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Effect of dye concentration on electron injection efficiency in nanocrystalline TiO2 films sensitized with N719 dye
Chemical Physics Letters 511 (2011) 336–339
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Effect of dye concentration on electron injection efficiency in nanocrystalline TiO2 films sensitized with N719 dye Ryuzi Katoh ⇑, Kaori Yaguchi, Akihiro Furube National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2011 In final form 16 June 2011 Available online 21 June 2011
a b s t r a c t The efficiency of electron injection from excited N719 dye to nanocrystalline TiO2 films was studied in films with several different dye loads by means of time-resolved microwave conductivity measurements. High-efficiency electron injection was observed in all films. The adsorption data could be fitted to the Langmuir-type isotherm, and the absorption spectra of N719 adsorbed on the TiO2 films did not vary with dye loading. These findings suggest that no dye aggregation occurred on the films even at full coverage condition. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction N719 dye, cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 - dicarboxylato)ruthenium(II) bis-tetrabutylammonium (Ru(dcbpy)2 (NCS)2 2TBA), is a well-known sensitizing dye for solar cells. Dye-sensitized solar cells (DSSCs) consisting of N719 dye adsorbed on nanocrystalline TiO2 films show high solar-energy-to-electricity-conversion efficiency (g > 10% under AM 1.5 irradiation) [1]. Although DSSCs prepared with newly synthesized Ru-complex dyes [2] and organic dyes [3] as sensitizers have been studied extensively, N719 dye is still used one of the best sensitizers. Various methods have been proposed for improving the performance of DSSCs. One important issue is the aggregation of the sensitizer dyes on nanocrystalline semiconductor films. Several studies have shown that solar cell performance is reduced at high dye loads [4–6]. It has been argued this reduction might be due to dye aggregation on TiO2 surfaces, although no strong evidence for aggregation has been presented. Some investigators have reported that aggregation reduces electron injection efficiency (Uinj). For example, Hirata found that Uinj decreases as the concentration of dye on the surface increases, and they attributed the decreased efficiency to multilayer adsorption of dye on the surface [5]. Moser and co-workers compared N3/TiO2 films with N719/TiO2 films, in which aggregation is expected to be reduced by the bulky tetrabutylammonium groups, and found that dye aggregation reduces the electron injection rate in the N3/TiO2 films [6]. Recently, Pellnor et al. carefully studied the difference in the dynamics of electron injection in N3/TiO2 and N719/TiO2 DSSCs by varying experimental conditions such as the observed wavelength and the surface den-
sity of the dye [7]. Although these investigators pointed out that the difference in injection dynamics between N3/TiO2 and N719/ TiO2 was due mainly to a difference in temporal spectral change, they observed that for N3/TiO2 films, the injection dynamics were affected by the density of dye on the surface. Therefore, additional careful studies, especially quantitative evaluation of Uinj, are needed for clarification of the effect of dye loading on electron injection. Transient absorption (TA) spectroscopy is a powerful tool for evaluation of Uinj [8,9]. However, conventional TA spectroscopy in the visible region detects only the dye cation produced by electron injection. Thus, estimation of the absorption coefficient of the dye cation leads to uncertainty in the estimate of the injection efficiency. To eliminate this uncertainty, we have developed experimental techniques to observe electrons injected in TiO2. TA spectroscopy in the near-IR region (1000–3000 nm) gives a signal from electrons in TiO2. Unfortunately, some dye cations also absorb strongly in this region [10]. An alternative technique is timeresolved microwave conductivity (TRMC) measurement, which permits the detection of conducting electrons only. Using TRMC, systematically evaluated the efficiency of electron injection [11,12]. Here we studied N719 dye-sensitized nanocrystalline TiO2 films under several dye-loading conditions by measuring adsorption isotherms and absorption spectra of N719 adsorbed on TiO2 films, and we used TRMC to evaluate Uinj. On the basis of our results, we discuss the effect of aggregation.
2. Experimental ⇑ Corresponding author. Present address: Nihon University, College of Engineering, Department of Chemical Biology and Applied Chemistry, Tamura, Koriyama, Fukushima 963-8642, Japan. Fax: +81 29 861 4517. E-mail addresses: [email protected], [email protected] (R. Katoh). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.06.046
N719 dye was purchased from Solaronix and used as received. An organic paste containing TiO2 nanoparticles (Solaronix, T/SP) was screen-printed onto glass slides, and nanocrystalline films
337
R. Katoh et al. / Chemical Physics Letters 511 (2011) 336–339
were prepared by calcination of the printed substrates for 1 h at 800 K. The resulting films had an area of 2 cm2 (2 1 cm) and a thickness of about 5 lm. The films were immersed for 12 h in an ethanol solution of N719 at various concentrations and were then dried in air and used immediately for TRMC measurements. All measurements were carried out just after films were dried, to minimize the effect of dye degradation. We determined the number of dye molecules adsorbed on the surface of the films by acquiring absorption spectra of the solutions obtained by rinsing the dried films with 0.1 M aqueous NaOH. Absorption spectra of the N719 dye nanocrystalline films were also obtained. All spectra were measured with a conventional absorption spectrophotometer (Shimadzu, UV-3100PC). For TRMC measurements, a home-made apparatus was used [12]. Microwaves were generated by an oscillator based on a Gunn diode (Nakadai, MGS-15B, 200 mW) equipped with a varactor diode for tuning the frequency of the microwaves from 8.5 to 10 GHz (X-band). The microwaves travelled toward the sample cavity through a circulator. The Q-value of the cavity under the present experimental conditions was about 200, and the time resolution was about 20 ns. An optical parametric oscillator (OPO) system (Spectra Physics, MOPO-SL) excited by a Nd3+:YAG laser (Spectra Physics, Pro-230-10) was used to generate 524-nm light. The duration of each laser pulse was about 10 ns. Reflected microwaves were detected with a diode (NEC, IN23WE). Signals were processed with a digital oscilloscope (Tektronix, TDS5032) and analyzed with a computer.
3. Results and discussion We determined the number of N719 dye molecules adsorbed on the TiO2 surface by observing the absorption spectra of the 0.1 M aqueous NaOH solutions obtained by rinsing the dye molecules from the dye-sensitized films. A plot of the absorbance of the rinse solutions as a function of the dye concentration in the solutions used to prepare the films indicated that the number of adsorbed dye molecules increased with increasing dye concentration in the solution and eventually reached a saturation value (Figure 1). The results were fitted by a Langmuir-type isotherm (solid line). The coverage of dye (h) on the surface can be expressed as [A]obs/ [A]full, where [A]obs is the absorbance of the rinse solutions, and [A]full is the absorbance at full coverage on the surface (saturation
value). Accordingly, the Langmuir-type isotherm can be expressed as:
h¼
½Aobs K½dye ¼ ½Afull 1 þ K½dye
ð1Þ
where K is the adsorption coefficient (M1), and [dye] is the dye concentration in the solutions used to prepare the dye-sensitized films. Using K = 5 103 M1 and [A]full = 0.7, we generated a curve obtained from Eq. (1) that accurately reproduced the experimental data. As we reported previously, the adsorption isotherm of N3 dye, an analog of N719, deviates from the Langmuir-type isotherm at low dye concentrations in the solution, and we tentatively explained this deviation by speculating that the effective pH in the nanopores was higher than the pH of the bulk solution [13]. In the present experiments, the dye concentrations in the solutions were higher than those in the previous study, and therefore no such deviation was observed. The area (S) occupied by a single N719 dye molecule on the TiO2 surface can be estimated from the peak absorbance (A) and the absorption coefficient (e) in alkaline solution (e = 1.35 104 dm3 mol1 cm1 at 500 nm). We estimated the area occupied by the dye under full-coverage conditions (Sfull) to be 1 nm2 by using a porosity value of 0.5 and a particle diameter of 10 nm. This occupied area is the same as the molecular size of N719 (1 nm2) [14,15], which indicates that N719 dye molecules were densely adsorbed on the TiO2 surface and thus that N719 adsorbed on the TiO2 surface as a dense monolayer film. To determine whether aggregation occurred as the dye concentration was increased, we obtained absorption spectra of N719/ TiO2 films prepared from 0.1, 0.5, and 1 mM solutions of N719 dye. The spectra were clearly similar to one another (Figure 2). Aggregation of organic molecules has been probed by observing shifts in their absorption spectra. In general, H-type aggregation, in which transition dipoles are parallel, results in blue-shifted spectra, whereas J-type aggregation, in which transition dipoles are oriented head-to-tail, results in red-shifted spectra [1]. The spectra in Figure 2 showed no evidence of strong interactions between N719 dye molecules adsorbed on the TiO2 surface. We used TRMC measurements to evaluate the efficiency of electron injection in the films prepared from the three solutions. As previously reported [11,12], when no decay occurs during the response time of the cavity, the product of Uinj and the mobility of the charge carriers (l is obtained from the maximum TRMC signal intensity, i.e., change of microwave power [DP/P)MAX]:
0.8 1
K = 5 x 103 M-1 [A]full = 0.7
0.6
COOR
0.5
Normalized Absorbance
Peak absorbance
0.7
COOR
0.4
HOOC
N NCS
N Ru
0.3
N HOOC
0.2
NCS N
COOR R = TBA (tetrabutylammonium)
0.1
HOOC
0.8
N NCS
N Ru N
0.6 0.4
NCS N
HOOC
COOR R = TBA (tetrabutylammonium)
0.1 mM 0.5 mM
0.2
1 mM
0 0
0.2
0.4
0.6
0.8
1
1.2
[N719] / mM
0 400
500
600
700
800
Wavelength / nm Figure 1. Peak absorbance of the rinse solutions as a function of dye concentration in the solutions used to prepare the dye-sensitized films. The solid line represents the fitting results based on the Langmuir-type isotherm.
Figure 2. Absorption spectra of N719/TiO2 films prepared from 0.1, 0.5, and 1 mM N719 solutions. The spectra were normalized at the peak.
R. Katoh et al. / Chemical Physics Letters 511 (2011) 336–339
0.08 0.1 0.06
0.04 0.1 mM (Iex = 0.14 mJcm-2)
0.02
0.5 mM (Iex = 0.07 mJcm-2) 1 mM (Iex = 0.07 mJcm-2)
0 -0.05
0
0.05
0.1
0.15
0.2
Time / micro sec
(-ΔP/P)MAX / (Iex (1-10 -A) ) [mJ -1cm2]
ex
-A
TRMC signal / (I (1-10 )) [mJ-1 cm2]
338
1 mM 0.5 mM
Figure 3. TRMC signal intensity (DP/P) divided by the number of absorbed photons [Iex(1–10A)] recorded at an excitation density of ca. 5 1017 cm3. Excitation intensities are also shown.
Uinj l ¼
C A
Iex ð1 10 Þ
ð
DP Þ P MAX
0.1 mM
0.01
ð2Þ
1016
1017
1018
1019
Excitation density / cm-3 where C is a constant, Iex is the intensity of excitation light, and A is the absorbance at the excitation wavelength. Iex(1–10A) represents the number of absorbed photons. We prepared plots of TRMC signals versus time for the three films observed at an excitation density of ca. 5 1017 cm3 (typical plots are shown in Figure 3). Signal intensity was divided by the number of absorbed photons [Iex(1–10A)]. Although the excitation density dependence of the signal intensity will be discussed below, the fact that similar (DP/P)MAX values were obtained at the given excitation density means that Uinj did not depend on dye loading. This also indicates that l did not depend on dye loading, suggesting that the number of traps is not affected by the dye loading. Note that the decay rate of TRMC signal of the film with a low dye load was slower than that of the film with a high dye load. Although the origin of the decay rate for TRMC signal intensity is still under debate [11,12,16], the decay profile reflects the rate of recombination between electrons and dye cations, the rate of electron trapping into deeper levels, or both. The results shown in Figure 3 suggest that dyes adsorbed on the surface of TiO2 affected the reactivity of electrons in the TiO2. For nanocrystalline TiO2 films, Uinjl is sensitive to the density of electrons generated in the TiO2 particles [11,12,17]. At lower electron densities, almost all the generated electrons are effectively trapped, and therefore their effective mobility becomes very small; whereas at higher electron densities, many electrons can move freely because the traps are ‘filled’ with other electrons (trap filling effect). Thus, a large Uinjl can be expected at higher electron densities. At higher electron densities, Uinjl decreases by efficient secondary charge recombination. Accordingly, the bell-shaped dependence of plots of (DP/P)MAX divided by the number of absorbed photons [Iex(1–10A)] versus excitation density can be observed (Figure 4). The similarity of the plots for the three N719/TiO2 films clearly shows that the efficiency of electron injection did not depend on dye loading. The performance of solar cells prepared from electrodes with high dye loads is reported to be suppressed [6], and it has been argued that the suppression is due to aggregation of dyes on the surface of TiO2. However, the results shown in Figure 4 clearly indicate that the electron injection efficiency did not depend on the surface concentration of the dye, even at high dye loading, which suggests
Figure 4. Plot of (DP/P)MAX divided by the number of absorbed photons [Iex(1– 10A)] as a function of excitation density.
that there was no aggregation. This result is consistent with our other experimental findings: that the adsorption isotherm could be fitted by the Langmuir-type isotherm and that the absorption spectra of N719 adsorbed on TiO2 films did not depend on dye loading. The reason for the discrepancy between our results and previous results is unclear. One possibility is that the effect of additives, such as 4-tert-butylpyridine (tBP) and Li ions, on the performance is suppressed when the surface is fully covered with sensitizer dyes. We recently observed that the effect of 4-tert-butylpyridine on electron injection efficiency is sensitive to dye coverage on the TiO2 surface [18]. Another possibility is microcrystal formation in the nanopores of the TiO2 films. Dye-sensitized electrodes for DSSCs are prepared by immersing the electrodes in dye solutions that are near the solubility limit, and therefore the dye solution is likely to be super-saturated. Supersaturation may lead to microcrystallization of dye in the nanopores of the films, and the microcrystals may suppress solar cell performance because they absorb light and hinder the diffusion of redox mediators in the nanopore network of the nanocrystalline TiO2 films. In summary, we used TRMC to evaluate the efficiency of electron injection from excited N719 dye to nanocrystalline TiO2 films in several different dye-loaded films and found no marked difference in Uinj between the films. This result suggests N719 did not aggregate on the TiO2 surface, which is consistent with our findings that the adsorption isotherm could be fitted to a Langmuir-type isotherm and that the absorption spectra of N719 adsorbed on the TiO2 films did not depend on the dye loading. These results indicate that N719 dyes are unlikely to form aggregates under ordinary adsorption conditions. Acknowledgement This research was supported by Strategic International Cooperative Program (Research Exchange Type), Japan Science and Technology Agency (JST).
R. Katoh et al. / Chemical Physics Letters 511 (2011) 336–339
References [1] M.K. Nazeeruddin et al., J. Am. Chem. Soc. 127 (2005) 16835. [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595. [3] H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 42 (2009) 1809. [4] S. Kambe, K. Murakoshi, T. Kitamura, Y. Wada, S. Yanagida, H. Kominami, Y. Kera, Sol. Energy Mater. Sol. Cells 61 (2000) 427. [5] N. Hirata, Ph.D. Thesis, Imperial College, London, UK, 2005. [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] T. Yoshihara et al., J. Phys. Chem. B 108 (2004) 2643. [9] R. Katoh, M. Kasuya, S. Kodate, A. Furube, N. Fuke, N. Koide, J. Phys. Chem. C 113 (2009) 20738.
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[10] R. Katoh, A. Furube, S. Mori, M. Miyashita, K. Sunahara, N. Koumura, K. Hara, Energy Environ. Sci. 2 (2009) 542. [11] R. Katoh, A. Huijser, K. Hara, T.J. Savenije, L.D.A. Siebbeles, J. Phys. Chem. C111 (2007) 10741. [12] R. Katoh, A. Furube, M. Kasuya, N. Fuke, N. Koide, L. Han, Sol. Energy Mater. Sol. Cell 93 (2009) 698. [13] R. Katoh, K. Yaguchi, M. Murai, S. Watanabe, A. Furube, Chem. Phys. Lett. 497 (2010) 48. [14] V. Shklover, Y.E. Ovchinnikov, L.S. Braginsky, S.M. Zakeeruddin, M. Grätzel, Chem. Matter. 10 (1998) 2533. [15] A. Sasahara, C.L. Pang, H. Onishi, J. Phys. Chem. B 110 (2006) 4751. [16] T.J. Savenije, A. Huijser, M.J.W. Vermeulen, R. Katoh, Chem. Phys. Lett. 461 (2008) 93. [17] J.E. Kroeze, T. Savenije, J.M. Warman, J. Am. Chem. Soc. 126 (2004) 7608. [18] R. Katoh, N. Fuke, A. Furube, N. Koide, Chem. Phys. Lett. 489 (2010) 202.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Effect of dye concentration on electron injection efficiency in nanocrystalline TiO2 films sensitized with N719 dye Ryuzi Katoh ⇑, Kaori Yaguchi, Akihiro Furube National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
a r t i c l e
i n f o
Article history: Received 28 March 2011 In final form 16 June 2011 Available online 21 June 2011
a b s t r a c t The efficiency of electron injection from excited N719 dye to nanocrystalline TiO2 films was studied in films with several different dye loads by means of time-resolved microwave conductivity measurements. High-efficiency electron injection was observed in all films. The adsorption data could be fitted to the Langmuir-type isotherm, and the absorption spectra of N719 adsorbed on the TiO2 films did not vary with dye loading. These findings suggest that no dye aggregation occurred on the films even at full coverage condition. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction N719 dye, cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 - dicarboxylato)ruthenium(II) bis-tetrabutylammonium (Ru(dcbpy)2 (NCS)2 2TBA), is a well-known sensitizing dye for solar cells. Dye-sensitized solar cells (DSSCs) consisting of N719 dye adsorbed on nanocrystalline TiO2 films show high solar-energy-to-electricity-conversion efficiency (g > 10% under AM 1.5 irradiation) [1]. Although DSSCs prepared with newly synthesized Ru-complex dyes [2] and organic dyes [3] as sensitizers have been studied extensively, N719 dye is still used one of the best sensitizers. Various methods have been proposed for improving the performance of DSSCs. One important issue is the aggregation of the sensitizer dyes on nanocrystalline semiconductor films. Several studies have shown that solar cell performance is reduced at high dye loads [4–6]. It has been argued this reduction might be due to dye aggregation on TiO2 surfaces, although no strong evidence for aggregation has been presented. Some investigators have reported that aggregation reduces electron injection efficiency (Uinj). For example, Hirata found that Uinj decreases as the concentration of dye on the surface increases, and they attributed the decreased efficiency to multilayer adsorption of dye on the surface [5]. Moser and co-workers compared N3/TiO2 films with N719/TiO2 films, in which aggregation is expected to be reduced by the bulky tetrabutylammonium groups, and found that dye aggregation reduces the electron injection rate in the N3/TiO2 films [6]. Recently, Pellnor et al. carefully studied the difference in the dynamics of electron injection in N3/TiO2 and N719/TiO2 DSSCs by varying experimental conditions such as the observed wavelength and the surface den-
sity of the dye [7]. Although these investigators pointed out that the difference in injection dynamics between N3/TiO2 and N719/ TiO2 was due mainly to a difference in temporal spectral change, they observed that for N3/TiO2 films, the injection dynamics were affected by the density of dye on the surface. Therefore, additional careful studies, especially quantitative evaluation of Uinj, are needed for clarification of the effect of dye loading on electron injection. Transient absorption (TA) spectroscopy is a powerful tool for evaluation of Uinj [8,9]. However, conventional TA spectroscopy in the visible region detects only the dye cation produced by electron injection. Thus, estimation of the absorption coefficient of the dye cation leads to uncertainty in the estimate of the injection efficiency. To eliminate this uncertainty, we have developed experimental techniques to observe electrons injected in TiO2. TA spectroscopy in the near-IR region (1000–3000 nm) gives a signal from electrons in TiO2. Unfortunately, some dye cations also absorb strongly in this region [10]. An alternative technique is timeresolved microwave conductivity (TRMC) measurement, which permits the detection of conducting electrons only. Using TRMC, systematically evaluated the efficiency of electron injection [11,12]. Here we studied N719 dye-sensitized nanocrystalline TiO2 films under several dye-loading conditions by measuring adsorption isotherms and absorption spectra of N719 adsorbed on TiO2 films, and we used TRMC to evaluate Uinj. On the basis of our results, we discuss the effect of aggregation.
2. Experimental ⇑ Corresponding author. Present address: Nihon University, College of Engineering, Department of Chemical Biology and Applied Chemistry, Tamura, Koriyama, Fukushima 963-8642, Japan. Fax: +81 29 861 4517. E-mail addresses: [email protected], [email protected] (R. Katoh). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.06.046
N719 dye was purchased from Solaronix and used as received. An organic paste containing TiO2 nanoparticles (Solaronix, T/SP) was screen-printed onto glass slides, and nanocrystalline films
337
R. Katoh et al. / Chemical Physics Letters 511 (2011) 336–339
were prepared by calcination of the printed substrates for 1 h at 800 K. The resulting films had an area of 2 cm2 (2 1 cm) and a thickness of about 5 lm. The films were immersed for 12 h in an ethanol solution of N719 at various concentrations and were then dried in air and used immediately for TRMC measurements. All measurements were carried out just after films were dried, to minimize the effect of dye degradation. We determined the number of dye molecules adsorbed on the surface of the films by acquiring absorption spectra of the solutions obtained by rinsing the dried films with 0.1 M aqueous NaOH. Absorption spectra of the N719 dye nanocrystalline films were also obtained. All spectra were measured with a conventional absorption spectrophotometer (Shimadzu, UV-3100PC). For TRMC measurements, a home-made apparatus was used [12]. Microwaves were generated by an oscillator based on a Gunn diode (Nakadai, MGS-15B, 200 mW) equipped with a varactor diode for tuning the frequency of the microwaves from 8.5 to 10 GHz (X-band). The microwaves travelled toward the sample cavity through a circulator. The Q-value of the cavity under the present experimental conditions was about 200, and the time resolution was about 20 ns. An optical parametric oscillator (OPO) system (Spectra Physics, MOPO-SL) excited by a Nd3+:YAG laser (Spectra Physics, Pro-230-10) was used to generate 524-nm light. The duration of each laser pulse was about 10 ns. Reflected microwaves were detected with a diode (NEC, IN23WE). Signals were processed with a digital oscilloscope (Tektronix, TDS5032) and analyzed with a computer.
3. Results and discussion We determined the number of N719 dye molecules adsorbed on the TiO2 surface by observing the absorption spectra of the 0.1 M aqueous NaOH solutions obtained by rinsing the dye molecules from the dye-sensitized films. A plot of the absorbance of the rinse solutions as a function of the dye concentration in the solutions used to prepare the films indicated that the number of adsorbed dye molecules increased with increasing dye concentration in the solution and eventually reached a saturation value (Figure 1). The results were fitted by a Langmuir-type isotherm (solid line). The coverage of dye (h) on the surface can be expressed as [A]obs/ [A]full, where [A]obs is the absorbance of the rinse solutions, and [A]full is the absorbance at full coverage on the surface (saturation
value). Accordingly, the Langmuir-type isotherm can be expressed as:
h¼
½Aobs K½dye ¼ ½Afull 1 þ K½dye
ð1Þ
where K is the adsorption coefficient (M1), and [dye] is the dye concentration in the solutions used to prepare the dye-sensitized films. Using K = 5 103 M1 and [A]full = 0.7, we generated a curve obtained from Eq. (1) that accurately reproduced the experimental data. As we reported previously, the adsorption isotherm of N3 dye, an analog of N719, deviates from the Langmuir-type isotherm at low dye concentrations in the solution, and we tentatively explained this deviation by speculating that the effective pH in the nanopores was higher than the pH of the bulk solution [13]. In the present experiments, the dye concentrations in the solutions were higher than those in the previous study, and therefore no such deviation was observed. The area (S) occupied by a single N719 dye molecule on the TiO2 surface can be estimated from the peak absorbance (A) and the absorption coefficient (e) in alkaline solution (e = 1.35 104 dm3 mol1 cm1 at 500 nm). We estimated the area occupied by the dye under full-coverage conditions (Sfull) to be 1 nm2 by using a porosity value of 0.5 and a particle diameter of 10 nm. This occupied area is the same as the molecular size of N719 (1 nm2) [14,15], which indicates that N719 dye molecules were densely adsorbed on the TiO2 surface and thus that N719 adsorbed on the TiO2 surface as a dense monolayer film. To determine whether aggregation occurred as the dye concentration was increased, we obtained absorption spectra of N719/ TiO2 films prepared from 0.1, 0.5, and 1 mM solutions of N719 dye. The spectra were clearly similar to one another (Figure 2). Aggregation of organic molecules has been probed by observing shifts in their absorption spectra. In general, H-type aggregation, in which transition dipoles are parallel, results in blue-shifted spectra, whereas J-type aggregation, in which transition dipoles are oriented head-to-tail, results in red-shifted spectra [1]. The spectra in Figure 2 showed no evidence of strong interactions between N719 dye molecules adsorbed on the TiO2 surface. We used TRMC measurements to evaluate the efficiency of electron injection in the films prepared from the three solutions. As previously reported [11,12], when no decay occurs during the response time of the cavity, the product of Uinj and the mobility of the charge carriers (l is obtained from the maximum TRMC signal intensity, i.e., change of microwave power [DP/P)MAX]:
0.8 1
K = 5 x 103 M-1 [A]full = 0.7
0.6
COOR
0.5
Normalized Absorbance
Peak absorbance
0.7
COOR
0.4
HOOC
N NCS
N Ru
0.3
N HOOC
0.2
NCS N
COOR R = TBA (tetrabutylammonium)
0.1
HOOC
0.8
N NCS
N Ru N
0.6 0.4
NCS N
HOOC
COOR R = TBA (tetrabutylammonium)
0.1 mM 0.5 mM
0.2
1 mM
0 0
0.2
0.4
0.6
0.8
1
1.2
[N719] / mM
0 400
500
600
700
800
Wavelength / nm Figure 1. Peak absorbance of the rinse solutions as a function of dye concentration in the solutions used to prepare the dye-sensitized films. The solid line represents the fitting results based on the Langmuir-type isotherm.
Figure 2. Absorption spectra of N719/TiO2 films prepared from 0.1, 0.5, and 1 mM N719 solutions. The spectra were normalized at the peak.
R. Katoh et al. / Chemical Physics Letters 511 (2011) 336–339
0.08 0.1 0.06
0.04 0.1 mM (Iex = 0.14 mJcm-2)
0.02
0.5 mM (Iex = 0.07 mJcm-2) 1 mM (Iex = 0.07 mJcm-2)
0 -0.05
0
0.05
0.1
0.15
0.2
Time / micro sec
(-ΔP/P)MAX / (Iex (1-10 -A) ) [mJ -1cm2]
ex
-A
TRMC signal / (I (1-10 )) [mJ-1 cm2]
338
1 mM 0.5 mM
Figure 3. TRMC signal intensity (DP/P) divided by the number of absorbed photons [Iex(1–10A)] recorded at an excitation density of ca. 5 1017 cm3. Excitation intensities are also shown.
Uinj l ¼
C A
Iex ð1 10 Þ
ð
DP Þ P MAX
0.1 mM
0.01
ð2Þ
1016
1017
1018
1019
Excitation density / cm-3 where C is a constant, Iex is the intensity of excitation light, and A is the absorbance at the excitation wavelength. Iex(1–10A) represents the number of absorbed photons. We prepared plots of TRMC signals versus time for the three films observed at an excitation density of ca. 5 1017 cm3 (typical plots are shown in Figure 3). Signal intensity was divided by the number of absorbed photons [Iex(1–10A)]. Although the excitation density dependence of the signal intensity will be discussed below, the fact that similar (DP/P)MAX values were obtained at the given excitation density means that Uinj did not depend on dye loading. This also indicates that l did not depend on dye loading, suggesting that the number of traps is not affected by the dye loading. Note that the decay rate of TRMC signal of the film with a low dye load was slower than that of the film with a high dye load. Although the origin of the decay rate for TRMC signal intensity is still under debate [11,12,16], the decay profile reflects the rate of recombination between electrons and dye cations, the rate of electron trapping into deeper levels, or both. The results shown in Figure 3 suggest that dyes adsorbed on the surface of TiO2 affected the reactivity of electrons in the TiO2. For nanocrystalline TiO2 films, Uinjl is sensitive to the density of electrons generated in the TiO2 particles [11,12,17]. At lower electron densities, almost all the generated electrons are effectively trapped, and therefore their effective mobility becomes very small; whereas at higher electron densities, many electrons can move freely because the traps are ‘filled’ with other electrons (trap filling effect). Thus, a large Uinjl can be expected at higher electron densities. At higher electron densities, Uinjl decreases by efficient secondary charge recombination. Accordingly, the bell-shaped dependence of plots of (DP/P)MAX divided by the number of absorbed photons [Iex(1–10A)] versus excitation density can be observed (Figure 4). The similarity of the plots for the three N719/TiO2 films clearly shows that the efficiency of electron injection did not depend on dye loading. The performance of solar cells prepared from electrodes with high dye loads is reported to be suppressed [6], and it has been argued that the suppression is due to aggregation of dyes on the surface of TiO2. However, the results shown in Figure 4 clearly indicate that the electron injection efficiency did not depend on the surface concentration of the dye, even at high dye loading, which suggests
Figure 4. Plot of (DP/P)MAX divided by the number of absorbed photons [Iex(1– 10A)] as a function of excitation density.
that there was no aggregation. This result is consistent with our other experimental findings: that the adsorption isotherm could be fitted by the Langmuir-type isotherm and that the absorption spectra of N719 adsorbed on TiO2 films did not depend on dye loading. The reason for the discrepancy between our results and previous results is unclear. One possibility is that the effect of additives, such as 4-tert-butylpyridine (tBP) and Li ions, on the performance is suppressed when the surface is fully covered with sensitizer dyes. We recently observed that the effect of 4-tert-butylpyridine on electron injection efficiency is sensitive to dye coverage on the TiO2 surface [18]. Another possibility is microcrystal formation in the nanopores of the TiO2 films. Dye-sensitized electrodes for DSSCs are prepared by immersing the electrodes in dye solutions that are near the solubility limit, and therefore the dye solution is likely to be super-saturated. Supersaturation may lead to microcrystallization of dye in the nanopores of the films, and the microcrystals may suppress solar cell performance because they absorb light and hinder the diffusion of redox mediators in the nanopore network of the nanocrystalline TiO2 films. In summary, we used TRMC to evaluate the efficiency of electron injection from excited N719 dye to nanocrystalline TiO2 films in several different dye-loaded films and found no marked difference in Uinj between the films. This result suggests N719 did not aggregate on the TiO2 surface, which is consistent with our findings that the adsorption isotherm could be fitted to a Langmuir-type isotherm and that the absorption spectra of N719 adsorbed on the TiO2 films did not depend on the dye loading. These results indicate that N719 dyes are unlikely to form aggregates under ordinary adsorption conditions. Acknowledgement This research was supported by Strategic International Cooperative Program (Research Exchange Type), Japan Science and Technology Agency (JST).
R. Katoh et al. / Chemical Physics Letters 511 (2011) 336–339
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