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titania bilayer structure photoelectrode
Journal of Colloid and Interface Science 358 (2011) 562–566
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Dye-sensitized solar cells based on multiwalled carbon nanotube–titania/titania bilayer structure photoelectrode Wei-Jhih Lin, Chun-Tsung Hsu, Yu-Chen Tsai ⇑ Department of Chemical Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 20 December 2010 Accepted 9 March 2011 Available online 15 March 2011 Keywords: Dye-sensitized solar cells Multiwalled carbon nanotubes TiO2 nanoparticles
a b s t r a c t Dye-sensitized solar cells (DSSCs) were fabricated using multiwalled carbon nanotube (MWCNT)–TiO2 nanocomposite as a light scattering layer. Morphology of the MWCNT–TiO2 film was investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM and TEM images demonstrate that MWCNTs and TiO2 nanoparticles can be dispersed with chitosan. Internal resistance in the DSSC was characterized by electrochemical impedance spectroscopy (EIS). EIS results reveal a decrease in the charge resistance of electrolyte/dye/MWCNT–TiO2/TiO2 interface with increasing MWCNT content up to 3 wt% which leads to an improvement in the photovoltaic performance. Compare with a nanocrystalline TiO2 single-layer cell, the DSSC based on the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode shows 100% increase in solar-to-electric energy conversion efficiency, which is attributed to the inclusion of MWCNTs in TiO2 matrix. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Dye-sensitized solar cell (DSSC) is an area of intense investigation since the remarkable work of O’Regan and Grätzel [1]. A nanocrystalline TiO2 film was combined with ruthenium–polypyridine complex dye and an overall conversion efficiency of 10% was achieved on this system [2]. Despite the success of achieving acceptable performance, the effort to further improve the photovoltaic conversion efficiency of DSSCs remains a challenge [3–8]. In a DSSC, sunlight is absorbed by dye molecules, in which photoexcited electrons are injected to the conduction band of the nanocrystallite. A recombination may take place between the injected electrons and the dye cation or redox couple. Electron trapping in the nanocrystallite is also a mechanism that causes energy loss [9,10]. Over the past few years, it has been expected that fabricating a secondary layer based on large-size particles on top of a first layer could lead to enhanced photon absorption. This method is to enhance the scattering of the incident light to boost the light to electricity conversion by combining with light scattering layer into the photoelectrode layer of DSSCs. Many attempts with an additional scattering layer to photoelectrodes have been made in order to develop high photovoltaic conversion efficiency of DSSCs. These methods include the use of multilayer structure of TiO2 particles with different size [11], a layer of electrodeposition of closely-
⇑ Corresponding author. Fax: +886 4 22854734. E-mail address: [email protected] (Y.-C. Tsai). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.03.031
packed submicron TiO2 [12], a layer of nano-embossed hollow spherical TiO2 [13], and a layer of TiO2–ZrO2 composite [14]. However, the introduction of larger particles into photoelectrode will unavoidably lower the internal surface area of the photoelectrode film. This serves to counteract the enhancement effect of light scattering on the optical absorption, whereas the incorporation of an additional layer of TiO2 may lead to an undesirable increase in the electron diffusion length and, consequently, increase the recombination rate of photoexcited electrons. Therefore, the development of scattering layer in DSSCs for enhancement of the photovoltaic conversion efficiency seems very attractive. In recent studies, TiO2 nanotubes [15] and ZnO nanowires [16] were used to promote electron transfer and to reduce charge recombination in DSSCs. However, these nanostructures seem to have some insufficient internal surface areas, which limit the photovoltaic conversion efficiency. Carbon nanotubes (CNTs) have attracted great attention in improving photovoltaic performance of DSSCs because of their high electrical conductivity, chemical stability, high surface area, and tubular structure [17–25]. High electron affinity at CNTs can be used to act as electron collector and to enhance carrier mobility in DSSCs. The major barrier for the application of CNTs in DSSCs is the insolubility of CNTs in most solvents. To avoid this problem, CNTs need to be pre-treated before mixing them with TiO2. However, pre-treatment of CNTs may have some side effects such as sidewall defects. Too many formations of defects may alter the properties of CNTs such as increasing electrical resistance [26,27]. The method of solubilizing CNTs with noncovalent bonding can preserve the CNT properties and their original length.
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In this study, we demonstrate the fabrication of multiwalled carbon nanotube (MWCNT)–TiO2 nanocomposite for use as a secondary layer in DSSCs to improve the photovoltaic conversion efficiency. A noncovalent method to disperse MWCNTs and TiO2 in aqueous solution with chitosan was used. In the secondary layer, TiO2 clusters formed in the presence of MWCNT and MWCNT acted as an efficient conduit for electron transfer that can enhance the photovoltaic conversion efficiency of the DSSC. The parameters that could affect the photovoltaic performance of the DSSCs were evaluated and the photoelectrochemical characteristics were described and discussed.
FTO
TiO2 FTO
MWCNT-TiO2
2. Experimental
TiO2
2.1. Reagents
FTO Two types of TiO2 nanoparticles were purchased from Degussa (P25, 80% anatase) and Aldrich (<25 nm, 99.7% anatase). The MWCNTs (TECO Nanotech Co., Ltd., Taiwan) with an outer diameter in the range 20–40 nm and a length of up to several micrometers were synthesized by an electric arc discharge method. Chitosan (75–85% deacetylation) and 4-tert-butylppridine (TBP) were obtained from Aldrich. I2 was purchased from Merck. H2PtCl6 and LiI were purchased from Alfa Aesar. Polyethylene glycol (Mw = 16,000–24,000) and 3-methoxypropionitril (MPN) were obtained from Fluka. Cis-(NCS)2-Ru(II)-bis(2,20 -bipyridine-4,40 -dicarboxylate) dye (N719) and 1-propyl-2,3-dimethylimidazolium iodide (DMPII) were purchased from Solaronix. The reagents were used as purchased without any further pretreatment. All solutions were prepared with demineralized and filtered water of resistivity not less than 18 MX cm taken from a Milli-Q water purification system (Milli-Q, USA). 2.2. Apparatus Field emission scanning electron microscopy (FESEM) images were obtained using a JSM-6700F (JEOL, Japan). Transmission electron microscopy (TEM) image was achieved using a JEM-1200CX II (JEOL, Japan). Photocurrent–voltage (J–V) characteristics were measured with a Keithley 2400 source meter under illumination from a solar simulator composed of a 500 W Xe lamp and an AM 1.5 filter (Oriel). Light intensity was calibrated with a silicon photodiode. Electrochemical impendence measurement was performed with an Autolab PGSTAT30 Electrochemical Analyzer with FRA2 module (Eco Chemie, Netherlands) under AM 1.5 (100 mW cm2) illumination and in the dark. The frequency range explored was from 0.01 to 65,000 Hz. 2.3. Preparation of the MWCNT–TiO2/TiO2 bilayer photoelectrode The schematic drawing of the general concept for fabrication MWCNT–TiO2/TiO2 bilayer photoelectrode is shown in Fig. 1. Nanocrystalline TiO2 (P25, 25 nm) paste was prepared with 2 ml of 5 wt% TiO2 and mixed with polyethylene glycol (PEG) at a 0.25 of PEG/TiO2 ratio with the aid of ultrasonic agitation for 1 h. The paste was coated by using the doctor blade technique onto fluorine-doped SnO2 glass substrates (FTO, Solaronix) and annealed at 450 °C for 30 min in the air to obtain a nanocrystalline TiO2 film. The thickness of the nanocrystalline TiO2 film was ca. 3.4 lm which was measured by FESEM (not shown). A 1 wt% chitosan solution was prepared by dissolving 20 mg chitosan in 2 ml aqueous solution (pH 1). The solution was filtered through an 11 lm diameter filter paper (Whatman, England) to obtain a clear solution and then was stored in 4 °C. A 3 mg of MWCNTs and 0.1 g TiO2 (<25 nm, 99.7% anatase) were dispersed in a 2 ml of 1 wt%
Fig. 1. Schematic drawing of the general concept for fabrication MWCNT–TiO2/TiO2 bilayer photoelectrode.
chitosan aqueous solution (pH 1) with the aid of ultrasonic agitation for 1.5 h. The MWCNT–TiO2 paste was coated by using the doctor blade technique onto the prepared nanocrystalline TiO2 (P25, 25 nm) film and annealed at 450 °C for 30 min in the air to obtain a MWCNT–TiO2/TiO2 bilayer film. The thickness of the MWCNT–TiO2 film was varied from 0.6 to 2.4 lm which was determined by FESEM (not shown). 2.4. Preparation of the DSSC based on the MWCNT–TiO2/TiO2 bilayer photoelectrode The above prepared photoelectrode had a 0.25 cm2 active area which was immersed in ethanol containing 0.5 mM N719 for 24 h. The counter electrode was prepared by spin-coating of H2PtCl6 solution (7 mM in ethanol solution) onto FTO and heated at 400 °C for 15 min. The DSSCs were sealed with sealing material, SX1170 (Solaronix). The electrolyte consisted of 0.1 M LiI, 0.05 M I2, 0.6 M DMPII, and 0.5 M TBP in MPN. 3. Results and discussion 3.1. MWCNT–TiO2 nanocomposite film characterization by FESEM and TEM The MWCNT–TiO2 nanocomposite was used as a scattering layer in this study to improve the photovoltaic performance of DSSCs. Different materials can be combined to produce excellent composite materials. Therefore, the homogeneity for the MWCNT–TiO2 nanocomposite is a key factor in the success of the proposed DSSC based on the MWCNT–TiO2/TiO2 bilayer structure photoelectrode. The SEM images of TiO2 (<25 nm, 99.7% anatase) film and MWCNT (3 wt%)–TiO2 nanocomposite are shown in Fig. 2a and b, respectively. It demonstrates that MWCNTs can be dispersed homogeneously in TiO2 matrix. The individual MWCNT within MWCNT–TiO2 nanocomposite can be seen in Fig. 2b. To further investigate on the formation of MWCNT–TiO2 nanocomposite, TEM was performed. The TEM image of MWCNT–TiO2 nanocomposite is shown in Fig. 3. It can be observed that TiO2 nanoparticles are covered on the surface of MWCNT. It is expected that the electrons are injected from the excited dye molecules into TiO2 nanoparticles and then transferred through a MWCNT conduit to generate photocurrent. The electron-accepting ability of MWCNTs thus offers an opportunity to facilitate electron transport and increase photovoltaic conversion efficiency. From the MWCNT–TiO2
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3.2. Photovoltaic performance of the DSSC based on the MWCNT–TiO2/ TiO2 bilayer structure photoelectrode
a
b
MWCNT Fig. 2. FESEM images of the (a) TiO2 (<25 nm, 99.7% anatase) film and (b) MWCNT (3 wt%)–TiO2 nanocomposite.
To evaluate the photovoltaic performance of the MWCNT–TiO2/ TiO2 bilayer structure configuration, the J–V characteristics of DSSC based on nanocrystalline TiO2 single-layer and MWCNT–TiO2/TiO2 bilayer with different MWCNTs contents are shown in Fig. 4. The MWCNT–TiO2/TiO2 bilayer structure photoelectrode with 3 wt% MWCNT achieved the highest short-circuit current density (Jsc) and photovoltaic conversion efficiency (g), whereas nanocrystalline TiO2 single-layer photoelectrode presented the lowest Jsc and photovoltaic conversion efficiency. Table 1 summarizes the open-circuit voltage (Voc), Jsc, fill factor (FF), and photovoltaic conversion efficiency for all photoelectrodes. The values of photovoltaic conversion efficiency were 2.60% and 3.28% for nanocrystalline TiO2 singlelayer photoelectrode and MWCNT (0 wt%)–TiO2/TiO2 bilayer structure photoelectrode, respectively. The greater photovoltaic conversion efficiency obtained at the MWCNT (0 wt%)–TiO2/TiO2 bilayer structure photoelectrode indicates that the bilayer structure is able to enhance the photon absorption. This result also could be due to higher surface area available for dye adsorption at the MWCNT– TiO2/TiO2 bilayer structure. The photovoltaic conversion efficiency of the DSSC employing the MWCNT (0.1 wt%)–TiO2 single-layer photoelectrode (1.05%, not shown) was smaller than that with the TiO2 single-layer photoelectrode. The decrease may be attributed to the aggregation of MWCNTs in the TiO2 film. The values of FF and photovoltaic conversion efficiency increased in the presence of MWCNT in the MWCNT–TiO2/TiO2 bilayer structure photoelectrode and reached maximum at 3 wt% MWCNT. The values of FF increased from 0.608 with TiO2 single-layer photoelectrode to 0.708 with MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode. Consequently, the MWCNT (3 wt%)–TiO2/TiO2 bilayer photoelectrode increased the photovoltaic conversion efficiency by 100% in comparison with the nanocrystalline TiO2 single-layer photoelectrode. The results demonstrate that the MWCNT–TiO2/TiO2 bilayer structure photoelectrode exhibits efficient light harvesting as well as the acceleration of electron transfer. 3.3. Electrochemical impedance spectroscopy (EIS) of the DSSC based on the MWCNT–TiO2/TiO2 bilayer structure photoelectrode For the DSSC based on the MWCNT–TiO2/TiO2 bilayer structure photoelectrode, injected electrons transfer out of the MWCNT–
-2
Current density (mA cm )
f
Fig. 3. TEM image of the MWCNT (3 wt%)–TiO2 nanocomposite.
10
a 5
0 0.0
nanocomposite morphology, the prepared MWCNT–TiO2 nanocomposite can be used for application in DSSC in the hope to enhance the electron transport and photovoltaic conversion efficiency.
0.2
0.4
0.6
0.8
Voltage (V) Fig. 4. Photocurrent–voltage characteristics of the DSSCs based on the (a) nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with (b) 0, (c) 1, (d) 2, (e) 3, and (f) 4 wt% MWCNTs.
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W.-J. Lin et al. / Journal of Colloid and Interface Science 358 (2011) 562–566 Table 1 Results for the photovoltaic performance of the DSSCs based on the nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with different MWCNT contents. Photoelectrode
Voc (V)
Jsc (mA cm2)
FF
g (%)
TiO2 MWCNT(0 wt%)–TiO2/TiO2 MWCNT(1 wt%)–TiO2/TiO2 MWCNT(2 wt%)–TiO2/TiO2 MWCNT(3 wt%)–TiO2/TiO2 MWCNT(4 wt%)–TiO2/TiO2
0.617 0.619 0.627 0.634 0.646 0.637
6.91 8.57 9.88 10.82 11.42 11.52
0.608 0.618 0.632 0.672 0.708 0.656
2.60 3.28 3.92 4.61 5.22 4.89
TiO2 in the secondary layer and into the underlying TiO2 layer to generate a photocurrent. To investigate the improvement of photovoltaic performance of the MWCNT–TiO2/TiO2 bilayer structure photoelectrode, EIS was carried out. EIS is a powerful technique to characterize internal resistances in DSSCs and a suitable equivalent electrical circuit has been developed to interpret the results [28]. The Nyquist plots of the DSSCs based on the nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer with different MWCNTs contents photoelectrode measured under 100 mW cm2 at an applied potential bias of 0.65 V are shown in Fig. 5. Three typical semicircles are observed in the measured frequency regions of 10 mHz to 65 kHz at both of the nanocrystalline TiO2 single-layer and MWCNT–TiO2/TiO2 bilayer photoelectrodes. The semicircles in the high, intermediate, and low frequency ranges are attributed to impedance related to the charge transfer process at the electrolyte/Pt interface, the charge transfer at the electrolyte/dye/MWCNT–TiO2/TiO2 interface, and the Nernstain diffusion of I =I 3 in the electrolyte, respectively [19]. The Nyquist plots indicated that the values of charge resistance determined from the real impedance component (Z0 ) of intermediate frequency side semicircle were 23.2, 22.1, 18.4, 15.5, 13.2, and 15.0 X for the electrolyte/dye/TiO2 and electrolyte/dye/MWCNT– TiO2/TiO2 with 0, 1, 2, 3, and 4 wt% MWCNT, respectively. The charge resistance decreased with increasing the MWCNT contents in the range of 0–3 wt% in the MWCNT–TiO2/TiO2 bilayer structure photoelectrode because of the increase of conductance in the MWCNT–TiO2/TiO2 bilayer structure. The increase of charge resistance at the MWCNT (4 wt%)–TiO2/TiO2 bilayer structure is presumably attributed to the charge recombination. The Nyquist plots of the DSSCs based on the nanocrystalline TiO2 single-layer
and MWCNT–TiO2/TiO2 bilayer with different MWCNTs contents photoelectrodes measured in the dark at an applied potential bias of 0.65 V are shown in Fig. 6. The values of charge recombination resistance estimated from the real impedance component (Z0 ) of intermediate frequency side semicircle were 64.8, 66.2, 72.3, 84.4, 106.7, and 90.2 X for the electrolyte/dye/TiO2 and electrolyte/dye/MWCNT–TiO2/TiO2 with 0, 1, 2, 3, and 4 wt% MWCNT, respectively. The high recombination charge resistance obtained at the electrolyte/dye/MWCNT (3 wt%)–TiO2/TiO2 in the dark means that the charge recombination between injected electrons and electron accepters in the redox electrolyte is low. From the EIS of the electrolyte/dye/MWCNT–TiO2/TiO2 interface resistances under illumination and in the dark, it indicated that a decrease in the charge transfer resistance and a increase in the charge recombination of the DSSCs after the inclusion of MWCNT (0–3 wt%) in MWCNT–TiO2 nanocomposite were obtained. However, subsequent increase in MWCNT content (4 wt%) in MWCNT–TiO2 nanocomposite does not help to increase the photovoltaic conversion efficiency. This behavior is consistent with the J–V results in Table 1. The series resistances determined in the high frequency range >1 MHz in the DSSCs were 32.2, 31.2, 26.1, 23.1, 21.3, and 22.6 X for the TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 with 0, 1, 2, 3, and 4 wt% MWCNTs bilayer photoelectrode, respectively. The increase of MWCNTs up to 3 wt% within MWCNT–TiO2 matrix leads to a reduction in series resistance and corresponding improvement in the FF. The high FF of the DSSC based on the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode is due to a lower series resistance [19]. From the EIS results we can conclude that MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode improves the Voc, Jsc, and FF to achieve a higher photovoltaic performance. 3.4. Effect of film thickness of MWCNT (3 wt%)–TiO2 layer on the photovoltaic performance To investigate the effect of film thickness of MWCNT (3 wt%)– TiO2 layer on the photovoltaic performance of the DSSC based on the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode, the J–V characteristics was conducted by using four different thicknesses (0.6, 1.2, 1.8, and 2.4 lm) of MWCNT (3 wt%)–TiO2 layers. The thickness of the nanocrystalline TiO2 layer was kept at ca. 3.4 lm. The photovoltaic performance of the DSSC based on MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrodes with four different thicknesses of MWCNT (3 wt%)–TiO2 is listed in Table
10
40
-Z' (ohm)
-Z' (ohm)
60
5
20
0 10
20
30
40
50
60
70
Z' (ohm)
0 0
40
80
120
160
Z' (ohm) Fig. 5. Nyquist plots of the DSSCs based on (j) nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with (w) 0, (d) 1, (N) 2, (.) 3, and () 4 wt% MWCNTs under AM 1.5 light illumination (100 mW cm2).
Fig. 6. Nyquist plots of the DSSCs based on (j) nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with (w) 0, (d) 1, (N) 2, (.) 3, and () 4 wt% MWCNTs in the dark.
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Table 2 Results for the photovoltaic performance of the DSSCs based on the MWCNT (3 wt%)– TiO2/TiO2 bilayer structure photoelectrodes with four different film thicknesses of MWCNT (3 wt%)–TiO2 layers. Film thickness of MWCNT (3 wt%)–TiO2 (lm)
Voc (V)
Jsc (mA cm2)
FF
0.6 1.2 1.8 2.4
0.643 0.646 0.643 0.635
9.38 11.42 13.24 12.28
0.683 0.708 0.687 0.681
single-walled CNT. The use of MWCNT is based on the low cost of the fabrication. The proposed MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode might be beneficial to develop high photovoltaic conversion efficiency of DSSCs.
g (%) 4.12 5.22 5.85 5.31
2. The Voc and FF are almost unaffected by changes in the film thickness of the MWCNT (3 wt%)–TiO2 layer. The Jsc increased from 9.38 to 13.24 mA cm2 (from 0.6 to 1.8 lm), and then decreased to 12.28 mA cm2 at a thickness of 2.4 lm. An increased Jsc with increasing film thickness in the range of 0.6–1.8 lm is attributed to more dye molecules adsorbed onto TiO2 nanoparticles. By increasing the film thickness of the MWCNT (3 wt%)–TiO2 up to 2.4 lm, the charge recombination between injected electrons and I 3 ions in the electrolyte become more significant and could influence the conductor phase within the MWCNT–TiO2 matrix [11]. This makes the lower Jsc at the thicker MWCNT (3 wt%)–TiO2 film. As a result, the photovoltaic conversion efficiency increased with increasing the film thickness of the MWCNT (3 wt%)–TiO2 layer from 0.6 to 1.8 lm. The film thickness of the MWCNT (3 wt%)– TiO2 at 1.8 lm achieved the high conversion efficiency of 5.85%. Clearly this effect is linked to the improvement in the Jsc. 4. Conclusion In this study, a secondary layer composed of MWCNT–TiO2 nanocomposite for DSSC application was demonstrated. This process constitutes a simple and versatile protocol for the noncovalent dispersion of MWCNTs and TiO2 in chitosan aqueous solution. The charge transfer resistance of the electrolyte/dye/MWCNT–TiO2/ TiO2 interface was reduced by increasing the MWCNTs up to 3 wt% in the MWCNT–TiO2 matrix. With a 3.4 lm nanocrystalline TiO2 layer as photoelectrode and an additional 1.8 lm MWCNT (3 wt%)–TiO2 layer, a photovoltaic conversion efficiency of 5.85% was achieved using a I =I 3 electrolyte to regenerate the dye N719. The photovoltaic performance of the DSSC with the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode is 100% higher than that of the device with a nanocrystalline TiO2 single-layer film as the photoelectrode. The enhanced photovoltaic conversion efficiency is correlated with efficient light harvesting, the promotion of electron transfer from the adsorbed dyes to the working electrode, and the suppression of charge recombination between the injected electrons and the dye cation or redox couple. It should be emphasized that the improvement of the photovoltaic conversion efficiency should be obtained with both MWCNT and
Acknowledgment The authors wish to thank the National Science Council, Taiwan for financial support. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Dye-sensitized solar cells based on multiwalled carbon nanotube–titania/titania bilayer structure photoelectrode Wei-Jhih Lin, Chun-Tsung Hsu, Yu-Chen Tsai ⇑ Department of Chemical Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 20 December 2010 Accepted 9 March 2011 Available online 15 March 2011 Keywords: Dye-sensitized solar cells Multiwalled carbon nanotubes TiO2 nanoparticles
a b s t r a c t Dye-sensitized solar cells (DSSCs) were fabricated using multiwalled carbon nanotube (MWCNT)–TiO2 nanocomposite as a light scattering layer. Morphology of the MWCNT–TiO2 film was investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM and TEM images demonstrate that MWCNTs and TiO2 nanoparticles can be dispersed with chitosan. Internal resistance in the DSSC was characterized by electrochemical impedance spectroscopy (EIS). EIS results reveal a decrease in the charge resistance of electrolyte/dye/MWCNT–TiO2/TiO2 interface with increasing MWCNT content up to 3 wt% which leads to an improvement in the photovoltaic performance. Compare with a nanocrystalline TiO2 single-layer cell, the DSSC based on the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode shows 100% increase in solar-to-electric energy conversion efficiency, which is attributed to the inclusion of MWCNTs in TiO2 matrix. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Dye-sensitized solar cell (DSSC) is an area of intense investigation since the remarkable work of O’Regan and Grätzel [1]. A nanocrystalline TiO2 film was combined with ruthenium–polypyridine complex dye and an overall conversion efficiency of 10% was achieved on this system [2]. Despite the success of achieving acceptable performance, the effort to further improve the photovoltaic conversion efficiency of DSSCs remains a challenge [3–8]. In a DSSC, sunlight is absorbed by dye molecules, in which photoexcited electrons are injected to the conduction band of the nanocrystallite. A recombination may take place between the injected electrons and the dye cation or redox couple. Electron trapping in the nanocrystallite is also a mechanism that causes energy loss [9,10]. Over the past few years, it has been expected that fabricating a secondary layer based on large-size particles on top of a first layer could lead to enhanced photon absorption. This method is to enhance the scattering of the incident light to boost the light to electricity conversion by combining with light scattering layer into the photoelectrode layer of DSSCs. Many attempts with an additional scattering layer to photoelectrodes have been made in order to develop high photovoltaic conversion efficiency of DSSCs. These methods include the use of multilayer structure of TiO2 particles with different size [11], a layer of electrodeposition of closely-
⇑ Corresponding author. Fax: +886 4 22854734. E-mail address: [email protected] (Y.-C. Tsai). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.03.031
packed submicron TiO2 [12], a layer of nano-embossed hollow spherical TiO2 [13], and a layer of TiO2–ZrO2 composite [14]. However, the introduction of larger particles into photoelectrode will unavoidably lower the internal surface area of the photoelectrode film. This serves to counteract the enhancement effect of light scattering on the optical absorption, whereas the incorporation of an additional layer of TiO2 may lead to an undesirable increase in the electron diffusion length and, consequently, increase the recombination rate of photoexcited electrons. Therefore, the development of scattering layer in DSSCs for enhancement of the photovoltaic conversion efficiency seems very attractive. In recent studies, TiO2 nanotubes [15] and ZnO nanowires [16] were used to promote electron transfer and to reduce charge recombination in DSSCs. However, these nanostructures seem to have some insufficient internal surface areas, which limit the photovoltaic conversion efficiency. Carbon nanotubes (CNTs) have attracted great attention in improving photovoltaic performance of DSSCs because of their high electrical conductivity, chemical stability, high surface area, and tubular structure [17–25]. High electron affinity at CNTs can be used to act as electron collector and to enhance carrier mobility in DSSCs. The major barrier for the application of CNTs in DSSCs is the insolubility of CNTs in most solvents. To avoid this problem, CNTs need to be pre-treated before mixing them with TiO2. However, pre-treatment of CNTs may have some side effects such as sidewall defects. Too many formations of defects may alter the properties of CNTs such as increasing electrical resistance [26,27]. The method of solubilizing CNTs with noncovalent bonding can preserve the CNT properties and their original length.
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In this study, we demonstrate the fabrication of multiwalled carbon nanotube (MWCNT)–TiO2 nanocomposite for use as a secondary layer in DSSCs to improve the photovoltaic conversion efficiency. A noncovalent method to disperse MWCNTs and TiO2 in aqueous solution with chitosan was used. In the secondary layer, TiO2 clusters formed in the presence of MWCNT and MWCNT acted as an efficient conduit for electron transfer that can enhance the photovoltaic conversion efficiency of the DSSC. The parameters that could affect the photovoltaic performance of the DSSCs were evaluated and the photoelectrochemical characteristics were described and discussed.
FTO
TiO2 FTO
MWCNT-TiO2
2. Experimental
TiO2
2.1. Reagents
FTO Two types of TiO2 nanoparticles were purchased from Degussa (P25, 80% anatase) and Aldrich (<25 nm, 99.7% anatase). The MWCNTs (TECO Nanotech Co., Ltd., Taiwan) with an outer diameter in the range 20–40 nm and a length of up to several micrometers were synthesized by an electric arc discharge method. Chitosan (75–85% deacetylation) and 4-tert-butylppridine (TBP) were obtained from Aldrich. I2 was purchased from Merck. H2PtCl6 and LiI were purchased from Alfa Aesar. Polyethylene glycol (Mw = 16,000–24,000) and 3-methoxypropionitril (MPN) were obtained from Fluka. Cis-(NCS)2-Ru(II)-bis(2,20 -bipyridine-4,40 -dicarboxylate) dye (N719) and 1-propyl-2,3-dimethylimidazolium iodide (DMPII) were purchased from Solaronix. The reagents were used as purchased without any further pretreatment. All solutions were prepared with demineralized and filtered water of resistivity not less than 18 MX cm taken from a Milli-Q water purification system (Milli-Q, USA). 2.2. Apparatus Field emission scanning electron microscopy (FESEM) images were obtained using a JSM-6700F (JEOL, Japan). Transmission electron microscopy (TEM) image was achieved using a JEM-1200CX II (JEOL, Japan). Photocurrent–voltage (J–V) characteristics were measured with a Keithley 2400 source meter under illumination from a solar simulator composed of a 500 W Xe lamp and an AM 1.5 filter (Oriel). Light intensity was calibrated with a silicon photodiode. Electrochemical impendence measurement was performed with an Autolab PGSTAT30 Electrochemical Analyzer with FRA2 module (Eco Chemie, Netherlands) under AM 1.5 (100 mW cm2) illumination and in the dark. The frequency range explored was from 0.01 to 65,000 Hz. 2.3. Preparation of the MWCNT–TiO2/TiO2 bilayer photoelectrode The schematic drawing of the general concept for fabrication MWCNT–TiO2/TiO2 bilayer photoelectrode is shown in Fig. 1. Nanocrystalline TiO2 (P25, 25 nm) paste was prepared with 2 ml of 5 wt% TiO2 and mixed with polyethylene glycol (PEG) at a 0.25 of PEG/TiO2 ratio with the aid of ultrasonic agitation for 1 h. The paste was coated by using the doctor blade technique onto fluorine-doped SnO2 glass substrates (FTO, Solaronix) and annealed at 450 °C for 30 min in the air to obtain a nanocrystalline TiO2 film. The thickness of the nanocrystalline TiO2 film was ca. 3.4 lm which was measured by FESEM (not shown). A 1 wt% chitosan solution was prepared by dissolving 20 mg chitosan in 2 ml aqueous solution (pH 1). The solution was filtered through an 11 lm diameter filter paper (Whatman, England) to obtain a clear solution and then was stored in 4 °C. A 3 mg of MWCNTs and 0.1 g TiO2 (<25 nm, 99.7% anatase) were dispersed in a 2 ml of 1 wt%
Fig. 1. Schematic drawing of the general concept for fabrication MWCNT–TiO2/TiO2 bilayer photoelectrode.
chitosan aqueous solution (pH 1) with the aid of ultrasonic agitation for 1.5 h. The MWCNT–TiO2 paste was coated by using the doctor blade technique onto the prepared nanocrystalline TiO2 (P25, 25 nm) film and annealed at 450 °C for 30 min in the air to obtain a MWCNT–TiO2/TiO2 bilayer film. The thickness of the MWCNT–TiO2 film was varied from 0.6 to 2.4 lm which was determined by FESEM (not shown). 2.4. Preparation of the DSSC based on the MWCNT–TiO2/TiO2 bilayer photoelectrode The above prepared photoelectrode had a 0.25 cm2 active area which was immersed in ethanol containing 0.5 mM N719 for 24 h. The counter electrode was prepared by spin-coating of H2PtCl6 solution (7 mM in ethanol solution) onto FTO and heated at 400 °C for 15 min. The DSSCs were sealed with sealing material, SX1170 (Solaronix). The electrolyte consisted of 0.1 M LiI, 0.05 M I2, 0.6 M DMPII, and 0.5 M TBP in MPN. 3. Results and discussion 3.1. MWCNT–TiO2 nanocomposite film characterization by FESEM and TEM The MWCNT–TiO2 nanocomposite was used as a scattering layer in this study to improve the photovoltaic performance of DSSCs. Different materials can be combined to produce excellent composite materials. Therefore, the homogeneity for the MWCNT–TiO2 nanocomposite is a key factor in the success of the proposed DSSC based on the MWCNT–TiO2/TiO2 bilayer structure photoelectrode. The SEM images of TiO2 (<25 nm, 99.7% anatase) film and MWCNT (3 wt%)–TiO2 nanocomposite are shown in Fig. 2a and b, respectively. It demonstrates that MWCNTs can be dispersed homogeneously in TiO2 matrix. The individual MWCNT within MWCNT–TiO2 nanocomposite can be seen in Fig. 2b. To further investigate on the formation of MWCNT–TiO2 nanocomposite, TEM was performed. The TEM image of MWCNT–TiO2 nanocomposite is shown in Fig. 3. It can be observed that TiO2 nanoparticles are covered on the surface of MWCNT. It is expected that the electrons are injected from the excited dye molecules into TiO2 nanoparticles and then transferred through a MWCNT conduit to generate photocurrent. The electron-accepting ability of MWCNTs thus offers an opportunity to facilitate electron transport and increase photovoltaic conversion efficiency. From the MWCNT–TiO2
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3.2. Photovoltaic performance of the DSSC based on the MWCNT–TiO2/ TiO2 bilayer structure photoelectrode
a
b
MWCNT Fig. 2. FESEM images of the (a) TiO2 (<25 nm, 99.7% anatase) film and (b) MWCNT (3 wt%)–TiO2 nanocomposite.
To evaluate the photovoltaic performance of the MWCNT–TiO2/ TiO2 bilayer structure configuration, the J–V characteristics of DSSC based on nanocrystalline TiO2 single-layer and MWCNT–TiO2/TiO2 bilayer with different MWCNTs contents are shown in Fig. 4. The MWCNT–TiO2/TiO2 bilayer structure photoelectrode with 3 wt% MWCNT achieved the highest short-circuit current density (Jsc) and photovoltaic conversion efficiency (g), whereas nanocrystalline TiO2 single-layer photoelectrode presented the lowest Jsc and photovoltaic conversion efficiency. Table 1 summarizes the open-circuit voltage (Voc), Jsc, fill factor (FF), and photovoltaic conversion efficiency for all photoelectrodes. The values of photovoltaic conversion efficiency were 2.60% and 3.28% for nanocrystalline TiO2 singlelayer photoelectrode and MWCNT (0 wt%)–TiO2/TiO2 bilayer structure photoelectrode, respectively. The greater photovoltaic conversion efficiency obtained at the MWCNT (0 wt%)–TiO2/TiO2 bilayer structure photoelectrode indicates that the bilayer structure is able to enhance the photon absorption. This result also could be due to higher surface area available for dye adsorption at the MWCNT– TiO2/TiO2 bilayer structure. The photovoltaic conversion efficiency of the DSSC employing the MWCNT (0.1 wt%)–TiO2 single-layer photoelectrode (1.05%, not shown) was smaller than that with the TiO2 single-layer photoelectrode. The decrease may be attributed to the aggregation of MWCNTs in the TiO2 film. The values of FF and photovoltaic conversion efficiency increased in the presence of MWCNT in the MWCNT–TiO2/TiO2 bilayer structure photoelectrode and reached maximum at 3 wt% MWCNT. The values of FF increased from 0.608 with TiO2 single-layer photoelectrode to 0.708 with MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode. Consequently, the MWCNT (3 wt%)–TiO2/TiO2 bilayer photoelectrode increased the photovoltaic conversion efficiency by 100% in comparison with the nanocrystalline TiO2 single-layer photoelectrode. The results demonstrate that the MWCNT–TiO2/TiO2 bilayer structure photoelectrode exhibits efficient light harvesting as well as the acceleration of electron transfer. 3.3. Electrochemical impedance spectroscopy (EIS) of the DSSC based on the MWCNT–TiO2/TiO2 bilayer structure photoelectrode For the DSSC based on the MWCNT–TiO2/TiO2 bilayer structure photoelectrode, injected electrons transfer out of the MWCNT–
-2
Current density (mA cm )
f
Fig. 3. TEM image of the MWCNT (3 wt%)–TiO2 nanocomposite.
10
a 5
0 0.0
nanocomposite morphology, the prepared MWCNT–TiO2 nanocomposite can be used for application in DSSC in the hope to enhance the electron transport and photovoltaic conversion efficiency.
0.2
0.4
0.6
0.8
Voltage (V) Fig. 4. Photocurrent–voltage characteristics of the DSSCs based on the (a) nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with (b) 0, (c) 1, (d) 2, (e) 3, and (f) 4 wt% MWCNTs.
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W.-J. Lin et al. / Journal of Colloid and Interface Science 358 (2011) 562–566 Table 1 Results for the photovoltaic performance of the DSSCs based on the nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with different MWCNT contents. Photoelectrode
Voc (V)
Jsc (mA cm2)
FF
g (%)
TiO2 MWCNT(0 wt%)–TiO2/TiO2 MWCNT(1 wt%)–TiO2/TiO2 MWCNT(2 wt%)–TiO2/TiO2 MWCNT(3 wt%)–TiO2/TiO2 MWCNT(4 wt%)–TiO2/TiO2
0.617 0.619 0.627 0.634 0.646 0.637
6.91 8.57 9.88 10.82 11.42 11.52
0.608 0.618 0.632 0.672 0.708 0.656
2.60 3.28 3.92 4.61 5.22 4.89
TiO2 in the secondary layer and into the underlying TiO2 layer to generate a photocurrent. To investigate the improvement of photovoltaic performance of the MWCNT–TiO2/TiO2 bilayer structure photoelectrode, EIS was carried out. EIS is a powerful technique to characterize internal resistances in DSSCs and a suitable equivalent electrical circuit has been developed to interpret the results [28]. The Nyquist plots of the DSSCs based on the nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer with different MWCNTs contents photoelectrode measured under 100 mW cm2 at an applied potential bias of 0.65 V are shown in Fig. 5. Three typical semicircles are observed in the measured frequency regions of 10 mHz to 65 kHz at both of the nanocrystalline TiO2 single-layer and MWCNT–TiO2/TiO2 bilayer photoelectrodes. The semicircles in the high, intermediate, and low frequency ranges are attributed to impedance related to the charge transfer process at the electrolyte/Pt interface, the charge transfer at the electrolyte/dye/MWCNT–TiO2/TiO2 interface, and the Nernstain diffusion of I =I 3 in the electrolyte, respectively [19]. The Nyquist plots indicated that the values of charge resistance determined from the real impedance component (Z0 ) of intermediate frequency side semicircle were 23.2, 22.1, 18.4, 15.5, 13.2, and 15.0 X for the electrolyte/dye/TiO2 and electrolyte/dye/MWCNT– TiO2/TiO2 with 0, 1, 2, 3, and 4 wt% MWCNT, respectively. The charge resistance decreased with increasing the MWCNT contents in the range of 0–3 wt% in the MWCNT–TiO2/TiO2 bilayer structure photoelectrode because of the increase of conductance in the MWCNT–TiO2/TiO2 bilayer structure. The increase of charge resistance at the MWCNT (4 wt%)–TiO2/TiO2 bilayer structure is presumably attributed to the charge recombination. The Nyquist plots of the DSSCs based on the nanocrystalline TiO2 single-layer
and MWCNT–TiO2/TiO2 bilayer with different MWCNTs contents photoelectrodes measured in the dark at an applied potential bias of 0.65 V are shown in Fig. 6. The values of charge recombination resistance estimated from the real impedance component (Z0 ) of intermediate frequency side semicircle were 64.8, 66.2, 72.3, 84.4, 106.7, and 90.2 X for the electrolyte/dye/TiO2 and electrolyte/dye/MWCNT–TiO2/TiO2 with 0, 1, 2, 3, and 4 wt% MWCNT, respectively. The high recombination charge resistance obtained at the electrolyte/dye/MWCNT (3 wt%)–TiO2/TiO2 in the dark means that the charge recombination between injected electrons and electron accepters in the redox electrolyte is low. From the EIS of the electrolyte/dye/MWCNT–TiO2/TiO2 interface resistances under illumination and in the dark, it indicated that a decrease in the charge transfer resistance and a increase in the charge recombination of the DSSCs after the inclusion of MWCNT (0–3 wt%) in MWCNT–TiO2 nanocomposite were obtained. However, subsequent increase in MWCNT content (4 wt%) in MWCNT–TiO2 nanocomposite does not help to increase the photovoltaic conversion efficiency. This behavior is consistent with the J–V results in Table 1. The series resistances determined in the high frequency range >1 MHz in the DSSCs were 32.2, 31.2, 26.1, 23.1, 21.3, and 22.6 X for the TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 with 0, 1, 2, 3, and 4 wt% MWCNTs bilayer photoelectrode, respectively. The increase of MWCNTs up to 3 wt% within MWCNT–TiO2 matrix leads to a reduction in series resistance and corresponding improvement in the FF. The high FF of the DSSC based on the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode is due to a lower series resistance [19]. From the EIS results we can conclude that MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode improves the Voc, Jsc, and FF to achieve a higher photovoltaic performance. 3.4. Effect of film thickness of MWCNT (3 wt%)–TiO2 layer on the photovoltaic performance To investigate the effect of film thickness of MWCNT (3 wt%)– TiO2 layer on the photovoltaic performance of the DSSC based on the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode, the J–V characteristics was conducted by using four different thicknesses (0.6, 1.2, 1.8, and 2.4 lm) of MWCNT (3 wt%)–TiO2 layers. The thickness of the nanocrystalline TiO2 layer was kept at ca. 3.4 lm. The photovoltaic performance of the DSSC based on MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrodes with four different thicknesses of MWCNT (3 wt%)–TiO2 is listed in Table
10
40
-Z' (ohm)
-Z' (ohm)
60
5
20
0 10
20
30
40
50
60
70
Z' (ohm)
0 0
40
80
120
160
Z' (ohm) Fig. 5. Nyquist plots of the DSSCs based on (j) nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with (w) 0, (d) 1, (N) 2, (.) 3, and () 4 wt% MWCNTs under AM 1.5 light illumination (100 mW cm2).
Fig. 6. Nyquist plots of the DSSCs based on (j) nanocrystalline TiO2 single-layer photoelectrode and MWCNT–TiO2/TiO2 bilayer photoelectrodes with (w) 0, (d) 1, (N) 2, (.) 3, and () 4 wt% MWCNTs in the dark.
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Table 2 Results for the photovoltaic performance of the DSSCs based on the MWCNT (3 wt%)– TiO2/TiO2 bilayer structure photoelectrodes with four different film thicknesses of MWCNT (3 wt%)–TiO2 layers. Film thickness of MWCNT (3 wt%)–TiO2 (lm)
Voc (V)
Jsc (mA cm2)
FF
0.6 1.2 1.8 2.4
0.643 0.646 0.643 0.635
9.38 11.42 13.24 12.28
0.683 0.708 0.687 0.681
single-walled CNT. The use of MWCNT is based on the low cost of the fabrication. The proposed MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode might be beneficial to develop high photovoltaic conversion efficiency of DSSCs.
g (%) 4.12 5.22 5.85 5.31
2. The Voc and FF are almost unaffected by changes in the film thickness of the MWCNT (3 wt%)–TiO2 layer. The Jsc increased from 9.38 to 13.24 mA cm2 (from 0.6 to 1.8 lm), and then decreased to 12.28 mA cm2 at a thickness of 2.4 lm. An increased Jsc with increasing film thickness in the range of 0.6–1.8 lm is attributed to more dye molecules adsorbed onto TiO2 nanoparticles. By increasing the film thickness of the MWCNT (3 wt%)–TiO2 up to 2.4 lm, the charge recombination between injected electrons and I 3 ions in the electrolyte become more significant and could influence the conductor phase within the MWCNT–TiO2 matrix [11]. This makes the lower Jsc at the thicker MWCNT (3 wt%)–TiO2 film. As a result, the photovoltaic conversion efficiency increased with increasing the film thickness of the MWCNT (3 wt%)–TiO2 layer from 0.6 to 1.8 lm. The film thickness of the MWCNT (3 wt%)– TiO2 at 1.8 lm achieved the high conversion efficiency of 5.85%. Clearly this effect is linked to the improvement in the Jsc. 4. Conclusion In this study, a secondary layer composed of MWCNT–TiO2 nanocomposite for DSSC application was demonstrated. This process constitutes a simple and versatile protocol for the noncovalent dispersion of MWCNTs and TiO2 in chitosan aqueous solution. The charge transfer resistance of the electrolyte/dye/MWCNT–TiO2/ TiO2 interface was reduced by increasing the MWCNTs up to 3 wt% in the MWCNT–TiO2 matrix. With a 3.4 lm nanocrystalline TiO2 layer as photoelectrode and an additional 1.8 lm MWCNT (3 wt%)–TiO2 layer, a photovoltaic conversion efficiency of 5.85% was achieved using a I =I 3 electrolyte to regenerate the dye N719. The photovoltaic performance of the DSSC with the MWCNT (3 wt%)–TiO2/TiO2 bilayer structure photoelectrode is 100% higher than that of the device with a nanocrystalline TiO2 single-layer film as the photoelectrode. The enhanced photovoltaic conversion efficiency is correlated with efficient light harvesting, the promotion of electron transfer from the adsorbed dyes to the working electrode, and the suppression of charge recombination between the injected electrons and the dye cation or redox couple. It should be emphasized that the improvement of the photovoltaic conversion efficiency should be obtained with both MWCNT and
Acknowledgment The authors wish to thank the National Science Council, Taiwan for financial support. This work is supported in part by the Ministry of Education, Taiwan under the ATU plan.
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