Solar Energy Materials & Solar Cells 95 (2011) 3472–3481

Contents lists available at SciVerse ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

Ljubljan

E-m

journal homepage: www.elsevier.com/locate/solmat

POSS based ionic liquid as an electrolyte for hybrid electrochromic devices

M. Colovic a, I. Jerman a,b, M. Gaberscek a,b, B. Orel a,b,n

a National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Sloveniab CO-NOT, Hajdrihova 19, Ljubljana, Slovenia

a r t i c l e i n f o

Article history:

Received 1 June 2011

Accepted 8 August 2011Available online 6 September 2011

Keywords:

POSS ionic liquids

Hybrid electrochromic device

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.08.009

esponding author at: National Institute of Ch

a, Slovenia. Tel.: þ386 1 4760 276; fax: þ38

ail address: boris.orel@ki.si (B. Orel).

a b s t r a c t

The main objective of this study was to broaden the assortment of I�/I3� redox ionic liquids using

polyhedral oligomeric silsesquioxanes (POSS) acting as nanobuilding blocks for the construction of

functionalized 1,3-alkylimidazolium iodide solid (melting temperature 150–200 1C) and room tem-

perature (RT) ionic liquids.

The structural characteristics of the synthesised final ionic liquids and the corresponding inter-

mediates were determined using 1H, 29Si NMR and infrared spectroscopic measurements. Raman

spectra were next reported, in order to demonstrate the presence of polyiodides formed after the

addition of iodine and the formation of redox electrolytes. Ionic conductivity values obtained from the

impedance (EIS) spectra were determined in the temperature interval from room temperature up to

100 1C. Finally, a hybrid electrochromic cell was constructed from room temperature MePrImþIx� IO7 T8

POSS (x¼1, 1.2, 3 and 5) ionic liquids encapsulated between a lithiated WO3 working and Pt counter-

electrode, and colouring–bleaching changes assessed for cells cycled up to 1000 repetitive cycles.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Ionic liquids [1] have already found applications in variouselectrochemical systems as robust and non-volatile electrolytes.For specific applications, such as dye sensitised photoelectro-chemical (DSPEC) [2], hybrid electrochromic (hybrid EC) [3–5]and photoelectrochromic (PEC) cells [6–8], in which room tem-perature ionic liquids create sealing problems, gel [9] or, evenbetter, solid or condensed ionic liquid-based electrolytes [10] thatcombine the non-volatility, temperature stability and high con-ductivity of room temperature ionic liquids with the mechanicalintegrity of solid electrolytes, are preferable [11].

Hybrid EC cells exhibit some special features, such as simpleconstruction (ion-storage layer is not needed [4]) and self-erasing oftheir colour state [5], the latter being important for aircraft ECwindows for safety reasons. The first relatively large hybrid ECwindow, constructed from 30�30 cm2 large segments, has onlyrecently been reported [12]. Semi-solid electrolyte based on hydro-phobic room temperature dialkylimidazolium iodide ionic liquidcondensed with nano silica particles has been used. For ‘‘all-solidstate’’ hybrid EC cells, by analogy with standard battery type EC cells[3,13] it would be best to have a redox electrolyte with thermo-plastic properties [14], for example those employed in some

ll rights reserved.

emistry, Hajdrihova 19, 1000

6 1 4259 244.

commercial EC systems such as helmet visors [15]. Accordingly,the main objective of this study was to broaden the assortment ofI�/I3

� redox solid (melting temperature 150–200 1C) and roomtemperature (RT) ionic liquids suitable as electrolytes for the hybridEC cells. We focused here on dialkylimidazolium iodide ionic liquid,which served as a basic compound for functionalization withpolyhedral oligomeric silsesquioxane (POSS).

POSS have already been used by Maitra and Wunder [16–19] forthe preparation of Liþ conductors based on polyethylene oxide(PEO)—functionalized with eight POSS units with incorporatedlithium salt. Similarly, Tanaka et al. [20], recently reported thesynthesis of a room temperature ionic liquid consisting of anoctacarboxy POSS anion [POSS-(COO�)8] and imidazolium cation[i.e., 1-butyl-3-methyl imidazolium], but an imidazolium basediodide ionic liquid with imidazolium cation functionalized on just asingle corner of POSS, as shown in Fig. 1, has not yet been reported.

POSS are specific compounds characterised by a cage-likestructure (1�3 nm in size) resembling in this respect organicallyfunctionalized nanosized particles of SiO2 [21,22]. POSS are themost ordered product of hydrolytic condensation of alkyltrialk-oxysilanes and consist of a silica core (SiO3/2) and organic corona(R). The variety of organic groups (R0, R00, R000), which are located atthe corners of the silsesquioxane polyhedra, gives an enormousnumber of heteroleptic POSS (R0xR00yR000z SiO1.5)8, (xþyþz¼8) withmultifunctional properties.

Specifically, in this study we decided to prepare monofunctio-nalized (Imþ I� R7 (SiO1.5)8) instead of octameric imidazoliumiodide POSS ionic liquids (Imþ I�)8 (SiO1.5)8, by analogy with the

Fig. 1. Reaction scheme for synthesis of 1-methyl-3-propyl hepta(i-butyl) octasilsesquioxane imidazolium iodide (MePrImþ I� IB7 T8 POSS) (3a), 1-(2-(2-(2-

methoxyethoxy)ethoxy)ethyl) 3-propyl hepta(i-butyl) octasilsesquioxane imidazolium iodide (PEOPrImþ I� IB7 T8 POSS (3b)).

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–3481 3473

majority of reported cases, in which POSS-based materials havebeen prepared with POSS side chains [23–27] or as end-groups[28–31]. By variation of the substituent on the remaining sevencorners of the POSS, solid or room temperature iodide ionicliquids could be made, due to the variously strong Van der Waalsinteractions.

Hepta(i-butyl)-T7-trisilanol (IB7(OH)3 T7 POSS (1)) and hepta(i-octyl)-T7-trisilanol open-cage POSS (IO7(OH)3 T7 POSS (10)) (Fig. 1)were used as commercial starting compounds (www.Hybrid.Plastics.com) because of defined structure and the possibility of applyingthe corner-capping method [32], which already enables the pre-paration of many POSS derivatives: nanostructured hybrid organic-inorganic composites [30], modifying agents [33], additives forproviding common organic polymers with added value, such asthermal resistance, electric insulating property, water repellence,weather resistance, anti-flammability [34,35], and as dispersantsand anti-soiling additives for solar selective paint coatings [36].

In this study, in addition to the synthesis of POSS functiona-lized imidazolium iodide based ionic liquids, we have alsodescribed their structural properties, which were inferred fromanalysis of the corresponding infrared and 29Si NMR spectra.Correlations between the structure of various POSS moleculesand their infrared and 29Si NMR spectra features have alreadybeen reported in sufficient detail [37], which enabled theexpected structure of the synthesised compounds to be conceivedfrom the collected data.

Conductivity is one of the various properties that should beknown about POSS functionalized imidazolium iodide ionicliquids. Solid (or solidified [10]) ionic liquid based electrolytesalways exhibit lower conductivities than their corresponding

liquid counterparts. However, at least for iodide based ionicliquids with I–/I3

– redox couples, this drawback can be partiallycompensated by the presence of polyiodides (I2nþ1

– ) [38,39].Polyiodides provide an additional charge exchange mechanism(i.e., Grottus mechanism) [40,41], whereby electron hopping andpolyiodides bond exchange are coupled. The Grotthus mechanismwas first proposed to exist for solid I–/I3

– electrolytes [42] and itsexistence has also been suggested for room temperature ionicliquids [1,2]. More directly, a relay type mechanism was con-ceived by Kawano and Watanabe [43], who showed that theapparent diffusion coefficient (Dapp) depends on the concentra-tion of iodine (i.e., redox couple). It has been shown for 1-methyl-3-propylimidazolium iodide (MPImþI�) ionic liquid that theaddition of iodine increases conductivity up to 5 times, i.e., from5�10�3 to 35�10�3 S/cm, which has been correlated to theformation of MPImþ Ix

� (x¼3 and 5) containing I3� and I5

� ions [44]identified from the Raman spectra. Due to the importance ofpolyiodides for the conductivity of iodide based ionic liquids, weidentified them from the measured Raman spectra of PEOPrImþ

I� IB7 T8 POSS (Fig. 1) containing different amounts of iodine,giving nominally PEOPrImþ Ix

� IB7 T8 ionic liquids with x¼1.2,3 and 5. In the next step, this enabled a correlation of theirpresence with the conductivity of the electrolytes determinedfrom EIS measurements. We deliberately focused on analysis ofthe high frequency arc, leaving out more detailed analysis, whichmay possibly have given information about the charge transferreactions at the electrode /electrolytes interface and the conduc-tivity ascribed to the polyiodides within the ionic liquid matrix.

The infrared spectra of the synthesised iodide ionic liquidswere used in this study in order to elucidate the interactions

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–34813474

between the ions and specific parts of the imidazolium cation,where interactions with the counter ions occur. From our pre-vious infrared spectra studies of room temperature 1-methyl-3-propylimidazolium iodide ionic liquid /iodine mixtures withnominal of MPImþ Ix

� with 1rxr5 [45], we inferred that thepolyiodides weaken the interactions between the imidazoliumcations and polyiodides. The interactions are quite specific andare limited to the aromatic C–H groups of the imidazolium cation.Accordingly, in this study, the corresponding information wasused for discovering whether the Colombic interactions amongthe large polyiodides and the imidazolium cations could also bedescribed in a similar manner in the solid PEOPrImþ Ix

� IB7 T8

POSS (x¼1, 1.2, 3 and 5), in order to conceive the relaxations ofsupermolecular aggregates, which have been already establishedfor solid PF6

� and its liquid BF4 imidazolium analogue [46,47].In the last part of the study, colouring/bleaching changes of the

hybrid EC device constructed of WO3 and the Pt counter-electrodewere investigated and, instead of using solid POSS based electro-lytes, room temperature MePrImþ Ix

� IO7 T8 POSS electrolyteswere incorporated between the two electrodes.

The main reasons for the deliberate use of room temperatureionic liquid were, firstly, to avoid the adhesion problems encoun-tered when a solid electrolyte is combined with electrochromicfilms and, secondly, to show the effect of the added iodine (Ix,x¼1, 1.2, 3 and 5) on the transmittance of the device.

2. Experimental

2.1. Measurements techniques

IR spectra measurements were performed on a Brucker ModelIFS 66/S spectrometer. The samples were deposited on Si wafersand transmission spectra recorded.

Raman spectra were recorded on a Horiba Yobin Ivon LabRAMHR 800 mm focal length instrument with He–Ne (632.8 nm) laserexcitation source. Using a 10� lens, the laser beam was focusedon a spot approximately 1 mm in diameter and the collectedscattered light was passed through a spectrometer onto a CCDdetector.

1H and 29Si NMR spectra were obtained in CDCl3 on a Varian300 MHz spectrometer, model Unity INOVA. Chemical shifts arereported in ppm relative to CHCl3 (d 7.28, 1H) and tetramethylsi-lane (d 0.00, 29Si).

The conductivity value (s in S/cm) of all synthesised POSSbased ionic liquids was measured by the complex impedancespectroscopy (EIS) technique. The impedance/admittance plotswere drawn using a computer interfaced Autolab PGSTAT30potentiostat–galvanostat with FRA module in a cell with platinumelectrodes. The conductivity was calculated by the equation s¼d/SR,where 1/R is the conductance to be determined from the admit-tance plots, d is the thickness of the sample and S is the cross-sectional area of each electrode. The solid samples were preparedin the form of pellets for the temperature dependant conductivity

MeOO

OHMsCl

Et3N

N NHN

THF, hea

CH3O

OOMs

Fig. 2. Reaction scheme of synthes

measurements. Pellets of the POSS based ionic liquids with adiameter of 7 mm and a thickness around 1.5 mm were fabricatedby compressing at 4 Mpa with Teflon plate covers and they werethen sandwiched between two parallel, 5 mm diameter, gold-coated electrodes. The ion conductivities of the samples weremeasured in a chamber under flowing nitrogen by the acimpedance method, over the frequency range from 106 to 1 Hzusing Solartron 1260 Impedance analysis.

2.2. Materials and synthesis

The following purchased materials were used without furtherpurification: isobutyltrimethoxysilane (IBTMS; manufacturerABCR, 97%), 3-iodopropyltrimethoxysilane (IPTMS; manufacturerABCR, 97%), phosphazene base P1-t-Bu (Aldrich) and tetrahydro-furan (THF; Merck, 499%), methanol (Merck, 499%), diethyle-neglycol monomethylether (Merck), methanesulfonyl chloride(Fluka), triethylamine (Merck), dichloromethane (Aldrich),sodiumhydride (NaH, Fluka, 60% in oil), imidazole (Fluka) and1-methyl imidazol (Fluka). THF was used as fresh distilled in thepresence of benzophenone and metallic sodium. All reactionswere performed in standard glassware. Sputtered WO3 films onFTO glass were used as a working electrode and obtained fromDControl Glas GmbH & Co. Sputtered Pt films (2 nm) on FTO glasswere obtained from the Fraunhofer Institute for Solar Energysystems ISE, Freiburg and used as a counter electrode.

Synthesis of (3-iodopropyl)hepta(i-butyl)octasilsesquioxane

(IodoPr IB7 T8 POSS (2) (Fig. 1)) and (3-iodopropyl)hepta(i-octy-

l)octasilsesquioxane (IodoPr IO7 T8 POSS (the same procedure asfor compound 2)).

In a dry nitrogen atmosphere, 3-iodopropyltrimethoxysilane(14.51 g, 50 mmol) was added dropwise over a ten-minute periodto a solution mixture of hepta(i-butyl)-T7-trisilanol POSS (1)(39.58 g, 50 mmol), phosphazene base P1-t-Bu (Aldrich) (1.74 g,5 mmol) and 450 mL of dry THF at �15 1C. The reaction solutionwas continuously stirred overnight. After the end-capping reac-tion was completed, the solution was neutralised with glacialacetic acid to pH 7 and the solvent was evaporated to 30% of thestarting reaction solution. The same amount of methanol wasadded to the corresponding solution and the formed precipitatewas filtered, washed several times with methanol and then driedin a vacuum in order to obtain a white solid. The product wasfurther purified by re-crystallisation in THF and methanol to yieldcolourless crystals (36.88 g, 37 mmol, 74% yield). The (3-iodopro-pyl)hepta(i-octyl)octasilsesquioxane (IodoPr IO7 T8 POSS (20)) wassynthesised following the same procedure as above except thatIO7(OH)3 T7 POSS was used as the starting compound.

Synthesis of mono alkylated (dietyleneglycol monomethyl ether)imidazole (Fig. 2).

The synthesis started with the preparation of diethyleneglycolmonomethylether monomesylate, as a good alkylation reagent,from diethyleneglycol monomethylether (DEGME). NaH (11.24 g,282 mmol) was added in portions to a solution mixture of dry THF(300 ml) and imidazole (15.33 g, 225 mmol) and the solution

CH3O

OOMs

CH3O

ON N

aH

t

is of monoalkylated imidazole.

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–3481 3475

heated at 50 1C for one hour. In the next step of the reaction,diethyleneglycol monomethylether monomesylate was added formonoalkylation of imidazol in a nitrogen atmosphere. The solu-tion mixture was continuously stirred under reflux for 6 h and thereaction product was then cooled to room temperature andfiltered for the separation of beige solid sodiuimidazolate. Thefiltrate was concentrated on a Rotavapor. Purification was doneby dissolving the obtained brownish oil product in methanol,followed by extraction of impurities with hexane. The methanolsolution of the product was concentrated in a Rotavapor. The nextpurification step was filtration through a short column of silica gelwith DCM/MeOH (20:1). Clear light brown oil was obtained afterremoving solvents on a rotavapor and drying in vacuum. Productstructure and purity were confirmed by IR and NMR spectroscopy(1.88 g, 2.09 mmol), 93% yield.

Synthesis of 1-(2-(2-(2-methoxyethoxy)ethoxy)ethyl) 3-pro-pyl hepta(i-butyl) octasilsesquioxane imidazolium iodide(PEOPrImþ I� IB7 T8 POSS) (3b) (Fig. 1).

In a dry nitrogen atmosphere, a solution mixture of toluene(90 ml) and IodoPr IB7 T8 POSS (2) (9.85 g, 10 mmol) was added tothe equivalent of mono alkylated (dietyleneglycol monomethylether) imidazole (b) (1.7 g, 10 mmol) (Fig. 1). The solutionmixture was continuously stirred at 100 1C for 24 h and thereaction was cooled at room temperature. The solid productwas evaporated, dried in a vacuum and used as an electrolyte.

The corresponding i-octyl POSS analogues were made asdescribed above, using IodoPr IO7 T8 POSS and mono alkylatedmethyl imidazole.

3. Results and discussion

3.1. Thermal properties

POSS are thermally stable compounds showing degradationabove 300 1C [48,49]. The melting temperature (Mp) can belowered by the substitution of flexible long chains attached tothe cube corners, which prevent close packing. Thermogravim-metric measurements revealed that the monofunctionalizedIodoPr IB7 T7 POSS melted at 240 1C, which was lower than forthe cube-like IB8 T8 POSS, which melts at 265 1C [49]. The lengthof the chain also affected the melting temperatures of thesynthesised ionic liquids; PEOPrImþ I� IB7 T8 POSS melted at155 1C, while MePrImþ I� IB7 T8 POSS melted at 200 1C (Fig. 3).

-100

Mp [155°C]

Glass Transition [-16°C]

Hea

t Flo

w [m

W]

Temperature [°C]

MePrIm IO7 POSS MePrIm IB7 POSS PEOPrImI IB7 POSS

Mp [200°C]

4

-50 0 50 100 150 200

Fig. 3. Results of DSC analysis for PEOPrImþ I� IB7 T8 POSS (Mp¼155 1C),

MePrImþ I� IB7 T8 POSS (Mp¼200 1C) and MePrImþ I� IO7 T8 POSS (Tg¼�16 1C) .

For [POSS-COO–] [BMimþ] ionic liquid, Tanaka et al. [20]reported a melting temperature of 23 1C. They attributed thelowering of the melting temperature to the presence of largeglobular POSS entities, which reduces the packing density andisolates the distal ion pairs. Even though the corresponding ionicliquids could not be directly compared to ours, we attributed theliquid state of MePrImþ I� IO7 T8 POSS ionic liquid compared tothe other two solid POSS-based ionic liquids (PEOPrImþ I� IB7 T8

POSS and MePrImþI� IB7 POSS) to the much larger size of theisooctyl substituted POSS. It seemed that the POSS–POSS interac-tions prevailed and governed the state of the ionic liquid, whilethe influence of the chain length did not have such a strong effecton the melting temperature.

All the investigated ionic liquids started to show degradationabove 250 1C (not shown here), agreeing with the fact that rigidsilica moieties enhance thermal stability [20].

3.2. Structural studies

3.2.1. 29Si NMR spectra

In order to confirm the structure of the compounds shown inFig. 1, 29Si NMR spectra of IodoPr IB7 T8 POSS and finalMePrImþ I� IB7 T8 POSS, PEOPrImþI� IB7 T8 POSS andMePrImþ I� IO7 T8 POSS compounds were recorded (Fig. 4).Examination of the spectra revealed that the intermediate IodoPrIB7 T8 POSS and the final PEOPrImþ I� IB7 T8POSS compoundswere practically identical, showing signals only in the T3 region[32], which enabled the conclusion that they consisted of onlycage-like products. The presence of three signals at �67.4, �67.8and �69.1 ppm in the T3 region, with a relative intensity ratio of3:4:1, was expected, due to the different substituents on thecorners of the POSS cage [37,50].

1H spectra (Fig. 5) of the PEOPrImþ I� IB7 T8 POSS provided theimportant information that dialkylation of imidazol had beenachieved. This was inferred from the existence of the signalsattributed to the imidazolium ring protons at d¼10.13 ppm (t),d¼7.69 ppm (t) and d¼7.15 ppm, which are shifted in compar-ison to the corresponding signals for monoalkylated imidazol, dueto the dispersion of the positive charge on the 1,3-N dialkylatedimidazolium ring, which causes changes in the magnetic sur-roundings of the corresponding protons. (Fig. 5) [51].

-50

D

C

B

-68.

8-6

7.8

-67.

5

A

δ [ppm]

Inte

nsity

-55 -60 -65 -70 -75

Fig. 4. 29Si NMR of IodoIB7 T8 POSS (A), PEOPrImþ I� IB7 T8 POSS (B), MePrImþI�

IB7 T8 POSS (C) and MePrImþ I� IO7 T8 POSS (D).

00

200

400

600

800

Inte

nsity

δ [ppm]2 4 6 8 10

Fig. 5. 1H NMR of PEOPrImþ I� IB7 T8 POSS.

4000

DC

B

3140

ν ringC

-H

3072

ν ringC

-C

1567

2908

2869

2951

1112

ν sSi-O

-Si

ν asS

i-O-S

i

Abs

orba

nce

Wavenumber [cm-1]

CH

2, CH

3

482

0.4

A

3500 3000 2500 2000 1500 1000 500

Fig. 6. IR spectra of of IodoIB7 T8 POSS (A), PEOPrImþ I� IB7 T8 POSS (B),

MePrImþ I� IB7 T8 POSS (C) and MePrImþ I� IO7 T8 POSS (D).

300

0.0

0.2

0.4

0.6

0.8

1.0

297

- I- 3

Abs

orba

nce

Wavelength [nm]

PEOPrIm +I- IB7 T8 POSS

PEOPrIm +I- IB7 T8 POSS+I2 PEOPrIm +I- IB7 T8 POSS+2I2

400 500 600 700 800

Fig. 7. UV–vis spectra of PEOPrImþI� IB7 T8 POSS with the addition of iodine

(molar ratio 1:0, 1:1 and 1:2) in CCl4.

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–34813476

3.2.2. Infrared spectra

The infrared signature of the IB7(OH)3 T7 POSS molecule is wellestablished [36,37] and was obtained in this study from themeasured infrared transmission spectra [36,52] (Fig. 6). Whilethe most typical vibrations of the IB7(OH)3 T7 POSS are attributedto the presence of the silanol moieties at �3250 cm�1 and at893 cm�1, the most prominent band of the trisilanol silsesquiox-ane cage appeared at 1118 cm�1 [36] and was accompanied byCH2 (2927, 2903 cm�1) and CH3 (2954, 2871 and 2817 cm�1)stretching [53] and corresponding deformational modes (1461,1350, 1228 and 839 cm�1) [54]. Examination of the infraredspectra revealed that closing the silsesquioxane cage shifted thestretching Si–O–Si to 1108 cm�1, suggesting a change in the cagestrains after corner-capping. The possible existence of the ladder-like structure of PEOPrImþI� IB7 T8 POSS (3b) could be rejected,since the spectra of the latter are characterised by a strong bandat �1030–1059 cm�1 [37,55]. The characteristic Si–O–Si band forPEOPrImþIx

� IB7 T8 POSS (x¼1) appeared at 1109 cm�1 and, aswould be expected, was similar to that for the related IB7(OH)3 T7

POSS. The similarity of the corresponding frequencies indicatedthat the POSS cage had not been fragmented during any proces-sing. As expected, the silanol band at 893 cm�1 disappeared from

the spectra of PEOPrImþ Ix� IB7 T8 POSS (x¼1) but all the other

CH2 and CH3 stretching [53] and corresponding deformationalmodes (1461, 1350, 1228, and 839 cm�1) were still seen in thespectra, while the band at 1561 cm�1 (imidazolium ring mode)and the corresponding C–H stretching ring modes at 3130 and3065 cm�1 signalled the presence of the imidazolium ring [44].

3.3. Identification of polyiodides

3.3.1. UV–vis spectra

Before considering the Raman spectra, UV–vis spectra ofPEOPrImþ I� IB7 T8 POSS with various amounts of iodine (molarratio 1:0, 1:1 and 1:2) in CCl4 are presented in Fig. 7. As expected,the absorption bands attributed to the triiodides at 297 and367 nm [56] were noted in the spectra, while the weak absorptionband at 524 nm was ascribed to the higher polyiodides, i.e., I5

�

[57]. It should be noted that PEOPrImþ I� IB7 T8 POSS can bedissolved in various solvents, including butyrolactone, propylenecarbonate, sulfolane and other aprotic solvents, which are used aselectrolytes, and the corresponding mixtures with iodine andlithium salts. The room temperature MePrImþI� IO7 T8 POSSionic liquid readily dissolved lithium salts and is compatible withother organic and aprotic organic electrolytes.

3.3.2. Raman spectra

Raman spectra of polyiodides are complex and studies of themhave a long tradition. Polyiodides appear in various materials andsystems, including iodide ionic liquids. For the 1-methyl-3-pro-pyl-imidazolium iodide (MPImþ Ix

�; 1oxo5) room temperatureionic liquid polyiodides Ix

� (x¼3 and 5) and 2I�yI2, adductsformed in the presence of iodine (0 molr I2 r2 mol) (Fig. 8A)[44,45], while for MPImþIx

�; (1rxr2.5), the existence of I3� and

2I�yI2 adducts was inferred from the symmetric stretching ns

(I3�) band at 141 cm�1 [39] and corresponding adduct band at

148 cm�1, the spectra of MPImþ Ix�; (3oxo5) revealed an addi-

tional ns (I5�) band at 165 cm�1 which, together with the weaker

bands at 111 and 141 cm�1, signalled the presence of linear anddiscrete I5

� ions [58–60].The Raman spectra of PEOPrImþ Ix

� IB7 T8 POSS (x¼1, 1.2, 3 and5) exhibited similar features (Fig. 8B), in spite of the fact that theyare solids. Predictably, PEOPrImþIx

� IB7 T8 POSS without addediodine (i.e., x¼1, not shown here) showed only a weak bandattributed to I3

� ions. As expected, the triiodide band at111 cm�1, together with the tetraiodide adduct mode at

50

141

172

111

Rel

ativ

e in

tens

ityRaman shift [cm-1]

4000 147

x=1.2

x=2.5

x=3

x=5

100 150 200

Fig. 8. Raman spectra of MPImþ Ix� (A) (Refs. [66,44]) and PEOPrImþ Ix

� IB7 T8 POSS (B) with the addition of different amounts of iodine.

Fig. 9. Electrochemical impedance spectra of PEOPrImþ Ix� IB7 T8 POSS without

(x¼1) and with various amounts of added iodine (x¼1.2, 3, 5).

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–3481 3477

147 cm�1, appeared in the spectra of PEOPrImþ Ix� IB7 T8 POSS

(x¼1.2); the latter band became gradually red-shifted to 141 cm�1

for x¼2.5 and x¼3 as more discrete I3� ions formed. Finally, the

bands at 111, 141, and the new band at 172 cm�1 became visiblefor PEOPrImþ Ix

� IB7 T8 POSS for x¼5. The latter band appeared athigher frequencies than expected for symmetric I5

� [38,44], sug-gesting the presence of various I–IyI3

� adducts and not just pureI5� species. However, the Raman spectra consistently showed a

gradual build-up of polyiodides responsible for the establishmentof a polyiodide based network and the concurrent Grotthus relaycharge transport in iodine reached PEOPrImþ Ix

� IB7 T8 POSS (x41).The Raman spectra unequivocally revealed the presence of

specific types of polyiodides, agreeing with electrochemical studies[43,45,61], showing that they accelerates the redox charge trans-port in room temperature ionic liquids and in similar systems, suchas crystal electrolyte ionic liquids [62,63], ionic liquids gelled bythe addition of nanoparticles [64] and also in polymer electrolytes[57]. Expectedly, not all systems showed exactly the same Ramanspectra. For example, the vibrational bands in the Raman spectra ofLiI/hydroxypropionitrile [57] for x¼0.1 exhibit bands at 146 and113 cm�1, while in the case of polymerised imidazolium iodidewith added SiO2 nanoparticles, bands at 111, 145 and 165 cm�1

appeared for higher I2 concentrations [65] but bands at 111 and145 cm�1 [41] and 150 and 170 cm�1 [66] have been reported forionic liquid gelled with small molecular gelators. Apparently, evenminor changes in the amount of added iodine (i.e., Ix) markedlychanged the corresponding vibrational bands, as shown in Fig. 8A[44]. However, the measured Raman spectra (Fig. 6) were con-clusive, confirming the co-existence of I3

–, I5– species and I–IyI–

adducts with an increasing nominal amount of iodine in solidPEOPrImþ I� IB7 T8 POSS ionic liquid.

Accordingly, the ionic conductivities of various POSS basediodide ionic liquids containing different polyiodides originatingfrom added iodine were determined as shown below.

3.4. Ionic conductivity of PEOPrImþ Ix� IB7 T8 POSS

Due to the observed changes of the Raman spectra of ionicliquids containing different amounts of added iodine and because

of the expected presence of the Grotthus charge transportmechanism, the variation of the ionic conductivities of thesynthesised ionic liquids was determined from the correspondingEIS spectra. The EIS spectra of PEOPrImþ Ix

� IB7 T8 POSS (Fig. 9)consisted of a high frequency arc and a low frequency feature, theshape of which varied with the addition of iodine. We estimatedthe physical meaning of individual impedance features in the EISspectra from the relaxation times (inverse value of arc peakfrequency) or, similarly, from the typical capacitance valuesassociated with the given arc. For example, the capacity due tothe high frequency arc in PEOPrImþI� IB7 T8 POSS (x¼1) wasabout 6�10�11 F, which is typical of bulk charge storage (i.e., dueto the dielectric properties of a bulk sample). We thus attributed

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–34813478

the corresponding resistance to the resistive properties of thebulk material between the electrodes. The calculated conductivityof this material is thus about 1.7�10�7 S/cm�1. Conversely, thelow frequency arc of PEOPrImþ Ix

� IB7 T8 POSS (x¼1) showed acapacity of about 2�10�6 F, which is a typical value for inter-facial charge storage between the sample and adjacent electrodes.The high-frequency arc changed very little with the addition of I2

to PEOPrImþ Ix� IB7 T8 POSS (x¼1.2). Its capacitance was almost

the same (5�10�11 F) and the specific conductivity slightlyincreased (2�10�7 S/cm�1). With the addition of a higheramount of I2 to PEOPrImþ Ix

� IB7 T8 POSS (x¼3 or 5), the lowfrequency response changed dramatically in both size and shape,while the typical capacitances remained in the region typical forinterfaces (in the order of 10�6 F). Accordingly, we assumed thatthe added I2 improved the contact between the sample and theelectrodes, hence the drop in impedance values at lowfrequencies.

The results of the EIS measurements of POSS-based ionicliquids (Fig. 9) and the corresponding temperature dependenceof their conductivities (s in S/cm Fig. 10), measured from roomtemperature up to 100 1C, revealed that (i) all solid (MePrImþ Ix

�

IB7 T8 POSS (x¼3 and 5), PEOPrImþIx� IB7 T8 POSS (x¼1, 3 and 5))

and room temperature MePrImþ Ix� IO7 T8 POSS (x¼1 and 3) ionic

liquids exhibited room temperature s values that were muchlower than those of room temperature MPImþ I–

x (x¼1) ionicliquid [44] and (ii) that the addition of iodine and the concurrentpresence of polyiodides significantly enhanced the correspondings values.

The highest s values were noted for solid PEOPrImþ Ix� IB7 T8

POSS (x¼5), which were close to 10�5 S/cm. As expected, thecorresponding s values were lower for the same ionic liquids withless iodine (i.e. x¼1 and 3). MePrImþ Ix

� IO7 T8 POSS (x¼3)exhibited s values that were close to those of PEOPrImþIx

� IB7

T8 POSS (x¼1 and 3) but the s values of MePrImþ Ix� IB7 T8 POSS

(x¼5) were lower than those of PEOPrImþ Ix� IB7 T8 POSS (x¼5).

We attributed this to the presence of the longer chain attached tothe POSS cube, making the structure of the resulting electrolytemore flexible. The s values for the room temperature MePrImþ Ix

�

IO7 T8 POSS (x¼1 and 3) ionic liquids were found between theconductivities of the solid analogues but, surprisingly, the corre-sponding s values did not surpass those of PEOPrImþIx

� IB7 T8

POSS (x¼3 or 5) and MePrImþ Ix� IB7 T8 POSS (x¼5) with added

iodine. This indicated that the bulky isooctyl POSS present inMePrImþ Ix

� IO7 T8 POSS hindered collision among the I3– and I5

–

3.4

-8

-7

-6

-5

-4

-3

MePrIm IO POSS I

MePrIm IO POSS I

PEOPrIm IB POSS I

PEOPrIm IB POSS I

PEOPrIm IB POSS I

MePrIm IB POSS I

MePrIm IB POSS I

log

σ [S

cm-1

]

1000/T [K-1]

MPIm I

3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6

Fig. 10. Temperature dependence of ionic conductivity for room temperature

MePrImþ I� and MePrImþ Ix� IO7 T8 POSS (x¼1 and 3) and solid MePrImþ Ix

� IB7 T8

POSS (x¼1, 3 and 5), PEOPrImþ Ix� IB7 T8 POSS (x¼1, 3 and 5) ionic liquids.

ions but complete blocking of the collisions did not take place asin some organic I–/I3

–electrolytes, such as polyethylene glycoldimethylether (Mw�500) [61] and LiI/3-hydroxypropionitrile(1:4) redox electrolyte used for DSPEC cells [57] with which theconductivity did not actually increase with the addition of iodine.

It should also be noted that the increase in the s valuesobserved for liquid MePrImþ Ix

� IO7 T8 POSS (x¼1 and 3) wasmuch smaller i.e., from 5.2�10�8 to 2.2�10�8 S/cm, than theconductivity enhancement observed for the solid POSS ionicliquids with the addition of iodine, by a few orders of magnitude.However, the increase was significantly higher with respect tothat reported for room temperature MePrImþ Ix

– (for x¼1 up to 5)[44], with which the s values increased up to 5 times, i.e., from5�10�3 to 35�10�3 S/cm. We inferred from these results thatthe contribution of the Grotthus relay type conductivity mechan-ism was smaller for room temperature than for similar solid ionicliquids. To sum up, the presence of polyiodides generated by theaddition of iodine had a pronounced effect on the conductivitiesof the solid ionic liquids. The increase in the conductivity valueswas quite large compared to that reported for other solid iodidebased ionic liquids [42]. The Grotthus effect definitely contributedsignificantly to the observed conductivity of the synthesised solidPOSS-based ionic liquid redox electrolytes.

3.5. Cation–anion interactions from infrared spectra.

Since polyiodides form an interpenetrating and connected net-work in many solid polyidodes with a well expressed crystallinestructure [38], anion–cation interactions are likely to be present.The corresponding interactions have actually already beenreported for room temperature MPImþIx

� (1rxr5) ionic liquids,by examination of the corresponding infrared spectra [44], provid-ing information about the interactions between the polyiodidesand specific groups of the imidazolium cation [67] (Fig. 11).

The infrared spectra of PEOPrImþ Ix� IB7 T8 POSS (x¼1, 1.2, 2.5,

3 and 5) shown in Fig. 9 revealed similar changes as have beenreported for room temperature MPImþIx

� (1rxr5) ionic liquids[44]. The spectra of PEOPrImþIx

� IB7 T8 POSS (x¼1) exhibited anaromatic C–H (Imþ) stretching band at �3132 cm�1, whichshifted to 3137 cm�1 with increasing x; however, its intensityincreased relative to the so-called ‘‘Cl interaction band’’ [68]. Thelatter band shifted from 3066 cm�1 (x¼1) to 3077 cm�1 (x¼2.5)and remained unchanged for higher x. At the same time, a newband at 3105 cm�1 appeared for x42.5 and its intensityincreased with increasing x.

The experimental data showed clearly that the extent of inter-actions changed with the presence of polyiodides. From the blueshifts of the frequencies of the C–H (Imþ) stretching bands, weinferred that the strength of H bond interactions was weaker in thepresence of polyiodides. This could be explained by the diminishingstrength of Colombic interactions among large charged polyiododeanions. Moreover, the observed splitting of the ‘‘Cl interaction band’’and the appearance of a new band at 3066–3077 cm�1 indicatedthe presence of non-equivalent C–H ring groups in the presence ofpolyiodides. A decrease in the effective symmetry of the imidazo-lium rings could also be inferred from the presence of a newshoulder band at 1580 cm�1 observed in the spectra ofPEOPrImþ Ix

� IB7 T8 POSS for x¼5 (not shown here).

3.6. Electrochromic device with MePrImþ I� IO7 T8 POSS

Electrochromic glazing allows control of solar energy gainsand daylighting in buildings and cars [69]. Basically, threedifferent EC configurations exist: solution, hybrid and batterytypes [3]. The hybrid EC type that was studied here consisted ofan active electrochromic layer, i.e., electrochemically lithiated

3250x=5

x=3

x=2.5

x=1

Abs

orba

nce

Wavenumber [cm-1]

I increase

x=1.2

3132

3137

3105

3077

3066

0.05

3200

0.0

0.1

0.2

0.3

0.4

0.5

x=5

x=3

x=2.5

x=1

Abs

orba

nce

Wavenumber [cm-1]

I increase

x=1.2

3200 3150 3100 3050 3000 3000 2800 2600

Fig. 11. Infrared spectra of PEOPrImþ Ix� IB7 T8 POSS with x¼1, 1.25, 2.5, 3 and 5.

Fig. 12. Examples of hybrid EC cells with MePrImþ Ix� IO7 T8 POSS electrolytes:

x¼1, 1.2, 3 and 5.

Fig. 13. Transmittance spectra of hybrid electrochromic cells with Pt on FTO as a

counter electrode for potentials �1.0 V and 1.7 V for Ix–, x¼1, 1.2, 3, 5.

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–3481 3479

WO3 (LixWO3), which was in contact via an I�/I3� redox electro-

lyte with a nanocrystalline Pt electrode. The hybrid EC systemrequires a continuous current to maintain the WO3 film in itscoloured, i.e., W in Wþ5, state (Fig. 10) because an I�/I3

� redoxelectrolyte enables an exchange of electrons via the Grotthusrelay type electron transport mechanism [43,61].

The charge-transfer resistance at the WO3I (I�/I3�) redox

electrolyte interface is quite high, contrasting with the muchlower electron charge-transfer resistance at the Pt counter-electrode, where the facile oxidation of I� to I3

� takes place [70].At both electrodes, therefore, the electron transfer processesare kinetically controlled, enabling WO3 films to maintain thecoloured state [12].

A hybrid electrochromic (HEC) cell was constructed by encap-sulating highly viscous, i.e., room temperature MePrImþIx

� IO7 T8

POSS (x¼1, 1.2, 3 and 5) redox electrolytes between an electro-chemically lithiated WO3 working electrode facing a nanocrystal-line Pt counter-electrode Fig. 12.

The transmittance spectra of the hybrid EC cell withMePrImþ Ix

� IO7 T8 POSS (x¼1.2, 3 and 5) are shown in Fig. 13.As expected, due to the absence of added iodine, the Hybrid ECcell in a bleached state exhibited the highest transmission in thespectral region lo400 nm but the corresponding colouring/bleaching changes were small, agreeing with low conductivitystemming from the lack of a sufficient amount of I3

– (Fig. 13).Hybrid EC cells in their bleached state employing electrolyteswith added iodine (Ix, x¼1.2, 3 and 5) showed a red-shiftedtransmission edge with increasing Ix. This was expected becauseof the presence of polyiodides, absorption of which (297 and

367 nm [38,56], Fig. 7) extends to the visible spectral region. Thetransmission maxima varied because the thickness of the electro-lyte layer was not the same in the investigated hybrid EC cells(Fig. 13).

The stability of cells was tested up to 1000 repetitive cycles.Since cells were not sealed, the stability seemed to be adequatefor the further development of a hybrid EC cell with I�/I3

� redoxelectrolytes based on a POSS modified imidazolium iodide liquidelectrolytes. The construction of a hybrid EC cell with a solidPOSS-based ionic liquid electrolyte is planned for future.

4. Conclusions

This study demonstrated the successful modification of1,3-alkyl imidazolium iodide ionic liquids by polyhedral oligo-meric silsesquioxanes acting as structural modifiers, leading tosolid and room temperature ionic liquids. 29Si NMR and infraredspectra confirmed the attachment of POSS molecules on theimidazolium ring via propyl groups, while the 1H NMR spectraof PEOPrImþI� IB7 T8 POSS provided the important informationthat dialkylation of the imidazol had been achieved. The chosen

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–34813480

synthesis route gave well defined compounds achieved due to theuse of trisilanol open T7 POSS molecules as starting compounds,instead of employing octameric POSS molecules and modifyingthem with imidazolium pendant groups. The yield of reactionswas up to 97%, even though a three step preparation route wasemployed

The most important finding of this work is that the particularproperties of synthesised solid (PEOPrImþI� IB7 T8 POSS) androom temperature (MePrImþIx

� IO7 T8 POSS) ionic liquids thatwere studied in detail were very similar to room temperature1,3-alkyl imidazolium iodide ionic liquids: the addition of iodineled to the formation of polyiodide species, responsible for theestablishment of a Grotthus relay type electron transportmechanism, demonstrated by examination of the correspondingRaman spectra, while we inferred from the infrared spectra adecreasing strength of interactions between the CH groups on theimidazolium cations with polyiodides. As expected, the conduc-tivity of the solid ionic liquids was in the range of 10�7 S/cm butincreased up to two decades with the formed polyiodidesalthough, surprisingly, the MePrImþ Ix

� IO7 T8 POSS analogues,despite the added iodine, remained in the same range. The latterroom temperature ionic liquid was used for construction of ahybrid electrochromic device showing adequate colouring/bleaching changes. We plan to make all-solid-state hybrid ECcells in future, due to the reversible solid–liquid transition ofPEOPrImþI� IB7 T8 POSS (155 1C) and the ability of the meltedionic liquid to form after cooling to ambient temperature trans-parent thin conductive films, a prerequisite for the construction ofhybrid EC cells with transmitted light modulation.

Acknowledgements

This work was supported by the Slovenian Research Agency(Programme P2-0213 and Project M2-0104), the CO- NOT (ProjectM2-0104) Programme and received funding from the EuropeanCommunity’s Seventh Framework Programmes (FP7) under GrantAgreement no. 200431 (Innoshade). M.C. thanks the Ministry ofHigher Education, Science and Technology for a Ph.D. Grant. Theauthors wish to thank to Dr. Andreas Georg from FraunhoferInstitute for Solar Energy Research (Freiburg, DE) for providingsputtered Pt counter electrodes.

References

[1] P. Bonhote, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Gratzel,Hydrophobic, highly conductive ambient-temperature molten salts, Inor-ganic Chemistry 35 (1996) 1168–1178.

[2] N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonhote, H. Pettersson,A. Azam, M. Gratzel, The performance and stability of ambient temperaturemolten salts for solar cell applications, Journal of the Electrochemical Society143 (1996) 3099–3108.

[3] R.D. Rauh, Electrochromic windows: an overview, Electrochimica Acta 44(1999) 3165–3176.

[4] B. Orel, A.S. Vuk, R. Jese, P. Lianos, E. Stathatos, P. Judeinstein, P. Colomban,Development of sol–gel redox I-3(-)/I- electrolytes and their application inhybrid electrochromic devices, Solid State Ionics 165 (2003) 235–246.

[5] A.S. Vuk, M. Gaberscek, B. Orel, P. Colomban, In situ resonance Ramanspectroelectrochemical studies of a semisolid redox (I-3(-)/I-) electrolyteencapsulated in a hybrid electrochromic cell, Journal of the ElectrochemicalSociety 151 (2004) E150–E161.

[6] A. Hauch, A. Georg, S. Baumgartner, U.O. Krasovec, B. Orel, New photoelec-trochromic device, Electrochimica Acta 46 (2001) 2131–2136.

[7] A. Hauch, A. Georg, U.O. Krasovec, B. Orel, Comparison of photoelectrochro-mic devices with different layer configurations, Journal of the Electrochemi-cal Society 149 (2002) H159–H163.

[8] G. Leftheriotis, G. Syrrokostas, P. Yianoulis, Development of photoelectro-chromic devices for dynamic solar control in buildings, Solar Energy Materi-als and Solar Cells 94 (2010) 2304–2313.

[9] E. Stathatos, V. Jovanovski, B. Orel, I. Jerman, P. Lianos, Dye-sensitised solarcells made by using a polysilsesquioxane polymeric ionic fluid as redoxelectrolyte, Journal of Physical Chemistry C 111 (2007) 6528–6532.

[10] H. Ohno, Electrochemical Aspects of Ionic Liquids, Wiley-Interscience, Hobo-ken, NJ, 2005, pp. 347–374.

[11] D. Baril, C. Michot, M. Armand, Electrochemistry of liquids vs. solids: polymerelectrolytes, Solid State Ionics 94 (1997) 35–47.

[12] A. Georg, A. Georg, Electrochromic device with a redox electrolyte, SolarEnergy Materials and Solar Cells 93 (2009) 1329–1337.

[13] R. Baetens, B.P. Jelle, A. Gustavsen, Properties, requirements and possibilitiesof smart windows for dynamic daylight and solar energy control in buildings:a state-of-the-art review, Solar Energy Materials and Solar Cells 94 (2010)87–105.

[14] A. Azens, E. Avendano, J. Backholm, L. Berggren, G. Gustavsson, R. Karmhag,G.A. Niklasson, A. Roos, C.G. Granqvist, Flexible foils with electrochromiccoatings: science, technology and applications, Materials Science and Engi-neering B-Solid State Materials for Advanced Technology 119 (2005)214–223.

[15] C.G. Granqvist, Oxide electrochromics: why, how, and whither, Solar EnergyMaterials and Solar Cells 92 (2008) 203–208.

[16] P. Maitra, S.L. Wunder, POSS based electrolytes for rechargeable lithiumbatteries, Electrochemical and Solid State Letters 7 (2004) A88–A92.

[17] P. Maitra, S.L. Wunder, Oligomeric poly(ethylene oxide)-functionalized sil-sesquioxanes: interfacial effects on Tg, Tm, and DHm, Chemistry of Materials14 (2002) 4494–4497.

[18] H.J. Zhang, S. Kulkarni, S.L. Wunder, Polyethylene glycol functionalizedpolyoctahedral silsesquioxanes as electrolytes for lithium batteries, Journalof the Electrochemical Society 153 (2006) A239–A248.

[19] H.J. Zhang, S. Kulkarni, S.L. Wunder, Blends of POSS-PEO(n¼4)(8) and highmolecular weight poly(ethylene oxide) as solid polymer electrolytes forlithium batteries, Journal of Physical Chemistry B 111 (2007) 3583–3590.

[20] K. Tanaka, F. Ishiguro, Y. Chujo, POSS ionic liquid, Journal of AmericanChemical Society 132 (2010) 17649–17651.

[21] J.D. Lichtenhan, J.J. Schwab, Y.-Z. An, W. Reinert, M.J. Carr, F.J. Feher, R.Terroba, Q. Liu, Process for the Formation of Polyhedral Oligomeric Silses-quioxanes, US 6972312, 2005.

[22] F.J. Feher, Polyhedral oligosilsesquioxanes and heterosilsesquioxanes, in:B. Arkles, G.L. Larson (Eds.), Silicon Compounds: Silanes and Silicones, Gelest,Inc., Morrisville, 2004, pp. 55–71.

[23] C.-M. Leu, Y.-T. Chang, K.-H. Wei, Polyimide-side-chain tethered polyhedraloligomeric silsesquioxane nanocomposites for low-dielectric film applica-tions, Chemistry of Materials 15 (2003) 3721–3727.

[24] C. Zhang, F. Babonneau, C. Bonhomme, R.M. Laine, C.L. Soles, H.A. Hristov,A.F. Yee, Highly porous polyhedral silsesquioxane polymers. synthesis andcharacterization, Journal of American Chemical Society 120 (1998)8380–8391.

[25] H. Xu, S.-W. Kuo, J.-S. Lee, F.-C. Chang, Preparations, thermal properties, and tgincrease mechanism of inorganic/organic hybrid polymers based on polyhe-dral oligomeric silsesquioxanes, Macromolecules 35 (2002) 8788–8793.

[26] B.X. Fu, W.H. Zhang, B.S. Hsiao, M. Rafailovich, J. Sokolov, G. Johansson,B.B. Sauer, S. Phillips, R. Balnski, Synthesis and characterization of segmentedpolyurethanes containing polyhedral oligomeric silsesquioxanes nanostruc-tured molecules, High Performance Polymers 12 (2000) 565–571.

[27] T.S. Haddad, J.D. Lichtenhan, Hybrid organic–inorganic thermoplastics:styryl-based polyhedral oligomeric silsesquioxane polymers, Macromole-cules 29 (1996) 7302–7304.

[28] K. Ohno, S. Sugiyama, K. Koh, Y. Tsujii, T. Fukuda, M. Yamahiro, H. Oikawa,Y. Yamamoto, N. Ootake, K. Watanabe, Living radical polymerization bypolyhedral oligomeric silsesquioxane-holding initiators: precision synthesisof tadpole-shaped organic/inorganic hybrid polymers, Macromolecules 37(2004) 8517–8522.

[29] G. Cardoen, E.B. Coughlin, Hemi-telechelic polystyrene-poss copolymers asmodel systems for the study of well-defined inorganic/organic hybridmaterials, Macromolecules 37 (2004) 5123–5126.

[30] K.-M. Kim, D.-K. Keum, Y. Chujo, Organic–inorganic polymer hybrids usingpolyoxazoline initiated by functionalized silsesquioxane, Macromolecules 36(2003) 867–875.

[31] B.-S. Kim, P.T. Mather, Amphiphilic telechelics incorporating polyhedraloligosilsesquioxane: 1 synthesis and characterization, Macromolecules 35(2002) 8378–8384.

[32] V. Ervithayasuporn, X. Wang, Y. Kawakami, Synthesis and characterization ofhighly pure azido-functionalized polyhedral oligomeric silsesquioxanes(POSS), Chemical Communications (2009) 5130–5132.

[33] D.M. Fox, P.H. Maupin, R.H. Harris, J.W. Gilman, D.V. Eldred, D. Katsoulis,P.C. Trulove, H.C. De Long, Use of a polyhedral oligomeric silsesquioxane(POSS)-imidazolium cation as an organic modifier for montmorillonite,Langmuir 23 (2007) 7707–7714.

[34] Y. Morimoto, K. Watanabe, N. OOtake, J. Inagaki, K. Yoshida, K. Ohguma,Silsesquioxane Derivatives and Process for Production Thereforeof, ChissoCorporation, US, 2007.

[35] K. Ito, M. Yamahiro, K. Watanabe, Silsesquioxane Derivative having Func-tional Group, Chisso Corporation, 2006.

[36] I. Jerman, M. Kozelj, B. Orel, The effect of polyhedral oligomeric silsesquiox-ane dispersant and low surface energy additives on spectrally selective paintcoatings with self-cleaning properties, Solar Energy Materials and Solar Cells94 (2010) 232–245.

[37] D.B. Cordes, P.D. Lickiss, F. Rataboul, Recent developments in the chemistry ofcubic polyhedral oligosilsesquioxanes, Chemical Reviews 110 (2010)2081–2173.

M. Colovic et al. / Solar Energy Materials & Solar Cells 95 (2011) 3472–3481 3481

[38] P.H. Svensson, L. Kloo, Synthesis, structure, and bonding in polyiodide andmetal iodide–iodine systems, Chemical Reviews 103 (2003) 1649–1684.

[39] P. Deplano, J.R. Ferraro, M.L. Mercuri, E.F. Trogu, Structural and Ramanspectroscopic studies as complementary tools in elucidating the nature ofthe bonding in polyiodides and in donor-I-2 adducts, Coordination ChemistryReviews 188 (1999) 71–95.

[40] T. Ueki, M. Watanabe, Macromolecules in ionic liquids: progress, challenges,and opportunities, Macromolecules 41 (2008) 3739–3749.

[41] W. Kubo, K. Murakoshi, T. Kitamura, S. Yoshida, M. Haruki, K. Hanabusa,H. Shirai, Y. Wada, S. Yanagida, Quasi-solid-state dye-sensitised TiO2 solarcells: effective charge transport in mesoporous space filled with gel electro-lytes containing iodide and iodine, The Journal of Physical Chemistry B 105(2001) 12809–12815.

[42] H. Stegemann, A. Rohde, A. Reiche, A. Schnittke, H. Fullbier, Room-tempera-ture molten polyiodides, Electrochimica Acta 37 (1992) 379–383.

[43] R. Kawano, M. Watanabe, Equilibrium potentials and charge transport of anI-/I-3(-) redox couple in an ionic liquid, Chemical Communications (2003)330–331.

[44] I. Jerman, V. Jovanovski, A.S. Vuk, S.B. Hocevar, M. Gaberscek, A. Jesih, B. Orel,Ionic conductivity, infrared and Raman spectroscopic studies of 1-methyl-3-propylimidazolium iodide ionic liquid with added iodine, ElectrochimicaActa 53 (2008) 2281–2288.

[45] V. Jovanovski, B. Orel, I. Jerman, S.B. Hocevar, B. Ogorevc, Electrochemical andin-situ Raman spectroelectrochemical study of 1-methyl-3-propylimidazo-lium iodide ionic liquid with added iodine, Electrochemistry Communica-tions 9 (2007) 2062–2066.

[46] J. Dupont, P.A.Z. Suarez, Physico-chemical processes in imidazolium ionicliquids, Physical Chemistry Chemical Physics 8 (2006) 2441–2452.

[47] C.S. Consorti, P.A.Z. Suarez, R.F. de Souza, R.A. Burrow, D.H. Farrar, A.J. Lough,W. Loh, L.H.M. da Silva, J. Dupont, Identification of 1,3-dialkylimidazoliumsalt supramolecular aggregates in solution, The Journal of Physical ChemistryB 109 (2005) 4341–4349.

[48] A. Fina, H.C.L. Abbenhuis, D. Tabuani, G. Camino, Metal functionalized POSSas fire retardants in polypropylene, Polymer Degradation and Stability 91(2006) 2275–2281.

[49] C. Bolln, A. Tsuchida, H. Frey, R. Mulhaupt, Thermal properties of thehomologous series of 8-fold alkyl-substituted octasilsesquioxanes, Chemistryof Materials 9 (1997) 1475–1479.

[50] A. Tsuchida, C. Bolln, F.G. Sernetz, H. Frey, R. Mulhaupt, Ethene and propenecopolymers containing silsesquioxane side groups, Macromolecules 30(1997) 2818–2824.

[51] J.E. Bara, C.J. Gabriel, E.S. Hatakeyama, T.K. Carlisle, S. Lessmann, R.D. Noble,D.L. Gin, Improving CO2 selectivity in polymerised room-temperature ionicliquid gas separation membranes through incorporation of polar substitu-ents, Journal of Membrane Science 321 (2008) 3–7.

[52] H.Z. Liu, S.I. Kondo, R. Tanaka, H. Oku, M. Unno, A spectroscopic investigationof incompletely condensed polyhedral oligomeric silsesquioxanes (POSS-mono-ol, POSS-diol and POSS-triol): hydrogen-bonded interaction andhost–guest complex, Journal of Organometallic Chemistry 693 (2008)1301–1308.

[53] R. Maoz, J. Sagiv, D. Degenhardt, H. Mohwald, P. Quint, Hydrogen-bondedmultilayers of self-assembling silanes: structure elucidation by combinedFourier transform infra-red spectroscopy and X-ray scattering techniques,Supramolecular Science 2 (1995) 9–24.

[54] L.J. Bellamy, The infrared spectra of complex molecules, Advances in InfraredGroup Frequencies, vol. 2, Methuen, London, 1980, pp. 276–283.

[55] M. Handke, B. Handke, A. Kowalewska, W. Jastrzebski, New polysilsesquiox-ane materials of ladder-like structure, Journal of Molecular Structure 924–926 (2009) 254–263.

[56] Z. Kebede, S.E. Lindquist, Donor–acceptor interaction between non-aqueoussolvents and I-2 to generate I-3(-), and its implication in dye sensitised solarcells, Solar Energy Materials and Solar Cells 57 (1999) 259–275.

[57] H. Wang, X. Liu, Z. Wang, H. Li, D. Li, Q. Meng, L. Chen, Effect of iodineaddition on solid-state electrolyte LiI/3-hydroxypropionitrile (1:4) for dye-sensitised solar cells, Journal of Physical Chemistry B 110 (2006) 5970–5974.

[58] E.M. Nour, L.H. Chen, J. Laane, Far-infrared and Raman-spectroscopic studiesof polyiodides, Journal of Physical Chemistry 90 (1986) 2841–2846.

[59] R.C. Teitelbaum, S.L. Ruby, T.J. Marks, A resonance Raman-iodine Mossbauerinvestigation of the starch-iodine structure—aqueous-solution and iodinevapor preparations, Journal of American Chemical Society 102 (1980)3322–3328.

[60] M.A. Tadayyoni, P. Gao, M.J. Weaver, Application of surface-enhanced raman-spectroscopy to mechanistic electrochemistry—oxidation of iodide at goldelectrodes, Journal of Electroanalytical Chemistry 198 (1986) 125–136.

[61] R. Kawano, M. Watanabe, Anomaly of charge transport of an iodide/tri-iodideredox couple in an ionic liquid and its importance in dye-sensitised solarcells, Chemical Communications (2005) 2107–2109.

[62] N. Yamanaka, R. Kawano, W. Kubo, N. Masaki, T. Kitamura, Y. Wada,M. Watanabe, S. Yanagida, Dye-sensitised TiO2 solar cells using imidazo-lium-type ionic liquid crystal systems as effective electrolytes, Journal ofPhysical Chemistry B 111 (2007) 4763–4769.

[63] N. Yamanaka, R. Kawano, W. Kubo, T. Kitamura, Y. Wada, M. Watanabe,S. Yanagida, Ionic liquid crystal as a hole transport layer of dye-sensitisedsolar cells, Chemical Communications (2005) 740–742.

[64] T. Katakabe, R. Kawano, M. Watanabe, Acceleration of redox diffusion andcharge-transfer rates in an ionic liquid with nanoparticle addition, Electro-chemical and Solid-State Letters 10 (2007) F23–F25.

[65] J. Shi, L. Wang, Y. Liang, S. Peng, F. Cheng, J. Chen, All-solid-state dye-sensitisedsolar cells with alkyloxy-imidazolium iodide ionic polymer/SiO2

nanocomposite electrolyte and triphenylamine-based organic dyes, TheJournal of Physical Chemistry C 114 (2010) 6814–6821.

[66] Z. Huo, S. Dai, C. Zhang, F. Kong, X. Fang, L. Guo, W. Liu, L. Hu, X. Pan, K. Wang,Low molecular mass organogelator based gel electrolyte with effectivecharge transport property for long-term stable quasi-solid-state dye-sensi-tised solar cells, The Journal of Physical Chemistry B 112 (2008)12927–12933.

[67] S. Katsyuba, P. Dyson, E. Vandyukova, A. Chernova, A. Vidis, Molecularstructure, vibrational spectra, and hydrogen bonding of the ionic liquid1-ethyl-3-methyl-1H-imidazolium tetrafluoroborate, Helvetica Chimica Acta87 (2004) 2556–2565.

[68] A. Elaiwi, P.B. Hitchcock, K.R. Seddon, N. Srinivasan, Y.-M. Tan, T. Welton,J.A. Zora, Hydrogen bonding in imidazolium salts and its implications forambient-temperature halogenoaluminate(III) ionic liquids, Journal of theChemical Society, Dalton Transactions (1995) 3467–3472.

[69] Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amster-dam, 1995, pp. 9–12.

[70] A. Hauch, A. Georg, Diffusion in the electrolyte and charge-transfer reactionat the platinum electrode in dye-sensitised solar cells, Electrochimica Acta 46(2001) 3457–3466.