explained as being due to multi-layer dye adsorption in highly loaded films. In fact, the Sd in these highly loaded samples was estimated to be smaller than 1 nm2, which is the size of the dye molecule. Accordingly, the situation of Hirata’s study is different from the present study; namely, we conducted our studies under less than full coverage density conditions.
3.3. Effect of tBP on the efficiency of electron injection To understand the effect of tBP on the electronic structure of N719 dye on a TiO2 surface, the TA spectra of N719/TiO2 films in a 1 M tBP solution of acetonitrile were measured (Fig. 3). These spectra were normalized at the peak. No spectral shape change was observed following the addition of tBP, suggesting that it did not change the electronic structure. Note that the relative efficiency of the electron injection as a function of tBP concentration can be evaluated by observing the TA signal at a particular wavelength. The inset in Fig. 4 shows the time profile of the TA signal for ti = 0.5 h film recorded at 800 nm. The absorbance decreases in the presence of tBP, indicating suppression of the electron injection efficiency. Decay of these signals reflects charge recombination and the effect of tBP on the recombination kinetics has been discussed previously [16]. The absolute value of the efficiency of the electron injection Uinj can be estimated by using the values of the absorbance change due to the generation of the oxidized form of the sensitizer dye (DAox) and its molar absorption coefficients eox:
1
36 h
tBP 0M tBP 1M
0.8 0.6 0.4 0.2 0 650
700
750 800 850 900 Wavelength / nm
950 1000
Fig. 3. Transient absorption (TA) spectra, after normalization at the peaks, of N719/ TiO2 films prepared with different immersion times ti = 0.5, 2, and 36 h in acetonitrile and in 1 M tBP solution recorded 20–40 ls after excitation.
Uinj ¼
DAox
eox ð1 TÞN0
ð1Þ
where T and N0 represent the transmittance of excitation light and the number of incident excitation photons per unit area, respectively. Earlier we estimated the value of eox for N3 dye at the peak to be 6000 mol1 dm3 cm1 [10]. Assuming that N719 has the same value, which is reasonable because the molecular structure of N3 dye is very similar to that of N719, we estimate the electron injection efficiency to be about 0.8. Based on our previous experiments, this is a reasonable value [10,14]. This high efficiency also indicates that there is no efficient fast recombination between an injected electron and a parent dye-cation. By comparing the absorbance in tBP solution at time = 0 ls with that in neat acetonitrile, the relative electron injection efficiency Urel inj ¼ Uinj ðtBPÞ=Uinj ð0Þ as a function of tBP concentration can be obtained (Fig. 4). Urel inj decreases with increasing tBP concentration and tends to saturate at higher tBP concentration. For the film with ti = 0.5 h, the electron injection efficiency decreased by 33% in a 1 M tBP solution. As discussed above, the electronic structure of N719 dye is not affected by the presence of tBP. This suggests that the suppression of the efficiency of electron injection is due to the increase in ECB by the adsorption of tBP onto the TiO2 surface. Note that the tBP effect is more pronounced for films with lower dye loading. This clearly indicates that the adsorption of tBP onto the surface is protected by densely adsorbed N719 dye on the surface. To discuss the effect of dye coverage in detail, all data points of Urel inj shown in Fig. 4 are plotted as a function of surface density
R. Katoh et al. / Chemical Physics Letters 489 (2010) 202–206
completely protect the association of tBP on the TiO2 surface at the given concentration. The Urel inj obtained by extrapolating the data to ds = 0 gives the efficiency without protection by dye adsorption on the surface at the given concentration of tBP. It seems that the change in the efficiency is proportional to the logarithm of the concentration of tBP. It is known that ECB of TiO2 is proportional to the logarithm of the concentration of protons, i.e., pH [30]. If the effect of tBP occurs in the same manner, this suggests Uinj is proportional to ECB. The change in ECB by the addition of 1 M tBP can be estimated to be about 0.2 eV from the observed VOC change [21–26]. Thus, a linear relationship between Uinj and log [tBP] is not surprising because we observed that Uinj changes linearly within a very small energy region of free energy change (DG) for various dye-sensitized nanocrystalline ZnO films [13]. Notably, the electron injection efficiency was still affected by the tBP in the higher concentration region tested here (0.3–1 M). This concentration at 1 M corresponded to a 1:19 M ratio between tBP and MeCN. These results indicate that the adsorption of tBP on the surface of TiO2 was not effective.
36 h
2h 0.8 0.5 h 0.6 0.5 h
in MeCN (0M tBP)
Absorbance
Relative efficiency of electron injection
1
0.4
205
in 1M tBP
0.2 0
0 -0.2
0
0.2
50 Time / μs
0.4
0.6
4. Conclusion
100
0.8
1
1.2
[tBP] / M Fig. 4. Relative electron injection efficiency Urel inj ¼ Uinj ðtBPÞ=Uinj ð0Þ as a function of tBP concentration of the films prepared with different immersion times ti = 0.5, 2, and 36 h. Inset: A typical transient absorption (TA) signal of the film prepared by immersion time ti = 0.5 h in acetonitrile and in 1 M tBP solution observed at 800 nm.
The effect of tBP on the photo-induced electron injection process in N719-sensitized nanocrystalline TiO2 films was studied by TA spectroscopy. We evaluated the electron injection efficiency as a function of dye concentration and tBP. We found that the electron injection efficiency is suppressed by the addition of tBP. This tBP effect is suppressed in films with higher dye loading. This suggests that sensitizer dye densely adsorbed onto the surface can protect the surface from association with tBP. These results imply that the effect of dye coverage should be optimized to obtain high performance in DSSCs. Acknowledgment
1
We gratefully acknowledge financial support from the New Energy and Industrial Technology Development Organization (NEDO).
0.9 0.1 M
Φinj
References 0.8 0.3 M
0.7 1M
0.5 h 2h 36 h
0.6
0
0.1
0.2
0.3
0.4
0.5
ds / nm-2 . Fig. 5. Relative electron injection efficiency Urel inj ¼ Uinj ðtBPÞ Uinj ð0Þ as a function of density of dye adsorbed on the surface.
ds = 1/Sd of dye on the surface (Fig. 5). At ds = 0.45 nm2 obtained by the extrapolation of the lines to Urel inj ¼ 1, no change of the efficiency of electron injection is expected. This density corresponds to Sd = 2.2 nm2, which is larger than the estimated size of an N3 dye molecule (1 nm2 [28,29]). Although direct comparison between these values is not easy as mentioned above, N719 dyes
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