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Electrochimica Acta 55 (2010) 2777–2781

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

pplication of poly(acrylic acid-g-gelatin)/polypyrrole gel electrolyte in flexibleuasi-solid-state dye-sensitized solar cell

inghua Lia,b, Jihuai Wua,b,∗, Ziying Tanga,b, Yaoming Xiaoa,b, Miaoliang Huanga,b, Jianming Lina,b

The Key Laboratory for Functional Materials of Fujian Higher Education, Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, ChinaInstitute of Materials Physical Chemistry, Huaqiao University, Quanzhou, 362021, China

r t i c l e i n f o

rticle history:eceived 22 September 2009eceived in revised form8 December 2009ccepted 28 December 2009

a b s t r a c t

A novel gel polymer electrolyte based on poly(acrylic acid-g-gelatin)/polypyrrole with conductivity of14.1 mS cm−1 was prepared. Based on the gel electrolyte, a flexible quasi-solid-state dye-sensitized solarcell was fabricated by using a low-temperature filming technique. Owing to high conductivity and thecatalytic function of polypyrrole for I−/I3

− redox reaction for the gel electrolyte, the flexible quasi-solid-state dye-sensitized solar cell showed a light-to-electric energy conversion efficiency of 1.28%, under a

vailable online 11 January 2010

eywords:lexibleuasi-solid-state dye-sensitized solar cellel electrolyte

simulated solar light irradiation with intensity of 100 mW cm−2 (AM 1.5).© 2010 Elsevier Ltd. All rights reserved.

oly(acrylic acid-g-gelatin)olypyrrole

. Introduction

Since the first report by O’Regan and Gratzel in 1991 [1], dye-ensitized solar cell (DSSC) has attracted a great deal of interestsver the past decades due to its potential for converting solar lightnto electricity and low cost. Up to now, the highest light-to-electriconversion efficiency of about 11% for liquid electrolyte DSSC haseen achieved [2]. In recent years, more attentions focus on flexibleSSCs, aiming at increasing the flexibility and reducing the weights well as the overall device thickness of DSSCs [3–5]. However, these of liquid electrolytes in flexible DSSCs displays some practicalroblems such as leakage of the liquid electrolyte, desorption ofhe sensitized dye and toxicity. Besides, the instability against long-erm use because of incomplete sealing of the electrolyte solutionas also restricted the development of the DSSCs. Scientists haveried to develop practical, high-performance DSSC modules withigh efficiency, good stability and easy processability.

To improve the stability of DSSCs, many researches have beenone to replace the liquid electrolytes with solid-state or quasi-olid-state type charge transport materials [6–9], but the efficiencynd stability of the DSSC in an aqueous phase were low. On the

∗ Corresponding author at: Institute of Materials Physical Chemistry, Huaqiao Uni-ersity, Fengze Area, Quanzhou, Fujian 362021, China. Tel.: +86 595 22693899;ax: +86 595 22693999.

E-mail address: jhwu@hqu.edu.cn (J. Wu).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.12.072

other hand, conducting polymers have a low-cost, high conduc-tivity, transparency, good stabilities, high specific capacitances(100–300 F/g), and catalytic activity for I−/I3− reaction, which is agood candidated material in DSSC [10–13]. Polypyrrole is a bet-ter conductive polymer, which has been employed to constructpolypyrrole/TiO2 anode or solid electrolyte, the photovoltaic per-formance of cell is improved [14–16]. However, there are only fewreports on the application of this conductive polymer in flexiblequasi-solid-state DSSCs.

In this paper, a porous poly(acrylic acid-g-gelatin)/polypyrrolewas synthesized by aqueous solution graft copolymerization andin situ polymerization of polypyrrole. Using the polymer as host,a polymer gel electrolytes with high conductivity of 14.1 mS cm−1

were prepared and a flexible quasi-solid-state DSSC was fabricatedby a low-temperature filming technique. The electrical property ofthe gel electrolyte and photoelectrical performance of the flexibleDSSC are also discussed.

2. Experimental

2.1. Materials

Acrylic acid and pyrrole monomers were distilled prior touse. Gelatin, a mixture of water-soluble macromolecular pro-tein made from porcine skin (300 Bloom), was used as received.Ammonium peroxydisulfate (APS) was a radical initiator for thesynthesis of poly(acrylic acid-g-gelatin) hybrid. N,N′-methylene

2 ca Act

bwsawnm(5Pwt

2e

biagimtdpstms(angwIat

2

3wctT

2

dbcIa5T1Naa

ttw

778 Q. Li et al. / Electrochimi

isacrylamide (NMBA), as a crosslinker for preparing the hybrid,as purified by recrystallization from 66 wt% ethanol/water

olution. cis-Di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4-carboxyliccid-4-tetrabutyl ammonium carboxylate) ruthenium (II) (N719)as purchased from Solaronix SA, and used as received. TiO2anoparticles (P25) were purchased from Degussa Co. Ltd., Ger-any. Tin-doped indium oxide coated poly(ethylene naphthalate)

ITO/PEN, 12–15 � cm−2) and Pt coated ITO/PEN (Pt-ITO/PEN,� cm−2) sheets were all purchased from Japan. The ITO/PEN andt-ITO/PEN substrates were ultrasonically cleaned with deionizedater, ethanol and acetone to remove impurities before using. All

he chemicals were of analytical reagent grade.

.2. Preparation of poly(acrylic acid-g-gelatin)/polypyrrole gellectrolyte

A mixed solution of acrylic acid, NMBA and gelatin was madey agitating acrylic acid of 10 g, gelatin of 3 g and NMBA of 0.01 g

n 12.5 ml deionized water for 30 min. Initiator APS of 0.1 g wasdded to the mixed solution consisting NMBA, acrylic acid andelatin. A graft copolymerization reaction took place under stirringn nitrogen atmosphere at 80 ◦C. After completion of the copoly-

erization reaction, the system was cooled to room temperature,he resultant product was filtered and then immersed in excesseionized water to remove any impurities and oligomers. Theroduct was vacuum dried at 60 ◦C for more than 12 h to a con-tant weight. The dried poly(acrylic acid-g-gelatin) was immersedhe mixed solution of N-methyl pyrrolidine (NMP) and pyrrole

onomer for adsorbing pyrrole monomer adequately. Then, thewollen poly(acrylic acid-g-gelatin) gel was immersed in a iodine0.05 M) acetonitrile solution at 4 ◦C for 24 h, which resulted inn in situ polymerization reaction of pyrrole monomers in the 3Detwork of poly(acrylic acid-g-gelatin) to form poly(acrylic acid-g-elatin)/polypyrrole. The poly(acrylic acid-g-gelatin)/polypyrroleas immersed in a liquid electrolyte composed of 0.1 M KI, 0.01 M

2, 20 vol.% NMP and 80 vol.% acetonitrile. After soaking adequately,black color poly(acrylic acid-g-gelatin)/polypyrrole gel electrolyte

hus was obtained.

.3. Preparation of TiO2 colloid

A predetermined amount of P25 powder was heated at 450 ◦C for0 min. Then, the pretreated P25 powder was added to deionizedater and ethanol solution with a mole ratio of 1:1:5 in an auto-

lave, the mixture was autoclaved at 180 ◦C under vigorous stirringo form a white milky slurry, the autoclaving action enables theiO2 particles disperse completely without any deposition.

.4. Fabrication of flexible DSSC

Nanoporous TiO2 films were prepared by the following proce-ures: the TiO2 colloid was dropped on the clean ITO/PEN substratey a doctor scraping technique [17–19]. The TiO2 film thickness wasontrolled by the thickness of adhesive tape around the edges ofTO/PEN substrate. The process was repeated for three times to formTiO2 film with thickness of 12 �m. The TiO2 film was treated with0 mM TiCl4 aqueous solution and washed with deionized water.hen, the TiO2 film was sintered by firing the ITO/PEN substrate at00 ◦C for 30 min. The resultant TiO2 film was immersed in a dye-719 ethanol solution (2.5 × 10−4 M) for 24 h to absorb the dyedequately. After the film was dried and irradiated 15 min [20,21],

flexible dye-sensitized TiO2 electrode thus was obtained.

A flexible DSSC was assembled by injecting an electrolyte intohe aperture between the TiO2 film electrode (anode electrode) andhe Pt-ITO/PEN electrode (counter electrode). The two electrodesere clipped together and cyanoacrylate adhesive was used as a

a 55 (2010) 2777–2781

sealant to prevent the electrolyte solution from leaking. Epoxy resinwas used for further sealing of the flexible DSSC.

2.5. Measurement and characterization

The conductivity of poly(acrylic acid-g-gelatin)/polypyrrolegel electrolyte was measured with a Pocket Conductivity Meter(HANNA8733, Hanna Instruments, Atlanta, USA). Micromorphol-ogy of TiO2 film was observed by using a scanning electronmicroscope (SEM; S-5200, Hitachi, Tokyo, Japan). The powderedsamples were identified by IR spectroscopy on a Nicolet Impact410 FTIR spectrophotometer using KBr pellets. The sample washeated in flowing nitrogen (99.9% purity, 100 ml min−1) at a heat-ing rate of 15 ◦C min−1 from room temperature to 650 ◦C untilthe gel sample was completely consumed. Cyclic voltammetry(CV) of samples was measured in a three-electrode one com-partment cell with poly(acrylic acid-g-gelatin) or poly(acrylicacid-g-gelatin)/polypyrrole as working electrode, Pt foil counterelectrode and an Ag/AgCl reference electrode dipped in an ace-tonitrile solution of 10 mM LiI, 1 mM I2 and 0.1 M LiClO4. Thephotovoltaic test of flexible DSSC was carried out by measuring theJ–V character curves using CHI660B electrochemical measurementsystem under a simulated solar light illumination of 100 mW cm−2

(AM 1.5) from a 100 W xenon arc lamp (XQ-500W, Shanghai Pho-toelectricity Device Company, China) in ambient atmosphere.

3. Results and discussion

3.1. Synthesis of poly(acrylic acid-g-gelatin) polymer

Acrylic acid and gelatin were simultaneously crosslinked in ahomogeneous aqueous solution by using APS as a radical initia-tor and NMBA as a crosslinker. The �-C atom connecting –CO– or–NH– is easy to lose a hydrogen atom to form carbon free radi-cal, which is prone to graft to monomers and copolymerize. Fig. 1shows the mechanism of the crosslink of acrylic acid and gelatin inthe presence of APS and NMBA. The sulfate anion radical that pro-duces from thermal decomposition of APS abstracts hydrogen fromthe C–H groups in amino acids to form the carbon free radical on thesubstrates. The graft copolymerization formation can be carried outin two ways [22]: (a) the carbon radicals on the gelatin backbonesresult in active centers capable of initiating free radical reactionswith NMBA to form a gel. (b) Self-crosslink of the free radicals ontoacrylic acid-g-gelatin results in crosslink points to produce gel [23].

Liu et al. [24] reported that water molecules in DSSC decreasedthe light-to-electric energy conversion efficiency. Therefore, thehydrophobic property of gel electrolyte is very important. Becauseof the hydrophilic –COOH groups and hydrophobic alkyl group (–R)in poly(acrylic acid-g-gelatin), the copolymer shows amphiphilicproperties. So, the hydrophobic pyrrole monomers can be adsorbedinto the network of poly(acrylic acid-g-gelatin) polymer, the pyr-role monomers are in situ polymerized to form polypyrrole underthe oxidation and induction of I2 at 4 ◦C. The introduction ofpolypyrrole is significant for the improvement of light-to-electricenergy conversion efficiency [16,25,26].

3.2. FTIR spectra of polymer gel electrolyte

The FTIR spectra of poly(acrylic acid-g-gelatin) and poly(acrylicacid-g-gelatin)/polypyrrole are shown in curves (a) and (b) in Fig. 2.

In the spectrum of poly(acrylic acid-g-gelatin), the broad absorp-tion band at 3177 cm−1 is attributed to N–H stretching. Band at2947 cm−1 is attributed to C–H stretching in –CH2 group. Fromthe report by Tang et al. [27], it can be concluded that the shift ofabsorption peak (amide I in gelatin) from 1664 cm−1 to 1730 cm−1,

Q. Li et al. / Electrochimica Acta 55 (2010) 2777–2781 2779

(acry

ia

t

Fg

Fig. 1. The synthesis of poly

ndicating the possible graft copolymerization of poly(acrylic acid)nd gelatin.

In the FTIR spectrum of poly(acrylic acid-g-gelatin)/polypyrrole,he absorption peak at 1650 cm−1 is for C C stretching, the peak

ig. 2. FTIR spectra of (a) poly(acrylic acid-g-gelatin) and (b) poly(acrylic acid-g-elatin)/polypyrrole polymer.

lic acid-g-gelatin) polymer.

at 1456 cm−1 is for C–C stretching. The peak at 1300 cm−1 is forC–N stretching and the peak at 790 cm−1 is the result of C–H

stretching in contraposition of benzene ring. Whereas the inten-sity ratio of the absorption peaks at 1650–1456 cm−1 (I1650/I1456)is related to conjugate degree of polypyrrole chains [28]. The detec-tion of absorption peaks of polypyrrole reveals the introduction ofpolypyrrole, which is significant for improvement of the light-to-electric energy conversion efficiency of DSSC.

3.3. Appearance of the polymer gel electrolytes

The SEM photograph of the poly(acrylic acid-g-gelatin) gelelectrolyte is shown in Fig. 3a. As a typical poly(acrylicacid-g-gelatin) polymer, the electrostatic repulsion among thehydrophilic carboxylic and ionized carboxylate groups locatedin poly(acrylic acid-g-gelatin) backbones causes an expansionof the polymer three-dimensional (3D) network. Fig. 3b showsthe photographs of (i) poly(acrylic acid-g-gelatin) polymer (ii)poly(acrylic acid-g-gelatin) gel electrolyte and (iii) poly(acrylicacid-g-gelatin)/polypyrrole gel electrolyte. Under the osmotic

pressure, liquid electrolyte (composed of 0.1 M KI, 0.01 M I2,20 vol.% NMP and 80 vol.% acetonitrile) is absorbed in the net-work of poly(acrylic acid-g-gelatin) (i) to form a brown-redcolor poly(acrylic acid-g-gelatin) gel electrolyte (ii). Similar to thepoly(acrylic acid-g-gelatin) gel electrolyte, the poly(acrylic acid-g-

2780 Q. Li et al. / Electrochimica Acta 55 (2010) 2777–2781

F tograe

gpc

3

etttaciicta1gIgcht[

au

Fge

(3) [31].

ig. 3. (a) SEM images of poly(acrylic acid-g-gelatin) polymer and (b) digital pholectrolyte and (iii) poly(acrylic acid-g-gelatin)/polypyrrole gel electrolyte.

elatin)/polypyrrole gel electrolyte is obtained by immersing theolymer into the liquid electrolyte, the gel electrolyte shows a blackolor, which is due to the introduction of polypyrrole.

.4. Influence of temperature on the conductivity of electrolytes

The ln � versus 1/T plots for the poly(acrylic acid-g-gelatin) gellectrolyte and poly(acrylic acid-g-gelatin)/polypyrrole gel elec-rolyte are shown in Fig. 4. From the figure, it can be seen thathe conductivities of the samples increase with the increase ofemperature. It is known that the polymer matrix is amorphousnd has a large amount of free-volume cages. These free-volumeages increase with an increase in temperature just as describedn the free-volume model [29]. The free volume increases withncrease of temperature that enhances the mobility of polymerhains and ions dissolved in polymer matrix. At room temperature,he conductivities for poly(acrylic acid-g-gelatin) and poly(acryliccid-g-gelatin)/polypyrrole gel electrolytes are 10.2 mS cm−1 and4.1 mS cm−1, respectively. The conduction of poly(acrylic acid--gelatin) gel electrolyte comes from the ionic conductivity of−/I3− system, whereas the conduction of poly(acrylic acid-g-elatin)/polypyrrole is attributed to the cooperation of ioniconduction of I−/I3− and electronic conduction of polypyrrole. Itas been reported that the increase of conductivity is favorable forhe improvement of light-to-electric energy conversion efficiency

30].

In Fig. 4, the dependence of conductivity on temperature islmost linear. Therefore, the Arrhenius equation (Eq. (1)) can besed to describe the conductivity–temperature behaviors of the gel

ig. 4. Temperature dependence of the conductivity of (a) poly(acrylic acid--gelatin)/polypyrrole gel electrolyte and (b) poly(acrylic acid-g-gelatin) gellectrolyte.

phs of (i) poly(acrylic acid-g-gelatin) polymer (ii) poly(acrylic acid-g-gelatin) gel

electrolytes.

ln �(T) = − Ea

RT+ ln A (1)

where Ea is the activation energy, R the molar gas constant, A is aconstant, and T is the absolute temperature.

According to Fig. 4 and Eq. (1), the activation energies (Ea)of poly(acrylic acid-g-gelatin)/polypyrrole and poly(acrylic acid-g-gelatin) gel electrolytes are calculated as 10.32 kJ mol−1 and10.85 kJ mol−1, respectively. The similar activation energies indi-cate the similar ionic conduction mechanism and the free-volumefor poly(acrylic acid-g-gelatin)/polypyrrole and poly(acrylic acid-g-gelatin) systems. However, owing to the introduction ofpolypyrrole and electronic conduction, the former shows a higherconductivity (14.1 mS cm−1) than the later (10.2 mS cm−1).

3.5. Cyclic voltammetry in I−/I3− system

Fig. 5 compares cyclic voltammograms of I−/I3− systemusing poly(acrylic acid-g-gelatin) and poly(acrylic acid-g-gelatin)/polypyrrole as electrodes at a scan rate of 50 mV s−1

and [I−]/[I3−] = 10/1. More negative pair is assigned to redoxreaction (2) and more positive one is assigned to redox reaction

I3− + 2e− = 3I− (2)

3I2− + 2e− = 2I3− (3)

Fig. 5. Cyclic voltammograms for (a) poly(acrylic acid-g-gelatin)/polypyrrole and(b) poly(acrylic acid-g-gelatin) electrodes 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.

Q. Li et al. / Electrochimica Act

Fig. 6. Photocurrent–voltage characteristics curves of flexible quasi-solid-stateDSSCs based on (a) poly(acrylic acid-g-gelatin)/polypyrrole gel electrolyte and (b)poly(acrylic acid-g-gelatin) gel electrolyte.

Table 1Photoelectric properties of the quasi-solid-state DSSCs with different electrolytes.

Gel electrolytea Jsc (mA cm−2) Voc (mV) FF � (%)

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Poly(acrylic acid-g-gelatin) 1.99 608 0.70 0.85Poly(acrylic acid-g-gelatin)/polypyrrole 2.76 667 0.70 1.28

a Gel electrolyte contains 0.1 M KI, 0.01 M I2, 20 vol.% NMP and 80 vol.% acetoni-rile.

In DSSC, electrons are injected to photoxidized dye from− ions in electrolyte, and the produced I3− are reduced onhe counter electrode. Fig. 5 shows a much higher currentensity of the I3− reduction peak for the poly(acrylic acid--gelatin)/polypyrrole, and there is no reduction peak for theoly(acrylic acid-g-gelatin), indicating a faster reaction rate foroly(acrylic acid-g-gelatin)/polypyrrole than for poly(acrylic acid--gelatin) ones. Thus, the poly(acrylic acid-g-gelatin)/polypyrroleas a higher electrocatalytic activity in I−/I3− redox reaction.

.6. Photovoltaic characterization of flexible DSSC

The photocurrent versus photovoltage curves of flexible DSSCsased on (a) the poly(acrylic acid-g-gelatin)/polypyrrole gel elec-rolyte and (b) poly(acrylic acid-g-gelatin) gel electrolyte are shownn Fig. 6. Under a simulated solar light irradiation with intensity of00 mW cm−2 (AM 1.5), the photoelectric parameters of DSSCs suchs short circuit current density (Jsc), open circuit voltage (Voc), fillactor (FF) and the light-to-electric energy conversion efficiency (�)ere measured and summarized in Table 1.

The light-to-electric energy conversion efficiencies forhe flexible quasi-solid-state DSSCs with poly(acrylic acid-g-elatin)/polypyrrole gel electrolyte and poly(acrylic acid-g-gelatin)el electrolyte are 0.85% and 1.28%, respectively. The improvementf photoelectric performances of flexible DSSC with poly(acrylic

cid-g-gelatin)/polypyrrole gel electrolyte mainly comes fromwo aspects: (i) the introduction of polypyrrole increases theonductivity of gel electrolyte, which speeds the mobility of chargearriers in the electrolyte and increases the photocurrent; (ii)olypyrrole catalyzes the I−/I3− redox reaction on the counter

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a 55 (2010) 2777–2781 2781

electrode, decreases the overpotential and enhances the opencircuit voltage. Therefore, the flexible DSSC with poly(acrylicacid-g-gelatin)/polypyrrole gel electrolyte has predominantphotoelectric performance.

4. Conclusion

Porous poly(acrylic acid-g-gelatin)/polypyrrole was synthe-sized by aqueous solution graft copolymerization and in situpolymerization of polypyrrole. Using the polymer as host, a poly-mer gel electrolytes with high conductivity of 14.1 mS cm−1 wasprepared. Based on the electrolyte, a flexible quasi-solid-state dye-sensitized solar cell was assembled by using a low-temperaturefilming technique. Owing to high conductivity and the catalyticfunction of polypyrrole for I−/I3− system for poly(acrylic acid-g-gelatin)/polypyrrole gel electrolyte, the flexible quasi-solid-statedye-sensitized solar cell showed a high light-to-electric energyconversion efficiency of 1.28%, under a simulated solar light irradi-ation with intensity of 100 mW cm−2 (AM 1.5).

Acknowledgements

The authors acknowledge jointly financial support by theNational High Technology Research and Development Program ofChina (No. 2009AA03Z217), the National Natural Science Foun-dation of China (Nos. 90922028 and 50842027), and SpecializedResearch Fund for the Doctoral Program of Chinese Higher Educa-tion (No. 20060385001).

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