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Synthesis and Photovoltaic Properties of Side-Chain Liquid-Crystal Click Polymers for Dye-Sensitized Solar-Cells Application Jin Su Park, Yeol Ho Kim, Myungkwan Song, Chul-Hyun Kim, Md. Anwarul Karim, Jae Wook Lee, Yeong-Soon Gal, Pankaj Kumar, Shin-Woong Kang,* Sung-Ho Jin*

2 Current density> mA/cm @

SCLCPs are synthesized using ‘‘click chemistry’’. The resulting polymers, P1 and P2, have good solubilities and molecular-weight distributions. Their Mw and polydispersities are in the ranges of 26.7–8.4 103 g mol1 and 1.99–1.29, respectively. DSC and POM studies reveal that -12 both polymers exhibit liquid-crystalline behavior. P1 and P2 are found to display blue emission. DSSCs are fabri-10 cated using P1 and P2 as matrices for electrolytes. The -8 maximum PCE of the P1- and P2-based polymer electro-6 lytes is 4.11% (at 1 sun). This synthesis route has again -4 P1 proven to be a useful synthetic methodology for fabriP2 -2 PAN cating SCLCPs that are promising materials for device 0 applications. 2

0.0

0.2

0.4

0.6

Bias[V]

Introduction Since the time they were first described in 1991 by O’Regan and Gra ¨tzel,[1] dye-sensitized solar cells (DSSCs) have aroused intensive interest, owing to their low cost, simple S.-H. Jin, J. S. Park, Y. H. Kim, M. Song, C.-H. Kim, Md. A. Karim Department of Chemistry Education, Interdisciplinary Program of Advanced Information and Display Materials, and Center for Plastic Information System, Pusan National University, Busan 609-735, Korea Fax: þ8251 581 2348; E-mail: [email protected] S.-W. Kang, P. Kumar Department of BIN Fusion Technology, Chonbuk National University, Jeonju 561-756, Korea Fax: þ8263 270 4254; E-mail: [email protected] J. W. Lee Department of Chemistry, Dong-A University, Busan 604-714, Korea Y.-S. Gal Polymer Chemistry Lab., Kyungil University, Hayang 712-701, Korea

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preparation and their application in devices that can potentially be used to resolve global-warming and environmental-pollution problems.[2] DSSCs based on liquid electrolytes currently achieve a power-conversion efficiency (PCE) of about 11%.[3] However, potential problems caused by liquid electrolytes such as leakage and volatilization of the organic solvents stand as critical hurdles to their long-term performance and practical use. As a result, systems based on solid-state and quasi-solid-state electrolytes, including organic hole conductors, inorganic p-type semiconductors and polymer electrolytes, have been proposed as alternatives to liquid electrolytes.[4] Among these, polymer electrolytes are considered to be one of the most-promising classes of substitutes for liquid electrolytes in the fabrication of practical DSSCs. This is a consequence of their meritorious features, which include high ionic conductivity, good interfacial filling properties and relatively high long-term stability.[5] One of the ultimate goals of polymer chemists is to synthesize multifunctional polymers that have welldefined properties and that are ideally suited to specific

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DOI: 10.1002/macp.201000264

Synthesis and Photovoltaic Properties of Side-Chain Liquid-Crystal Click . . .

applications. Control of the properties of the polymers can often be achieved by manipulation of the polymer architecture, which is in turn governed by the polymer’s topology, composition, functionality (e.g., the variety in functional groups and the degree of functionalization), the form of the molecular-weight distributions (e.g., narrow, broad, multimodal) and the morphology. All of these features, with the possible exception of the morphology, in principle, are controllable by utilizing appropriate polymerization processes. Recently, ‘‘click chemistry’’, introduced by Sharpless[6] and his colleagues, has led to a paradigm shift in the way synthetic problems in polymer and material science are addressed. Click reactions have gained great attention, mainly as a result of their high specificities, quantitative yields and near-perfect chemoselectivities in the presence of most functional groups. The most-popular reaction used in click chemistry is Huisgen’s dipolar cycloaddition of azides and alkynes, leading to 1,2,3-triazole-linked products. This methodology has been applied widely in the fields of organic chemistry,[7] supramolecular chemistry,[8] drug discovery,[9] bioconjugation,[10] materials science,[11–14] and dye-sensitized solar cells (DSSCs).[15–17] Currently, 1,3-dipolar-cycloaddition-based click chemistry has also been the focus of efforts aimed at the synthesis of liquid-crystal polymers.[18] However, the preparation of side-chain liquid-crystal polymers (SCLCPs) by using click chemistry has not been explored. Liquid-crystal polymers (LCPs) have advantageous features that result from a combination of anisotropic and attractive bulk properties, as well as new possibilities for polymer processing. In the family of LCPs, SCLCPs are of special interest owing to their potential application in numerous areas, such as datastorage systems, nonlinear optical systems, and pyro-, piezo-, and ferroelectric devices.[19–21] In an extension of our earlier work,[15,16] we have developed a new methodology for the synthesis of SCLCPs that relies on the use of click chemistry and have fully characterized the properties of the resulting polymers. Also, for the first time, the SCLCPs created in this way have been employed as polymer matrices in DSSCs, having the configuration: SnO2:F/TiO2/N719 dye/polymer electrolyte/Pt. Finally, the performances of the new DSSCs have been compared to systems based on polyacrylonitrile (PAN). The results of these efforts are described below.

Experimental Part Materials All of the reagents used were purchased from Sigma–Aldrich Co. and used without further purification. The solvents were purified using normal procedures and were handled in a moisture-free atmosphere. Column chromatography was carried out using silica Macromol. Chem. Phys. 2010, 211, 2464–2473 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

gel (Merck, 250–430 mesh). Conventional Schlenk techniques were used and the reactions were carried out under a N2 atmosphere unless otherwise noted.

Characterization The 1H NMR spectra were recorded on a Bruker AM-300 spectrometer and the chemical shifts were recorded in ppm units with chloroform as an internal standard. The absorption and photoluminescence (PL) spectra were measured using a Shimadzu UV3100 UV-vis spectrometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively. The solid-state-emission measurements were carried out by supporting each film on a quartz substrate that was mounted to receive front-face excitation at an angle of < 458. Each polymer film was excited with several portions of the visible spectrum from a xenon lamp. The molecular weights and polydispersities of the polymers were determined by gelpermeation chromatography (GPC) using a Pl gel 5 mm MIXED-C column on an Agilent 1100 series liquid-chromatography system with tetrahydrofuran (THF) as the eluent and calibration with polystyrene standards. Thermal analyses were carried out on a Mettler Toledo TGA/SDTA 851e, DSC 822e analyzer under an N2 atmosphere at a heating rate of 10 8C min1. Polarized optical microscopy (POM) (Axiolab Pol, Carl Zeiss Co.) was used for observation of the thermotropic behavior and texture of monomers and polymers at 100 pixels.

Synthesis of the Monomers 2,7-Diazido-9,9-dioctylfluorene (M1),[15] bis(4-methoxy-40 -hexyloxybiphenyl) dipropargyl malonate (M2)[22,23] and bis(4-cyano40 -hexyloxybiphenyl)dipropargyl malonate (M3)[24] were synthesized according to the reported literature with slight modifications.

Synthesis of SCLCPs by Click Chemistry In a flame-dried Schlenk flask, diazide- and diacetylene-based monomers (1:1 equiv.) and sodium L-ascorbate (10 mol-%) were dissolved in THF (2–3 mL) under a N2 atmosphere and triethylamine (TEA) (0.2–0.3 mL) was added as a ligand.[15] The flask was flushed with N2 for 20–30 min. The mixture was frozen and evacuated three times; this was followed by the addition of CuSO4 5H2O (5 mol-%) under a flow of N2 atmosphere. The polymerization mixture was stirred at 30–35 8C for 48 h. The THF was removed under vacuum and the mixture was dissolved in chloroform and washed with an aqueous NH4OH solution followed by water. NH4OH or KCN solutions are better to remove the copper metal completely from the resulting polymer. To ensure environmental friendliness, we used an NH4OH solution and washed several times followed by water. The organic layer was separated and the solvent was removed. The resulting polymer was precipitated into methanol. In the above procedure, the reaction will not proceed if TEA is not added, even after 10 d at room temperature. By adding TEA and increasing the reaction temperature, polymers with reasonable molecular weights could be synthesized by click chemistry.

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Polymer 1 (P1): [M1 R M2]

Brown solid. 1H NMR (CDCl3, 300 MHz): d ¼ 8.02 (s), 7.89 (s), 7.83 (s) 7.77 (t), 7.59 (m), 6.95 (m), 4.15 (d), 3.97 (m) 3.48 (s), 3.18 (t), 2.62 (s), 2.08 (s), 1.73–1.05 (m), 0.87–0.62 (m). (C78H92N8O6)n: Calcd C 75.70, H 7.49, N 9.05, O 7.76; Found C 74.95, H 7.42, N 8.97, O 7.69.

In Equation (1), FF ¼ Pmax/(Jsc Voc) ¼ (Jmax Vmax)/(Jsc Voc), Pout is the output electrical power of the device under illumination, and Pin is the intensity of the incident light (e.g., in W m2 or mW cm2). Voc is the open-circuit voltage, Jsc is the short-circuit current density, and the fill factor (FF) is calculated from the values of Voc, Jsc, and the maximum-power point, Pmax. All of the fabrication steps and characterization measurements were carried out in an ambient environment without a protective atmosphere. While measuring the current-density–voltage (J–V) curves for the DSSCs, a black mask was used and only the effective area of the cell was exposed to light irradiation. The data reported in this paper were confirmed by making each type of device more than 5 times.

Fabrication and Measurement of DSSCs

Measurement of Diffusion Coefficient

The DSSCs were fabricated as follows. Screen-printable nanocrystalline-TiO2 (nc-TiO2) pastes were prepared using ethyl cellulose (Aldrich), lauric acid (Fluka) and terpineol (Fluka) as described elsewhere.[25] The prepared nc-TiO2 paste was coated on a fluorinedoped tin oxide (FTO) conducting glass (TEC8, Pilkington, 8 V cm2, glass thickness of 2.3 mm), dried in air at ambient temperature for 5 min and sintered at 500 8C for 30 min. The thicknesses of the annealed films were measured using an Alpha-step IQ surface profiler (KLA Tencor). For dye adsorption, the annealed nc-TiO2 electrodes were immersed in absolute ethanol containing 0.5 103 M N719 dye (Ru[LL’(NCS)2], L ¼ 2,20 -bypyridyl-4,40 -dicarboxylic acid, L’ ¼ 2,20 -bypyridyl-4,40 -ditetrabutylammonium carboxylate) for 24 h at ambient temperature. Pt counter-electrodes were prepared by thermal reduction of the thin film formed from 7 103 M H2PtCl6 in 2-propanol solution at 400 8C for 20 min. The dye-adsorbed nc-TiO2 electrode and Pt counter-electrode were assembled using 60 mm-thick surlyn (Dupont 1702). The optimized polymer electrolyte consisted of tetrabutylammonium iodide (TBAI) (0.48 M), 1-propyl-3-methylimidazolium iodide (PMII) (0.79 M), iodine (I2) (0.23 M), ethylene carbonate (EC) (6.8 M), propylene carbonate (PC) (1.9 M), polymer matrix (P1, P2, and PAN) (0.05 g, 0.016 g, and 0.005 g), and acetonitrile (5.91 M). To ensure homogeneity, the mixture was stirred continuously at 80 8C for 24 h. After cooling to room temperature, the stable, viscous polymer electrolyte was used as a redox electrolyte for the DSSCs. The polymer electrolytes were filled between two electrodes using a vacuum pump through tiny filling holes in a hotplate. A uniform polymer-electrolyte layer was formed in the cells after cooling to room temperature. The active areas of the dye-coated TiO2 films were measured using an image-analysis program equipped with a digital-microscope camera (Moticam 1000). The performances of the DSSCs were measured using a calibrated AM 1.5G solar simulator (Orel 300 W simulator, model 81150) with a light intensity of 100 mW cm2, adjusted using a standard photovoltaic (PV) reference cell (2 cm 2 cm monocrystalline silicon solar cell, calibrated at the National Renewable Energy Laboratory (NREL), Colorado, USA) and a computer-controlled Keithley 236 sourcemeasure unit. The power-conversion efficiency (PCE) (h) of a solar cell is given by Equation (1):

The diffusion-coefficient measurements were performed using thin-layer cells.[26] The two electrodes of the cells were made of platinized, transparent, conducting oxide-coated glass. The platinization was carried out by doctor blend and sintering. The electrodes were separated by a distance of about 60 mm using sealant. Due to large variations of the electrode distance, this distance had to be determined for every cell used in the diffusion-coefficient measurements. The polymer electrolytes were filled between the two electrodes after sealing using a vacuum pump through tiny filling holes in a hotplate. A uniform polymer-electrolyte layer was formed at room temperature. Impedance spectroscopy was conducted using WEIS500 ZMAN version 2.0 programs. Impedance measurements of the thinlayer cells were conducted over a frequency range of about 40 kHz to 0.5 mHz. The finite Warburg impedance (Ws) only includes diffusion of the I 3 ion, because the I ion is like a supporting electrolyte, and the diffusion coefficients of both species have nearly the same order of magnitude. The Ws is given by Equation 2:

1

Yellow solid. H NMR (CDCl3, 300 MHz): d ¼ 8.03 (s), 7.67 (m), 7.45 (m), 6.93 (m), 4.02 (t), 3.85 (t), 3.23 (s), 2.08 (s), 1.73–1.07 (m), 0.79– 0.64 (m). (C78H98N6O8)n: Calcd. C 75.09, H 7.92, N 6.74, O 10.26; Found C 74.34, H 7.85, N 6.07, O 10.17.

Polymer 2 (P2): [M1 R M3]

h ¼ Pout =Pin ¼ ðJsc Voc Þ FF=Pin

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(1)

Z ðvÞ ¼ RD

pffiffiffiffiffiffiffiffiffiffi tanh ivtD pffiffiffiffiffiffiffiffiffiffi ivtD

(2)

In Equation (2), RD has the dimension V; the parameter tD is related to the diffusion coefficient and to the Nernst distance, d, to the electrode, where the concentration of the diffusion-limiting species is constant. For thin-layer cells, this constraint is fulfilled for a distance of d ¼ l/2 and for tD as follows in Equation (3), yielding the diffusion coefficient of the I 3 ion: 2 l= 2 d2 ¼ ; CI3 ðd; tÞ ¼ constant tD ¼ DI3 DI3

(3)

Results and Discussion The side-chain liquid-crystal polymers (SCLCPs) P1 and P2 were prepared by using the sequences outlined in Scheme 1. The fluorene-based diazide monomer M1 and the diacetylene-based side-chain-mesogen-

DOI: 10.1002/macp.201000264


N3 +

N3 C8H17

Meo

O(H2C)6O2C

C8H17

CO2(CH2)6O

OMe

M2

M1 N N

N3

THE, TEA, CuSO4 5H2O

C8H17

Na-L-ascorbate, 35oC, 48h

N

C8H17 n CO2(CH2)6O

O(H2C)6O2C

MeO

OMe

P1

N3 +

N3 C8H17

NC

O(H2C)6O2C

CO2(CH2)6O

C8H17

M3

M1

N N

N3 C8H17

THE, TEA, CuSO4 5H2O Na-L-ascorbate, 35oC, 48h

CN

NC

N

C8H17 O(H2C)6O2C

n CO2(CH2)6O

CN

P2

Scheme 1. Synthesis of SCLCPs (P1 and P2).

containing monomers M2 and M3 have been reported previously.[15,22–24] However, monomers M2 and M3 were tried, for the first time, to synthesize SCLCPs by click chemistry. To introduce the desired 1,4-disubstituted 1,2,3-triazole ring units as linkers in the polymer backbone, 2,7-diazido-9,9-dioctylfluorene was utilized as a common azide monomer for the click coupling reactions with the diacetylene monomers, bis(4-methoxy-40 -hexyloxybiphenyl)dipropargyl malonate (M2) and bis(4-cyano-40 hexyloxybiphenyl)dipropargyl malonate (M3), promoted using copper sulfate. A small quantity of TEA was employed in these processes to enhance the activity of the Cu catalyst. During the polymerization reactions, the mixtures became progressively more viscous and homogeneous without the formation of detectable precipitates. In order to improve the purity and photovoltaic performance of P1 and P2, the precipitated polymers were further subjected to multiple Soxhlet extractions with methanol and were finally extracted with chloroform. By using this procedure, highly purified polymers with narrow polydispersities were obtained. The resulting polymers, P1 and P2, were found to be completely soluble in various organic solvents, Macromol. Chem. Phys. 2010, 211, 2464–2473 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

such as chloroform, chlorobenzene, THF, toluene, and xylene. A summary of the polymerization results and optical properties of P1 and P2 is given in Table 1. The weight-average molecular weights (Mw ) and the polydispersities of P1 and P2 were found to be 26 700 g mol1 and 1.99 and 8 400 g mol1 and 1.29, respectively. The GPC results show that these polymers had a reasonable molecular weight and relatively low polydispersity indices, which may be due to the better solubility properties of the materials involved in the polymerization reactions. The structures of the monomers, and of P1 and P2 were assigned using 1H NMR spectroscopy and elemental analysis. 1H NMR-spectroscopy analysis clearly indicated the disappearance of the peak at 2.08 ppm, assigned to the acetylenic proton signals and the appearance of a new peak at 8.0 ppm attributed to the vinylic proton of the 1,2,3triazole moiety. The other peaks in the spectra were consistent with those expected for the proposed structures of the SCLCPs. The thermal transitions of monomers, and of P1 and P2, were measured using DSC analysis, performed under a N2

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Table 1. Polymerization results and optical properties of P1 and P2.

PDIa)

M w a)

Polymer

Abs (solution)

Abs (filmb))

PL (solution)

PL (filmb))

nm

nm

nm

nm

g mol1 P1

26.7 103

1.99

287, 325

286, 326

367

366, 384

P2

3

1.29

319

304, 321

370

366

8.4 10

a)

The Mw and the polydispersity index (PDI) of the polymers were determined by GPC using polystyrene standards; b)Measured as a thin film on a quartz substrate.

atmosphere, as shown in Figure 1–2. All of the DSC heating and cooling scans were performed at the same rate, 10 8C min1, and all show identical phase sequences in both cycles, with sharp peaks appearing at the transition temperatures., The DSC curve obtained from the heating

a)

and cooling cycles for monomer M2 is shown in Figure 1a. In the heating phase, two well-separated transitions, corresponding to liquid-crystal smectic A and smectic to nematic phases were observed at 48 and 70 8C, respectively. However, on cooling, a considerable change takes place: the upper, cooling curve displays a single nematic phase transition with a slight variation in the temperature at

M2 M3

Heat flow

Heat flow

a)

40

60

80

100

120

-20

0

Temperature oC

b)

40

60

80

100

Temperature o C

b)

P2

Heat flow

Heat flow

P1

40

60

80

100

120

140 o Temperature C

160

180

Figure 1. DSC thermograms of M2 (a) and P1 (b) at a scanning rate of 10 8C min1.

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20


-20

0

20

40

60

80

100

Temperature OC

Figure 2. DSC thermograms of M3 (a) and P2 (b) at a scanning rate of 10 8C min1.

DOI: 10.1002/macp.201000264


66 8C. The DSC curve for P1, for its heating and cooling cycles, is shown in Figure 1b. In the heating cycle, P1 displays a transition from the crystalline to the smectic phase at a temperature of 43 8C and in the cooling cycle, the sample shows crystalline-phase properties below a transition temperature of 59 8C. This result is probably a consequence of the high viscosity and rigid backbone of P1. In Figure 2, the DSC curves obtained from the heating and cooling cycles of M3 and P2 are displayed. The curves show that the phase transitions are in the same temperature range in both cycles, taking place at approximately 5 8C for M3 and 21 8C for P2. The liquid-crystal phases and transition temperatures of the monomers and polymers, observed using DSC, were confirmed by POM. The behavior of the samples under thermal conditions was probed in order to assess the relationships that exist between the temperaturedependence characteristics and the phase transition. The peaks of the phase transitions in the DSC curves of the samples (Figure 1–2) were found to be in good agreement with each other, within the measurement precision of the optical POM experiments. The POM textures of the liquid-crystal phase of monomers M2 and M3, and of polymers P1 and P2, are shown in Figure 3–6. The phase morphology of M2

(Figure 3) and the POM textures (Figure 3a–c) show that the smectic liquid-crystal phase exists below 70 8C. However on cooling, the POM textures (Figure 3d–f) show a single nematic phase with a slight variation in the transition temperature at 66 8C, results that are in good agreement with those obtained from the DSC measurements. In both the heating and cooling cycles, the POM textures (Figure 4a–h) of P1 show the same results, as predicted using the DSC thermograms; that is to say, isotropic transition temperatures at 105 and 90 8C, respectively. As shown in Figure 4f, the POM texture shows crystalline-phase properties below a transition temperature at 60 8C, which agrees well with the DSC curve in the cooling cycle of the sample, which had a transition temperature at 59 8C. Furthermore, it is interesting to note that after supercooling of a sample of M3, the POM textures (Figure 5a–e) display a wide smectic phase range from room temperature

Figure 3. Polarized optical micrographs of M2 at different temperatures, (a–c) in heating and (d–f) in cooling, at a magnification of 20.

Figure 4. Polarized optical micrographs of P1 at different temperatures, (a–d) in heating and (e–h) in cooling, at a magnification of 20.


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Figure 5. Polarized optical micrographs of M3 at different temperatures, (a–e) in heating and (f–j) in cooling, at a magnification of 20.

to an isotropic temperature of 75 8C in the heating cycle. However, upon cooling, the temperature for the isotropic to liquid-crystal-phase transition (Figure 5f–j) was found to be approximately 5.5 8C. The results from repeated POM analysis of heating and cooling to 20 8C were again found to agree well with the DSC results. Figure 6(a–h) shows the POM images of polymer P2. The sample shows similar textures in both the heating and cooling cycles from room temperature to isotropic temperature and vice versa, suggesting the absence of any significant phases transitions. As shown in Figure 6c and 6g, the phase transition from liquid crystal to isotropic and isotropic to liquid crystal starts at 100 8C and the sample is completely changed to the isotropic phase at a temperature of 107 8C. It is expected that the smectic phase exists below 21 8C. Results from repeated POM analysis of heating and cooling

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Figure 6. Polarized optical micrographs of P2 at different temperatures, (a–d) in heating and (e–h) in cooling, at a magnification of 20.

were again found to agree well with the DSC results, as the DSC curves in Figure 2b obtained from the heating and cooling cycles of P2 did not show any transition other then at 21 8C. The absorption and PL spectra of P1 and P2, measured in both the solution and film states, yielded the optical properties that are summarized in Table 1. The UV-vis absorption spectra of P1 and P2 in the solution state have absorption maxima at 325 and 319 nm, respectively (Figure 7a). The absorption spectrum of P2 is dominated by the fluorene chromophore in the polymer main chain. The PL spectra of P1 and P2 in chloroform were similar, in that both show blue-colored light emission at between 367 and 370 nm (Figure 7b), which again corresponds to fluorene-based emission. In order to prove the possibility of using P1 and P2 as polymer electrolytes in DSSCs, we fabricated DSSC devices with a configuration of SnO2:F/TiO2/N719 dye/

DOI: 10.1002/macp.201000264


a)

-12

P1 P2 2 Current density> mA/cm @

-10 -8 -6 -4

0 2

300

400 500 Wavelength (nm)

b) Normalized PL intensity (a.u.)

400

500 Wavelength (nm)

0.0

0.2

0.4

0.6

Bias[V]

600

Figure 8. Current-density–voltage characteristics of the DSSCs with polymer electrolytes at 1 sun (100 mW cm2).

P1 P2

600

Figure 7. UV-vis (a) and PL spectra (b) of P1 and P2 in chloroform solution.

polymer electrolyte/Pt. Current-density–voltage (J–V) curves of SnO2:F/TiO2/N719 dye/polymer electrolyte/Pt devices using P1, P2 and PAN as the polymer matrix for the electrolyte, under AM 1.5G illumination (100 mW cm2), are shown in Figure 8. The photovoltaic properties of the DSSCs are summarized in Table 2. The DSSCs exhibited photovoltaic performances with power-conversion efficiencies (PCEs) of 4.11, 3.20 and 3.43% for the P1-, P2- and PAN-based polymer electrolytes, respectively. Among the three DSSCs investigated, the one fabricated using the P1-based polymer electrolyte gave the highest PCE of 4.11% (Voc ¼ 0.63 V, Jsc ¼ 10.03 mA cm2, FF ¼ 64.6). The better photovoltaic performance of this DSSC is a consequence of the good molecular-weight distribution and liquid-crystal behavior of P1, which enables the polymer electrolyte to penetrate easily into the dye-adsorbed nc-TiO2 Macromol. Chem. Phys. 2010, 211, 2464–2473 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

P1 P2 PAN

-2

electrode. Furthermore, the lower PCE values of the P2and PAN-based polymer electrolytes can be attributed to the lower miscibility between the polymer-electrolyte components. The DSSC employing the P1-based polymer electrolyte gave a high current density of 10.03 mA cm2 as a result of the high diffusion coefficient and the properties of the redox couple that leads to regeneration of the oxidized sensitizers and the smooth operation of the redox process. Impedance spectra are shown in Figure 9 and the diffusion coefficients of the I 3 ions are summarized in Table 2. Owing to their lower diffusion coefficients, the P2- and PAN-based polymer electrolytes have photovoltaic performances that exhibit low current densities, fill factors and PCEs. Additionally, the reason for this behavior relates to the existence of insufficient redox reactions with sensitizers and an impediment to charge transfer at the counterelectrode due to the low diffusion coefficients. Moreover, the P2- and PAN-based polymer electrolytes show lower PCEs than the P1-based analog, which also can be attributed to the lower miscibility between the polymer-electrolyte components.

Table 2. Photovoltaic properties of the DSSCs comprising the P1-, P2- and PAN-based polymer electrolytes.

Polymer

DI3

Jsc

Voc

FF

PCE

cm2 s1

mA cm2

V

%

%

P1

6.13 106

10.03

0.63

64

4.11

P2

6

5.46 10

7.96

0.62

64

3.20

PAN

1.25 106

9.40

0.63

57

3.43

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2.0

2.0

P2 1.5

1.0

1.0

z''/Ω

z''/Ω

P1 1.5

0.5

0.0

0.5

15.5

16.0

16.5

17.0

17.5

18.0

0.0 19

18.5

20

21

z''/Ω

22 23 z''/Ω

24

25

26

2.0 PAN

z''/Ω

1.5

1.0

0.5

0.0 17.0

17.5

18.0

18.5

19.0

19.5

20.0

20.5

21.0

z''/Ω

Figure 9. Nyquist plots for the P1-, P2- and PAN-based polymer electrolytes.

Conclusion In this study, a new series of SCLCPs were synthesized by utilizing 1,4-disubstituted 1,2,3-triazole units, introduced through the use of click chemistry. The mesomorphic phase behaviors of the monomers, as well as of the polymers, were investigated using DSC and POM techniques and found to be in good agreement with each other. The thermal properties of the polymers were investigated and found to be dependent on the structural and electronic parameters as well as on the molecular weight. PL spectra showed that the polymers exhibit blue emission. For the first time, these SCLCPs were applied in the fabrication of DSSCs, which displayed high PCEs. The maximum PCE of the click polymers probed was found to be 4.11%, in the system using P1 as the electrolyte component, with the configuration of SnO2:F/TiO2/N719 dye/polymer electrolyte/Pt. CuI-catalyzed 1,3-dipolar cycloaddition chemistry has again proven to be a useful synthetic methodology in routes for the fabrication of SCLCPs that are promising materials for device applications. We believe that the results presented above will facilitate the generation of new functional materials by click chemistry. Further studies aimed at the development of new electrolytes to enhance DSSC performance are currently underway.

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Acknowledgements: This work was supported by a National Research Foundation of Korea (NRF) grant funded from the Ministry of Education, Science and Technology (MEST) of Korea (No. M10600000157-06J0000-15710) and the Basic Science Research Program, through a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (No. 2010-0001842). This work was also supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2009-0082141).

Received: May 15, 2010; Revised: July 22, 2010; Published online: October 15, 2010; DOI: 10.1002/macp.201000264 Keywords: click chemistry; dye-sensitized solar cells; polymer electrolytes; side-chain liquid-crystal polymers

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DOI: 10.1002/macp.201000264


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