Electrochemistry Communications 10 (2008) 1299–1302

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Electrochemistry Communications

journal homepage: www.elsevier .com/locate /e lecom

Application of microporous polyaniline counter electrode for dye-sensitizedsolar cells

Qinghua Li, Jihuai Wu *, Qunwei Tang, Zhang Lan, Pinjiang Li, Jianming Lin, Leqing FanInstitute of Materials Physical Chemistry, Huaqiao University, Fengze Area, Quanzhou, Fujian 362021, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 May 2008Received in revised form 27 June 2008Accepted 27 June 2008Available online 5 July 2008

Keywords:Dye-sensitized solar cellsPolyanilineCounter electrodeMicroporousityCyclic voltammogram

1388-2481/$ - see front matter � 2008 Published bydoi:10.1016/j.elecom.2008.06.029

* Corresponding author. Tel.: +86 595 22693899; faE-mail address: jhwu@hqu.edu.cn (J. Wu).

An inexpensive microporous polyaniline (PANI) is used as a substitute for platinum to construct the coun-ter electrode in dye-sensitized solar cells (DSSCs). The PANI counter electrode with microporosity and asize diameter of about 100 nm possesses lower charge-transfer resistance and higher electrocatalyticactivity for the I�3 =I� redox reaction than Pt electrode does. The overall energy conversion efficiency ofthe DSSC with PANI counter electrode reaches 7.15%, which is higher than that of the DSSC with Pt coun-ter electrode. The excellent photoelectric properties, simple preparation procedure and inexpensive costallow PANI electrode to be a credible alternative for DSSCs.

� 2008 Published by Elsevier B.V.

1. Introduction

Since the first report of a dye-sensitized solar cell (DSSC) in1991 by O’Regan and Gratzel [1], this system has aroused a lot ofinterests over the last decade due to its high efficiency, low costand simple preparation procedure [2–4]. In general, the DSSC con-sists of a dye-sensitized porous nanocrystalline TiO2 film electrode,a redox electrolyte, and a platinized counter electrode to collectelectrons and catalyze the redox couple regeneration. Platinizedcounter electrode is an expensive component in DSSC [5]. Recently,in order to reduce the production cost of DSSCs, attempts on vari-ous carbon materials have been made to replace Pt [6,7]. However,the conversion efficiency of the DSSCs based on the carbon elec-trode was relatively low due to the poor catalytic activity for I�3reduction and lower carbon conductivity.

Conducting polymers are promising candidates for counterelectrode materials used in DSSCs, because of their unique proper-ties, such as inexpensiveness, high-conductivity, good stability,and catalytic activity for I�3 reduction [8,9]. However, there arefew reports about using conducting polymers as counter electrodematerials in DSSCs. The polyaniline (PANI) is one of the most inten-sively studied conducting polymers during the last decade, due toits easy synthesis, high-conductivity, good environmental stabilityand interesting redox properties [10,11]. Up to now, no publicationhas reported on using PANI as counter electrode for DSSC. In thispaper, PANI nanoparticles are used to construct counter electrodes

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for DSSCs and it is expected that the photoelectric performances ofDSSC with PANI electrode could be improved.

2. Experimental

2.1. Preparation of PANI

The PANI was synthesized by an aqueous oxidative polymeriza-tion reaction with perchloride acid as a dopant in the presence ofammonium persulfate [11]. About 50 mL of aniline monomer, puri-fied by a vacuum distillation, was added into 100 mL of perchlorideacid solution and 100 mL of 1 M ammonium persulfate was addedslowly into. Keep the whole system in the dark at 0 �C for at least4 h, the solution’s color changed gradually to dark green. The emer-ald sediment was filtered, collected, and rinsed adequately with anenough amount of methanol, perchloride acid and distilled water.Then, it was subjected to ultrasonic irradiation with a power of100 W for 2 h. After sonication, the emerald material was filteredand dried at 85 �C to remove residue solvent. The resultant emer-ald material was obtained with a yield of 60%. Scheme 1 showsthe polymerization reaction equation.

2.2. Assembling of DSSCs

0.5 g of PANI was slowly added into 50 mL of 10 wt.% TritonX-100 solution. And the mixture was subjected to ultrasonic irradi-ation for 15 min to form an even suspension. The PANI was depos-ited on the surface of the pretreated FTO glass (sheet resistance10 X cm�2, Hartford Glass Co. USA) [12] by the ‘‘dip-tugging”

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Fig. 2. Cyclic voltammograms for PANI electrode and Pt electrode at a scan rate of50 mV s�1 in 10 mM LiI, 1 mM I2 acetonitrile solution containing 0.1 M LiClO4 as thesupporting electrolyte.

APS NH

NH N NNH2

NH

NH

N NHClO4

ClO4 HHClO4 +

-

+ -

Scheme 1. The synthesis of PANI.

1300 Q. Li et al. / Electrochemistry Communications 10 (2008) 1299–1302

method at room temperature, then the PANI was solidified on thegrass by heating at 150 �C for 2 h. Thus, a PANI electrode wasprepared.

Nanoporous TiO2 film was prepared and a DSSC with PANI elec-trode was assembled according to our previous reports [13,14].

2.3. Measurement and characterization

Micrograph of PANI electrode was observed by using a scanningelectron microscope (Hitachi S-5200, JAPAN). The cyclic voltamme-try (CV) of samples was measured in a three-electrode electro-chemical cell using a CHI660 C potentiostat with the PANIworking electrode, a Pt-foiled counter electrode and an Ag/AgClreference electrode dipped in an acetonitrile solution of 10 mMLiI, 1 mM I2 and 0.1 M LiClO4. The measurement of CV was per-formed using CHI660B electrochemical measurement system (scancondition: 50–200 mV s�1). The photovoltaic test of DSSC was car-ried out by measuring [15] the J–V character curves under simu-lated AM 1.5 solar illumination at 100 mW cm�2 from a 100 Wxenon arc lamp (XQ-500 W, Shanghai Photoelectricity Device Com-pany, China) in ambient atmosphere.

3. Results and discussion

3.1. Morphology and compositions of PANI

Fig. 1 shows the SEM image of PANI electrode. It is obvious thatPANI nanoparticles have been separated with a size diameter of100 nm. The surface of the electrode clearly exhibits a porousstructure, which benefits the improvement of electrocatalyticactivity for I�3 =I� redox reaction. On the other hand, the micropo-rous structure of PANI is ready for the adsorption of the liquid elec-trolyte by trapping the liquid in the microporomerics [4].

Fig. 1. SEM image of polyaniline nanoparticles with magnification of 10,000 times,insert figure is the magnification of 80,000 times.

3.2. Electrochemical properties of PANI counter electrode

Fig. 2 compares the cyclic voltammograms of I�3 =I� system ofthe PANI electrode and the Pt electrode. The more negative peakis assigned to redox reaction Eq. (1) and the more positive one isassigned to redox reaction Eq. (2) [16].

3I� � 2e� ¼ I�3 ð1ÞI�3 þ 2e� ¼ 3I�: ð2Þ

In DSSC, electrons are injected into photo-oxidized dye from I� ionsin the electrolyte (Eq. (1)), and the produced I�3 ions are reduced on thecounter electrode (Eq. (2)). Fig. 2 shows a much larger current densityof the I�3 reduction peak for the PANI electrode than that for the Pt elec-trode. This suggests a faster reaction rate on the PANI electrode thanthat on the Pt electrode. In other words, the charge-transfer resistance(RCT) for the I�3 =I� redox reaction is lower on the PANI electrode com-pared with the Pt electrode under the same conditions [6].

Fig. 3a shows consecutive CVs of PANI electrode. On successivescans, the peak currents density changes with the scan rate. It indi-cates that the PANI nanoparticles are tightly on the FTO glass sur-face. Both redox peak currents show good linear relationship withthe cycle times, as shown in Fig. 3b. Therefore, it also indicates thatthe PANI film is uniform and homogeneous [17].

Fig. 4a shows CVs of the I�3 =I� system on the PANI electrode withdifferent scan rates. It can be found that the absolute values of catho-dic peak currents are almost the same as those of the correspondinganodic peak currents. The cathodic peak gradually and regularlyshifts to the negative direction and the corresponding anodic peakshifts to the positive direction with an increasing scan rate. Fig. 4billustrates a relationship between the cathodic and anodic peak cur-rents and the square root of the scan rate. The good linear relation-ship with various scan rates indicates the diffusion limitation ofthe redox reaction on PANI electrode, which may be connected withthe transport of iodide species out of the PANI electrode surface[8,18]. This phenomenon shows the adsorption of iodide species islittle affected by the redox reaction on the PANI electrode surface,and suggests there is no specific interaction between I�3 =I� redoxcouple and PANI electrode as in the case of Pt electrode [8].

3.3. Photoelectric performance of DSSC with PANI electrode

The photocurrent–voltage curves of the DSSCs with PAIN counterelectrode or Pt counter electrode were measured under irradiation

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Fig. 3. (a) Consecutive ten cyclic voltammograms of I2/I� system for PANI electrode in the acetonitrile solution containing 0.1 M LiClO4 as the supporting electrolyte and10 mM LiI, 1 mM I2 as the redox couple, and Pt foil as working electrode and t = 50 mV s�1. (b) The relationship between the cycle times and the redox peak currents for PANIelectrode.

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Fig. 4. (a) Cyclic voltammograms for the PANI electrode in acetonitrile solution of 0.1 M LiClO4, 10 mM LiI, 1 mM I2 with different scan rates (from inner to outer: 50, 100, 150,and 200 mV s�1, respectively). (b) Relationship between all the redox peak currents and scan rates.

Q. Li et al. / Electrochemistry Communications 10 (2008) 1299–1302 1301

of 100 mW cm�2. And the photoelectric parameters of DSSCs such asshort circuit photocurrent density (JSC), open circuit voltage (VOC), fillfactor (FF) and the overall energy conversion efficiency (g) are listedin Table 1. Compared with the DSSC with Pt electrode, all photoelec-tric parameters of the DSSC with PANI electrode are higher. Theoverall energy conversion efficiency of the DSSC with PANI electrodereaches 7.15%, which is higher than that of the DSSC with Ptelectrode.

The improvement of photoelectric performances of DSSC withPANI counter electrode mainly comes from three aspects. Firstly,

Table 1Effects of the counter electrodes on the photoelectric properties of DSSCs

Electrode JSC (mA cm�2) VOC (mV) FF g (%)

PANI 14.60 714 0.69 7.15Pt 14.47 701 0.68 6.90

Condition: Liquid electrolyte contains 0.1 M KI, 0.01 M I2, 0.6 M tetrabutylammo-nium iodide and acetonitrile.

the counter electrode covered with porous PANI nanoparticlesengenders a large active surface area on the electrode and excellentstability by trapping liquid electrolyte in the microporomerics. Thephotoelectric performance of DSSCs can be improved with the in-crease of surface area of the counter electrode, [6,8] especially, itcan benefit the increase of the I�3 =I� redox reaction rate on the PANIelectrode. Secondly, the small charge-transfer resistance RCT at theinterface between electrolyte and the electrode for the I�3 =I� redoxreaction is favorable to electron transport and photocurrent den-sity’s enhancement. Thirdly, the higher electrocatalytic activity ofPANI for I�3 =I� redox reaction results in the increasing of open cir-cuit voltage. Therefore, the DSSC with PANI counter electrode haspredominant photoelectric performance.

4. Conclusions

In summary, PANI nanoparticles are prepared and coated on aconducting FTO glass to construct a PANI counter electrode fordye-sensitized solar cells. Scanning electron microscope imagesshow that PANI nanoparticles with a size diameter about 100 nm

1302 Q. Li et al. / Electrochemistry Communications 10 (2008) 1299–1302

and microporocity are covered on the FTO glass uniformly andtightly. The increase of the surface area of PANI electrode makesfor improving the catalytic activity and traping liquid electrolytesin DSSCs. Cyclic voltammograms of I�3 =I� system measurements re-veal that the PANI electrode has a lower charge-transfer resistanceand higher electrocatalytic activity for I�3 =I� redox reaction than Ptelectrode does. The overall energy conversion efficiency of theDSSC with PANI counter electrode is 7.15%, which is higher thanthat of the DSSC with Pt counter electrode under the same condi-tions. The excellent photoelectric properties, simple preparationprocedure and inexpensive cost allow PANI electrode to be a cred-ible alternative counter electrode used in DSSCs.

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