A Comparative Study of Gel Polymer Electrolytes based on PVDF-HFP and Liquid Electrolytes Containing Imidazolinium Ionic Liquids with Different Carbon Chain Lengths in DSSCs

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Solar Energy Materials & Solar Cells 91 (2007) 1467–1471

A comparative study of gel polymer electrolytes based on PVDF-HFP

and liquid electrolytes, containing imidazolinium ionic liquids of

different carbon chain lengths in DSSCs

$

Vembu Suryanarayanan

a

, Kun-Mu Lee

b

, Wen-Hsien Ho

c

, Hung-Chang Chen

c

,

Kuo-Chuan Ho

a,b,

a_{Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan} b_{Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan} c_{Department of Product Development, Taiwan Textile Research Institute, Tucheng 23674, Taiwan}

Available online 9 May 2007

Abstract

The photoelectrochemical characteristics of titanium dioxide (TiO2)-based dye-sensitized solar cells (DSSCs) containing gel polymer

electrolyte (GPE) and organic liquid electrolyte (OLE) were studied in detail. GPE was prepared by adding poly(vinyidene ﬂuoride-co-hexaﬂuoro propylene) (PVDF-HFP) to imidazolinium ionic liquids (IILs) of the type, 1-methyl-3-alkyl imidazolinium iodides (alkyl is

CnH2n+1, where n ¼ 3–10) in methoxy propionitrile (MPN) and the OLE contained the above molten salt in MPN. The IILs were

synthesized in the laboratory and characterized by1H nuclear magnetic resonance spectroscopy (NMR). The conductivities (s) of both

GPE and OLE decrease with increase in chain length (n) of the alkyl group of IILs; however, the effect is more drastic in the former case. The performance of the DSSCs containing OLE increases with the increase in alkyl chain length of IIL from C3 to C7, whereas, there is a linear decrease in the efﬁciency of the DSSCs incorporated with GPE containing IIL of alkyl chain length from C3 to C10. The change in

short circuit current density (JSC) determines the cell efﬁciency as the VOCof the DSSCs remains almost the same with increase of alkyl

chain length of IILs for both the electrolytes. The change in JSCvalues and the consistency of the VOCof the DSSCs for both the

electrolytes may be explained on the basis of increase in viscosity of IILs from C3 to C10 and the dominating role of the 4-tertiary butyl pyridine (TBP), respectively, on the phenomenon of charge recombination.

Keywords: Dye-sensitized solar cells; Gel polymer electrolyte; Imidazolinium iodides; Ionic liquid; PVDF-HFP

1. Introduction

Because of the possible low production cost and the expected high-energy conversion efﬁciency, dye-sensitized solar cells (DSSCs) with nanocrystalline TiO2, organic dyes and an electrolyte become a large-scale prevalent material for light harvesting [1]. These solar cells usually employ

liquid electrolytes containing I

/I3 redox couple as supporting electrolyte in order to reduce the dye cation, generated by the injection of the photo-excited electron. However, as far as long-term stability of the solar cell is considered, use of the liquid electrolytes must be avoided as the organic solvents evaporate slowly leading to decrease in cell performance. Hence, in order to improve the long-term stability of the solar cell, gel polymer electrolytes (GPEs) such as poly(acrolynitrile) [2–4], poly(ethylene glycol) [5], poly(oligoethylene glycol methacrylate) [6], poly(siloxane-co-ethylene oxide) [7], poly(butylacrylate) [8] and poly(-vinyidene ﬂuoride-co-hexaﬂuoro propylene) (PVDF-HFP)

[9,10] with different plasticizers have been employed. Among them, PVDF-HFP shows relatively high ionic

www.elsevier.com/locate/solmat

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This paper S4-P36 presented at the International Materials Research Congress (IMRC), Symposium 4-Photovoltaics, Solar Energy Materials and Thin Films, August 20-24, 2006, Cancun, Mexico.

Corresponding author. Department of Chemical Engineering, Na-tional Taiwan University, Taipei 10617, Taiwan. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040.

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conductivities at room temperature and they are stable in the presence of TiO2and Pt nanoparticles.

Imidazolinium iodides have been used as liquid[11–13]

and quasi-solid-state electrolytes[11,12,14–16]in the solar cells. They contribute to enhancement of conductivity of I/I3

electrolyte and their multilayer adsorption on the TiO2surface leads to the enhancement in electron diffusion coefﬁcient[17]. In this study, the photovoltaic performance of the DSSCs fabricated with GPE and organic liquid electrolyte (OLE), both incorporated with imidazolinium ionic liquids (IILs) of the type 1-methyl-3-alkyl imidazo-linium iodides (alkyl is CnH2n+1, where n ¼ 3–10) in methoxy propionitrile (MPN), have been compared under identical experimental conditions. This study brings a direct comparison on the performance of DSSCs contain-ing different chain length of IILs in both liquid and gel systems and this is different from other Refs.[9,10]. 2. Experimental section

2.1. Materials

Anhydrous LiI, I2, poly(ethylene glycol) (PEG), 4-tertiary butyl pyridine (TBP) and MPN were obtained from Merck. Tetrabutylammonium iodide (TBAI, +98%) and titanium (IV) isopropoxide (+98%) were purchased from Acros and used as received. The N3 dye was the commercial product obtained from Solaronix S.A., Au-bonne, Switzerland.

1-Alkyl-3-methylimidazolium iodide (Scheme 1) [18]

were prepared by the quaternization reaction of 1-methylimidazole (Fluka) with an equimolar amount of the corresponding alkyl iodide (RI) (Fluka) at 120 1C for 12 h in cyclohexane, where R ¼ CnH2n+1 (n ¼ 3–10). The products were washed with diethyl ether to remove remains of the starting materials and dried under vacuum at 75 1C for 4 h, and the purity was conﬁrmed by 400 M Hz1H FT nuclear magnetic resonance spectroscopy (NMR) instru-ment (Bruker Avance) in CDCl3.

2.2. Preparation of TiO2thin films and the cell assembly The preparation of TiO2 precursor and the electrode fabrication were carried out based on the previous literature [19] except after autoclave treatment, where the solution was concentrated to 13 wt% and two types of pastes, namely, P1 and P2 were prepared by the addition of 30 wt% (with respect to TiO2) of poly (ethylene glycol) (PEG) having molecular weights of 20,000 and 200,000,

respectively, to the above solution in order to control the pore diameters and to prevent the film from cracking during drying [20]. P2 was coated over P1 on a fluorine-doped tin-oxide (FTO) glass (Rsh¼25 O/&, Sinonar Corp., Hsinchu, Taiwan), using glass rod. This bilayer TiO2 film is an efficient light-harvesting material, as confirmed from the previous study carried out in this laboratory[20].

An active area of 0.25 cm2 was selected from sintered electrode and the electrodes were immersed in 3 104M solution of cis-di(thiocyanato)bis(2,20_{-bipyridyl-4,4}0

-dicar-boxylate)ruthenium (II) (N3) containing acetonitrile and tertiary butanol (in the volume ratio of 1:1) for 24 h. The OLE and GPE contain an optimized electrolyte composi-tion of 0.6 M of IILs/0.2 M LiI/0.08 M of I2/0.5 M of TBP and 5 wt% PVDF-HFP/0.8 M of IILs/0.12 M of I2/0.2 M of TBP in MPN, respectively. The speciﬁed composition of the electrolyte was sprayed on both the dye-immobilized photoanode and Pt-sputtered (thickness of Pt is 100 nm) FTO glass and they are physically sandwiched together using cell holders.

2.3. Instrumentation

The photoelectrochemical characterizations of the DSSCs were carried out by using an AM 1.5 simulated light radiation. The light source was a 450 W Xe lamp (Oriel, #6266) equipped with a water-based IR ﬁlter and AM 1.5 ﬁlter (Oriel, #81075). Conductivity measurement of solution was performed by electrochemical impedance spectroscopy (EIS) with two Pt electrodes conductance cell where the area of each Pt electrode is 1 cm2 and the distance between the two electrodes is 1 cm. The cell constant is 0.55 cm1, as calibrated from the standard aqueous KCl solution. Photoelectrochemical characteris-tics of the DSSCs were recorded with a potentiostat/ galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands) under constant light illumination of 100 mW/cm2.

The photovoltage transients of assembled devices were recorded with a digital oscilloscope (LeCroy, model LT322). Pulsed laser excitation was applied by a fre-quency-doubled Q-switched Nd:YAG laser (Spectra-Phy-sics laser, model Quanta-Ray GCR-3-10) with 2 Hz repetition rate at 532 nm and 7 ns pulse width at half height. The beam size was larger than 0.25 cm2to cover the area of the device with incident energy of 1 mJ/cm2. The average lifetime of electron can be estimated approximately by ﬁtting a decay of the open-circuit voltage transient with

N N H3C + RI 120°C in cyclohexane N N I -H3C R Scheme 1.

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exp (t/te), where t is the time and te is an average time constant before recombination.

3. Results and discussion

Fig. 1 shows the photovoltaic characteristics of the DSSCs based on GPE and OLE containing IILs of different carbon chain lengths. From Fig. 1a, it is under-standable that open-circuit voltages of the DSSCs (VOC) do not vary very much by incorporating IILs of different carbon chain lengths (C3–C10). (VOCchanges from 0.69 to 0.71 V for OLEs and 0.65 to 0.68 V for GPEs.) The short-circuit densities (JSC) of the DSSCs varied significantly for IILs containing carbon chain length from C3 to C10. For example, the JSC value of the DSSCs based on OLE increases with increase in alkyl group of IIL from C3 to C8 and then decreases, whereas, a linear decrease in JSC is observed for GPE-containing IIL with alkyl group from C3 to C10 (Fig. 1b). On the other hand, there was only little variation in fill factors (FF) of the DSSCs when the chain length of alkyl groups of IIL was varied for both OLE and GPE (Fig. 1c). In the similar manner, the efficiencies of the DSSCs based on GPE decrease from C3 to C10, whereas, a linear increase and decrease on the performances of the

DSSCs were noted for the IILs from C3–C7 to C8–C10, respectively, with OLE (Fig. 1d).

It is well known that the VOCvalues of the DSSCs are related to the difference between the Fermi level for electrons in the TiO2electrode and the redox potential of I/I3

. Actually, the increase of alkyl chain length would increase electron density in the TiO2by suppressing charge recombination, causing the increase of Fermi level.Fig. 2a

shows the photovoltage transients induced by pulse laser (532 nm) radiation for TiO2 electrodes in different pure IILs and Fig. 2b brings the correlation between electron lifetime (te) and viscosity of the pure IILs used in this study. Though there is an increase of electron recombina-tion lifetime from C3 to C10, presence of 4-tertirary butyl pyridine (TBP) nulliﬁes this effect. Because of this reason, there is not much difference in the VOC of the DSSCs containing both OLE and GPE[18].

This characteristic variation in the JSC (Fig. 1b) and conversion efﬁciency (Fig. 1d) of the DSSCs containing IIL may be correlated with different conductivities and viscosities of individual IIL. The relationship between the conductivity of both GPE and OLE with different IILs vs. the viscosity of pure IIL is shown inFig. 3. It is well known that the IILs are highly viscous liquids and their viscosities increase with increase in chain length of alkyl group. This is

6 8 10 12 14 16 18 20 OLE GPE C9 C8 C7 C6 C5 C4 C3 JSC (mA cm -2) OLE GPE C10 C9 C8 C7 C6 C5 C4 C3 VOC (V) _0.7 0.8 0.6

Imidazolinium ionic liquids Imidazolinium ionic liquids

C10 4 5 6 7 8 9 10 OLE GPE C9 C8 C7 C6 C5 C4 C3 η (%) OLE GPE C9 C8 C7 C6 C5 C4 C3 Fill Factor 0.70 0.65 0.60 0.55 0.50

Imidazolinium ionic liquids C10

a

b

c

d

Fig. 1. Photovoltaic characteristics such as (a) VOC(b) JSC(c) FF, and (d) Z of the DSSCs containing OLE and GPE incorporated with IILs of different carbon chain lengths from C3 to C10.

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3 6 7 10 11 0 2 4 6 8 10 12 14 16 Electron lifetime (ms) 0 400 800 1200 1600 2000 80 60 40 20 0 -10 -5 0 5 10 15 20 25 30 Time (ms) Photovoltage (mV) C3 C4 C5 C6 C7 C8 C9 C10

Viscosity of pure ionic liquid (cp)

9 8 Chain length (C number)

4 5

Fig. 2. (a) Photovoltage transients induced by pulse laser (532 nm) radiation for the TiO2electrodes in the GPE with IILs containing different alkyl chain lengths (C3–C10). (b) Relationship between electron lifetimes in the TiO2electrode and the viscosity of IILs under the same condition.

0 4 8 12 16 20 C9 C8 C7 C6 C5 C4 C3 Conductivity (mS cm -1) 800 1000 1200 1400 1600 1800 2000 2200 GPE OLE

Imidazolinium ionic liquids

C10

Viscosity of pure ionic liquids (cp)

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because of high van der Waals forces of attraction functioning between the molecules and this lowers diffu-sion of ions. Since the diffudiffu-sion of ions in a liquid depends on its viscosity, the conductivity of the liquid decreases with increase in the viscosity. FromFig. 3, it is noted that IIL containing propyl group (C3) shows maximum conductivity in both the GPE and OLE and the con-ductivity decreases from C3 to C10. Naturally, higher conductivity values are obtained for OLE than the GPE irrespective of the different IILs.

From the previous literature, it has been observed that the diffusion coefﬁcient of electrons injected in a TiO2 electrode (De) increases as a result of the multilayer adsorption of imidazolinium cations[21]. When the chain length of alkyl group in IIL increases, the carbon–carbon will have more chance to encounter, the van der Waals forces will increase in the presence of the longer chain length. The aggregation of these imidazolinium cations will be more and this leads to high Devalue. As a consequence of this, the JSCvalue of the DSSC also increases. Though the conductivity decreases with increases in the chain length of IIL, both the effect of te and Deovercome this. Consequently, the conversion efﬁciency of the DSSCs-containing OLE increases from C3 to C7. However, for GPE containing IILs from C3 to C10, a linear decrease in the performance of the DSSCs is observed. This may be correlated with the weak adsorption of imidazolinium cations on the TiO2electrode as a result by trapping of ILL in polymer cages formed by the host matrix in GPE. 4. Conclusions

A comparative study on the photoelectrochemical characteristics of the DSSCs-containing PVDF-HFP as GPE and OLE both incorporated with different IILs containing different carbon chain lengths from C3 to C10 in MPN was carried out in this work. The conductivities (s) of both GPE and OLE decrease with increase in chain length of the alkyl group of IILs. The VOCof the DSSCs remains almost stable with increase of alkyl chain length of ILLs for both the electrolytes and this is related with the dominating role of the TBP on the charge recombination behavior. The performance of the DSSCs based on OLE increases with the addition of IILs from C3 to C7, whereas, there is a linear decrease in the efﬁciency of the DSSCs incorporated with GPE electrolytes containing IILs of different carbon chain lengths from C3 to C10. This may be explained on the basis of increase in both electron lifetime and the diffusion coefﬁcient of electron, and is in consistent with increase in viscosity of IILs.

Acknowledgments

This work was ﬁnancially supported by the Academia Sinica, Taipei, Taiwan, the Republic of China, under Grant AS-94-TP-A02. This work was partially supported by the Taiwan Textile Research Institute (TTRI), Tucheng City, Taipei County, Taiwan. We also want to thank Professor King-Chuen Lin and his research group mem-bers, of Department of Chemistry, National Taiwan University, for the help in making the pulsed laser apparatus available to us.

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