Gel polymer electrolytes for dye sensitised solar cells: a review

Gel polymer electrolytes for dye sensitised solar cells: a review O. A. Ileperuma* Dye sensitised solar cells (DSCs) based on mesoporous nanostructured TiO2 have attracted attention as an alternative to silicon photovoltaics. However, the use of a liquid electrolyte causes practical problems of leakages and its volatilisation, desorption and photodegradation of the dye, corrosion of the platinum secondary electrode and ineffective sealing of cells for long term applications in solar panels. Some of the attempts to solve this problem include the use of polymer electrolytes, gelling agents and both organic and inorganic hole conductors. A common feature of all such alternatives is the notable reduction in efficiency of the resultant solar cells primarily, due to the lower mobility of the iodide species through the solid or quasi-solid medium and imperfect wetting of pores with the electrolyte. The conversion efficiencies with the gel polymer electrolytes typically do not exceed 5%, but even at low efficiencies, these cells may become viable alternatives to the organic liquid containing Gratzel type cells due to improved stability and better sealing ability. The dyes are less liable to undergo photocorrosion and desorption, and the corrosion effects on the Pt electrode are lower. Some commonly used polymer electrolytes include poly(ethylene oxide), poly(propylene oxide), poly(acrylonitrile), poly(methyl methacrylate), poly(vinyl chloride) and poly(vinylidene fluoride). Plasticisers employed are ethylene carbonate, propylene carbonate, dimethyl carbonate, c-butyl lactone and diethylene carbonate. In addition, inert fillers, such as TiO2, SiO2, etc., are added to enhance ionic conductivity and stability at the interface with electrodes. These self-sealing gel polymer electrolytes have reported efficiencies of about 2–5%. Recently, the poly(acrylonitrile) based gel polymer electrolytes are reported to give an efficiency of 7?5% for the TiO2/Ru-N719 system under AM 1?5 illumination. Incorporation of TiO2 nanoparticles increases the efficiency of a DSC employing poly(vinylidene fluoride-cohexafluoropropylene) from 5?72 to 7?18% where the standard liquid electrolyte gave an efficiency of 7?01%. The nanoparticles here are assumed to reduce recombination at the interface of dyed electrode/electrolyte. This review will cover only the application of gel polymer electrolytes for dye sensitised TiO2 systems in quasi-solid state solar cells and will not cover the closely related areas of gelated electrolytes where the standard liquid electrolyte is gelated with gelling agents and conducting polymers used as hole conductors. Keywords: Dye sensitisation, Solar cells, Gel polymer electrolytes

This paper is part of a special issue on Solar Energy

Introduction Dye sensitised solar cells (DSCs) based on mesoporous TiO2 have attracted attention owing to their ease of production from relatively impure materials as low cost alternatives to silicon photovoltaics.1–3 This type of solar cell is composed of a nanoporous TiO2 electrode sensitised by a ruthenium bipyridine dye, a redox electrolyte and a platinum counter electrode. Photo-excited electrons on

University of Peradeniya, Peradeniya, Sri Lanka *Corresponding author, email [email protected]

ß 2013 W. S. Maney & Son Ltd. Received 30 April 2012; accepted 26 July 2012 DOI 10.1179/1753555712Y.0000000043

the dye are transferred to the conduction band of the TiO2, and the oxidised dye cation is regenerated by the redox couple, which in turn gets reduced at the platinum secondary electrode, completing the cycle and generating a photocurrent. The use of a liquid electrolyte causes practical problems of leakages and its volatilisation, desorption and photodegradation of the dye, corrosion of the platinum secondary electrode and ineffective sealing of cells for long term applications in solar panels. Some of the attempts to solve this problem include the use of polymer electrolytes,4–6 gelling agents7 and both organic8 and inorganic9 hole conductors. A common feature of all such alternatives is the notable reduction in efficiency of

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Gel polymer electrolytes for dye sensitised solar cells

the resultant solar cells primarily due to the lower mobility of the iodide species through the solid or quasi-solid medium and imperfect wetting of pores with the electrolyte. The conversion efficiencies with the gel polymer electrolytes typically do not exceed 5%, but even at low efficiencies, these cells may become viable alternatives to the organic liquid containing Gratzel type cells due to improved stability and better sealing ability. The dyes are less liable to undergo photocorrosion and desorption, and the corrosion effects on the Pt electrode are lower. This review will cover only on the application of gel polymer electrolytes for Ru dye sensitised TiO2 systems in quasisolid state solar cells and will not cover the closely related areas of gelated electrolytes where the standard liquid electrolyte is gelated with gelling agents and also those examples where conducting polymers are used as hole conductors.

General aspects of polymer electrolytes A polymer electrolyte may be defined as a membrane or a similar medium that possesses transport properties similar to liquid ionic media. There are obvious advantages with polymer electrolytes such as the absence of internal shorting, leakage of electrolytes and noncombustible reaction products at the electrolyte surface existing in the liquid electrolytes. There are two basic types of polymer electrolytes: the ‘dry solid’ type includes poly(ethylene oxide) (PEO) based lithium ion conductors that typically show conductivities of 1028 S cm21, while the second category called the ‘gel’ or ‘plasticised’ polymer electrolytes have semi-solid character with much higher ionic conductivities of the order 1025–1023 S cm21. Gel polymer electrolytes or plasticised polymer electrolytes offer the advantages of the cohesive properties of a solid along with the diffusive nature of liquids. They offer good contacting properties and pore filling in the porous TiO2 electrode and also show high conductivity in the film, owing to free flow of ions through trapped electrolytes. A gel is formed between the polymer backbone and the plasticiser, ensuring the non-volatility of the electrolyte. Tables 1 and 2 give the properties of some commonly employed polymer framework materials and plasticisers respectively.10

Composite electrolytes refer to polymer electrolytes with inert fillers, such as Al2O3, TiO2, ZrO2, SiO2 and Al2O3. These have the effect of enhancing ionic conductivity and improve the stability at the interface with electrodes. The purpose of this review is to evaluate several polymer hosts used as ion conducting media in dye sensitised solar cells (DSSCs). For a general account on conducting polymer electrolytes for lithium batteries, an excellent review is available.10

Some commonly used gel polymer electrolytes for Ru dye sensitised TiO2 solar cells Poly(ethylene oxide) Ionic conductivity of alkali metal salts in PEO and its use in lithium batteries prompted research activity in this area. Poly(ethylene oxide) alone is amorphous in character and offers very low ionic conductivity from about 1028–1026 S cm21 in the temperature range of 40–100uC. Batteries that function on lithium ion conductivity generally employ large relatively immobile anions. In the case of DSSCs, which function on the conductivity of anions (iodide/triiodide), exactly the opposite approach, namely, larger cations, are used to enhance anionic conductivity. Several approaches have been employed to enhance ionic conductivity of PEO based electrolytes as efficient conducting media for DSSC applications. While PEO alone gave only a very low efficiency of ,0?6% for a Ru– N3 dye sensitised TiO2,11 the use of plasticisers, ethylene carbonate (EC) and propylene carbonate (PC) gave efficiencies of 2?9 and 3?6% respectively for the PEO2000 and PEO1500 segments.12 One striking feature here, as well as with many other polymer electrolytes, is the higher VOC values obtained compared to that of the wet cell. The polymer chains can block recombination sites and retard reactions of photogenerated electrons with triiodide ions, thereby enhancing VOC values. However, the wet cell efficiency, which these researchers obtained, was quite low at 4?2%, and hence, there is scope for further improvement of this system. The ionic conductivity of PEO with plasticisers depends on the plasticiser content,

Table 1 Some commonly used polymer framework materials Polymer framework

Repeat unit

Glass transition temperature/uC

Melting point/uC

Poly(ethylene oxide) Poly(propylene oxide) Poly(acrylonitrile) Poly(methyl methacrylate) Poly(vinyl chloride) Poly(vinylidene fluoride) Poly(vinylidene fluoride-hexafluoropropylene)

–{CH2CH2O}n– –{CH(–CH3)CH2O}n– –{CH2–CH(–CN)}n– –{CH2C(–CH3)(–COOCH3)}n –{CH2CHCl}n– –{CH2CF2}n– –(CH2CF2)x {CF2CF(CF3)}y

264 260 125 2105 85 240 290

65 … 317 … … 171 135

Table 2 Physical properties of some organic solvents commonly used as plasticisers Plasticiser

MP/uC

BP/uC

Density/g cm23

Dielectric constant

Molecular weight

Dimethyl carbonate Diethyl carbonate c-Butyllactone Propylene carbonate Ethylene carbonate

2.4 243.0 243.3 248.8 36.4

90 126 204 242 248

1.06 0.975 1.128 1.205 1.321

3.12 2.82 39.0 66.14 89.78

90.08 118.13 86.09 102.09 88.06

MP: Melting point; BP: Boiling point

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and an abrupt increase of ionic conductivity by two orders of magnitude was observed when the EC content was increased from 60 to 70 wt-%.13 Another approach to enhance ionic conductivity of PEO systems is by incorporating co-monomers or nanoparticle fillers. For example, poly(epichlorohydrinco-ethylene oxide) with 9%NaI and 0?9%I2 gave an ionic conductivity of 1?761024 S cm21 and an efficiency of 2?6% at 10 mW cm22.14 The use of nanofillers prevents crystal formation and consequently enhances the mobility of redox couples. For example, incorporation of TiO2 into the PEO/LiI/I2 system prevents crystal formation and yields an efficiency of 4?2% at 65?6 mW cm22 in a DSSC. Another important aspect of these systems is the interfacial contact between the polymer electrolyte and the photo-electrode surface. In order for the electrolyte to penetrate deeper into the pores of the semiconductor, radius of gyration of polymer chains should be adequately balanced by the pore diameter,13 and this leads to the other approach called the ‘oligomer approach’ to prepare quasi-solid state DSSCs. Oligo-polyethylene glycol (PEG) along with KI/I2 cross-linked with bifunctional glutaraldehyde results in solar cells of efficiency 3?64% at AM1?5 irradiation, and this high efficiency is attributed to enhanced interfacial contact and the deeper penetration of the electrolyte since the smaller coil size of the oligomer matches the average pore sizes in the TiO2 structure. Hybrid polymer compositions involving polyacrylic acid–polyethylene glycol have been fabricated and give solar cell efficiencies of 3?1% under AM1?5 irradiation.15 While polyacrylic acid is a superabsorbent of water, it does not readily absorb organic liquids, and hence, it is modified with the amphiphilic PEG. This readily absorbs the liquid electrolyte and gives a highly stable gel polymer electrolyte and a solar cell efficiency of 3?2% at a light intensity of 100 mW cm22. Lowering of efficiency can be attributed to the resistance losses arising from the thicker polymer membrane sandwiched between the electrodes compared to a thin liquid film. The use of PEO and poly(propylene oxide) as framework materials to solidify room temperature ionic liquids in DSCs has been reported to give efficiencies of ,5% compared to a liquid electrolyte cell that only gave a 5?3% efficiency. Lower conductivity and high viscosity have been used to explain the trend, and the authors claim16 that ionic conductivity has no relationship to solar cell efficiency with this particular system. This is ascribed to the fact that ionic conductivity arises due to both the cation and the anion, and solar cell function depends only on the conductivity of the iodide anion. These results mean that anion diffusion, and hence conductivity, is governed by solvent diffusion rather than the amount of polyether added.

Poly(acrylonitrile) (PAN) polymers Poly(acrylonitrile) offers a homogeneous hybrid electrolyte film where the salts and the plasticisers are molecularly dispersed.10 Poly(acrylonitrile) host structure is inactive in ionic conduction, but provides a matrix for structural stability. A typical polymer electrolyte composition of PAN (12%), EC (40%), PC (40%) and LiClO4 has a conductivity as high as 261023 S cm21 at room temperature, and this type of polymer electrolyte has been employed in Li–Mn batteries.17 The first report of the use of a solid polymer


electrolyte in a DSC can be attributed to Searson18 who employed a PAN polymer with EC, PC and acetonitrile as plasticisers. With NaI/I2 redox couple, an ionic conductivity of 4?561023 S cm21 and an energy efficiency of 4?4% at 30 mW cm22 were obtained. Lower efficiency compared to the liquid electrolyte is attributed to low incident photoconversion efficiency values brought about by incomplete wetting of the TiO2 film by the polymer electrolyte. It is also not clear whether this procedure gives a free standing film, since gelation reportedly required 3 days for completing the setting process and the cell had to be sealed with epoxy. Ileperuma and Dissanayake first reported the use of an acetonitrile free gel polymer electrolyte of PAN, along with EC and PC as plasticisers for the N3 dye sensitised TiO2 solar cells with 2?99% efficiency.5,6,19 Here, these low molecular weight organic solvents decrease chain interactions in the polymer network and hence impart increased amorphous character to the resulting electrolyte. The ionic conductivities of such polymer electrolytes reach values in excess of 1023 S cm21 and approach the conductivities in the liquid state. These quasi-solids keep a large percentage by mass of liquid electrolyte. The electrolyte compositions were selected based on the results developed for lithium batteries, where the proportions of the plasticisers and the polymer have been optimised. In the above report, an efficiency of 2?99% was obtained for a PAN based electrolyte where the PAN/ PC/EC ratio was optimised for ionic conductivity, while the I2/I2 molar ratio was maintained at a fixed value of 0?70 : 0?05 (mol L21/mol L21). The iodide salt used here was tetra-n-propyl ammonium iodide, and this ratio was selected on the basis of the ratio of I2/I2 generally employed in the organic liquid electrolytes used for wet DSSCs. In polymer electrolytes, larger cations of a homologous series tend to interact stronger with the polymer matrix due to viscous forces, thereby enhancing anionic conductivity.20 Adsorbed cations have been shown to have an effect on the kinetics of the reduction of the oxidised dye by I2 back electron transfer in DSSCs.21 Adsorption of cations on the TiO2 particles controls the energy of the conduction band and affects the injection of electrons from photoexcited dye molecules. This is due to the screening effects of the different cations on the movement of the electron through the nanoparticle network. When a bulkier cation, namely, tetra-n-butylammonium, is employed and the I2/I2 is optimised, at a redox couple molar ratio of 1?5 : 0?05 (mol L21/mol L21), a record high efficiency of 7?23% (JSC515?96 mA cm22, VOC50?751 V, FF50?62) was achieved.22 When the ionic liquid 1,2dimethyl-3-propyl imidazolium iodide was used as the iodide source, a slightly higher efficiency of 7?49% was obtained. These high values were realised on high efficiency TiO2 film electrodes fabricated by a unique spray technique employing two different kinds of nanoparticles of TiO2 of sizes 6 and 25 nm and typically having solar cell efficiencies exceeding 10%. The solar cell characteristics obtained for these DSCs with the acetonitrile based liquid electrolyte were as follows: JSC520?56 mA cm22, VOC50?739 V, FF50?66 and g510?06%.22 From the above data, it is evident that the observed difference in solar cell performance is mainly due to a lowering of the short circuit current


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density and the fill factor. This is the highest efficiency obtained for a quasi-solid electrolyte employing a gel polymer electrolyte. The high efficiency can be attributed to the high ionic conductivity of 4?461023 S cm21 of the optimised polymer electrolyte. Polyblend electrolytes containing alkali metal iodides show Arrhenius type conductivity temperature behaviour, which can be attributed mechanistically to ionic transport involving intermolecular ion hopping. For ionic species such as tetra-n-butylammonium iodide where the cation is very large, such hopping is unlikely and the conductivity–temperature variation in general shows curvature. The temperature dependence of the conductivity of polymer electrolytes involving large cations and where the ion transport is dominated by the mobility of solvent molecules is best described by the Vogel–Tamman–Fulcher equation23 s~AT {1=2 exp ({B=T{T0 ) where s is the conductivity, A is a constant that is proportional to the number of carrier ions, B is a constant and T0 is the temperature at which configurational entropy of the polymer becomes zero and is practically taken as the glass transition temperature Tg. The general applicability of this equation implies that ionic conductivity in the polymer is coupled to the flow behaviour of the plasticisers in the polymer matrix. In the case of lithium ion batteries that work on Li ion conductivity, this is achieved by having large anions. Similarly, where high iodide ion conductivity is desired, larger cations are preferable in order to obtain higher efficiencies in solar cells. In general, high permittivity solvents such as PC and EC or their mixtures bring about high conductivities of the order of 1023 S cm21 even at temperatures as low as 210uC. The maximum ionic conductivity is observed for an electrolyte where the molar ratio of I–/I251?50 : 0?05. Lowering of ionic conductivity at higher concentrations of iodide may be due to increased viscosity of the resultant electrolytes and also due to a reduction of the segmental motion of the polymer chains. There is no evidence of any separation of salts at concentrations as high as 1?5 mol L21. There were no liquids exuded from fabricated electrolytes after the photocurrent measurements were made, indicating that the electrolyte is well encapsulated in the polymer matrix. As such, these polymer membranes will act as self-sealants and will not have leakage and sealing problems normally encountered with organic liquid electrolytes. The VOC value, which is the difference between the Fermi level of TiO2 and the potential of the redox couple, depends mainly on the molar ratio of the I2/I2 couple. There are many reports24 that the VOC values increase when the liquid electrolyte is replaced by a quasi-solid electrolyte. A likely explanation is the reduction of the back electron transfer from the TiO2 film to the triiodide ion owing to the presence of a polymer film firmly attached to it surface and the slower diffusion of triiodide ions through the gel electrolyte. However, we observe only a marginal increase in the VOC values with the PAN system, since at the higher iodide concentrations used, the dark reaction due to reduction of triiodide ion by conduction band electrons accumulated on TiO2 increases, which results in a lowering of the VOC values. These effects combined

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yield VOC values comparable to those obtained for the liquid electrolyte cell. It was found that optimal solar cell performance is obtained when the I2/I2 ratio was 30 : 1 at concentrations of 1?50 and 0?05 mol L21 respectively. This is quite surprising since the concentrations of these two components generally used by other workers in the area of gel polymer electrolytes are 0?50 and 0?05 mol L21 at a ratio of 10 : 1. However, in at least one related study of a wet cell, the highest efficiencies were observed at a fairly high iodide ion concentration of 1?5 mol L21 when 1,2-dimethyl-3-n-propylimidazolium iodide was used and attributed to the strong adsorption of the 1,2-dimethyl-3-n-propylimidazolium iodide cation on the TiO2 surface.25 The quasi-solid electrolytes appear to require a higher concentration of the redox species in the electrolyte composition compared to the liquid electrolytes. Kubo et al.26 found relatively higher efficiencies for organic liquid free solidified pure ionic liquids using organic gelators, where the high efficiencies are attributed to a Grotthus type of mechanism for ionic conduction taking place through the polyiodide ions. Packing of polyiodide ions and the exchange of iodide through polyiodide chains is an attractive explanation to interpret these results where optimum efficiency is obtained at higher iodide concentrations. The decrease of VOC and the increase of JSC with temperature for DSSCs are now well established with optimum cell performances observed at y60uC.27,28 Increase in the cell performance after a few minutes of illumination where JSC increases substantially by y30% was also observed,22 while the other parameters showed a slight decrease. This can be attributed to a heating effect that increases conductivity owing to a lowering of viscosity and the slow pore filling by the viscous electrolytes. Similar effects have been noted29 for large area solar (625 cm2) cells with poly(methyl methacrylate) gel polymer electrolytes, where an initial decrease in the cell performance was observed followed by an unexpected recovery when left for several hours in the dark. The optimum efficiency of these types of solar cells is at a temperature of y60uC, which represents a practical temperature reached on roof top solar panels. Thus, the gel polymer electrolytes have a critical advantage over cells employing organic liquid electrolyte for outdoor applications. This example is also unique, since it is the first report of large area solar cells employing a polymer gel electrolyte, even though its efficiency is only y1%. Gel polymer electrolytes based on poly(acrylonitrileco-styrene)/NaIzI2 along with plasticisers EC and PC have been fabricated, giving an ionic conductivity of 2?461024 S cm21 and a solar cell efficiency of 2?75%. Here, it was shown that ionic conductivity decreases after the NaI concentration exceeds 0?5 mol L21, and this is explained on the basis of shrinkage of the polymer chains and a phase disengagement between polymer matrix and the liquid electrolyte.15 Development of a polymer electrolyte consisting of poly(n-butyl acrylate), NaI and I2 has been reported.30 The maximum ionic conductivity observed was much lower than other comparable systems, and the solar cell efficiencies are also considerably lower at 1?66% even under a low light intensity of 10 W cm22. Poor wetting ability and hence interfacial contact may be responsible for the observed trend.

Ileperuma

Matsumoto et al.31 used a related polymer, amethacryloyl-v-methoxyocta(oxyethylene) or 2-(2methoxyethoxy)ethyl acrylate, as base polymer and an ethylene glycol derivative as plasticiser, which exhibited an ionic conductivity of 2?6761023 S cm21. The energy conversion efficiency reported was 2?62%, which is 86?4% of the value for the liquid electrolyte. Here, the flexible structure of the polymer along with high plasticiser content is considered important for obtaining high ionic conductivity. Gel polymer electrolytes prepared by incorporating the standard acetonitrile based liquid electrolyte into a matrix polymer such as PAN, poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) reportedly give high ionic conductivities of the order of 1023 S cm21.32 However, these systems are sticky and soft due to the presence of liquid electrolyte, and in order to overcome this problem, a polymer membrane based on acrylonitrile–methyl methacrylate co-polymer has been developed where the film was first prepared33 and then soaked in the liquid electrolyte. An efficiency of 2?4% was obtained for this system, and the relatively low value of efficiency is attributed to the lower JSC value.

Poly(vinylidene fluoride) polymers Poly(vinylidene), PVDF and its co-polymer, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) have been used in many studies of polymer gel polymer electrolytes. Wang et al. prepared a series of DSCs by adding 5 wt-%PVDF-HFP to methoxy propionitrile based liquid electrolytes.34 However, at such low levels of polymer concentrations, the physical form is rather like glue instead of a free standing film, and sealing problems with the electrolytes still remain. This is a common problem associated with many so called quasisolid state films, which range from viscous liquids to glues to real free standing solids. Increasing the polymer content, on the other hand, causes a decrease in the ionic conductivity, and this is the dilemma faced by researchers working in this area. Various attempts have been made to alleviate this problem, and one such approach is to chemically cross-link the relatively non-polar PVDF polymer in a more polar polymer matrix such as PEG methyl methacrylate in a cross-linked reinforced network35 The efficiency obtained for a 11 mm film was 3?35%, which decreased considerably for thicker films. Another approach to enhance the efficiency of PVDFHFP gel polymer electrolytes is to incorporate TiO2 nanoparticles to form nanocomposite gel electrolytes.36 Introduction of TiO2 nanoparticles here increased the efficiency from 5?72 to 7?18%, which reached the same level of efficiency as the liquid electrolyte (7?01%). The nanoparticles here are assumed to reduce the charge recombination at the interface of dyed electrode/ electrolyte. This system shows excellent thermal stability, and there was only 10% loss in efficiency after thermal soaking at 60uC for 1000 h. This is one instance where the efficiency in a quasi-solid state exceeded that of the liquid state. Wang et al.37 reported a similarly constituted gel polymer electrolyte, where fumed silica was employed as the nanofiller, reaching the same efficiency as in the liquid state, while Usui et al. reported an example where the gel polymer electrolyte efficiency exceeded that of the corresponding liquid electrolyte.38 Recently, the use of an ultrathin PVDF-HFP membrane deposited on Ru N-719/TiO2 DSC, which gives a


DSC efficiency of 8?73%, has been reported.39 The efficiency of the corresponding liquid electrolyte DSC was 8?23%, and this enhancement can be explained on the basis of the increased VOC value arising from the passivation of traps on the TiO2 surface, thereby retarding recombination reactions. In a related study, when 10% by weight of TiO2 nanofiller was incorporated to the same membrane as in the previous example, an energy conversion efficiency of 10?40% compared to the value of 8?88% observed40 in the absence of added TiO2 was obtained. The increased efficiency can be attributed to the scattering effects of the TiO2 particles, which enhance the absorption of longer wavelength light. Incorporation of conductive carbon nanoparticles into PEO/P(VDF-HFP)/SiO2 nanocomposite polymer for DSSCs reportedly41 improves energy conversion efficiency to 4?27% from the original 3?87%. Here, the optimal carbon loading has been determined to be 5%, and beyond this, ionic conductivity decreases owing to the increase in crystallinity of the polymer and the decrease in free volume, which blocks ionic transport. The major drawback of gel polymer electrolytes is the reduced ionic mobility, and one approach to enhance ionic conduction has been to incorporate additives such as nanoparticles into the polymer matrix as discussed earlier. Another example is the introduction of nanoparticles of AlN, which possess high thermal conductivity and excellent mechanical properties.42 This increased porosity decreased the degree of crystallinity and decreased selfaggregation of the polymer. At an optimal concentration of 0?1 wt-%AlN, the efficiency obtained was 5?27% and the increase in JSC and efficiency is attributed to the higher diffusion coefficient of I2 in its electrolyte to 3?5261026 cm2 s21 compared to that of the electrolyte without AlN at 2?9761026 cm2 s21. Electrospun polymer nanofibres consisting of PVDFHFP blended with polystyrene have been applied43 in DSSCs to improve their performance, and the best results obtained were VOC50?76 V, JSC511?8 mA cm22, FF50?66 and g55?75%. In contrast, spin coated PVDFHFP blends showed only an efficiency of 1?43, and this is attributed to the higher resistance and poor I{ =I{ 3 activity between Pt and the electrolyte, giving a low value of the JSC. A similar system44 where the polymer was doped with a liquid crystal shows a regular morphology and enhanced ionic conductivity of 2?961023 S cm21 at room temperature. This showed a much higher power conversion efficiency with the DSSC parameters of VOC50?72 V, JSC514?62 mA cm22, FF50?64 and g5 6?82% at one sun intensity. These values are reported as comparable to those obtained with the conventional liquid electrolyte with VOC50?75 V, JSC5 14?71 mA cm22, FF50?65 and g57?17%.

Poly(acrylamide) polymers The advantage of this type of polymer is the presence of both amide and carbonyl groups in its structure, which may promote the interaction of the sensitised dye and the polymer gel matrix. The carbonyl groups can also coordinate to alkali metal cations, thereby enhancing iodide ion conductivity of the electrolyte and hence the DSC efficiencies. However, the reported44 efficiencies for a variety of plasticisers are ,3% at a light intensity of 60 mW cm22. One drawback of this system is the high viscosity of the electrolytic medium, which retards the movement of ions.


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Future outlook While a considerable amount of research studies have been carried out on gel polymer electrolytes, it is quite difficult to critically assess the work since most reported efficiencies have not been carried out on optimised TiO2 films. In some cases, efficiencies of the cells with liquid electrolyte reported are as low as y3%. There is scope, however, to improve the efficiencies of these cells compared to those reported for wet cells, and hence, they offer excellent promise as future solar cell materials. Polymer electrolytes such as PAN offer self-sealing films of exceptional stability and reaching efficiencies of .7%. The technology involves simple hot pressing, and these may become competitive for practical applications in large area solar cells. While this review was aimed at Ru dye sensitised TiO2 solar cells, the application of polymers to other more recently developed organic dyes is also promising. For example, the use of PAN based quasi-solid electrolyte based on plasticisers EC and PC, yielding an efficiency of 5?3% for an indoline dye sensitised TiO2 solar cell,45 has been reported.

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