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In situ oxidative polymerization of polyaniline counter electrode on ITO conductive glass substrate.

INTRODUCTION

Dye-sensitized solar cells (DSSCs) are one of the most promising photovoltaic devices owing to their high conversion efficiency and low cost [1-4]. The photovoltaic device is composed of a photoanodic electrode and a counter electrode. In general, an electrode composed of a platinum (Pt) thin film formed on a transparent conductive glass substrate by vacuum vapor deposition or sputtering is used as the counter electrode. However, the fabricating cost of platinized counter electrode was very expensive [5, 6]. Further, the size of the substrate was limited, so it was hard to form a platinum layer on the large area substrate.

Recently, in order to reduce the production cost of DSSC, conducting polymers are applied in counter electrode materials to replace platinized electrode. Polyaniline (PANI) is one of the most potential conducting polymers as counter electrodes due to its easy synthesis, high conductivity, and unique redox properties [7-10]. Among the previous research, it was reported that PANI was deposited as a thin layer on various materials, [11] like glass, [12-15] metals, [16] as well as on polymeric supports [17-22]. The coating of various materials with a layer of conductive polymer is achieved by using several methods, such as the spreading of a solution of conductive polymer on the surface of the material followed by the evaporation of the solvent, [23] the electropolymerization of the monomers on an electrode, [24-27] or the in situ chemical polymerization and then deposition on the surfaces of various materials immersed in the polymerization solution [28]. However, there is a few report on PANI film prepared by this method used as a counter electrode in DSSC. Wu and coworkers had researched on using PANI [5] or polypyrrole [6] as counter electrode in DSSCs. However, using the vertical dip-tugging method [12, 13] to prepare PANI electrode is difficult to obtain an even and uniform PANI film owing to the insolubility or poor solubility of PANI in nearly all solvents [29]. As a result, the catalytic properties and photoelectric properties of PANI counter electrode can be improved by changing the synthesis method.

Considering these problems, the method of in situ polymerization to prepare a compact PANI film on the surface of indium tin oxide (ITO) conductive glasses is a reasonable method. Hence, PANI films are deposited on the surface of ITO conductive glass by in situ chemical polymerization in this study, and the photoelectric performances of DSSC with PANI electrode are also discussed.

EXPERIMENTAL

Materials

Aniline (An, reagent grade) from Sinopharm Chemical Reagent Company was doubly distilled prior to use. Ammonium persulfate (APS), hydrochloric acid (HCl), iodine ([I.sub.2]), lithium perchlorate (LiCl[O.sub.4]), ethanol, and acetonitrile were also obtained from Sinopharm Chemical Reagent Company. ITO glasses from Institute of Plasma Physics (CAS) were used as the substrates. Titanium dioxide ([TiO.sub.2]) electrode and N719 dye were the commercial product purchased from Solaronix SA (Switzerland). Anhydrous lithium iodide (Lil), 4-tert-butylpyridine (4-TBP), 1-methyl-3-propylimidazolium iodide (MPII), Methoxypropionitrile were provided by Fluka Chemical Corporation. Platinized (Pt) counter electrode prepared on FTO conducting glass (Dyesol, Australia) was used in the contrast test.

Preparation of PANI

The PANI was synthesized by an in situ oxidative polymerization reaction with hydrochloric acid as a dopant in the presence of ammonium persulfate. Double distilled aniline (0.5 mol/L) was dissolved in hydrochloric acid (1 mol/L) solution and a piece of cleaned ITO conductive glass with an active area of 1 [cm.sup.2] was immersed in aniline solution. A plastic adhesive tape was fixed on the four sides of conducting glass sheet to restrict the thickness and area of PANI film. Subsequently, ammonium persulfate (0.5 mol/L) used as an oxidant was added slowly into aniline solution over a period of half an hour with constant stirring for the polymerization reaction. Afterward, the mixture was kept under magnetic stirring condition at room temperature (20[degrees]C) for 2 to 5 h, respectively. Finally, the resulting green PANT films with different film thickness controlled by the adhesive tape with different thickness were rinsed with deionized water and ethanol for several times and then dried in a vacuum at 60[degrees]C for 24 h.

Assembling of DSSCs

Nano-Ti[O.sub.2] colloid was dropped on the ITO glass plate by a doctor scraping technique to form a porous film. A drop of Ti[O.sub.2] paste was scraped onto the conducting glass by using a glass rod to form a smooth and uniform film, and then repeated for three times to form a thick Ti[O.sub.2] film. Then the Ti[O.sub.2] porous film was sintered by anneal at 450[degrees]C for 30 min. After cooling to 100[degrees]C, the Ti[O.sub.2] film was immersed in an ethanol solution of N719 dye (0.5 mmol/L) for 24 h. Finally, DSSC was assembled by injecting a drop of electrolyte with [I.sub.2] (0.05 mol/L), Lil (0.5 mol/L), MPII (0.4 mol/L), 4-tert-butylpyridine (0.5 mol/L) in methoxypropionitrile (5 mL) into the aperture between the Ti[O.sub.2] porous film electrode and the PANI electrode.

Measurement and Characterization

The scanning electron microscopy (SEM) image of the sample was performed using a SEM (LEO1550, GER). Fourier transform infrared spectroscopy (FTIR) spectrum of the PANI was recorded in the range of 500-4000 [cm.sup.-1] using FTIR spectroscopy (Perkin Elmer 1760, USA). Cyclic voltammogram (CV) of the sample was carried out in a three-electrode cell (PANI as a working electrode, Pt-foiled as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode) using a CHI660A electrochemical workstation (CH Instrument, CN). The electrochemical impedance spectroscopy (EIS) measurements were performed in a symmetric cell consisted of two identical counter electrodes with an electrochemical station (CHI660C, CH Instrument, China) at the frequency range of 0.1-[10.sup.5] Hz. The magnitude of the alternative signal was 10 mV. The Brunauer-Emmett-Teller (BET) surface area of the sample was measured on an ASAP 2010 analyzer (Micromeritics, USA). Photocurrent-voltage characteristics of the DSSCs were obtained by a Keithley model 2400 digital source meter using an Oriel 91192 solar simulator equipped with AM 1.5 filter and intensity of 100 mW/[cm.sup.2].

RESULTS AND DISCUSSION

Micro-Morphology of PANI

Figure 1 shows the SEM image of PANI nanoparticles coated on the surface of ITO glass by in situ oxidative polymerization. It can be seen clearly that PANI particles in a diameter range of 20-30 nm are covered on the ITO glass uniformly and tightly to exhibit a porous state, and little aggregation of PANI particles on the surface results in high specific surface area. Moreover, the microporous structure of PANI is contributed to the sufficient adsorption of the liquid electrolyte by trapping the liquid in the microporous film, which benefits for the improvement of electrocatalytic activity for [I.sub.3.sup.-]/[I.sup.-] redox reaction [5].

[FIGURE 1 OMITTED]

Molecular structure of PANI

Figure 2 shows the FTIR spectra of PANI deposited on the surface of conductive glass. Generally, the characteristic peaks of undoping PANI is at 1600 and 1510 (C--C stretching for quinoid and benzenoid groups), 1301 and 1176 (C--N) (KBr, thin film, [cm.sup.-1]) [30-34]. However, the main characteristic peaks of PANI with hydrochloric acid doping have intense tendency to move to lower wave numbers. Because the doping of protonic acid leads to the decline of electron cloud density and the raise of charge delocalization, this shift of the characteristic peaks appears in FTIR spectra. In particular, this red shift of the peaks at 1580 and 1140 [cm.sup.-1] are more obvious, corresponding to the stretching mode of C--C and C--N in quinoid form. These obvious changes suggest that PANI is mainly doped on nitrogen atom of quinoid structure, and the effect of charge delocalization in molecule chain results in more shift of the vibration peaks in quinoid structure.

[FIGURE 2 OMITTED]

Electrochemical Properties of PANI Counter Electrode

Figure 3 shows the CVs of [I.sub.3.sup.-]/[I.sup.-] system on the PANI electrode and Pt electrode. The current peak of the positive potential is assigned to oxidation reaction (Eq. 1) and the current peak of the negative potential is assigned to reduction reaction (Eq. 2) [35-39].

3[I.sup.-] - 2e [right arrow] [I.sub.3.sup.-] (1)

[I.sub.3.sup.-] + 2e [right arrow] 3[I.sup.-] (2)

Figure 3 shows the current peaks of [I.sub.3.sup.-]/[I.sup.-] redox reaction for PANI electrode are much larger than that for Pt electrode. This means a faster reaction rate of the redox reaction of [I.sub.3.sup.-]/[I.sup.-] on the PANI electrode than Pt electrode. In other words, the charge-transfer resistance for the [I.sub.3.sup.-]/[I.sup.-] redox reaction is lower on the PANI electrode compared with the Pt electrode.

[FIGURE 3 OMITTED]

In addition, it can be seen that the [I.sub.3.sup.-]/[I.sup.-] redox reaction is a quasi-reversible reaction on PANI electrode from Fig. 3. The [I.sub.3.sup.-]/[I.sup.-] redox potential can be calculated by Eqs. 3 and 4, and the results are presented in Table 1.

[E.sup.o] = [[[E.sub.pa] + [E.sub.pc]]/2] (3)

[E.sub.redox] = [E.sub.vac] = -e|[E.sub.SCE] + 4.742| (4)

[V.sub.oc] = [[[E.sub.CB] - [E.sub.redox]]/e] (5)

where [E.sup.0], [E.sub.pa], [E.sub.pc], and [E.sub.vac] represent standard electrode potential, the anodic peak potential, the cathodic peak potential and vacuum energy level, respectively. Moreover, open circuit voltage ([V.sub.oc]) depends on the energy levels between conduction band edge ([E.sub.CB]) of TiO[.sub.2] (-4.21eV) [40] and [I.sub.3.sup.-]/[I.sup.-] redox potential (Eredox) on counter electrode (Eq. 5). From Table 1, it can be seen that the redox potential of PANI electrode is lower than that of Pt electrode, thus the theoretical [V.sub.oc] value of DSSC with PANI electrode is higher than that with Pt electrode.
TABLE 1. Cyclic voltammograms peak potential data for PANI and Pt
electrode.

Electrode  [E.sub.pa](V) vs.  [E.sub.pc]  [E.sub.redox]  Theoretical
                 SCE           (V) vs.         (V)        [V.sub.oc]
                                 SCE                         (V)

PANI            0.515           -0.196       -4.880         0.670
Pt              0.378           -0.145       -4.858         0.648

PANI electrode prepared for 2 h at 20[degrees]C.


To estimate the electrochemical behavior of the PANI electrode, the current response obtained as a function of scan rate is presented in Fig. 4. The CV curves for PANI electrodes were at the potential from -1.0 to 0.8 V (vs. SCE) with the different scan rates of 20, 50, 100, 150, and 200 mV [s.sup.-1]. According to Randles-Sevcik equation, the reversible reaction peak current response obtained as a function of scan rate is given by Eq. 6.

[i.sub.p] = [kn.sup.3/2][AD.sup.1/2][v.sup.1/2]C (6)

where [i.sub.p], k, n, A, D, v, and c is peak current, the Randles-Sevcik constant (2.69 x [10.sub.5] As/[V.sub.m]mol), number of electrons exchanged, area of electrode, diffusion coefficient, scan rate, and concentration, respectively. From this equation, a linear relationship between the peak currents and the square root of the scan rate is shown. Then compared with the curves of redox peak currents at different scan rates in Fig. 4, both anodic and cathodic peak currents increased approximately linearly proportional to the square root of scan rate. Obviously, this good linearly relationship corresponds to the Randles-Sevcik equation, and it indicates that the [I.sub.3.sup.-]/[I.sup.-] redox reaction on PANI electrode surface is a reversible redox reaction with fast redox reaction rate and high charge-transport speed. In other words, this redox reaction process is controlled by the matter diffusion limitation, which is connected with transport of iodide species in the electrolyte [41].

[FIGURE 4 OMITTED]

Figure 5 shows the relationship between the redox peak currents of CV curve for PANI electrode and cycle times. The CV of [I.sub.3.sup.-]/[I.sup.-] system for PANI electrode was from -1.0 to 0.8 V (vs. SCE) at a scan rate of 20 mV/s. On successive scans, the redox peak currents density changed with the cycle times. It indicates that the PANI nanoparticles are coated tightly on the conducting glass surface. Both redox peak currents show good linear relationship with the cycle times, as shown in Fig. 5. Therefore, it also indicates that the PANI film is uniform and homogeneous.

[FIGURE 5 OMITTED]

Electrochemical Impedance Spectroscopy

To gain insight into the charge-transfer and catalytic properties of the PANI counter electrodes, the EIS analysis was earned out with the symmetric cell consisted of two identical PANI electrode with 5-[micro]m thickness and Pt electrode, respectively. The inter-electrode space was filled with electrolyte which was same as the one used in the assembly of DSSC.

The Nyquist plots of symmetric cells for these various counter electrodes are shown in Fig. 6 and the impedance values are listed in Table 2. The [R.sub.s] describes mainly the resistance of the two identical electrodes and the electrolytic resistance. The [R.sub.ct] measures the electrode's catalytic activity for reducing triiodide to iodine. The result shows that the [R.sub.ct] of Pt electrode is high to 170.7 [OMEGA]. By preparing a PANI layer onto ITO glass, the [R.sub.ct] dramatically decreases to 79.91 [OMEGA]. This illuminates that Pt electrode has poor catalytic activity for the reduction of triiodide ion. The contribution of the high surface area of PANI layer can considerably improve the catalytic activity of the counter electrodes. It is indicated that the counter electrode of PANI thin film has lower charge transfer resistance, and can efficiently induce the reduction of [I.sub.3.sup.-] to [I.sup.-] in the electrolyte.

[FIGURE 6 OMITTED]
TABLE 2. Impedance parameters of various electrodes estimated from EIS.

Electrode     [R.sub.s] ([OMEGA])  [R.sub.ct]([OMEGA])

PANI                 57.09                 79.91
Pt electrode         33.31                170.7

PANI electrode prepared for 2 h at 20[degrees]C.


BET Analysis of PANI

Table 3 displays the surface areas of PANI with different polymerization time, and the highest BET surface area is 53.1140 [m.sup.2]/g for polymerizing 2 h. It is clear that as the polymerization time increasing, the PANI particles grow up gradually, and the particle size becomes larger. As a result, the BET surface area was decreased with the synthesis time prolonging. Furthermore, owing to a large surface area and high porosity, the PANI film can adsorb more liquid electrolyte on the PANI electrode for [I.sub.3.sup.-]/[I.sup.-] redox reaction process, especially, may favor iodide ions accessing from the electrolyte into the surface of the electrode. Consequently, the photoelectric performance of PANI counter electrode with this high BET surface area will be assuredly enhanced to some extent.
TABLE 3. BET surface area, pore volume, and pore size of PANI.

Sample  BET surface area   Pore volume    Pore size (nm)
          ([m.sup.2]/g)   ([cm.sup.3]/g)

PANI 1       53.114          0.23181         174.578
PANI 2       44.972          0.18447         164.075
PANI 3       37.759          0.13909         150.542
PANI 4       36.957          0.12032         127.457

(PANI1, 2, 3, and 4 is PANI electrode prepared at 20[degrees]C for 2 to
5 h, respectively.


Photoelectric Performance of PANI Electrode With Different Thickness

Figure 7 shows the influence of film thickness on the photoelectric properties of PANI counter electrode in DSSC. Using the same photo anodic electrode and liquid electrolyte, the photoelectric properties of DSSC with PANI and Pt counter electrode are discussed in this study, and the photoelectric parameters are listed in Table 4. From Fig. 7, it is found that with the diminution of the PANI film's thickness, the short-circuit current density ([J.sub.sc]) and open circuit voltage ([V.sub.oc]) of DSSC is enhanced from 3.25 to 8.39 mA/[cm.sup.2] and from 544 to 668 mV, respectively. Because the reduction of thickness results in the rise of conductivity of PANI counter electrode, the electric resistance of DSSC with this thin PANI electrode becomes lower, and then open circuit voltage and current density are also increased. Moreover, the photoelectric performances of PANI counter electrode is contrasted with Pt counter electrode in Fig. 8. The result shows that using the same Ti[O.sub.2] anode and electrolyte, the photoelectric efficiency of DSSC is relative to the change of counter electrode. Thus, this improvement of the photoelectric efficiency is referred to the PANI electrode with the higher surface area, the higher electrocatalytic activity and the lower charge-transfer resistance, compared with Pt electrode. It can be concluded that the optimal film thickness of PANI was about 5 [micro]m, and the energy conversion efficiency of DSSC with this electrode reached 2.64%, which was higher than Pt electrode (1.75%). However, compared with other research, [6] this photo-electron conversion efficiency was relative low due to the poor self-made Ti[O.sub.2] photoanode, the new PANI counter electrode prepared by in situ oxidative synthesis method has more predominant photoelectric performance than Pt electrode. Therefore, the PANI counter electrode can replace the conventional Pt counter electrode for reducing the production cost of DSSC.
TABLE 4. Photovoltaic performance of DSSCs with PANI and Pt counter
electrode.

Electrode     [J.sub.sc]    [V.sub.oc] (mV)   FF   Efficiency (%)
           (mA/[cm.sup.2])

PANI 1           8.39             668        0.47      2.64
PANI 2           6.36             613        0.46      1.80
PANI 3           5.53             593        0.45      1.48
PANI 4           3.25             544        0.48      0.856
Pt               6.18             645        0.44      1.75

PANI electrode prepared for 2 h at 20[degrees]C, the thickness of PANI1
2, 3, and 4 is 5, 10, 15, and 20 [micro]m, respectively.


[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Stability of PANI Counter Electrode

In addition, the stability of DSSC with PANI counter electrode was primarily studied by monitoring the photo-current density-voltage characteristics with time, see Fig. 9. During the first 5 days, [V.sub.oc] slightly decreased due to the electrode-hole recombination loss, but both [J.sub.sc] and energy conversion efficiency of the DSSC increased resulting from the few micron thick PANI layer made up of nano-size powders, which can increase the adsorption of the liquid electrolyte, and enhance the [I.sub.3.sup.-] reduction near Ti[O.sub.2] electrode. Hence, this DSSC device shows its maximum energy conversion efficiency (2.88%) at the 5th day. However, extended storage negatively influenced on this device performance. After 10 days, [J.sub.sc] and [V.sub.oc] of the PANI counter electrode device decreased to 7.20 mA [cm.sup.-2] and 0.595 V, respectively, while FF increased to 0.634. The gradual drop in [J.sub.sc] and [V.sub.oc] resulted in a little decrease of efficiency, this could be the partial volatilization of liquid electrolyte used in the device. In the following days, it can be seen that the [V.sub.oc] was almost unchanged; the photocurrent and the energy conversation efficiency of DSSC trended to be stable, indicating the DSSC with PANI counter electrode had good stability. All these indicated that the in situ polymerized PANI with a porous and homogeneously structure were promising candidates the high performance DSSC.

[FIGURE 9 OMITTED]

CONCLUSION

In summary, the PANI counter electrode was successfully fabricated on conducting ITO glass by in situ chemical polymerization method. The results showed that PANI film with a porous state provided the high surface area for adsorbing more liquid electrolyte on the surface of the counter electrode. The energy conversion efficiency of DSSC with PANI counter electrode reached 2.64%, which was higher than that with Pt electrode. This improvement corresponds to larger surface area, higher electrocatalytic activity, and smaller charge-transfer resistance of PANI electrode. Therefore, the PANI counter electrode with the simple preparation procedure, low fabrication cost, and excellent catalytic properties will be a credible alternative of the counter electrode from DSSCs in the future.

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Qi Qin, Jie Tao, Yan Yang, Xiang Dong

College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, People's Republic of China

Correspondence to: Jie Tao; e-mail: taojie@nuaa.edu.cn

DOI 10.1002/pen.21858

Published online in Wiley Online Library (wileyonlinelibrary.com).

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