Carbon Composite

J. Phys. Chem. C 2010, 114, 11673–11679

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In Situ Preparation of a Flexible Polyaniline/Carbon Composite Counter Electrode and Its Application in Dye-Sensitized Solar Cells Huicheng Sun, Yanhong Luo, Yiduo Zhang, Dongmei Li, Zhexun Yu, Kexin Li, and Qingbo Meng* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box, 603, Beijing 100190, People’s Republic of China ReceiVed: April 3, 2010; ReVised Manuscript ReceiVed: June 2, 2010

A flexible composite electrode, which is composed of conducting polyaniline (PANI) as electroactive material and flexible graphite (FG) as conducting substrate, has been fabricated by in situ chemical polymerization to substitute for the expensive Pt counter electrode (CE) used in dye-sensitized solar cells (DSCs). The photovoltaic parameters of DSCs are strongly dependent on the oxidation state and the thickness of the PANI film. Higher photocurrent density and efficiency have been obtained by using emeraldine PANI compared to pernigraniline. The fabrication conditions, such as reaction time and initial monomer concentration, have been investigated to control the thickness of the PANI film. With initial monomer concentration of 0.3 M and reaction time of 60 min, an optimized PANI/FG composite CE with a PANI film thickness of 330 nm has been obtained. A DSC with the composite CE shows an overall conversion efficiency of 7.36%, which is comparable to 7.45% of that with Pt electrode under the same test condition. Facile charge-transfer and low sheet resistance of the composite electrode are suggested to be responsible for high performance of the DSC using such CE. Introduction Dye-sensitized solar cells (DSCs) have been attracting widespread attention because of their low cost, simple fabrication process, and relatively high efficiency of converting light to electricity.1-4 A typical DSC consists of three components: a dye-sensitized nanocrystalline titanium dioxide (TiO2) electrode, a counter electrode (CE), and an electrolyte usually containing a I-/I3- redox couple between the two electrodes. The most commonly used CEs in DSCs are fluorine-doped tin oxide (FTO) glass with Pt deposited on them.5,6 Although Pt exhibits excellent catalytic activity for I3- reduction and high conductivity, its high cost and limited reserve would prevent it from use in large-scale manufacture.7,8 Besides, problems also arise from the rigid nature of FTO glass substrates for CEs, such as relative high sheet resistance,9,10 high cost,11 and inconvenient transportation.12 Alternative cheap catalysts and substrates for CEs are required for further cost reduction while maintaining the good performance of DSCs. Inexpensive carbonaceous materials, such as carbon black,8 hard carbon sphere,13 and carbon nanotube,14 and conductive polymers, such as polypyrrole,15 polyaniline (PANI),16 and poly(3,4ethylenedioxythiophene),17-19 have been deposited on FTO glass as catalytic materials of CEs. Meanwhile, novel substrates such as metals, plastic foils, and graphite paper have been used to fabricate flexible CEs to achieve the requirement for portable electricity and high-throughput industrial roll-to-roll production.12,20 As a type of functional conductive polymers, PANI has been used for decades owing to its easy fabrication, high electric conductivity, and fast doping/dedoping nature. Electrodes fabricated by casting PANI onto FTO glass from its suspension have shown good catalytic activity for I3- reduction.16 However, complex processes and high-temperature treatments are required in the method mentioned above. Moreover, the rigid FTO glass * To whom correspondence should be addressed. Phone: +86-1082649242. Fax: +86-10-82649242. E-mail: [email protected].

substrate cannot fulfill the demand of a flexible device and rollto-roll fabrication. As an alternative to FTO glass substrate, we have recently reported the use of flexible graphite (FG) paper as substrate for fabricating pure carbon CEs.21 We have found that good conductivity of FG can benefit the fabricated DSC device with lower series resistance (Rs) and higher fill factor (FF). The low density (0.03 g · cm-2) and flexibility of FG is also a benefit for fabricating potable electricity and lowering the transportation cost for large-scale production. In addition, a site-selective interaction between PANI and carbon has also been reported by chemically polymerizing a PANI layer on carbon nanotube.22 This interaction is expected to enhance the adhesion of the PANI film on the substrate, which will ensure good performance and long time stability. Here, we report the application of a facile in situ chemical polymerization method to fabricate flexible a PANI/FG composite electrode at low temperature. Homogeneous and well-adhered PANI films have been obtained. The property of the PANI film has been optimized by studying the influences of reaction time, initial monomer concentrations, oxidation states, and types of doping acids on the performance of DSCs. Experimental Methods Aniline (Beijing Shiying Chemicals, 99.5%) was predistilled and stored in the fridge before use. Other chemicals and solvents were used as received (i.e., ammonium persulfate (APS), perchloride acid, and sulfuric acid). The in situ fabrication of a PANI/FG composite electrode was carried out according to published procedures, except for using aniline instead of aniline hydrochloride.23 A series of PANI/FG composite electrodes were prepared under different reaction conditions, for example, reaction time and initial monomer concentrations. Aniline was dissolved in perchloride acid (or sulfuric acid for the investigation of doping acid types) and placed in a 0 °C ice-water bath with FG immersed in the beaker. A saturated aqueous solution of ammonium persulfate was then added under vigorous stirring.

10.1021/jp1030015  2010 American Chemical Society Published on Web 06/15/2010

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The concentration of the acid was 1 M and the ratio of monomer and oxidant was kept at 1:1. After a certain reaction time, the FG was taken out, washed by 0.1 M perchloride acid solution, and dried under vacuum at 40 °C for 6 h. All reagents for DSCs (including the electrolytes and electrode materials) were used directly without further purification. The TiO2 photoanodes were fabricated on well-cleaned FTO glass (TEC-15, LOF) by using a screen printing technique. A 10 µm thick transparent layer with P25 (Degussa) particles was first printed on the FTO glass and further coated by a 5 µm thick scattering layer of 300 nm sized rutile TiO2 particles. The detailed preparation procedures and components of the TiO2 pastes for screen printing are the same as those reported in the literature.24 After screen printing, TiO2 layers were gradually heated to 450 °C and held for 30 min. Electrodes were further treated in TiCl4 solution, rinsed, dried, and heated to 500 °C in a hot air stream for 30 min. When the temperature was cooled to 80 °C, they were immersed into a 0.3 mM ethanol solution of N719 dye (RuL2(SCN)2 · 2H2O, L ) 2,2′-bipyridyl-4,4′dicarboxylic acid, Dyesol) overnight. The dye-sensitized TiO2 electrodes were rinsed with ethanol and dried in the air before measurement. For comparison, the Pt/FTO electrode was fabricated by thermal decomposition of H2PtCl6 (30 mM in isopropanol) on FTO glass at 385 °C for 30 min.25 The Pt loading was controlled to be 5 µg · cm-2. The bright Pt electrodes were prepared by sputtering metal Pt target on FTO glass. The electrolyte was composed of 0.6 M methylhexylimidazolium iodide, 0.1 M iodine, 0.5 M tert-butylpyridine, and 0.1 M lithium iodide in 3-methoxypropionitrile. An open sandwich-type cell was fabricated in the air by clamping the sensitized TiO2, a drop of electrolyte, and a CE with two clips. A mask with a window of 0.15 cm2 was also clipped on the TiO2 side to define the active area of the cell. For UV-vis absorption measurements, PANI films were obtained during the polymerization of aniline on FTO glass. The absorption spectra were recorded with a Shimadzu UV2500 spectrophotometer with uncoated FTO glass as a reference. The cyclic voltammetry (CV) was carried out by using a Pt wire as auxiliary electrode, an Ag/Ag+ electrode as reference electrode, and the fabricated PANI/FG, FG, or Pt/FTO as working electrode with a total exposure area of 1 cm2 in an acetonitrile solution containing 0.1 M LiClO4, 10 mM LiI, and 1 mM I2. Scan rates from 50 to 200 mV · s-1 were used and the data were acquired with an IM6ex electrochemical workstation (ZAHNER). The charge-transfer resistance (Rct) and the influence of sheet resistance on Rs of DSCs were determined by electrochemical impedance spectroscopy (EIS). EIS measurements were also performed with the IM6ex electrochemical workstation with the frequency range from 100 mHz to 100 kHz. The magnitude of the alternative signal was 10 mV. For photovoltaic testing, a solar light simulator (Oriel, 91192) was used to give an illumination of 100 mW · cm-2 (AM 1.5) on the surface of solar cells. The incident light intensity was measured with a radiant power/energy meter (Oriel, 70260) before each experiment. A potentiostat (Princeton Applied Research, Model 263A) was used to record the current-voltage plots. Results and Discussion Morphology and Film Thickness. Figure 1 shows the SEM surface morphologies of PANI films prepared with 0.1 M monomer concentration. The low-magnification image (Figure 1a) shows a layer of smooth film, which is incorporated with granular precipitates, has been deposited on the substrate. The grain size is in the range of 200 to 400 nm. Different from the

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Figure 1. SEM images of PANI films deposited with initial monomer concentration of 0.1 M: low magnification, scale bar is 5 µm (a), and high magnification, scale bar is 500 nm (b).

Figure 2. Cyclic voltammograms of Pt/FTO, bare FG, and PANI/FG electrode at a scan rate of 50 mV · s-1 in 10 mM LiI, 1 mM I2 acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte.

high thickness (10 µm) of the PANI electrode used in other works,16,26 the average film thickness is only about 250 nm for the PANI film with 0.1 M monomer concentration and 330 nm for 0.3 M. This is in good accordance with previous work,23,27 which suggested that a two-stage film formation was involved during the in situ polymerization. In the first stage, aniline cation radicals adsorb primarily on the substrate and give rise to a smooth film composed of PANI macromolecules anchored to the substrates. The secondary nucleation of PANI occurs on the already grown PANI film, resulting in granular surface morphology with the incorporation of PANI precipitates into the continuous layer. Further increasing the magnification reveals the porous nature of the continuous film (Figure 1b). The diameter of small granules composing the continuous layer is in the range of 20-50 nm, leading to high film surface areas and favoring good catalytic activities. Electrochemical Properties of PANI/FG Composite Electrodes. CV was applied to analyze the reaction kinetics of various electrodes in the I-/I3- electrochemical system. The results are shown in Figure 2. In the case of the Pt/FTO electrode, two pairs of redox peaks are observed. The left redox peaks correspond to eq 1 and the right ones correspond to eq 2.

3I- T I3- + 2e-

(1)

2I3- T 3I2 + 2e-

(2)

Differing from the electrochemical behavior of Pt/FTO electrode, the bare FG electrode exhibits high peak current density for the second redox reactions while the current of the first redox peaks is very small. This indicates a rather low electrochemical activity of FG for the reduction of I3-. As the second redox reactions would little affect the performance, DSCs with bare FG CEs show lower conversion efficiency (η), and

Polyaniline/Carbon Composite Counter Electrode

Figure 3. Cyclic voltammograms of the PANI/FG electrode under various scan rates. The inset shows the redox peak current of eq 1 as a function of scan rate.

Figure 4. Consecutive 20 cyclic voltammograms of the PANI/FG electrode at a scan rate of 50 mV · s-1. The inset shows the cathodic peak current of eq 1 versus cycle times.

will be discussed in the next part.28 After depositing a PANI film on FG, the first redox peaks reappear with high peak current density. This manifests a faster reaction rate of I3- reduction on the PANI film, and accounts for the high electrochemical activity of PANI. Comparing with the results reported by another group,16 the second redox peaks in the CV of PANI/FG are considered as coming from the FG substrate. Figure 3 shows the CV curves of PANI/FG electrodes in the I-/I3- system with different scan rates. The inset of Figure 3 illustrates a linear relationship between the redox peak current density and the square root of scan rates. This linear relationship indicates the redox reactions (eq 1) on the PANI/FG electrode are controlled by ionic diffusion in the electrolyte,17 which may be related to the transport of iodide species out of the PANI/ FG electrode surface. Figure 4 shows the consecutive CV curves of the PANI/FG electrode in I-/I3- solutions. With a 20 cycles test, the curves do not change significantly, and show stable peak current density (Figure 4, inset), indicating that the PANI film has been coated tightly and has excellent electrochemical stability. Previous analyses of EIS have suggested that consumption of power in the internal resistances of solar cells will reduce the FF and lower the energy conversion efficiency.29-31 The internal resistances of DSCs are mainly related to the sheet resistance of electrodes, the charge-transfer processes occurring at the CEs, the electron transfer at the TiO2/dye/electrolyte interface, and the carrier transport by ions within the electrolyte.29,32 In order to remove the impact of the photoanode, a dummy sandwich-like cell was applied for EIS measurement to directly

J. Phys. Chem. C, Vol. 114, No. 26, 2010 11675 investigate the sheet resistance and Rct of different CEs.6,31 The dummy cell (Figure 5a) was assembled by a piece of Pt/FTO electrode and a CE placed face-to-face, with the electrolyte used in full functional DSCs filling in the interspaces between them. The effective electrode area is 0.5 cm2. Nyquist plots (Figure 5b,c) and Bode plots (Figure 5d) of various cells using Pt/FTO electrode, bare FG electrode, and PANI/FG electrode as working electrodes are shown in Figure 5. The resistance at high frequency around 100 kHz, which is influenced by the sheet resistance of the electrode, can be determined as Rs. For the symmetric cell with two Pt/FTO electrodes, the Nyquist plot shows two semicircles in the frequency range from 100 mHz to 100 kHz. Correspondingly, two characteristic frequencies are observed in the Bode phase plot. The semicircles in the frequency regions of 1 kHz to 100 kHz and 100 mHz to 10 Hz are assigned to the impedance related to the charge transfer processes occurring at the Pt/electrolyte interface and the finite layer Nernst diffusion impedance within the electrolyte, respectively.31 When one Pt/FTO electrode was substituted by a PANI/FG (or FG) electrode, a new peak arose in the middle frequency region of 10 to 100 Hz in Bode phase plots, and a new semicircle was found in Nyquist plots correspondingly. The emergence of this semicircle in the middle frequency region should be assigned to the charge-transfer process on the PANI/FG (or FG) electrode/ electrolyte interface. The characteristic frequency of the PANI/ FG (or FG) electrode is lower than that of the Pt/FTO electrode, suggesting the charge-transfer process at the PANI/electrolyte (or FG/electrolyte) interfaces is slower than that at the Pt/ electrolyte interface. This may be ascribed to better catalytic activity of Pt than PANI. It should also be noticed that a strong overlapping occurs between the charge-transfer resistance on the Pt/FTO electrode surface and that on the FG electrode surface, making it difficult to distinguish the first two semicircles in the Nyquist plot of the FG electrode. Table 1 summarizes the impedance parameters of various electrodes obtained by fitting the experimental data with the equivalent circuit in Figure 6. The Rs of cells with PANI/FG or FG electrodes is less than 8 Ω, which is almost half that of the cell with two Pt/FTO electrodes. This is well matched with the fact that the sheet resistance of FG (ca. 4.2 × 10-2 Ω/0) is low enough to be neglected compared to the average sheet resistance of FTO glass (ca. 15 Ω/0). Thus, introducing FG as conducting substrate instead of FTO glass can greatly reduce the Rs of the fabricated DSCs, and alleviate the energy loss on CEs. However, despite the superior advantage of FG on the sheet resistance over FTO glass, the Rct of bare FG is quite large compared to that of PANI/FG and Pt electrodes, indicating lower catalytic activity for the reduction of I3- ion on FG electrodes surface. After covering the FG surface with a thin layer of PANI, the Rct reduced to 3.9 Ω · cm2, which approaches 1.7 Ω · cm2 for the PT/FTO electrode. This is in good accordance with the result of CV. The higher Q and lower n for the CPE of the PANI/FG electrode can also clearly reflect the porous nature of the composite electrode. Optimization for the Preparing Condition of PANI/FG Electrodes. The film thickness of catalytic materials on CE substrate strongly influences the FF and η of the fabricated DSCs.8 The mechanism of in situ polymerization of PANI on the substrate has been well studied previously, revealing a relationship between the film thickness and various reaction conditions.27 Herein we studied the effects of reaction time and monomer concentration on the film thickness as well as the photovoltaic performance of DSCs with PANI/FG CEs.

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Figure 5. The dummy sandwich-like cell for EIS measurements (a), Nyquist plots (b and c; c is the expanded range of the ordinate and abscissa from b), and Bode phase plots (d) of the cells with different CEs. Symbols: experimental results; solid lines: fitting results.

TABLE 1: EIS Parameters of the Dummy Cells with Different CEs and Pt/FTO Electrode electrodes

Rs (Ω)

Rct (Ω · cm2)

Q (S · cm-2 · s-n)

n

Pt/FTO PANI/FG FG

16.3 7.8 7.6

1.7 3.9 188

1 × 10-5 5.7 × 10-3 2.5 × 10-4

0.92 0.64 0.67

Reaction Time. Figure 7 illustrates the photovoltaic parameters of DSCs with PANI/FG CEs fabricated under different polymerization times. The monomer concentration is 0.1 M and the PANI/FG composite films are taken out from the reaction solution consecutively. At the very beginning (5 min), the cells based on PANI/FG CEs exhibit decreased short circuit current (Jsc), open circuit voltage (Voc), FF, and η compared with the cells using bare FG CEs, resulting in a drop of conversion efficiency from 3.84% to 2.88%. Then, Voc, FF, and η of the cells with PANI/FG CEs experienced a rapid increase within 30 min, followed by a moderate increase over 30 to 60 min. Finally, the photovoltaic parameters remained almost the same as the reaction time was further extended. Meanwhile, Jsc

Figure 6. Equivalent circuit diagram of the dummy cells. Rs: serial resistance; Rct(Pt): charge-transfer resistance of Pt/FTO; Rct(CE): charge-transfer resistance of CEs; CPE(Pt): constant phase element of Pt/FTO; CPE(CE): constant phase element of the CEs. The impendence of CPE is described as ZCPE ) Q(jω)-n (0 e n e 1). Q and n are frequency-independent parameters of the CPE.

Figure 7. Photovoltaic parameters of DSCs with PANI/FG CE prepared at various reaction times. Initial monomer concentration is 0.1 M and doping acid is perchloride acid.

remained at a lower value over 5 to 50 min and increased again at 60 min. FF is considered to be the dominant factor (from 0.35 to ca. 0.7) responsible for the increase in overall conversion efficiency of the DSCs by extending the reaction time. Conversion efficiency of the cells increases from 2.88% (5 min) to a highest efficiency of 6.58% at 60 min. The profiles of photovoltaic parameters versus reaction time depend greatly on the initial monomer concentration. Lower initial monomer concentration brings about a longer and more distinguishable growth of photovoltaic parameters, and vice versa. For instance, under a monomer concentration of 0.2 or 0.5 M, the growing course

Polyaniline/Carbon Composite Counter Electrode

Figure 8. UV-vis absorption spectra of PANI films deposited on FTO glass. The FTO glass was removed from the reaction system from 5 to 240 min. The initial monomer concentration is 0.1 M and the doping acid is perchloride acid.

of photovoltaic parameters will decrease to 20 min or less than 10 min, respectively. To explain the relation between the photovoltaic parameters and polymerization time, the process of the PANI film formation was monitored by UV-vis absorption spectra. Figure 8 shows the absorption of PANI films obtained at different reaction times with 0.1 M initial monomer concentration. At the very beginning of the polymerization, the absorption of the PANI film is very small and unable to be detected until the reaction time reaches about 25 min. Even we cannot obtain a clear UV-vis absorption at the early stage. MacDiarmid et al.33 have measured the potential-time profile of polymerization of aniline and indicate that full oxidized pernigraniline PANI is first formed after adding the oxidation agent into the reaction system. As shown in Figure 8, the shapes and positions of absorption peaks of PANI films obtained after 60 min remain almost the same, indicating the formation of the emeraldine form with similar film thickness. However, the absorption spectra of the PANI film obtained at 25 min shows a relatively low absorption in the region of 700-900 nm compared to emeraldine, suggesting the existence of the pernigraniline form. When the polymerization time increases from 25 to 60 min, the absorption of the PANI film at 700-900 nm quickly increases and the absorption at 500-600 nm decreases relatively, indicating that gradual transformation of the pernigraniline form to the emeraldine form takes place over 25 to 60 min. Since the Jsc remains at a relatively low level until 60 min, at which point all the pernigraniline form has been converted to the emeraldine form, it is reasonable to attribute the variation of Jsc to the pernigraniline form of PANI. To eliminate the effect of film thickness, the performances of DSCs are further investigated in the later part of this section by using emeraldine and pernigraniline electrodes with the same film thickness. The absorbance of the film increased with the increase of the reaction time, indicating that the film thickness grew as well. It has been reported23 that the film thickness is directly proportional to the absorbance around a wavelength of 400 nm. Figure 9 shows the plot of the absorbance of the PANI film at 380 nm and the cell efficiency against the different reaction times. There are three distinct periods during the whole reaction. At the very beginning of the induction period, the absorption and spectra change of the PANI film is very small until the reaction time reaches about 25 min. After that, the absorbance of the PANI film is linearly increased with reaction time up to an optimum value at 60 min. After 60 min, the absorbance remains almost constant due to the depletion of the reactants.

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Figure 9. Plots of cell efficiency and absorbance of the PANI film at λ ) 380 nm versus reaction time. The initial monomer concentration is 0.1 M and the doping acid is perchloride acid.

According to SEM images, the thickness of the PANI film obtained at 60 min is about 250 nm. Therefore, the film thicknesses at 25 and 30 min are calculated to be 16 and 50 nm, respectively. Figure 9 compares the profiles of cell efficiency and absorbance at 380 nm versus reaction time. A quick increase of cell efficiency is observed during the first 30 min from 2.88% to 6.05%. The rapid increase of cell efficiency in this period with slow film thickness growth rate suggests the formation of a PANI film with a high percentage of emeraldine state is crucial for good performance of the composite CE. A thin film (50 nm in thickness) can produce over 90% of the highest performance given by a thick film (250 nm in thickness), indicating good electrochemical activity of emeraldine PANI. This is also in good accordance with the CV studied previously. The highest cell efficiency of 6.58% is obtained at 60 min corresponding to the maximum film thickness of the PANI film. Oxidation State. As mentioned above, a thin emeraldine PANI film on FG can greatly increase the cell efficiency. The change of oxidation states in PANI has been proposed to be a factor to influence the performance of the electrodes. Considering the difficulty in determining the oxidation state of the PANI film by UV-vis absorption due to the extreme thinness of the PANI film formed in the induction period, we investigate the influence of the oxidation state on the performance of the PANI composite CE using thick PANI films. PANI/FG CE with pernigraniline form was obtained by immersing the prepared emeraldine PANI/FG (polymerization time is 60 min) into 1 M HClO4 solution containing 0.1 mM APS for 2 h. Undergoing such treatments, the emeraldine PANI on FG substrates will be partly converted into pernigraniline without changing the film thickness. After oxidation process, the UV-vis absorption spectra of the PANI show a characteristic absorption (maximum absorption at 560 nm) of pernigraniline, as shown in Figure 8. Figure 10 shows the current-voltage curves of cells using the composite PANI/FG CEs with different oxidation states. The Jsc, Voc, and FF of the device with emeraldine PANI/FG CE under full sunlight are 14.13 mA · cm-2, 0.650 V, and 0.716, respectively, yielding an overall conversion efficiency of 6.58%. In contrast, the photovoltaic parameters (Jsc, Voc, FF, and η) of the device with pernigraniline PANI/FG CE are 12.11 mA · cm-2, 0.616 V, 0.658, and 4.91%, respectively. Using pernigraniline PANI film as catalytic materials of CE results in a total loss of about 25% in the solar cell conversion efficiency. Therefore, it is important to synthesize emeraldine PANI as an electroactive material of CE in order to fabricate DSCs with high efficiency.

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Figure 10. Current-voltage curves of DSCs with an emeraldine PANI/ FG composite CE (a) and a pernigraniline PANI/FG composite CE (b). The initial monomer concentration is 0.1 M and the polymerization time is 60 min.

Figure 11. Photovoltaic parameters of DSCs using PANI/FG CEs prepared under various monomer concentrations. The doping acid is perchloride acid and the polymerization time is 60 min.

Monomer Concentration. Besides the reaction time, initial monomer concentration is another crucial factor that determines the thickness of the PANI film.26 Photovoltaic performances of DSCs using PANI/FG CEs prepared under various initial monomer concentrations are shown in Figure 11. When the initial monomer concentration increased from 0.05 to 0.3 M, all three parameters keep growing accordingly, leading to an efficiency improvement of about 1.4% from 6.0% to 7.36%. When the initial monomer concentration is further increased, however, a decline in the Jsc resulted in the same set for the conversion efficiency. The increase of η under monomer concentration below 0.3 M should occur thanks to the growing PANI film thickness. This suggestion is supported by the observation of improvement in Jsc, Voc, and FF when monomer concentration increases. Nevertheless, high monomer concentration not only brings about thick films, but also contributes to fast film formation, which increases the granular size on the film surface morphology. This will lead to high roughness of the film, improving the chargetransfer between the electrode and electrolyte. Another critical problem arises due to the exothermic character of the polymerization of PANI. The heat evolved dramatically at high

Sun et al.

Figure 12. Photocurrent-voltage characteristics of DSCs using optimized bright Pt, Pt/FTO, PANI/FG, and FG CEs.

TABLE 2: Photovoltaic Parameters Obtained from the Data Shown in Figure 12 electrodes

Jsc (mA · cm-2)

Voc (V)

FF

η (%)

bright Pt Pt/FTO PANI/FG FG

16.11 15.13 15.41 14.61

0.664 0.669 0.657 0.631

0.736 0.736 0.727 0.417

7.87 7.45 7.36 3.84

monomer concentration, raising the temperature of the reaction system to a harmful degree. When initial monomer concentration exceeds 0.3 M, the formation of PANI is quit fast and the quickly increased temperature turns the solution into gel and lowers the diffusion of the aniline molecule onto the surface of FG. This will reduce the film thickness of PANI and decrease the cell efficiency. It should be mentioned that the temperature profile is also dependent on other conditions involving heat dissipation, such as the volume and geometry of the reaction vessel. Finally, the optimized monomer concentration of 0.3 M is obtained. As shown in Figure 12 and Table 2, an optimized overall conversion efficiency of 7.36% has been obtained by the DSCs applying PANI/FG composite electrode fabricating with monomer concentration of 0.3 M and reaction time of 60 min. This efficiency is comparable to the one using Pt/FTO electrode (7.45%) and higher than that using pure FG electrode (3.84%). The advantage in Rs when using FG as conducting substrate compensates the slight insufficiency of catalytic activity of PANI compared with Pt, yielding high Jsc and FF of DSCs with PANI/ FG CEs. Since the sputtering bright Pt electrode can reflect the light, DSCs with bright Pt electrode give the highest Jsc (16.11 mA · cm-2) and η (7.87%). Types of Doping Acids. We also investigate the influence of doping acid types by preparing a PANI/FG composite electrode in the presence of H2SO4. All three photovoltaic parameters and cell conversion efficiency show similar values for both DSCs with HClO4 and H2SO4 doped PANI CEs. Our result is different from Li’s work,26 in which DSCs with the SO42- doped PANI CEs show the best performance (efficiency: 5.6%) while the cells with ClO4- doped PANI CEs give efficiency only less than 1%. Different composition and structures of PANI prepared with different methods may be the reason for different catalytic performance. The reason needs to be investigated further. Conclusions A new kind of flexible composite electrode has been fabricated by in situ polymerizing a thin layer of conductive

Polyaniline/Carbon Composite Counter Electrode PANI on a FG substrate. The electrochemical characterizations have confirmed that the composite electrode shows a good catalytic activity for I3- reduction and a reduced sheet resistance, which lead to high Jsc, FF, and final conversion efficiency for the fabricated DSCs. Further investigation reveals the relationship between the performance of the PANI/FG electrode and its synthesis conditions, such as reaction time, monomer concentrations, and doping acids. A thin layer (ca. 50 nm) of emeraldine PANI deposited on FG can promote the cell efficiency greatly, suggesting the excellent electrochemical activity comes from the nature of the emeraldine PANI. The type of doping acid (sulfuric acid or perchloride acid) has almost no influence on the photoelectron conversion performance in DSCs. An optimizing overall conversion efficiency of 7.36% has been obtained by using the composite CE with PANI film thickness of 330 nm. The facile chemical polymerization procedure, flexibility, low cost, and good photovoltaic properties allow the PANI/FG composite electrode to be a promising alternative CE used in future large-scale fabrication of DSCs. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20725311, 20673141, 20703063, 20721140647, and 20873178), the Ministry of Science and Technology of China (973 Project, No. 2006CB202606 and 863 Project, No. 2006AA03Z341), and the Knowledge Innovation Project of the Chinese Academy of Sciences. References and Notes (1) Oregan, B.; Grätzel, M. Nature (London) 1991, 353, 737–740. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (3) Frank, A. J.; Kopidakis, N.; van de Lagemaat, J. Coord. Chem. ReV. 2004, 248, 1165–1179. (4) Jayaweera, P. V. V.; Perera, A. G. U.; Tennakone, K. Inorg. Chim. Acta 2008, 361, 707–711. (5) Papageorgiou, N.; Maier, W. F.; Grätzel, M. J. Electrochem. Soc. 1997, 144, 876–884. (6) Papageorgiou, N. Coord. Chem. ReV. 2004, 248, 1421–1446. (7) Kay, A.; Grätzel, M. Sol. Energy Mater. Sol. Cells 1996, 44, 99– 117. (8) Murakami, T. N.; Ito, S.; Wang, Q.; Nazeeruddin, M. K.; Bessho, T.; Cesar, I.; Liska, P.; Humphry-Baker, R.; Comte, P.; Péchy, P.; Grätzel, M. J. Electrochem. Soc. 2006, 153, A2255–A2261. (9) Spath, M.; Sommeling, P. M.; van Roosmalen, J. A. M.; Smit, H. J. P.; van der Burg, N. P. G.; Mahieu, D. R.; Bakker, N. J.; Kroon, J. M. Prog. PhotoVoltaics 2003, 11, 207–220.

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