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Heterogeneous ruthenium dye adsorption on nano-structured TiO2 films for dye-sensitized solar cells
Current Applied Physics 10 (2010) S184–S187
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Heterogeneous ruthenium dye adsorption on nano-structured TiO2 films for dye-sensitized solar cells Kyung-Jun Hwang a, Sung-Hoon Jung a, Dong-Won Park b, Seung-Joon Yoo c, Jae-Wook Lee a,* a
Department of Chemical Engineering, Chosun University, Gwangju 501-759, Republic of Korea Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea c Department of Environmental and Chemical Engineering, Seonam University, Namwon 590-711, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 29 November 2008 Accepted 8 June 2009 Available online 11 November 2009 Keywords: Dye-sensitized solar cell N719 dye TiO2 Adsorption I–V curve
a b s t r a c t TiO2 particles of single-phase anatase nanocrystallites were prepared by the hydrolysis of titanium-tetraisopropoxide under acidic condition and characterized by XRD, FE-SEM, and BET analysis. The adsorption isotherms of dye molecule on TiO2 particles were obtained at three different temperatures (298.15, 313.15, 333.15 K) and the experimental data were correlated with Sips isotherm model. Also the isosteric enthalpies of dye adsorption were calculated by the Clausius–Clapeyron equation. The influence of heterogeneous adsorption of cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato)-ruthenium(II) bistetrabutylammonium dye (N719) on the energy conversion efficiency of solar cell was investigated on the basis of photocurrent–potential curves. The results showed that the conversion efficiency of dye-sensitized solar cell was highly dependent on the heterogeneous adsorption properties of N719 dye on TiO2 films. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon-type solar cells have been extensively studied because of their high performance and good stability. Nevertheless, the expensive fabrication cost is limiting factor for broad applications [1,2]. Compared to the conventional silicon-type cells, dye-sensitized solar cells (DSSCs) have been extensively investigated because they offer attractive advantages including low cost, less toxic manu- facturing, easy scale-up, light weight, and use of flexible panels compared to conventional p–n junction devices [3,4]. The light absorption was performed by a chemical adsorption of ruthenium complex dye on the surface of the nanocrystalline TiO2 particles. The absorbed light became excited on the dye molecules and the electrons were transferred into the TiO2 conduction band. The oxidized dye was subsequently reduced by electron donation from an electrolyte containing the iodide/triiodide redox system. The injected electron flowed through the semiconductor network to arrive at the back contact and then through the external load to the counter electrode coated with Pt on FTO (F-doped thin oxide) glass. At the counter electrode, the reduction of triiodide in turn regenerated iodide in turn, which completed the circuit [5,6]. It has been pointed out that further works on the development of nano-structured materials as well as the analysis of the electron transport dynamics should be conducted to en-
* Corresponding author. Fax: +82 62 232 2474. E-mail address: [email protected] (J.-W. Lee). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.11.068
hance the low energy conversion efficiency of DSSC. There have been many studies on the synthesis and characterization of TiO2 nanocrystallites as well as the development of dyes for DSSC. However, systematic studies on the influence of adsorption properties between dye molecules and titania films on the power conversion efficiency of DSSC are very limited. The aim of this work is to investigate the influence of Ru(II) dye adsorption properties on the conversion efficiency of DSSC. For this purpose, experimental and theoretical studies on the adsorption equilibrium studies as functions of solution pH and temperatures were conducted to control the adsorption amount and also understand the adsorption mechanism of N719 dye on nano-structured TiO2 particles. In addition, the solar cell performances including the overall conversion efficiency (geff), fill factor (FF), open-circuit voltage (Voc) and short-circuit current (Isc) of the dye-sensitized nanocrystalline TiO2 solar cells were investigated in accordance with the adsorption quantity of N719 dye.
2. Experiments 2.1. Fabrication and characterization of TiO2 films Porous TiO2 particle were prepared by the hydrolysis of titanium-tetraisopropoxide (TTIP, Aldrich). For the preparation of TiO2 electrode, TiO2 paste was prepared by mixing TiO2 particles with the aqueous solution of acetyl acetone, poly ethylene glycol, Triton X-100, and water. The mixture was ground using a Zr ball
S185
K.-J. Hwang et al. / Current Applied Physics 10 (2010) S184–S187
mill. A TiO2 electrode was fabricated by coating a precursor paste onto the fluorine-doped SnO2 conducting glass plates (FTO, 10 X cm2) using a doctor blade technique followed by heating it at 723.15 K for 30 min. The TiO2 film formed on the FTO glass was 0.64 cm2 in size. To fabricate the DSSCs, the prepared thin film electrode was immersed in the N719 dye (Solaronix Co.) solution of 5 104 mol/L at 353.15 K for different time (15, 30, 60, 330, 780 min) to control the adsorption amount. A Pt coated FTO glass electrode was prepared as a counter electrode. The Pt electrode was placed over the dye-adsorbed TiO2 electrode, and the edges of the cell were sealed with sealing sheet (SX 1170-60, Solaronix). The redox electrolyte consisting of 1,2-dimethyl-3-propylim idazolium iodide (Solaronix), LiI (Aldrich), I2 (Aldrich), and 4-tert-butylpyridine (4-TBP, Aldrich) and 3-metoxy propionitrile as a solvent, was introduced into the cell through the small holes and sealed with a small square of sealing sheet (Amosil, Solaronix). The crystallinity of synthesized TiO2 particles was characterized with an Xray diffractometer (XRD, Rigaku, D/MAX-1200). The film thickness and surface morphology were measured by field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4700). Nitrogen gas adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 automatic analyzer. 2.2. Adsorption experiments Adsorption equilibrium experiments were carried out by contacting a given amount of TiO2 with N719 dye solution of 0.01– 0.5 mM in a shaking incubator at different temperatures (298.15, 313.15, 333.15 K) and pH (3, 4, 7, 10). Solution pH was adjusted using HCl and NaOH and determined by using the pH meter (Orion, USA). Zeta potentials of TiO2 particle in the solution were measured using a zeta potential apparatus (Photal Otsuka ELS-8000). The adsorption capacity (q) of TiO2 particles was determined by measuring the dye concentrations before and after adsorption using a UV spectrophotometer (Shimadzu UV-160A). On the other hand, the adsorption capacity of TiO2 film was measured by completely desorbing the adsorbed dye molecules from TiO2 film using 0.1 M NaOH solution/ethanol (50/50 vol.%). 2.3. Photoelectrochemical measurements The photocurrent–voltage (I–V) curves were measured using a source measure unit under irradiation of white light from a 1000 W Xenon lamp (Thermo Oriel Instruments). The incident light intensity and the active cell area were 45 mW cm2 and 0.64 cm2, respectively. The I–V curves were used to calculate the short-circuit current (Isc), open-circuit voltage (Voc), fill factor (FF), and overall conversion efficiency (geff) of DSSC.
Fig. 1. (a) XRD patterns of the single-phase anatase nanocrystallites TiO2 and (b) FE-SEM image of TiO2 thin film.
Table 1 Physico-chemical properties of TiO2 particles. Property
Unit
Degussa P25
This work
Surface area (BET) Pore volume Average pore size Average particle size Density Purity
m2 g1 cm3 g1 Å nm g l1 %
43.2 0.19 69 Approx. 21 Approx. 130 >99.5a
63 0.14 83 Approx. 20 128 –
3. Results and discussion 3.1. TiO2 nanocrystalline particle and thin film The titania particles synthesized in this work showed singlephase anatase nanocrytallites without rutile. Fig. 1a shows XRD patterns of the synthesized TiO2 with and without heat treatment at 723.15 K for 30 min in air gas flow. The anatase peak intensities are increased with the calcinations temperature up to 723.15 K without any phase transformation from anatase to rutile. It has been known that the anatase TiO2-based solar cells exhibit better photovoltaic characteristics compared to the rutile TiO2-based solar cells because of higher surface area (i.e., higher amount of dyeadsorbed). For comparison, the physico-chemical properties of commercialized P25 and synthesized TiO2 are listed in Table 1. P25 (Degussa) was the mixture of anatase and rutile phases (8:2)
[7]. The surface area determined by nitrogen adsorption/desorption isotherms data was found to be 63 m2 g1 and the average pore size calculated by BJH method was 8.3 nm. A slight increase of surface area of TiO2 synthesized in this work compared to P25 is attributed to the development of mesopores. Fig. 1b exhibits the FE-SEM images of surface morphology and the cross-section of TiO2 thin films coated on FTO glass. The TiO2 spherical nanoparticles (ca. 10–20 nm) are well distributed and the film thickness was ca. 5 lm. 3.2. Adsorption study The adsorption characteristics (i.e., equilibrium) of N719 dye on the TiO2 were evaluated on the basis of adsorption equilibrium studies. N719 dye has two bipyridyl ligands with two carboxyl
S186
K.-J. Hwang et al. / Current Applied Physics 10 (2010) S184–S187 Table 2 Sips isotherm parameters of N719 adsorption on synthesized TiO2 for different temperature. Temperature (K)
pH
298.15 313.15 333.15
7 7 7
Parameters qm
b
n
E (%)
4.805 6.432 7.874
283.01 163.41 106.63
0.648 0.676 0.702
0.029 0.176 0.234
bate molecules and adsorbent lattice atoms, and it may be used as a measure of the energetic heterogeneity of a solid surface. For the heterogeneous adsorption system, the isosteric enthalpy curve varies with the surface loading. It has been recognized that surface heterogeneity may come from the energetic, structural, and geometric heterogeneity [9]. The isosteric enthalpies of dye adsorption were calculated by the Clausius–Clapeyron equation [11]:
qst ¼ R
@ ln C : @ð1=TÞ q
ð2Þ
where C is the concentration, T is the temperature, R is the gas constant, and qst is the isosteric enthalpy. The results showed that the isosteric enthalpies of adsorption with the surface loading increased slightly in the range from 2 kJ mol1 to 25 kJ mol1 (Fig. 2b). To compare the influence of solution pH, the adsorption amount data were obtained in terms of pH (3, 4, 7, 10) at higher temperature (333.15 K). As the results, higher adsorption capacity was observed at pH 4 (data not shown here). This result may come from the fact the isoelectric point of TiO2 adsorbing N719 dye is around pH 4. To verify this fact, zeta potential measurements of TiO2 before and after adsorption were measured in terms of solution pH (3–10). The zeta potential values ranged approximately between +20 and 40 mV regardless of the adsorption of N719 dye on TiO2. Thus, the isoelectric point of TiO2 was present around pH 3.5. Fig. 2. Adsorption isotherms of N719 on TiO2 particle in terms of temperatures (a) and isosteric heat of adsorption (b).
groups at the 4 and 40 position of the bipyridyl group. It has been known that the carboxyl group interacts with TiO2 surfaces through chemical bond formation to the surface, chelating or bridging modes, or physical adsorption [7,8,14]. Fig. 2a shows the adsorption isotherms for three different temperatures (298.15, 313.15, 333.15 K). The results show that the adsorption capacity increased with increasing temperature. The chemisorption usually requires activation energy which increases with increasing temperature. Finnie et al. [9] have suggested from the vibrational spectroscopic study of the coordination of Ru(II) dye to the surface of nanocrystalline titania that the chemical bonding structure of dye molecule is a bidentate chelate or bridging coordination to the TiO2 surface via two carboxylate groups per dye molecule. Among the well-known isotherms such as Langmuir, Freundlich and Sips isotherm models, the adsorption equilibrium data of N719 on TiO2 were correlated well with the Sips equation [10]:
q¼
qm bC
3.3. Photovoltaic performance of DSSC It is interesting to compare the amount of adsorbed dye with the photocurrent densities of corresponding DSSC. Prior to make the cell, the working electrodes were immersed in a 5 104 mol/L of N719 dye for different adsorption time (15, 30, 60, 330, 560 min) to control the adsorption amount. Fig. 3 shows the influence of adsorption amount on the graphs of photocur-
1=n
1 þ bC
1=n
:
ð1Þ
The isotherm parameters were determined by minimizing the mean percentage deviations between experimental and predicted amounts adsorbed, based on a modified Levenberg–Marquardt method (IMSL routine DUNSLF). The solid lines (Fig. 2a) are the predicted results with Sips isotherm parameters (Table 2). The isosteric enthalpy is a measure of the interaction between adsor-
Fig. 3. I–V curves of TiO2 films depending on the adsorption amount of N719 dye.
K.-J. Hwang et al. / Current Applied Physics 10 (2010) S184–S187 Table 3 Characteristics values of DSSCs. Adsorption time (min)
Adsorption amount (mol cm2)
Isc (mA cm2)
Voc (V)
FF (%)
g
15 30 60 330 780
0.25 108 1.05 108 3.67 108 7.29 108 7.35 108
1.4 3.3 6.3 8.5 7.3
0.58 0.62 0.63 0.62 0.66
59 47 42 45 45
1.1 2.2 3.8 6.5 4.8
(%)
rent–voltage (I–V) for nanocrystalline solar cell and the characteristic values of DSSCs obtained from these curves are summarized in Table 3. As expected, the order of the photocurrent densities increased with the adsorption amount (i.e., adsorption time) of N719. The high adsorption amount (i.e., increase in adsorption time up to 330 min) of TiO2 may give rise to the superior of DSSC owing to the low charge transfer resistance from the monolayer adsorption (Fig. 3a). However, in the case of higher immersion time (780 min), the photocurrent density deceased. This result may explain the formation of the agglomeration of dyes molecules on TiO2 surfaces (Fig. 3b) that causes the high charge transfer resistance [12–14]. To avoid the agglomeration of dye molecules, the dye adsorption should be conducted under a supercritical condition. It can be concluded from this work that the conversion efficiency of the dye-sensitized solar cell was highly dependent on the adsorption properties of N719 dye on TiO2 films. 4. Conclusions A colloidal TiO2 suspension was prepared by the hydrolysis of titanium-tetraisopropoxide. Also, the TiO2 film of single-phase anatase crystallites was formed on the FTO glass for a working electrode of DSSC. Adsorption equilibrium studies revealed that the adsorption of N719 on TiO2 nano particles was temperaturedependent and also pH-dependent. The maximum adsorption
S187
capacity was achieved at the isoelectric point of around pH 4. The adsorption isotherm data can be fitted with Sips isotherm. A dye-sensitized solar cell fabricated in this work in accordance with the adsorption amount gave an open-circuit voltage of 0.62V and short-circuits current density of 5.4 mA cm2 for an incident light intensity of 45 mW cm2. The power conversion efficiency of over 6.5% was achieved using the single-phase anatase crystallites synthesized in this work. It was also found that the conversion efficiency is highly influenced by the heterogeneous adsorption properties. Acknowledgment This study was supported by research funds from Chosun University, 2008. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] L.L. Kazmerski, J. Electron Spectrosc. Related Phenom. 150 (2006) 105. [3] T. Hoshikawa, T. Ikebe, M. Yamada, R. Kikuchi, K. Eguchi, J. Photochem. Photobiol. A 184 (2006) 78. [4] J. Xia, F. Li, C. Huang, J. Zhai, L. Jiang, Solar Energy Mater. Solar Cells 90 (2006) 944. [5] M. Kaneko, I. Okura, Photocatalysis–Science and Technology, Springer, Kodansha, 2002. [6] G. Hodes, Electrochemistry of Nanomaterials, Wiley-VCH, New York, 2001. [7] S. Kambe, K. Murakoshi, T. Kitamura, Y. Wada, S. Yanagida, H. Kominami, Y. Kera, Solar Energy Mater. Solar Cells 61 (2000) 427. [8] K. Murakoshi, G. Kano, Y. Wada, S. Yanagida, H. Miyazaki, M. Matsumoto, S. Murasawa, J. Electroanal. Chem. 396 (1995) 27. [9] K.S. Finnie, J.R. Bartlett, J.L. Woolfrey, Langmuir 14 (1998) 2744. [10] R.J. Sips, Chem. Phys. 16 (1948) 490. [11] W. Rudzinski, D.H. Everett, Adsorption of Gases on Heterogeneous Surfaces, Academic Press, London, 1992. [12] G. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids, Academic Press, London, 1999. [13] R. Katoh, A. Furube, A.V. Barzykin, H. Arakawa, M. Tachiya, Coordinat. Chem. Rev. 248 (2004) 1195. [14] K.J. Hwang, S.J. Yoo, S.S. Kim, J.M. Kim, W.G. Shim, S.I. Kim, J.W. Lee, J. Nanosci. Nanotechnol. 8 (10) (2008) 4976.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Heterogeneous ruthenium dye adsorption on nano-structured TiO2 films for dye-sensitized solar cells Kyung-Jun Hwang a, Sung-Hoon Jung a, Dong-Won Park b, Seung-Joon Yoo c, Jae-Wook Lee a,* a
Department of Chemical Engineering, Chosun University, Gwangju 501-759, Republic of Korea Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea c Department of Environmental and Chemical Engineering, Seonam University, Namwon 590-711, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 29 November 2008 Accepted 8 June 2009 Available online 11 November 2009 Keywords: Dye-sensitized solar cell N719 dye TiO2 Adsorption I–V curve
a b s t r a c t TiO2 particles of single-phase anatase nanocrystallites were prepared by the hydrolysis of titanium-tetraisopropoxide under acidic condition and characterized by XRD, FE-SEM, and BET analysis. The adsorption isotherms of dye molecule on TiO2 particles were obtained at three different temperatures (298.15, 313.15, 333.15 K) and the experimental data were correlated with Sips isotherm model. Also the isosteric enthalpies of dye adsorption were calculated by the Clausius–Clapeyron equation. The influence of heterogeneous adsorption of cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato)-ruthenium(II) bistetrabutylammonium dye (N719) on the energy conversion efficiency of solar cell was investigated on the basis of photocurrent–potential curves. The results showed that the conversion efficiency of dye-sensitized solar cell was highly dependent on the heterogeneous adsorption properties of N719 dye on TiO2 films. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon-type solar cells have been extensively studied because of their high performance and good stability. Nevertheless, the expensive fabrication cost is limiting factor for broad applications [1,2]. Compared to the conventional silicon-type cells, dye-sensitized solar cells (DSSCs) have been extensively investigated because they offer attractive advantages including low cost, less toxic manu- facturing, easy scale-up, light weight, and use of flexible panels compared to conventional p–n junction devices [3,4]. The light absorption was performed by a chemical adsorption of ruthenium complex dye on the surface of the nanocrystalline TiO2 particles. The absorbed light became excited on the dye molecules and the electrons were transferred into the TiO2 conduction band. The oxidized dye was subsequently reduced by electron donation from an electrolyte containing the iodide/triiodide redox system. The injected electron flowed through the semiconductor network to arrive at the back contact and then through the external load to the counter electrode coated with Pt on FTO (F-doped thin oxide) glass. At the counter electrode, the reduction of triiodide in turn regenerated iodide in turn, which completed the circuit [5,6]. It has been pointed out that further works on the development of nano-structured materials as well as the analysis of the electron transport dynamics should be conducted to en-
* Corresponding author. Fax: +82 62 232 2474. E-mail address: [email protected] (J.-W. Lee). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.11.068
hance the low energy conversion efficiency of DSSC. There have been many studies on the synthesis and characterization of TiO2 nanocrystallites as well as the development of dyes for DSSC. However, systematic studies on the influence of adsorption properties between dye molecules and titania films on the power conversion efficiency of DSSC are very limited. The aim of this work is to investigate the influence of Ru(II) dye adsorption properties on the conversion efficiency of DSSC. For this purpose, experimental and theoretical studies on the adsorption equilibrium studies as functions of solution pH and temperatures were conducted to control the adsorption amount and also understand the adsorption mechanism of N719 dye on nano-structured TiO2 particles. In addition, the solar cell performances including the overall conversion efficiency (geff), fill factor (FF), open-circuit voltage (Voc) and short-circuit current (Isc) of the dye-sensitized nanocrystalline TiO2 solar cells were investigated in accordance with the adsorption quantity of N719 dye.
2. Experiments 2.1. Fabrication and characterization of TiO2 films Porous TiO2 particle were prepared by the hydrolysis of titanium-tetraisopropoxide (TTIP, Aldrich). For the preparation of TiO2 electrode, TiO2 paste was prepared by mixing TiO2 particles with the aqueous solution of acetyl acetone, poly ethylene glycol, Triton X-100, and water. The mixture was ground using a Zr ball
S185
K.-J. Hwang et al. / Current Applied Physics 10 (2010) S184–S187
mill. A TiO2 electrode was fabricated by coating a precursor paste onto the fluorine-doped SnO2 conducting glass plates (FTO, 10 X cm2) using a doctor blade technique followed by heating it at 723.15 K for 30 min. The TiO2 film formed on the FTO glass was 0.64 cm2 in size. To fabricate the DSSCs, the prepared thin film electrode was immersed in the N719 dye (Solaronix Co.) solution of 5 104 mol/L at 353.15 K for different time (15, 30, 60, 330, 780 min) to control the adsorption amount. A Pt coated FTO glass electrode was prepared as a counter electrode. The Pt electrode was placed over the dye-adsorbed TiO2 electrode, and the edges of the cell were sealed with sealing sheet (SX 1170-60, Solaronix). The redox electrolyte consisting of 1,2-dimethyl-3-propylim idazolium iodide (Solaronix), LiI (Aldrich), I2 (Aldrich), and 4-tert-butylpyridine (4-TBP, Aldrich) and 3-metoxy propionitrile as a solvent, was introduced into the cell through the small holes and sealed with a small square of sealing sheet (Amosil, Solaronix). The crystallinity of synthesized TiO2 particles was characterized with an Xray diffractometer (XRD, Rigaku, D/MAX-1200). The film thickness and surface morphology were measured by field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4700). Nitrogen gas adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP 2010 automatic analyzer. 2.2. Adsorption experiments Adsorption equilibrium experiments were carried out by contacting a given amount of TiO2 with N719 dye solution of 0.01– 0.5 mM in a shaking incubator at different temperatures (298.15, 313.15, 333.15 K) and pH (3, 4, 7, 10). Solution pH was adjusted using HCl and NaOH and determined by using the pH meter (Orion, USA). Zeta potentials of TiO2 particle in the solution were measured using a zeta potential apparatus (Photal Otsuka ELS-8000). The adsorption capacity (q) of TiO2 particles was determined by measuring the dye concentrations before and after adsorption using a UV spectrophotometer (Shimadzu UV-160A). On the other hand, the adsorption capacity of TiO2 film was measured by completely desorbing the adsorbed dye molecules from TiO2 film using 0.1 M NaOH solution/ethanol (50/50 vol.%). 2.3. Photoelectrochemical measurements The photocurrent–voltage (I–V) curves were measured using a source measure unit under irradiation of white light from a 1000 W Xenon lamp (Thermo Oriel Instruments). The incident light intensity and the active cell area were 45 mW cm2 and 0.64 cm2, respectively. The I–V curves were used to calculate the short-circuit current (Isc), open-circuit voltage (Voc), fill factor (FF), and overall conversion efficiency (geff) of DSSC.
Fig. 1. (a) XRD patterns of the single-phase anatase nanocrystallites TiO2 and (b) FE-SEM image of TiO2 thin film.
Table 1 Physico-chemical properties of TiO2 particles. Property
Unit
Degussa P25
This work
Surface area (BET) Pore volume Average pore size Average particle size Density Purity
m2 g1 cm3 g1 Å nm g l1 %
43.2 0.19 69 Approx. 21 Approx. 130 >99.5a
63 0.14 83 Approx. 20 128 –
3. Results and discussion 3.1. TiO2 nanocrystalline particle and thin film The titania particles synthesized in this work showed singlephase anatase nanocrytallites without rutile. Fig. 1a shows XRD patterns of the synthesized TiO2 with and without heat treatment at 723.15 K for 30 min in air gas flow. The anatase peak intensities are increased with the calcinations temperature up to 723.15 K without any phase transformation from anatase to rutile. It has been known that the anatase TiO2-based solar cells exhibit better photovoltaic characteristics compared to the rutile TiO2-based solar cells because of higher surface area (i.e., higher amount of dyeadsorbed). For comparison, the physico-chemical properties of commercialized P25 and synthesized TiO2 are listed in Table 1. P25 (Degussa) was the mixture of anatase and rutile phases (8:2)
[7]. The surface area determined by nitrogen adsorption/desorption isotherms data was found to be 63 m2 g1 and the average pore size calculated by BJH method was 8.3 nm. A slight increase of surface area of TiO2 synthesized in this work compared to P25 is attributed to the development of mesopores. Fig. 1b exhibits the FE-SEM images of surface morphology and the cross-section of TiO2 thin films coated on FTO glass. The TiO2 spherical nanoparticles (ca. 10–20 nm) are well distributed and the film thickness was ca. 5 lm. 3.2. Adsorption study The adsorption characteristics (i.e., equilibrium) of N719 dye on the TiO2 were evaluated on the basis of adsorption equilibrium studies. N719 dye has two bipyridyl ligands with two carboxyl
S186
K.-J. Hwang et al. / Current Applied Physics 10 (2010) S184–S187 Table 2 Sips isotherm parameters of N719 adsorption on synthesized TiO2 for different temperature. Temperature (K)
pH
298.15 313.15 333.15
7 7 7
Parameters qm
b
n
E (%)
4.805 6.432 7.874
283.01 163.41 106.63
0.648 0.676 0.702
0.029 0.176 0.234
bate molecules and adsorbent lattice atoms, and it may be used as a measure of the energetic heterogeneity of a solid surface. For the heterogeneous adsorption system, the isosteric enthalpy curve varies with the surface loading. It has been recognized that surface heterogeneity may come from the energetic, structural, and geometric heterogeneity [9]. The isosteric enthalpies of dye adsorption were calculated by the Clausius–Clapeyron equation [11]:
qst ¼ R
@ ln C : @ð1=TÞ q
ð2Þ
where C is the concentration, T is the temperature, R is the gas constant, and qst is the isosteric enthalpy. The results showed that the isosteric enthalpies of adsorption with the surface loading increased slightly in the range from 2 kJ mol1 to 25 kJ mol1 (Fig. 2b). To compare the influence of solution pH, the adsorption amount data were obtained in terms of pH (3, 4, 7, 10) at higher temperature (333.15 K). As the results, higher adsorption capacity was observed at pH 4 (data not shown here). This result may come from the fact the isoelectric point of TiO2 adsorbing N719 dye is around pH 4. To verify this fact, zeta potential measurements of TiO2 before and after adsorption were measured in terms of solution pH (3–10). The zeta potential values ranged approximately between +20 and 40 mV regardless of the adsorption of N719 dye on TiO2. Thus, the isoelectric point of TiO2 was present around pH 3.5. Fig. 2. Adsorption isotherms of N719 on TiO2 particle in terms of temperatures (a) and isosteric heat of adsorption (b).
groups at the 4 and 40 position of the bipyridyl group. It has been known that the carboxyl group interacts with TiO2 surfaces through chemical bond formation to the surface, chelating or bridging modes, or physical adsorption [7,8,14]. Fig. 2a shows the adsorption isotherms for three different temperatures (298.15, 313.15, 333.15 K). The results show that the adsorption capacity increased with increasing temperature. The chemisorption usually requires activation energy which increases with increasing temperature. Finnie et al. [9] have suggested from the vibrational spectroscopic study of the coordination of Ru(II) dye to the surface of nanocrystalline titania that the chemical bonding structure of dye molecule is a bidentate chelate or bridging coordination to the TiO2 surface via two carboxylate groups per dye molecule. Among the well-known isotherms such as Langmuir, Freundlich and Sips isotherm models, the adsorption equilibrium data of N719 on TiO2 were correlated well with the Sips equation [10]:
q¼
qm bC
3.3. Photovoltaic performance of DSSC It is interesting to compare the amount of adsorbed dye with the photocurrent densities of corresponding DSSC. Prior to make the cell, the working electrodes were immersed in a 5 104 mol/L of N719 dye for different adsorption time (15, 30, 60, 330, 560 min) to control the adsorption amount. Fig. 3 shows the influence of adsorption amount on the graphs of photocur-
1=n
1 þ bC
1=n
:
ð1Þ
The isotherm parameters were determined by minimizing the mean percentage deviations between experimental and predicted amounts adsorbed, based on a modified Levenberg–Marquardt method (IMSL routine DUNSLF). The solid lines (Fig. 2a) are the predicted results with Sips isotherm parameters (Table 2). The isosteric enthalpy is a measure of the interaction between adsor-
Fig. 3. I–V curves of TiO2 films depending on the adsorption amount of N719 dye.
K.-J. Hwang et al. / Current Applied Physics 10 (2010) S184–S187 Table 3 Characteristics values of DSSCs. Adsorption time (min)
Adsorption amount (mol cm2)
Isc (mA cm2)
Voc (V)
FF (%)
g
15 30 60 330 780
0.25 108 1.05 108 3.67 108 7.29 108 7.35 108
1.4 3.3 6.3 8.5 7.3
0.58 0.62 0.63 0.62 0.66
59 47 42 45 45
1.1 2.2 3.8 6.5 4.8
(%)
rent–voltage (I–V) for nanocrystalline solar cell and the characteristic values of DSSCs obtained from these curves are summarized in Table 3. As expected, the order of the photocurrent densities increased with the adsorption amount (i.e., adsorption time) of N719. The high adsorption amount (i.e., increase in adsorption time up to 330 min) of TiO2 may give rise to the superior of DSSC owing to the low charge transfer resistance from the monolayer adsorption (Fig. 3a). However, in the case of higher immersion time (780 min), the photocurrent density deceased. This result may explain the formation of the agglomeration of dyes molecules on TiO2 surfaces (Fig. 3b) that causes the high charge transfer resistance [12–14]. To avoid the agglomeration of dye molecules, the dye adsorption should be conducted under a supercritical condition. It can be concluded from this work that the conversion efficiency of the dye-sensitized solar cell was highly dependent on the adsorption properties of N719 dye on TiO2 films. 4. Conclusions A colloidal TiO2 suspension was prepared by the hydrolysis of titanium-tetraisopropoxide. Also, the TiO2 film of single-phase anatase crystallites was formed on the FTO glass for a working electrode of DSSC. Adsorption equilibrium studies revealed that the adsorption of N719 on TiO2 nano particles was temperaturedependent and also pH-dependent. The maximum adsorption
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capacity was achieved at the isoelectric point of around pH 4. The adsorption isotherm data can be fitted with Sips isotherm. A dye-sensitized solar cell fabricated in this work in accordance with the adsorption amount gave an open-circuit voltage of 0.62V and short-circuits current density of 5.4 mA cm2 for an incident light intensity of 45 mW cm2. The power conversion efficiency of over 6.5% was achieved using the single-phase anatase crystallites synthesized in this work. It was also found that the conversion efficiency is highly influenced by the heterogeneous adsorption properties. Acknowledgment This study was supported by research funds from Chosun University, 2008. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] L.L. Kazmerski, J. Electron Spectrosc. Related Phenom. 150 (2006) 105. [3] T. Hoshikawa, T. Ikebe, M. Yamada, R. Kikuchi, K. Eguchi, J. Photochem. Photobiol. A 184 (2006) 78. [4] J. Xia, F. Li, C. Huang, J. Zhai, L. Jiang, Solar Energy Mater. Solar Cells 90 (2006) 944. [5] M. Kaneko, I. Okura, Photocatalysis–Science and Technology, Springer, Kodansha, 2002. [6] G. Hodes, Electrochemistry of Nanomaterials, Wiley-VCH, New York, 2001. [7] S. Kambe, K. Murakoshi, T. Kitamura, Y. Wada, S. Yanagida, H. Kominami, Y. Kera, Solar Energy Mater. Solar Cells 61 (2000) 427. [8] K. Murakoshi, G. Kano, Y. Wada, S. Yanagida, H. Miyazaki, M. Matsumoto, S. Murasawa, J. Electroanal. Chem. 396 (1995) 27. [9] K.S. Finnie, J.R. Bartlett, J.L. Woolfrey, Langmuir 14 (1998) 2744. [10] R.J. Sips, Chem. Phys. 16 (1948) 490. [11] W. Rudzinski, D.H. Everett, Adsorption of Gases on Heterogeneous Surfaces, Academic Press, London, 1992. [12] G. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids, Academic Press, London, 1999. [13] R. Katoh, A. Furube, A.V. Barzykin, H. Arakawa, M. Tachiya, Coordinat. Chem. Rev. 248 (2004) 1195. [14] K.J. Hwang, S.J. Yoo, S.S. Kim, J.M. Kim, W.G. Shim, S.I. Kim, J.W. Lee, J. Nanosci. Nanotechnol. 8 (10) (2008) 4976.