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Ionic liquid (1-methyl 3-propyl imidazolium iodide) with polymer electrolyte for DSSC application.

INTRODUCTION

Dye-sensitized solar cell (DSSC), as an alternate source in place of photo electrochemical solar cells, came in consideration after the novel work of Gratzel and co-workers (1), (2). The overall cell performance depends upon many factors, like nature of electrodes, dye, sensitizer, electrolytes, and so forth. Out of these, the electrolytes play an important role because the electrolytes with redox couple ([I.sup.-]/[I.sub.3.sup.-]) can influence the rereduction of the oxidized state of the dye as well as several other process in the DSSC (30. DSSC based on different electrolytes are available in literature (4), (5). Optimum device efficiencies to date have been obtained by using liquid electrolytes (2). Apart from good electrical conductivity and high efficiency in liquid electrolytes, the sealing of volatile liquid electrolytes in commercial stage still remains a critical issue which reduces the long term durability and performance of the cell. The other approach to replace this liquid electrolyte is the use of polymer gel electrolytes (6). However, it is well known that the polymer gel still retains a significant volume of liquid encapsulated in the gel pores, which gives large increase in vapor pressure as the temperature is raised and therefore the sealing of such cells creates problem. The poly(ethylene oxide) (PEO)-based electrolyte seems good alternative to remove these problems (7), (8). Apart from many advantages, the low ionic conductivity of PEO-polymer electrolyte limits its application in devices. Recently, ionic liquids (ILs) (room temperature molten salts) have received keen interest in electrochemical devices due to their unique properties such as high ionic conductivity, negligible vapor pressure, and nonvolatility (9), (10). In DSSC application. ILs have been found to play the role of both the iodide source as well as nonvolatile plasticizers (11). Despite their high conductivity. ILs are not preferred in device applications due to their liquid nature. One of the possible approaches is to develop solid polymer electrolyte films by incorporating IL in a suitable polymer matrix.

In this article, solid polymer electrolyte membranes have been prepared by incorporating 1-methyl 3-propyl imidazolium iodide (PMII) in PEO and examined for DSSC. The effect of humidity on ionic conductivity ([sigma]) and overall efficiency ([eta]) have also been discussed.

EXPERIMENTAL

Ionic Liquid Doped Solid Polymer Electrolyte Membranes

Polymer electrolyte membranes containing IL were prepared by the solution casting. In this method, the polymer (PEO, [M.sub.w] = 1 x [10.sup.6]) and desired amount of ionic liquid (PMII) as iodide source were dissolved in acetonitrile with continuous stirring to obtain a homogeneous viscous solution. The viscous polymer electrolyte solution was then pouted in polypropylene petri dishes and the solvent was allowed to evaporate slowly followed by vacuum drying. To make DSSC electrolyte redox couple ([I.sup-] / [I.sub.3.sup.-]), we put iodine in PEO:PMII polymer salt complex solution having maximum ionic conductivity. The iodide salt (PMII) to iodine ratio was fixed at 10/1 (w/w) (7). Ionic conductivities of these polymer electrolyte films were measured by impedance method using a die which contain pressure contacted stainless steel electrodes. The measurements were performed in a computer controlled Solartron bridge (SI 1287) interfaced with frequency response analyzer (1252A).

Most of the chemicals used in present study were purchased from Aldrich chemical, whereas ionic liquid PMII was purchased from Solaronix (Switzerland). The chemical structure of polymer (PEO) as well as IL (PMII) used in this work is shown in Fig. 1.

[FIGURE 1 OMITTED]

Dye Sensitized Solar Cell Assembly

DSSC with an active area of 0.175 [cm.sup.2] was fabricated on FTO glass covered with 10 [mu]m thick mesoporous Ti[O.sub.2] film sensitized with N-719 dye solution. The details of fabrication and characterization of mesoporous Ti[O.sub.2] electrode (photoelectrode) and DSSC assembly are published elsewhere (12). The surface morphology of porous Ti[O.sub.2] film and PEO:PMII polymer electrolyte films were examined with an atomic force microscope (AFM, Nanoscope III, operated in tapping mode). The Pt coated counter electrode was prepared by spin-coating [H.sub.2]Pt[Cl.sub.6] solution (0.05 mol [dm.sup.-3] in isopropyl alcohol) onto the FTO conductive glass and then sintering at 400[degrees]C for 30 min. Finally, the polymer/IL./[I.sub.2] electrolyte (maximum [sigma]) was cast on the photoelectrode followed by two step casting method (7) and sandwiched by platinized counter electrode. The DSSC assembly was dried under vacuum for two days and the cell performance was evaluated without any special sealing.

Photocurrent-voltage (J-V) characteristics of the DSSC were measured with Keithley 2400 source meter under one sun light intensity (100 mW [cm.sup.-1]) simulated by a solar simulator (Oriel. 91193 model equipped with 1000 W Xenon lamp) with an AMI.5 G filter.

RESULTS AND DISCUSSION

Surface Morphology Using AFM

To investigate the morphological feature of both Ti[O.sub.2] mesoporous film and IL-incorporated PEO polymer electrolyte films, we have taken AFM images in the tapping mode. Figure 2a, shows the three dimensional AFM plot of nanocrystalline mesoporous Ti[O.sub.2] film. The obtained morphologies confirmed high surface roughness factor (Rms 34.5 nm) which is known to be beneficial criterion for good efficient DSSC. In fact, such a high rough surface not only permits the adsorptions of greater number of dye molecules, but also helps in better "wetting" by the polymer film which finally results in perfect penetration of the ([I.sup.-]/[I.sub.3.sup.-] redox couple into the film pores (13). The Fig. 2b, shows the three dimensional AFM image of PEO:PMII polymer electrolyte films. It is observed that the surface morphology of PEO:PMII film showed high complexity and relatively low roughness (Rms 11.3 nm).

[FIGURE 2 OMITTED]

Effect of IL Doping on the Conductivity

To obtain the maximum value of ionic conductivity ([sigma]) by varying the amount of PMII ionic liquid, we have evaluated the values of [sigma] using complex impedance spectroscopy. The calculated values of room temperature (25[degrees]C) conductivity ([sigma]) at 68% relative humidity are listed in Table 1. The ionic conductivity of the PEO:PMII polymer electrolytes is always greater than pure PEO-polymer. where conductivity value lies between [10.sup.-8] to [10.sup.-9] S [cm.sup.-1] (14). The conductivity of polymer electrolytes increased with increasing IL concentration and attained maxima at 200 wt% (EO/[PMI.sup.+] = 2.8) where conductivity value reaches up to 3.29 X [10.sup.-4] S [cm.sup.-1]. This increase in ionic conductivity with increasing IL concentration is attributed to the enhanced concentration of free ions from IL. On further enhancement in IL concentration, the conductivity value goes down due to the aggregation of ions in polymer:salt complex (15). From Table 1, it is clear that the ionic conductivity of our polymer film having 200 wt% PMII ionic liquid is highest among all the polymer electrolyte films, and hence this film was selected for present DSSC application.
TABLE 1. Room temperature (25[degrees]C) ionic conductivity of PEO:
PMII polymer electrolyte systems at 68% relative humidity.

Composition (salt wt%) EO/[PMI.sup.+] Conductivity ([sigma]) (S
 [cm.sup.-1])

PEO:10 wt% PMII 57.0:1 1.20 x [10.sup.-6]
PEO:20 wt% PMII 28.5:1 4.47 x [10.sup.-6]
PEO:40 wt% PMII 14.3:1 5.72 x [10.sup.-6]
PEO:60 wt% PMII 9.5:1 9.33 x [10.sup.-6]
PEO:80 wt% PMII 7.1:1 3.52 x [10.sup.-5]
PEO:100 wt% PMII 5.7:1 7.46 x [10.sup.-5]
PEO:120 wt% PMII 4.8:1 9.05 x [10.sup.-5]
PEO:140 wt% PMII 4.1:1 1.65 x [10.sup.-4]
PEO:160 wt% PMII 3.6:1 2.19 x [10.sup.-4]
PEO:180 wt% PMII 3.2:1 2.69 x [10.sup.-4]
PEO:200 wt% PMII 2.8:1 3.29 x [10.sup.-4]
PEO:220 wt% PMII 2.6:1 2.76 x [10.sup.-4]
PEO:240 wt% PMII 2.4:1 1.84 x [10.sup.-4]
PEO:260 wt% PMII 2.2:1 1.63 x [10.sup.-4]
PEO:280 wt% PMII 2.0:1 9.23 x [10.sup.-5]
PEO:300 wt% PMII 1.9:1 6.62 x [10.sup.-5]


Effect of Humidity (Fractal Formation)

The relative humidity (RH) also plays a dominant role in conductivity and efficiency modification. To examine it, we have taken the optical photographs of the DSSC at different humidity. Figure 3 showed the photographs of the fabricated DSSC at 54% and 74% RH. It is clear that our DSSC is transparent (Fig. 3a),which is suggested to be beneficial for light harvesting (16). Further the doping of iodine in humid atmosphere (RH > 68% ) results to fractal like structure (Fig. 3b), whereas at low humidity it disappears (Fig. 3a). No fractal growth was observed till 68% RH. The doping of iodine into iodide matrix formed polyiodide (i.e. [I.sub.3.sup.-]) which may assist fractal formation (17). Similar patterns of fractal formation were already reported in literature for PEO:[NH.sub.4]I or PEO:NaI systems at higher humidity (17), (18). It has been reported that the higher humidity offers the right reducing environment for anoin-clustering to occur which further increases the crystallinity and reduces conductivity (17).

[FIGURE 3 OMITTED]

Effect of Humidity on Ionic Conductivity and Photovoltaic Performance

The DSSC performance with polymer electrolyte/IL/[I.sub.2] film having highest ionic conductivity at two different humidities are shown in Fig. 4. The calculated values of ionic conductivity ([sigma]), short circuit current density ([J.sub.se]), open circuit voltage ([V.sub.oc]), and fill factor (FF) are listed in Table 2. It is well known that in PEO: salt complex, the humidity play a dominant role in conductivity enhancement and vis-a-vis [J.sub.se] (3), (18). The obtained results (Table 2) showed good agreement. It is clear that DSSC at high humidity (68%) showed high conductivity and [J.sub.se] values which gives overall efficiency [eta] = 0.81% at 100 mW [cm.sub.-2]. At low humidity (54%), the efficiency was 0.52. This low efficiency value was expected as the conductivity of the film is quite low (~1 order of magnitude) in comparison with that at higher humidity. Further improvement in efficiency by retarding fractal formation is still continuing in our laboratory.

[FIGURE 4 OMITTED]
TABLE 2. Ionic conductivity ([sigma]) and photovoltaic parameters of
PEO: PMII/[I.sub.2] solid polymer electrolyte films at different
humidity.

Composition RH (%) [sigma] [J.sub.sc]
 (S [cm.sup.-1]) (mA [cm.sup.-2])

PEO:PMII/[I.sub.2] 68 3.56 x [10.sup.-4] 5.34
PEO:PMII/[I.sub.2] 54 6.56 x [10.sup.-5] 4.32

Composition [V.sub.oc] (V) FF (%) [eta] (%)

PEO:PMII/[I.sub.2] 0.40 38 0.81
PEO:PMII/[I.sub.2] 0.39 31 0.52


CONCLUSIONS

We have successfully developed PEO-based solid polymer electrolyte containing PMII ionic liquid. The doping of IL (PMII) assisted in conductivity enhancement due to the free charge carriers and the maximum ionic conductivity was 3.29 X [10.sup.-4] S [cm.sup.-1] at 200 wt% IL concentration. The surface morphologies using AFM confirmed the high roughness of films. A DSSC based on PEO:PMII/[I.sub.2] as electrolyte with highest ionic conductivity (3.56 X [10.sup.-4] S [cm.sup.-1]) showed an energy conversion efficiency of 0.81% at 100 mW [cm.sub.-2]. The relative humidity also played a dominant role in enhancing [sigma] and hence overall cell efficiency. Further doping of iodine in too much humid atmosphere (70% RH) formed "fractals" of [I.sub.3.sup.-] species.

REFERENCES

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(12.) P.K. Singh, K.W. Kim, K.I. Kim, N.G. Park, and H.W. Rhee, J Nanosci. Nanotechnoi, 8, 1 (2008).

(13.) G. Katsaros, T. Stergiopoulos, I.M. Arabatzis, K.G Papado-kostaki, and P. Falaras, J. Photochem. Photobiol A: Chem., 149, 191 (2002).

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Pramod K. Singh, Kang-Wook Kim, Hee-Woo Rhee

Department of Chemical and Biomolecular Engineering, Sogang University, Seoul, S. Korea

Correspondence to: Hee-Woo Rhee; e-mail: hwrhee@sogang.ac.kr Contract grant sponsor: Brain Korea 21 Project (Ministry of Education).

DOI 10.1002/pen.212l2

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Singh, Pramod K.; Kim, Kang-Wook; Rhee, Hee-Woo
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9SOUT
Date:May 1, 2009
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