ORIGINAL PAPER

Poly(vinyl chloride)-graft-poly(N-vinyl caprolactam) graftcopolymer: synthesis and use as template for porousTiO2 thin films in dye-sensitized solar cells

Rajkumar Patel & Sung Hoon Ahn & Won Seok Chi &Jong Hak Kim

Received: 23 June 2011 /Revised: 7 September 2011 /Accepted: 22 October 2011 /Published online: 11 November 2011# Springer-Verlag 2011

Abstract Poly(N-vinyl caprolactam) (PNVCL) side chainswere grafted to a poly(vinyl chloride) (PVC) backbonevia atom transfer radical polymerization. The synthe-sized PVC-g-PNVCL graft copolymer was templated forthe preparation of porous TiO2 thin films, which involveda sol–gel reaction and calcination process. The interactionof the carbonyl groups in the PVC-g-PNVCL with thetitania was revealed by FT-IR spectroscopy. X-ray diffrac-tion and transmission electron microscopy analysisshowed the formation of porous TiO2 thin films with theanatase phase. A series of porous TiO2 thin films withdifferent pore sizes and porosities was prepared byvarying the solution compositions and were used asphotoelectrodes in dye-sensitized solar cells (DSSC) witha polymer electrolyte. The DSSC performed best whenusing the TiO2 film with higher porosity, lower interfacialresistance, and longer electron life time. The highestenergy conversion efficiency, photovoltage (Voc), photo-current density (Jsc), and fill factor (FF) were 1.2%,0.68 V, 3.2 mA/cm2, and 0.57 at 100 mW/cm2, respec-tively, for the quasi-solid state DSSC with a 730-nm-thickTiO2 film.

Keywords Graft copolymer . Atom transfer radicalpolymerization . Polymer electrolyte . Dye-sensitized solarcell . Sol–gel

Introduction

A technologically interesting application of nanocrystallinetitania (TiO2) is the dye-sensitized solar cell (DSSC) thatwas introduced by O’Regan and Grazel in 1991 [1]. Anenergy conversion efficiency of about 11% has beenachieved in the DSSC using an organic, liquid-basedelectrolyte contaning triiodide/iodide, i.e., I3

−/I−, as a redoxcouple. Recently, solid- or quasi-solid state DSSCs havebeen developed as an alternative to liquid electrolytes usinghole conductors [2–6], gel electrolytes [7–9], and quasi-solid [10–12] or solid polymer electrolytes [13–15]. Thestructure and morphology of the TiO2 electrode areimportant factors in determining photoelectron transportand dye adsorption [16]. The physical properties of TiO2

mainly depend on the morphology, type, and size of itscrystallites.

There are several methods for synthesizing TiO2 nano-particles such as the hydrothermal process [17], electro-chemical method [18], surfactant-directed approaches [19],and sol–gel approaches [20, 21]. The hydrothermal processis carried out at higher temperatures and often results in theagglomeration of nanoparticles. Thus, stabilizing agents areused to prevent this agglomeration. Sol–gel processesare commonly used to fabricate targeted material sincetheir structural and morphological characteristics can beeasily controlled. In this method, the calcination step isthe most important in converting amorphous titania tocrystalline form and eliminating organic residue. Duringthe calcination process, however, nanoparticles arecommonly agglomerated. To avoid these hurdles,structure-directing agents such as surfactants or amphiphilicblock copolymers are often introduced to prepare well-defined, porous TiO2 films [22–30].

R. Patel : S. H. Ahn :W. S. Chi : J. H. Kim (*)Department of Chemical and Biomolecular Engineering,Yonsei University,262 Seongsanno, Seodaemun-gu,Seoul 120-749, South Koreae-mail: jonghak@yonsei.ac.kr

Ionics (2012) 18:395–402DOI 10.1007/s11581-011-0641-4

Block copolymers are of particular interest for thepreparation of nanoporous materials due to the abilityto precisely control their pore size, uniformity, andinterconnectivity. These are used as structure-directingagents for semiconductor nanoparticles [25], nanowires[26], porous carbon films [27], and nano thin films [28].Graft copolymers have also received attention as templatesfor the synthesis of porous inorganic films due to theirsimple synthesis procedures [29–32]. In our earlier reports,poly(vinylidene fluoride-co-chlorotrifluoroethylene)-graft-poly(oxyethylene methacrylate) (P(VDF-co-CTFE)-g-POEM)[31] and poly(vinyl chloride)-graft-poly(oxyethylene methac-rylate) (PVC-g-POEM) [32] were used as structure-directingagents to synthesize mesoporous TiO2 films.

In general, ethylene oxide moieties containing ethergroups are used as hydrophilic domains in amphiphiliccopolymer templates, e.g., polystyrene-block-poly(ethyleneoxide) (PS-b-PEO) [33] or polyisoprene-block-poly(ethyl-ene oxide) (PI-b-PEO) [34]. This is because the ethyleneoxide moieties can selectively confine the TiO2 precursorand its hydrolysis product. According to a previous paper[35], however, the interaction of transition metals withamide groups is stronger than that with ether groups. Amore favorable interaction may allow a precisely controlledsol–gel process to occur on the formation of a TiO2 film.

In this study, thus, poly(vinyl chloride)-graft-poly(N-vinylcaprolactam) (PVC-g-PNVCL) graft copolymer containingamide groups was synthesized using atom transfer radicalpolymerization (ATRP). The graft copolymer was combinedwith a TiO2 precursour, i.e., titania isopropoxide (TTIP) andcalcinated at high temperature to form porous TiO2 thin films.The TiO2 thin films were characterized by transmissionelectron microscopy (TEM), UV–visible spectroscopy, andX-ray diffraction (XRD). Finally, the porous TiO2 thin filmswere used as photoelectrodes for a quasi-solid state, dye-sensitized solar cell (DSSC) empolying a polymer electrolyte.

Experiment

Materials

Poly(vinyl chloride) (PVC, Mw=45,000 g/mol, Mn=22,000 g/mol), N-vinyl caprolactam (NVCL), 1,1,4,7,10,10-

hexamethyltriethylene tetramine (99%), copper(I) chloride(CuCl, 99%), fumed silica nanoparticles (SiO2, 14 nm), poly(ethylene glycol dimethyl ether) (PEGDME,Mn=500 g/mol),lithium iodide (LiI), iodine (I2), alumina (Al2O3), titanium(IV) bis(ethyl acetoacetato) diisopropoxide, chloroplatinicacid hexahydrate (H2PtCl6), titanium(IV) isopropoxide(97%), hydrogen chloride solution (HCl, 35 wt.%), andsodium hydroxide solution (NaOH, 0.1 N) were purchasedfrom Aldrich. Distilled water was obtained from a waterpurification system made by Millipore Corporation. 1-Methyl-3-propyl imidazolium iodide (MPII, C7H13N2I) andruthenium dye (535-bisTBA, N719) were purchased fromSolaronix, Switzerland. All solvents, such as tetrahydrofuran(THF), N-methyl pyrrolidone (NMP), acetonitrile (CH3CN,99.9%), butanol (C4H10O, 99.9%), 2-propanol (C3H7OH,99.9%), chloroform (CHCl3, 99.9%), methanol and ethanol(C2H6O, 99.9%), were purchased from J.T. Baker. Fluorine-doped tin oxide (FTO)-conducting glass substrate (TEC8,8 Ω/sq, 2.3 mm in thickness) was purchased from Pilkington,France.

Synthesis of PVC-g-PNVCL graft copolymer

Three grams of PVC was dissolved in 90 mL of NMP and18 g of NVCL was added to the PVC solution. Aftermixing in a round-bottom flask, 0.1 g of CuCl catalyst and0.3 mL of HEMTA ligand were added, followed by N2

purge for 30 min. Then, the temperature was raised to 90 °Cwhile stirring in order to homogenize the mixture and thereaction continued for 44 h. The resultant products werediluted with THF and passed through an activated Al2O3

column to remove the catalyst and then precipitated inmethanol. The products were purified by dissolving them inTHF and were then re-precipitated in methanol. The productswere dried completely overnight in a vacuum oven at 50 °C.

Preparation of counter electrodes

FTO coated with platinum were used as counterelectrode. The glasses were cleaned by sonication inisopropanol and then in chloroform. The counterelectrodes were prepared by spin coating 4 wt. %H2PtCl6 propanol solution onto the conductive FTO glassand sintering at 450 °C for 30 min.

Table 1 Composition of porousTiO2 films templated by thePVC-g-PNVCL graft copolymervia sol–gel reaction

Sample ID Polymer (g) THF (mL) TTIP (mL) HCl (mL) DI water (mL)

TiO2-1 0.05 1.5 0.2 0.1 –

TiO2-2 0.05 1.5 0.2 0.05 –

TiO2-3 0.05 2.0 0.2 0.1 –

TiO2-4 0.05 2.0 0.2 0.05 –

TiO2-5 0.05 1.5 0.2 0.1 0.05

396 Ionics (2012) 18:395–402

Preparation of porous TiO2 thin films

Transparent glass coated with conductive FTO was used asa photoelectrode. The neat glass was cleaned by sonicationin isopropanol and then in chloroform. The clean conductingsurface of the FTO glass was blocked by a layer of titanium(IV) bis (ethyl acetoacetato) diisopropoxide using spincoating, followed by sintering at 450 °C for 30 min. Then,0.1 mL of HCl (37%) was added slowly to 0.2 mL TTIPsolution in THF under vigorous stirring. Separately, 0.05 g ofPVC-g-PNVCL graft copolymer was dissolved in 1.5 mL ofTHF and slowly added to the TTIP/HCl/THF solution tomake a clear solution. The solution was aged by stirring at

ambient temperature for at least 3 h. Five differentcompositions were prepared (presented in Table 1). Thefilms were deposited onto an FTO conducting glass using anSMSS Delta 80BM spin coater at 2,000 rpm for 30 s. Uponcalcination at 500 °C for 30 min, the organic chemicals werecompletely removed to produce mesoporous TiO2 thin films.The mesoporous TiO2 layers were then sensitized with a10−4 mol dm−3 alcoholic ruthenium solution at 50 °C for 2 hin darkness. Finally, the dye-sensitized photoelectrodes wererinsed with absolute ethanol and dried in a vacuum oven.

Fabrication of DSSC

DSSCs with an active area of 0.4 cm2 were constructed bydrop-casting of the polymer electrolyte solution onto thephotoelectrode and then covering this with the counterelectrode according to the following procedure [32, 36–38].The polymer electrolyte solution was prepared by dissolvingPEGDME, SiO2, MPII, and I2 in THF. The mole ratio ofether oxygen to iodide salt was fixed at 20, and the iodinecontent was fixed at 10 wt. % with respect to the salt [37].

Scheme 1 Interaction between the PVC-g-PNVCL graft copolymerand TiO2 via sol–gel

4000 3500 3000 2500 2000 1500 1000

TiO2-2

TiO2-1

TiO2-5

TiO2-4

PVC-PNVCL

Ab

sorb

ance

(a.

u.)

Wavenumber (cm-1)

1679

1625

TiO2-3

3149

3236

Fig. 1 FT-IR spectra of the PVC-g-PNVCL/TTIP composite withdifferent compositions Fig. 2 FE-TEM images of porous TiO2 films: a TiO2-1 and b TiO2-3

Ionics (2012) 18:395–402 397

After casting the polymer electrolyte solution, the solventwas evaporated very slowly to allow penetration of theelectrolytes through the mesopores of the TiO2 layer. Bothelectrodes were then superposed and pressed between twoglass plates to build a thin electrolyte layer. The cells wereplaced in a vacuum oven for 1 day for complete evaporationof the solvent.

The photovoltaic performance, including the short-circuitcurrent (Jsc, mA/cm2), open-circuit voltage (Voc, V), fillfactor (FF), and overall energy conversion efficiency (η), wasmeasured using a Keithley Model 2400 source meter and a1,000 W xenon lamp (Oriel, 91193). The light washomogeneous to an area of 8×8 in.2 and its intensity wascalibrated with a Si solar cell (Fraunhofer Institute for

Solar Energy Systems, Mono-Si + KG filter, CertificateNo. C-ISE269) to 1 sunlight intensity (100 mW/cm2).The intensity was verified with a NREL-calibrated Si solarcell (PV Measurements Inc.).

Characterization

Fourier transfer infrared (FT-IR) spectra of the sampleswere collected with an Excalibur series FT-IR instrument(DIGLAB Co., Hannover, Germany) in the frequency rangeof 4,000–600 cm−1 with an attenuated total reflectionfacility. XRD measurements were carried out on a RigakuRINT2000 wide-angle goniometer with a Cu cathodeoperated at 40 kV and 300 mA. UV–visible spectroscopy

PVC-g-PNVCL Porous TiO2 film

Sol-gel

calcination

Scheme 2 Schematic represen-tation of the morphologies of thePVC-g-PNVCL graft copolymerand the TiO2 porous films

Fig. 3 SEM images of porous TiO2 film templated by the graft copolymer: a TiO2-1, b TiO2-2, c TiO2-4, and d cross-sectional image of theTiO2-1 film

398 Ionics (2012) 18:395–402

was performed with a spectrophotometer (Shimazu) in therange of 200 to 800 nm. After drop-casting of the dilutesolution onto a standard copper grid, energy-filteringtransmission electron microscope (EF-TEM) pictures wereobtained using a Philips CM30 microscope operating at300 kV. Morphological characterization of the porous TiO2

film was carried out using a field-emission scanningelectron microscope (FE-SEM, S-4700, Hitachi). Theporous TiO2 films were characterized by FE-TEM, XRD,and SEM.

Dye loading measurement

In order to measure the amounts of dye adsorbed onto theTiO2 photoelectrode, the N719 dye-sensitized TiO2 photo-electrode was dipped into 10 mL of a 10−2 M solution ofNaOH in an ethanol–H2O mixture (1:1). This mixture wasstirred until complete desorption of the N719 dye into theliquid took place. The volume of NaOH solution containingthe fully desorbed dye was carefully measured by UV–visible spectroscopy (Hewlett–Packard, Hayward, CA,USA). The absorption value at 515 nm was used to

calculate the number of adsorbed N719 dye moleculesaccording to the Beer-Lambert law:

A ¼ "lc ð1Þwhere A is the absorbance of UV–visible spectra at a 515-nmwavelength, the molar extinction coefficient of the dye at515 nm is ε=14,100/M cm, l is the path length of the lightbeam, and c is the concentration of the dye.

Results and discussion

Formation of porous TiO2 thin films

Scheme 1 shows the hydrogen bonding interaction betweenthe graft copolymer and TiO2. The specific interaction leadsto the selective affinity between TiO2 and the PNVCLdomains of the PVC-g-PNVCL graft copolymer, resultingin a specific film morphology formation [34, 39]. Thehydrogen bonding interaction between the hydroxyl groupof the TiO2 and the carbonyl group of the PNVCL chainswas investigated by FT-IR spectroscopy. Figure 1 shows theFT-IR spectra of PVC-g-PNVCL/TTIP with different

300 400 500 600 700

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Ab

sorb

ance

(a.

u.)

Wavelength (nm)

TiO2-1TiO

2-2

TiO2-3 TiO

2-4

TiO2-5

Fig. 5 UV–visible spectra of dye adsorption onto TiO2 photoelectrodes

Table 2 Photovoltaic performances of DSSCs fabricated using the porous TiO2 films templated by the PVC-g-PNVCL graft copolymer and thepolymer electrolyte consisting of PEODME/SiO2/MPII/I2 at 100 mW/cm2

TiO2 electrode Voc (V) Jsc (mA/cm2) FF η (%) R1 (Ω) R2 (Ω) Dye adsorption (nmol/cm2) Electron lifetime (ms)

TiO2-1 0.68 3.2 0.57 1.2 97 14 66.8 6.9

TiO2-2 0.59 3.3 0.58 1.1 125 44 64.2 6.3

TiO2-3 0.53 2.2 0.57 0.8 128 28 50.4 6.0

TiO2-4 0.60 2.5 0.55 0.8 136 16 25.4 5.6

TiO2-5 0.60 2.4 0.54 0.8 187 40 18.8 5.3

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

1

2

3

4

TiO2-5

TiO2-4

TiO2-3

TiO2-1

Ph

oto

curr

ent

den

sity

(m

A/c

m2 )

Voltage (V)

TiO2-2

Fig. 4 J–V curves for DSSCs fabricated using the porous TiO2 filmstemplated by the PVC-g-PNVCL graft copolymer and the polymerelectrolyte consisting of PEODME/SiO2/MPII/I2 at 100 mW/cm2

Ionics (2012) 18:395–402 399

weight ratios after drying at 50 °C for 24 h. The presence ofthe broad peak in TiO2-3 at 3,236 cm−1 is due to thestretching vibration of the Ti–OH groups. The hydrogenbonding interaction between the carbonyl group of thePNVCL chains and the hydroxyl group of the TiO2 resultedin a lowering of the stretching vibration peak of the carbonyl(−C = O) group from 1,679 to 1,625 cm−1. Similarly, thestretching vibration of the hydroxyl group at 3,236 cm−1 forTiO2-3 was shifted to the lower wavenumber of 3,149 cm−1

in the TiO2-2 sample. This indicates the presence of ahydrogen bonding interaction between the hydroxyl group ofthe TiO2 and the carbonyl group of the PNVCL moiety [40].

Figure 2 shows the FE-TEM images of the TiO2-1 andTiO2-3 samples. The darker spherical regions in the imagesrepresent the TiO2 nanoparticles formed after the hydrolysisand condensation of TTIP in the presence of THF/HCl/PVC-g-PNVCL with a size of 10–20 nm. With the higherviscosity and HCl concentration, TiO2-1 electrode hashigher porosity than the rest. Figure 3 shows the SEMimages of the TiO2 porous films templated by PVC-g-PNVCL graft copolymer, which involved a sol–gel processbased on the hydrolysis and condensation of TTIP andcalcination at 450 °C. The cylindrical, worm-like micellesformed around the PVC chains were burned duringcalcination to form pores. Thus, the morphologies of thePVC-g-PNVCL graft copolymer and TiO2 porous films canbe illustrated as in Scheme 2. As the HCl amount wasreduced (TiO2-2) or the THF amount was increased (TiO2-4), the size of the TiO2 nanoparticles was also reduced. Inthe TiO2-1 sample, the pore size was larger than 50 nm andwas reduced to less than 30 nm in the TiO2-2 sample. Byfurther dilution of the TiO2-1 sol–gel solution and byreducing the amount of acid by half, the pore size wasfurther reduced to less than 25 nm (TiO2-4). This is a resultof the reduced size of the micelle caused by the lowerpolymer concentration, lower hydrolysis, and the reducedcondensation rate resulting from the presence of less acid.Thus, the component in the sol–gel solution as well as themorphology of graft copolymer played a major role indetermining the TiO2 morphology. The reactivity for thesol–gel process was in turn governed by the viscosity aswell as the acid concentration because TTIP was selectivelyembedded into the hydrophilic PNVCL domains of thegraft copolymer through hydrogen bonding [33, 41]. Thecross-sectional thickness of the TiO2-1 film was determinedto be around 732 nm.

The structural pattern of crystallized TiO2 porous filmstemplated by PVC-g-PNVCL was investigated by XRDanalysis. The sharp crystalline peaks at 25.3°, 48.2°, and54.5° (2θ) are assigned to the (101), (200), and (211)planes, respectively, of the anatase TiO2 phase [34, 42]. Theremaining peaks arose from the bare FTO glass. This resultindicates that the structural changes and phase transforma-

tion of TiO2 to crystalline anatase occurred around 450 °C.According to the Scherrer equation [43], the crystallite sizeof the TiO2 particles was determined from the diffractionpeak broadening: D=Kλ/β cosθ, where D is the crystallitesize of the particles, λ is the X-ray wavelength, β is the fullwidth at half-maximum of the (101) crystalline peak, K=0.9 is a coefficient, and θ is the diffraction angle. Theaverage crystallite sizes of the TiO2 porous films werecalculated to be 17.5, 16.8, and 18.3 nm for the TiO2-1,TiO2-2, and TiO2-3 samples, respectively. The opticaltransmittance spectra of different TiO2 photoelectrodeswere also measured. The average transmittance of allelectrodes was above 90%, and the TiO2-1 electrode had atransmittance of above 95%.

Performance of DSSC

There are some advantages of a solid- or quasi-solid stateDSSC over a liquid-based device in terms of flexibility,

0 30 60 90 120 150 180 210 240

0

20

40

60

Imag

inar

y re

sist

ance

/oh

m

Real resistance/ohm

TiO2-1

TiO2-2

TiO2-3

TiO2-4 TiO

2-5

(a)

10-2 10-1 100 101 102 103 104 105 106-20

-10

0

10

20

30

40

50

60

Ph

ase/

deg

Frequency/Hz

TiO2-1

TiO2-2

TiO2-3

TiO2-4

TiO2-5

(b)

Fig. 6 a Nyquist plots and b Bode phase plots of DSSCs fabricatedusing porous TiO2 photoelectrodes and the polymer electrolyteconsisting of PEODME/SiO2/MPII/I2 at 100 mW/cm2

400 Ionics (2012) 18:395–402

lightness, and long-term stability. Thus, a polymer electro-lyte consisting of PEGDME, SiO2, MPII, and I2 wasemployed as a quasi-solid state electrolyte to fabricate thesolar cell, and the cell’s performance was measured usingcurrent–voltage test under illumination (presented inFig. 4). The characteristics of the DSSCs, such as Voc, Jsc,FF, and efficiency (η), are summarized in Table 2 andstrongly dependent on the morphology of the TiO2 film.The DSSC fabricated from the TiO2-1 film exhibited thehighest efficiency of 1.2% with a Jsc of 3.2 mA/cm2, Voc of0.68 V, and FF of 57%. This performance was also comparedwith the dye adsorption values (presented in Fig. 5 andTable 2). It was found that the dye adsorption of the TiO2-1photoelectrode was the highest, which is attributed to thehigher porosity and larger transmittance [44].

The internal resistances and electron transport kinetics inthe DSSCwere investigated using electrochemical impedancespectroscopy analysis. Figure 6a shows the Nyquist plots forthe DSSCs with different photoelectrodes measured at100 mW/cm2. All DSSC spectra exhibited three semicircles,which were assigned to electrochemical reactions at the Ptcounter electrode, the charge transfer at the TiO2/dye/electrolyte, and the Warburg diffusion process of I3

−/I−.The first semicircle in the high frequency range was smalland primarily related to the sheet resistance of the FTO. Itremained nearly constant regardless of the system. Thesecond semicircle was attributed to the impedance in thecounter electrode/redox electrolyte interface (R1). The largestsemicircle in the low frequency range represents theimpedance associated with the charge transfer between thedye-sensitized TiO2 and the electrolyte interface (R2). Thecharge transport resistance at the TiO2/dye/electrolyte inter-face increased from the TiO2-1 to the TiO2-5 electrode. Thisis consistent with the cell efficiency; the TiO2-1 electrodewith the lowest resistance showed the highest efficiencyamong the samples. Similarly, the characteristic frequencypeaks (10−2–106 Hz) in the Bode phase plots are shown inFig. 6b. The characteristic frequency (fmin) is related to theinverse of the recombination lifetime or electron lifetime (τr)in the TiO2 film [45–47] according to the following relation(τr=1/ωmin=1/2πfmin). This result shows that the TiO2-1photoelectrode has a longer electron lifetime of about 6.9 ms.This result is in agreement with the DSSC performanceresults showing that the TiO2-1 photoelectrode had thehighest efficiency, resulting from its higher porosity, lowerinterfacial resistance, and longer electron life time.

Conclusions

PVC-g-PNVCL graft copolymers with different graftingratios were synthesized by the ATRP method and charac-terized by FT-IR and NMR spectroscopy. The self-assembly

behavior of the graft copolymer was analyzed by TEManalysis. The porous TiO2 thin films were prepared bytemplating the PVC-g-PNVCL graft copolymer using asol–gel synthesis and calcination process. The structure andmorphology of the anataze TiO2 films were characterizedby XRD, TEM, and SEM analysis. The resultant porousTiO2 thin films were used as photoelectrodes for fabricatingquasi-solid state DSSCs, and the maximum energy conver-sion efficiency, Voc, Jsc, and FF reached 1.2%, 0.68 V,3.2 mA/cm2, and 0.57 at 100 mW/cm2, respectively, uponusing the 730-nm-thick TiO2 film with higher porosity,lower interfacial resistance, and longer electron life time.

Acknowledgements This work was supported by a NationalResearch Foundation (NRF) grant funded by the Korean government(MEST) through the Pioneer Research Center Program (2008–05103),the Ministry of Knowledge Economy (MKE) through the HumanResources Development of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) (20104010100500), and the MKEand Korea Institute for Advancement in Technology (KIAT) throughthe Workforce Development Program in Strategic Technology.

References

1. O'Reagan B, Grätzel M (1991) Nature 353:7372. Li B, Wang L, Kang B, Wang P, Qiu Y (2006) Sol Energy Mater

Sol Cell 90:543. Beltran EL, Prené P, Boscher C, Belleville P, Buvat P, Lambert S,

Guillet F, Boissière C, Grosso D, Sanchez C (2006) Chem Mater18:6152

4. Tan S, Zhai J, Wan M, Meng Q, Li Y, Jiang L, Zhu D (2004) JPhys Chem B 108:18693

5. Tan S, Zhai J, Xue B, Wan M, Meng Q, Li Y, Jiang L, Zhu D(2004) Langmuir 20:2934

6. Kroeze JE, Hirata N, Schmidt-Mende L, Orizu C, Ogier SD, CarrK, Gratzel M, Durrant JR (2006) Adv Funct Mater 16:1832

7. Wu J, Lan Z, Lin J, Huang M, Hao S, Sato T, Yin S (2007) AdvMater 19:4006

8. Xia J, Li F, Huang C, Zhai J, Jiang L (2006) Sol Energy Mater SolCell 90:944

9. Wang P, Zakeeruddin SM, Exnar I, Grätzel M (2002) ChemCommun 2972

10. Wang P, Zakeeruddin SM, Comte P, Exnar I, Gratzel M (2003) JAm Chem Soc 125:1166

11. Sakaguchi S, Ueki H, Kato T, Kadoa T, Shiratuchi R, TakashimaW, Kaneto K, Hayase S (2004) J Photochem Photobiol A: Chem164:117

12. Lu S, Koeppe R, Gunes S, Sariciftci NS (2007) Sol Energy MaterSol Cell 91:1081

13. Kim JH, Kang MS, Kim YJ, Won J, Park NG, Kang YS (2004)Chem Commun 14:1662

14. Kang MS, Kim JH, Won J, Kang YS (2007) J Phys Chem C111:5222

15. Kalaignan GP, Kang M, Kang YS (2006) Solid State Ionics177:1091

16. Taslim R, Rahman MYA, Salleh MM, Umar AA, Ahmad A(2010) Ion 16:639

17. Kotani Y, Matoda T, Matsuda A, Kogure T, Tatsumisago M,Minami T (2001) J Mater Chem 11:2045

18. Chettah H, Abdi D, Amardjia H, Haffar H (2009) Ion 15:169

Ionics (2012) 18:395–402 401

19. Yang PD, Zhao DY, Margolese DI, Chmelka BF, Stucky GD(1999) Chem Mater 11:2813

20. Lu YF, Ganguli R, Drewien CA, Anderson MT, Brinker CJ, GongWL, Guo YX, Soyez H, Dunn B, Huang MH, Zink JI (1997)Nature 389:364

21. Kumar RV, Raza G (2009) Ion 15:57922. Förster S, Antonietti M (1998) Adv Mater 10:19523. Kim HC, Jia X, Stafford CM, Kim DH, McCarthy TJ, Tuominen

M, Hawker CJ, Russell TP (2001) Adv Mater 13:79524. Urbas AM, Maldovan M, DeRege P, Thomas EL (2002) Adv

Mater 14:185025. Yeh SW, Wei KH, Sun YS, Jeng US, Liang KS (2003)

Macromolecules 36:790326. Fahmi AW, Braun HG, Stamm M (2003) Adv Mater 15:120127. Liang C, Hong K, Guiochon GA, Mays JW, Dai S (2004) Angew

Chem 116:578528. Melde BJ, Burkett SL, Xu T, Goldbach JT, Russell TP, Hawker CJ

(2005) Chem Mater 17:474329. Lee KJ, Park JT, Goh JH, Kim JH (2008) J Polym Sci A: Polym

Chem 46:391130. Koh JK, Seo JA, Koh JH, Kim JH (2009) Mater Lett 63:136031. Koh JH, Seo JA, Ahn SH, Kim JH (2010) Thin Solid Films

519:15832. Ahn SH, Koh JH, Seo JA, Kim JH (2010) Chem Commun

46:193533. Cheng YJ, Gutmann JS (2006) J Am Chem Soc 128:4658

34. Nedelcu M, Lee J, Crossland EJW, Warren SC, Orilall MC,Guldin S, Huttner S, Ducati C, Eder D, Wiesner U, Steiner U,Snaith HJ (2009) Soft Matter 5:134

35. Kim JH, Min BR, Won J, Joo SH, Kim HS, Kang YS (2003)Macromolecules 36:6183

36. Park JT, Roh DK, Patel R, Kim E, Ryu DY, Kim JH (2010) JMater Chem 20:8521

37. Roh DK, Park JT, Ahn SH, Ahn H, Ryu DY, Kim JH (2010)Electrochim Acta 55:4976

38. Ahn SH, Jeon H, Son KJ, Ahn H, Koh WG, Ryu DY, Kim JH(2011) J Mater Chem 21:1772

39. Martinez G, Gomez MA, Gomez R, Segura JL (2007) J Polym SciA: Polym Chem 45:5408

40. Li R, Nie K, Shen X, Wang S (2007) Mater Lett 61:136841. Alberius PCA, Frindell KL, Hayward RC, Kramer EJ, Stucky GD,

Chmelka BF (2002) Chem Mater 14:328442. Falaras P, Stergiopoulos PT, Tsoukleris DS (2008) Small 4:77043. Hamal DB, Klabunde KJ (2007) J Colloid Interf Sci 311:51444. Wang ZS, Kawauchi H, Kashima T, Arakawa H (2004) Coord

Chem Rev 248:138145. Schlichthorl G, Huang SY, Sprague J, Frank AJ (1997) J Phys

Chem B 101:814146. Schlichthorl G, Park NG, Frank AJ (1999) J Phys Chem B

103:78247. Kern R, Sastrawan R, Ferber J, Stangl R, Luther J (2002)

Electrochim Acta 47:4213

402 Ionics (2012) 18:395–402