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Electrochimica Acta 56 (2011) 8463– 8466

Contents lists available at ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

tability study of carbon-based counter electrodes in dye-sensitized solar cells

ao Hu, Bo-Lei Chen, Cheng-Hao Bu, Qi-Dong Tai, Feng Guo, Sheng Xu, Jun-Hua Xu, Xing-Zhong Zhao ∗

chool of Physics and Technology, Key Laboratory of Artificial Micro/Nano Structures of Ministry of Education, Wuhan University, Wuhan 430072, China

r t i c l e i n f o

rticle history:eceived 8 April 2011eceived in revised form 10 July 2011ccepted 10 July 2011

a b s t r a c t

This study describes a systematic investigation of the stability of a carbon/TiO2 counter electrode for usein dye-sensitized solar cells (DSSCs). In this system, nanoparticle additives were introduced by addingTi-hydrogel. The additives then bound carbon particles and enhanced the adhesion of carbon materials tothe conductive substrate. After introducing the Ti-hydrogel into the carbon paste, the carbon/Ti-hydrogel

vailable online 20 July 2011

eywords:i-hydrogelarbon/TiO2 counter electrodetabilityye-sensitized solar cell

composited counter electrode (HC-CE) showed a better conductivity and stability compared with that ofthe carbon counter electrode (C-CE), while the catalytic activity was not influenced. The device based onthe HC-CE showed superior power conversion efficiency (6.3%) and long-term stability over the devicebased on the C-CE (5.8%).

© 2011 Elsevier Ltd. All rights reserved.

. Introduction

Because of their potential low cost, easy fabrication and rea-onable energy conversion efficiency, dye-sensitized solar cellsDSSCs) have recently received significant attention as alterna-ives to silicon-based solar cells [1,2]. The rate of reductionI3− + 2e− → 3I−) for the counter electrode in conventional conduc-ive glasses, such as indium tin oxide (ITO) or fluorine-doped tinxide (FTO), without a catalyst is extremely slow, and thus, theounter electrode must be coated with catalytic material to accel-rate the reaction [3]. Platinum is an efficient catalyst for tri-iodideeduction, and a power conversion efficiency of more than 11% [4]as been achieved by using a platinum counter electrode [5,6].owever, requiring the use of this expensive noble metal might

imit the practical applications of this technology. Thus, extensiveesearch has been focused on finding a substitute for platinumounter electrodes in DSSCs [7–13].

Various carbonaceous materials, including carbon black [14],ctivated carbon [15], graphite [16], carbon nanotubes [17–19] andraphene [20], are low cost, corrosion-resistant and electricallyonductive materials that have the potential to replace platinumn DSSC counter electrodes. However, the poor stability of theseystems, caused by their poor adhesion to the substrate [21], is a

rucial issue when considering carbonaceous materials’ practicalpplications. TiO2 was first introduced into carbon paste by Lind-tröm et al. [22]. Since that time, carbon and TiO2 (Degussa P25)

∗ Corresponding author. Tel.: +86 27 87642784; fax: +86 27 68752569.E-mail address: xzzhao@whu.edu.cn (X.-Z. Zhao).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.07.035

composite DSSC counter electrodes have been prepared with anefficiency of 5.5% [23]. As an amorphous binder, Ti-hydrogel couldsupply a more sufficient three-dimensional network formed byTi–O bonds than could TiO2 (P25) powder in carbon paste, whichresults in a highly stable counter electrode. Furthermore, usingcarbon black and graphite composite paste could lead to a bet-ter conductivity and catalytic activity when compared with usingonly carbon black [24]. In this paper, the effect of small-particleadditives (Ti-hydrogel) on the stability of DSSCs based on carboncounter electrode (C-CE) was investigated.

2. Experimental

2.1. Preparation of carbon-based counter electrodes

To obtain the carbon paste, 1.5 g of carbon black (40 nm, TopVender, Beijing, China) and 2.25 g of graphite (30 �m, SCRC, China)were ground in a mortar with 20 mL of water and 0.5 mL ofacetylacetone (PC, 99.9%, anhydrous, SCRC, China). The Ti-hydrogelmodified carbon composite paste was obtained by grinding 1.5 gof carbon black and 2.25 g of graphite with 0.5 mL of acetylacetoneand 5 mL of Ti-hydrogel in a mortar. At this amount of Ti-hydrogel,the DSSC gives the highest efficiency.

The Ti-hydrogel was prepared as follows: a solution consisting of20 mL of Ti(OCH(CH3)2)4 and 4 mL of acetic acid was added drop-

wise to 100 mL of vigorously stirring water. After the addition of65% nitric acid (1.4 mL), the solution was stirred at 80 ◦C for 3 h.

The as-prepared pastes were coated on FTO-glass by doctor-blading. The substrates were heated in air at 450 ◦C for 30 min.

8464 H. Hu et al. / Electrochimica Acta 56 (2011) 8463– 8466

-CE. C

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activity of HC-CE toward the I−/I3− redox couple. The C-CE wasalso studied under the same conditions. Two pairs of redox waveswere observed on both CEs, which are explained as the oxidation

Fig. 1. Top view SEM images of (a) HC-CE and (b) C

.2. Fabrication of DSSCs

Dye sensitized TiO2 photoanodes were prepared accordingo the previously reported procedures [25]. The resulting TiO2hotoanodes were immersed in an anhydrous ethanol solution con-aining 5 × 10−4 M of purified N3 dye and kept at 60 ◦C for 12 h.

25 �m Surlyn film (Duport, Shanghai) was used to separate thehotoanode and the counter electrode and to seal the cell after thelectrolyte [26] was added.

.3. Characterization of counter electrodes

The surface morphology of the HC-CEs and C-CEs was moni-ored using a scanning electron microscope (SEM) (JEOL, 6700F,apan). Cyclic voltammetry (CV) was performed using a Pt plates the auxiliary electrode, an Ag/Ag+ electrode as the referencelectrode and C-CE and HC-CE as working electrodes, with a totalxposure area of 1 cm2 in an acetonitrile solution containing 10 mMiI, 1 mM I2 and 0.1 M LiClO4 as supporting electrolytes. The Ag/Ag+

lectrode consisted of Ag in contact with a soluble AgNO3 salthat was separated from the working electrode solution by aiaphragm. A scan rate of 50 mV/s was used, and the data werecquired with a CHI 660C (Shanghai, China) electrochemical sta-ion. Electrochemical impedance spectroscopy (EIS) measurementsnd photon-to-current conversion efficiencies of the DSSCs withhe C-CE and HC-CE were evaluated using a solar light simulatorOriel, 91192) as the light source. The intensity of the incident lightas calibrated by a Si-1787 photodiode (spectral response range:

20–730 nm). The active DSSC area was controlled at 0.25 cm2 by

mask. All of the measurements were taken under ambient con-itions. The devices were kept in the laboratory atmosphere andere only irradiated under the simulated sun light during the pho-

ovoltaic testing.

ross-section SEM images of (c) HC-CE and (d) C-CE.

3. Results and discussion

3.1. Characterization of the HC-CE

The top view SEM images of the pure carbon materials and thecarbon/Ti-hydrogel composited films are shown in Fig. 1a and b. Ahighly porous HC-CE catalyst layer is shown in Fig. 1a. After anneal-ing, TiO2 nanoparticles can bridge the carbon material’s surface,which could result in a good connection between the carbon blackand the graphite. Thus, I3− ions, which are only few angstroms wide,could easily diffuse into the pores to be reduced [27]. The cross-section SEM images of HC-CE and C-CE are also shown in Fig. 1cand d. TiO2 nanoparticles in HC-CE could serve as a binder becauseof their good adherence between the carbon materials and FTO sub-strate (see Fig. 2). Energy dispersive X-ray analysis (EDX) confirmedthe presence of titanium in the HC-CE (Fig. S1, Supporting Infor-mation). The two functions of the Ti-hydrogels, mentioned above,could result in better a conductivity (see Fig. S2, Supporting Infor-mation) and stabilities of HC-CE.

Cyclic voltammetry (CV) was used to investigate the catalytic

Fig. 2. Schematic illustration of the enhanced connection between the carbon mate-rials and the substrate caused by introducing Ti-hydrogel in the system.

H. Hu et al. / Electrochimica Acta 56 (2011) 8463– 8466 8465

Fig. 3. (a) Cyclic voltammograms for HC-CE and C-CE obtained at a scan rate of50 mV/s in a 10 mM LiI, 1 mM I2 acetonitrile solution containing 0.1 M LiClO4 astv

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12.63 mA/cm , an open circuit voltage (Voc) of 704 mV and a fillfactor of 0.71, resulting in a power conversion efficiency of 6.3%.This efficiency was much higher than that of C-CE, which was 5.8%.The higher Jsc of the HC-CE based DSSC was due to the lower Rs,

he supporting electrolyte. (b) Digital photograph of HC-CE and C-CE after cyclicoltammetry test.

nd reduction of I−/I3− and I2/I3−, respectively (see Fig. 3a) [15].he waves marked in green circles in Fig. 3 are the focus of ournalysis, as they indicate the CE’s catalytic activity for the reduc-ion of I3−/I−. The value of the peak current is largely determinedy mass transport. The potential difference between oxidation andeduction peak currents is indicative of electrocatalytic activity. Aeaction with the I3− mechanism has a potential difference of 68 mV12,15,20,28]. As a semiconducting additive, TiO2 could not pro-ide enough catalytic activity with carbon black for the reductionf I3−/I−. The catalytic activity of HC-CE was found to be almost theame as that of C-CE, as shown by the similar position of their peaks.owever, the stability of HC-CE is much better than that of C-CE.ome of the carbon films on the C-CE were peeled off after the CVest, as shown in Fig. 3b. This result suggests that the HC-CE couldxhibit a better stability while maintaining good catalytic activity.

In general, electrochemical impedance spectroscopy (EIS) anal-sis and equivalent circuit fitting of DSSCs provide an estimate forhe impedance in the cells [29–31]. The impedance of DSSCs is

ainly caused by charge transfer processes at the counter elec-rodes, the sheet resistance (Rs) of the substrates, the electronransfer at the TiO /dye/electrolyte interface and the ion trans-

2ort within the electrolyte [32]. To check the catalytic behaviornd the sheet resistance of the HC-CE and C-CEs, the charge trans-er resistance (Rct) at the counter electrodes and the Rs of the

Fig. 4. Nyquist plots of HC-CE and C-CE based DSSCs obtained under 100 mW/cm2,1.5 simulated irradiation with active area of 0.25 cm2. Shapes represent the experi-mental results, and the solid lines represent the fitting results.

devices were measured using EIS. Fig. 4 shows the EIS spectrameasured under 1 sun illumination with open circuit conditions.The semicircle at the highest frequency region, which describesthe electron transfer at the counter electrode/electrolyte interface,was the focus of our study. By fitting the measured data with theinset equivalent circuit, the Rct at the counter electrode/electrolyteinterface could be obtained. The Rs could be obtained directly fromthe onset of the first semicircle. Table 1 shows the Rct and Rs

values of HC-CE and C-CE in the devices. The Rct value of C-CEand HC-CE were approximately the same, which indicated thatthese two counter electrodes had almost the same catalytic activ-ity. This result agrees with the CV analysis. However, the Rs ofthe HC-CE was much lower than that of C-CE, which was due tothe enhanced connection between carbon materials, as mentionedabove.

3.2. J–V curves of the DSSCs

Fig. 5 shows the representative J–V characteristics of theDSSCs with C-CE and HC-CE under 1 sun illumination (AM 1.5,100 mW/cm2). The active area was 0.25 cm2. Table 1 shows theJ–V parameters of the counter electrodes in devices. The deviceemploying HC-CE gave a short circuit current density (Jsc) of

2

Fig. 5. J–V characteristics of DSSCs with C-CE and HC-CE measured under100 mW/cm2, AM 1.5 simulated irradiation with an active area of 0.25 cm2.

8466 H. Hu et al. / Electrochimica Acta 56 (2011) 8463– 8466

Table 1Electrochemical impedance spectroscopy and current–voltage parameters of DSSCs with C-CE and HC-CE.

Electrodes Rs (� cm2) Rct (� cm2) Voc (mV) Jsc (mA/cm2) Efficiency (%) Fill factor

C-CE 10.21 0.76 700

HC-CE 8.78 0.78 704

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ig. 6. Time-course changes of the normalized efficiencies (�) for the DSSCs with-CE and HC-CE. Photovoltaic data were obtained under AM 1.5 illumination of00 mW/cm2.

s mentioned in the EIS analysis, which resulted in the higher fillactor of the device.

.3. Stability of the DSSCs

The evolution of power conversion efficiencies over time forSSCs based on HC-CE and C-CE are presented in Fig. 6. Theoc, Jsc and fill factor versus time graphs are also presented (seeigs. S4–6, Supporting Information). An improved long-term sta-ility of the HC-CE DSSCs was observed as compared with thatf the C-CE device. From 0 to 100 h, the efficiencies of DSSCsith C-CE and HC-CE were slightly increased, which could be due

o the slow stabilization of electrolyte additives on the porouslms. The performance of the DSSCs based on C-CE shows aecrease of 82% in efficiency. However, HC-CE devices showedetter long-term stability. The stability tests corroborate the datarom SEM, CV and EIS analyses. The improved stability of DSSCsan be related to the functions of the Ti-hydrogel, which increaseshe connection between the carbon materials and the substratesee Fig. S3, Supporting Information). As for the device with C-E, the poor connection could result in a decrease of the DSSC’stability.

. Conclusions

In conclusion, a stable carbon-based counter electrode was pre-ared by adding a small-particle additive. The HC-CE was foundo have a better conductivity. Furthermore, the DSSC with theC-CE showed an improved photovoltaic performance and sta-ility compared with that with the C-CE. The best efficiency,.3%, was achieved in the DSSC with HC-CE, compared with the.8% efficiency of the device with the C-CE. The facile proce-

ure, low cost, admirable photovoltaic properties and increasedtability suggest that the HC-CE is a promising alternativeounter electrode to be used in future large-scale fabrication ofSSCs.

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Acknowledgements

We gratefully acknowledge the financial support of this workby the National Basic Research Program (No. 2011CB933300) ofChina and the National Science Fund for Talent Training in BasicScience (Grant No. J0830310). We also acknowledge the help of theNanoscience and Nanotechnology Center at Wuhan University forthe SEM measurements.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.electacta.2011.07.035.

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