In Situ Low Temperature Polymerization of Bismaleimide for Gel-Type

Jul 23, 2010 - Chuan-Pei Lee , Chen-Yu Chou , Chia-Yuan Chen , Min-Hsin Yeh , Lu-Yin Lin , R. Vittal , Chun-Guey Wu , Kuo-Chuan Ho. Journal of Power ...
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J. Phys. Chem. C 2010, 114, 13832–13837

In Situ Low Temperature Polymerization of Bismaleimide for Gel-Type Electrolyte for Dye-Sensitized Solar Cells Jian-Ging Chen,† Chia-Yuan Chen,‡ Chun-Guey Wu,*,‡ and Kuo-Chuan Ho*,†,§ Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617; Department of Chemistry, National Central UniVersity, Chung-Li, Taiwan 32001; and Institute of Polymer Science and Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617 ReceiVed: March 1, 2010; ReVised Manuscript ReceiVed: May 17, 2010

For the first time, gel-type dye-sensitized solar cells (DSSCs) were fabricated with gel electrolyte containing poly-1,1′-(methylenedi-4,1-phenylene)bismaleimide (PBMI) prepared by in situ polymerization of the corresponding monomer without an initiator at 30 °C. Furthermore, when 0.3 wt % of the exfoliated alkylmodified nanomica (EAMNM) was added into the gel electrolyte, the corresponding DSSC shows higher short-circuit current density (Jsc ) 17.08 mA/cm2) and efficiency (η ) 7.05%) than that with PBMI-gel electrolytes without EAMNM (Jsc ) 15.34 mA/cm2 and η ) 6.24%, respectively). 1. Introduction High-efficiency liquid-type dye-sensitized solar cells (DSSCs) using an organic liquid electrolyte containing an I3-/I- redox couple are under intensive investigation.1-3 However, liquidtype DSSCs are not very durable, especially at high temperatures, due to the volatile nature of the organic solvent.4 Therefore, the assembly of liquid-type DSSCs requires critical sealing techniques to reduce solvent leakage/evaporation. To solve these drawbacks, considerable efforts have been made to supersede the liquid electrolytes with solid or quasi-solid state electrolytes, such as plastic crystal electrolytes,5,6 organic holetransfer conducting polymers,7 polymer gel electrolytes,8,9 ionic liquid based electrolytes,10,11 and liquid electrolytes solidified with physically cross-linked gelators.12 In practice, quasi-solid polymer electrolytes have difficulty penetrating into the pores of TiO2 electrodes due to their high viscosity or large molecular size. However, this problem can be solved by thermal polymerization after the monomers have penetrated into the pores of the TiO2 electrode. For instance, DSSC with a photochemically stable fluorine polymer, poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP), solidified 3-methoxypropionitrile (MPN)-based liquid electrolyte has a conversion efficiency of 6%.13 In this aspect, chemically cross-linked gelators are adequate for in situ polymerization of DSSC electrolytes because the quaternization reaction occurs at high temperature even in the presence of iodine.14-17 Nevertheless, high molecular-weight gelators with high viscosity may have difficulty penetrating into the pores of TiO2, results in low photocurrents of the corresponding DSSCs. However, DSSC with 7.72% efficiency based on gel-type electrolytes containing chemically cross-linked gel electrolyte precursors, such as polypyridyl-pendant poly(amidoamine) dendritic derivatives (PPDDs), has been reported.18 * To whom correspondence should be addressed. (K.-C.H.) Tel: +886-22366-0739. Fax: +886-2-2362-3040. E-mail: [email protected]. (C.-G.W.) Tel: +886-3-422-7151ext. 65900. Fax: +886-3-422-7664. E-mail: t610002@ cc.ncu.edu.tw. † Department of Chemical Engineering, National Taiwan University. ‡ Department of Chemistry, National Central University. § Institute of Polymer Science and Engineering, National Taiwan University.

It was known that 1,1′-(methylenedi-4,1-phenylene)bismaleimide, commonly known as bismaleimide (henceforth abbreviated as BMI), polymerizes on heating via the condensation reaction shown below.19-21 Previous studies have also shown that homopolymerization involves free radicals, as the polymerization is affected by freeradical inhibitors.21 Alternatively, BMI is capable of homopolymerization at high temperature without free-radical initiator. In addition, BMI-based network polymers possess high glass transition temperature, high thermal stability, and good flexibility.22 The electron deficit of the maleimide double bond is more reactive toward nucleophilic or anionic reactants than classical ethylene bonds.20 This double bond can also undergo Diels-Alder or back Diels-Alder reaction, since it is an excellent diene attractor.23 It can also homopolymerize and copolymerize in solution or in the molten state after different types of priming (by radicals or anions).21-24 Interestingly, we find that BMI monomers incorporating in an ionic liquid electrolyte can be homopolymerized to form a gel even at temperature as low as 30 °C. Therefore, in this work, we explore the photovoltaic characteristics of DSSCs fabricated with ionic liquid (1-butyl-3-methylimidazolium iodide, abbreviated as BMImI) based gel-type electrolytes containing, poly-1,1′(methylenedi-4,1-phenylene)bismaleimide, PBMI prepared by in situ low-temperature polymerization of the corresponding monomer. Furthermore, our previous work has successfully demonstrated that poly (n-isopropylacrylamide) (PNIPAAm) based gel electrolyte incorporated an exfoliated montmorillonite (MMT) can improve the photovoltaic performance of the corresponding DSSC.25 Mica is a natural clay and belongs to the structural family known as the 2:1 phyllosilicates (Figure S1 (a) in the Supporting Information). Compared to nanoparticles, such as TiO2 and SiO2, nanomicas have high aspect ratios due to their thin platelet structure, resulting from an exfoliated process. Their crystal lattice consists of two-dimensional layers, where a central octahedral sheet of alumina or magnesia is fused to two external silica tetrahedron by the tip so that the oxygen ions of the octahedral sheet do also belong to the tetrahedral sheets.26 The layer thickness and lateral dimension of nanomica used here are around 1 nm and 300-600 nm, respectively. Stacking of layer silicate platelets creates a regular van der

10.1021/jp101859r  2010 American Chemical Society Published on Web 07/23/2010

Polymerization of Bismaleimide

Waals gap between the platelets, which is called the interlayer. Even though natural mica is hydrophilic, it would become hydrophobic and have good compatibility with organic electrolytes, if modified with organogroups. In this work, we incorporated EAMNM (Figure S1(b) in the Supporting Information) into PBMI gel-type electrolytes to form nanocomposite gel electrolytes. The effect of the EAMNM content on the cell performance was investigated and the EAMNM content was optimized. To further improve the cell performance, a porous TiO2 photoanode was prepared. 2. Experimental Section Anhydrous LiI (+98%), I2, poly(ethylene glycol) (PEG), 4-tert-butylpyridine (TBP) (96%), 1-butyl-3-methyl-imidazolium iodide (BMImI), propylene carbonate (PC), and acetonitrile (ACN) were obtained from Merck Co. Titanium(IV) isopropoxide (TTIP) and 1,1′-(methylenedi-4,1-phenylene)bismaleimide (BMI, 95%) were acquired from Aldrich Co. Guanidinium thiocyanate (GuSCN) was purchased from Acros Co. Fluorinedoped tin oxide (FTO) conducting glass plates (15 Ω/sq.) and ionomer resin (Surlyn, SX1170-25) were obtained from Solaronix S.A., Aubonne, Switzerland. The exfoliated alkylmodified nanomica (EAMNM; type: NM-933) was acquired from NanoMica Technology Co., Ltd., Taiwan. DSSC was constructed with platinum-deposited FTO conducting glass as a counter electrode. The composition of liquid electrolyte is as follows: 0.6 M BMImI, (0.01-0.4) M I2, 0.5 M TBP, 0.1 M GuSCN in PC, and ACN (1:1 in v/v). PBMI gel electrolyte was prepared by adding 6 wt % BMI (vs liquid electrolyte) into the liquid electrolyte. The PBMI/EAMNM composite gel electrolytes were prepared by adding 6 wt % BMI and different weight ratios (0-3 wt %) of EAMNM into the liquid electrolyte. In order to reduce the ionic diffusion resistance of gel-type electrolyte in TiO2 electrodes, incorporation of monodispersed PMMA into the TiO2 paste to form the micropore after sintering was considered. By using the in situ polymerization method, it is expected that the porous TiO2 film and gel electrolyte would have good physical contacts. The detailed steps for the preparation of the highly porous TiO2 film were described in our previous work.27 The highly porous TiO2 electrode with a thickness of 15 µm was prepared by deposition of TiO2 colloidal paste containing 350 nm monodispersed PMMA spherical particles at the optimal weight ratio27 of PMMA/TiO2 ) 3.75 on FTO by a doctor-blade method, followed by sintering at 500 °C for 30 min in air. After removing the monodispersed PMMA particles via sintering, TiO2 electrode with large pores was made. Using TiO2 electrode with larger pore size aims to increase the penetration of the BMI monomers and electrolyte. The crosssectional view of the highly porous TiO2 electrode was illustrated in Figure 1(a). The TiO2 electrode with 0.4 × 0.4 cm2 geometric area was immersed into the ACN/TBP mixture (volume ratio ) 1:1) containing 2 × 10-4 M dye for overnight. Here, a ruthenium

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supersensitizer coded CYC-B6S28 (molecular structure shown in Figure 1(b)), in which one of the bipyridine ligands was functionalized with thienyl-carbazole moiety instead of the criterion N3 dye, was used as the photon-to-current conversion center. Subsequently, the dye-sensitized TiO2 photoanode was rinsed with ACN and dried in air and then sandwiched with a thin transparent hot-melt ionomer resin of 25-µm thickness (Surlyn 1702, DuPont) and a preprepared Pt sputtered FTO glass as a counter electrode. After filling the electrolyte through one of the two small holes drilled on the counter electrode, the holes were covered with small squares of Surlyn 1702 and sealed completely with Torr Seal cement (Varian, MA, USA). Afterward, the cell was placed in the thermostatic chamber at 30 °C for 3 h to make a gel-type electrolyte DSSC. The cell was illuminated by an Oriel solar simulator (#6266) at AM 1.5 (Oriel, #81075) irradiation. The photoelectrochemical characteristics of the DSSCs were recorded with a potentiostat/ galvanostat (PGSTAT 30, Autolab, Eco-Chemie, The Netherlands). Electrochemical impedance spectra (EIS) were obtained by the above-mentioned potentiostat/galvanostat equipped with FRA2 module under constant light illumination of 100 mW/ cm2. The impedance spectra were analyzed by an equivalent circuit model for interpreting the characteristics of the DSSCs.29 Impedance parameters were determined by fitting of the impedance spectrum using Z-view software. The photocurrent action spectra of the DSSCs were measured by a monochromator (Oriel Instruments, model 74100). A symmetric cell consisted of two Pt-sputtered FTO conducting glasses, electrolyte, and a spacer (effective electrode area ) 0.25 cm2) was used to measure the ionic conductivities of the gel-type electrolytes.30,31 The cell constant was calibrated with standard KCl aqueous solution (ionic conductivity of 12.9 mS/cm) at 25 °C before the experiments. The frequency-dependent impedance was measured via a frequency response analyzer on 10 mV of modulation amplitude. Additionally, in continuous light-soaking tests the hermetic cells covered with a 2-mm thick layer of ultraviolet cutoff filter (item no. UVCUT 400, Rocoes, Taiwan) were irradiated at open-circuit condition at 55 °C in the light soaking chamber (1 sun, LSC, Dyesol, Australia). 3. Results and Discussions Wan et al. reported the anionic homopolymerization of N-phenylmaleimide via the transfer of protons in the presence of pyridine at 0 °C.24 Therefore, the formation mechanism for BMI-based cross-linked resin in the presence of TBP at anionic polymerization process can be speculated as Figure 2. TBP is a stronger base, when TBP mixes with BMI, TBP will be protonated to form cations and at the same time BMI deprotonated to be anions via proton transfer. The BMI anions are the initiators for BMI polymerization. Figure 3 shows the photocurrent density-voltage curves of DSSCs fabricated with a PBMI-gel electrolyte consisting of 0.6 M BMImI/0.1 M GuSCN/0.5 M TBP with various I2 concentrations (0.01-0.4 M) in ACN/PC (1:1 in vol. ratio) and 6 wt %

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of BMI monomer under 1.5 a.m., 100 mW/cm2 illumination. The inset of Figure 3 shows the Jsc, Voc, and the light-toelectricity conversion efficiency (η) vs the I2 concentration. The corresponding photovoltaic parameters of DSSCs and the roomtemperature ionic conductivities of the electrolyte with various I2 concentrations are summarized in Table 1. It is found that the conductivity of the electrolyte increases as the I2 concentration increases, reaching a maximum conductivity of 13.88 × 10-3 S · cm-1 at 0.1 M I2, and then decreases when I2 concentration is larger than 0.1 M. A similar tendency is observed in Jsc and η: Jsc increases from 8.23 to 15.5 mA/cm2 when the I2 concentration increases from 0.01 to 0.1 M, then decreases again. When the I2 concentration increases to 0.4 M, the Jsc decreases to 9.25 mA/cm2. However, Voc decreases gradually from 0.635 to 0.57 V when the I2 concentration increases from 0.01 to 0.4 M. The efficiency of the DSSC with this gel-type electrolyte reaches its maximum of 6.41% at 0.1 M I2 (see Table 1) and this finding is consistent with the reported literature32-34 whose optimum I2 concentration was 0.1 M. This phenomenon can be understood from the following theoretical consideration:35

Voc )

( kTe )ln( n k [I ] ) Iinj

cb et 3

cell efficiency from 6.41 to 7.02% when 0.3 wt % EAMNM is added to the PBMI gel-type electrolyte as listed in Table 2. Furthermore, the room-temperature ionic conductivity of the electrolyte initially increases as the EAMNM content increases, reaching a maximum conductivity of 15.36 × 10-3 S · cm-1 at 0.3 wt % of EAMNM, and then decreases when the EAMNM content is higher than 0.3 wt %. The EIS technique is also used to realize the effect of adding EAMNM (0.3 wt %) on the PBMI gel-type electrolyte. Surprisingly, the EIS spectra show that Rct2 and Rdiff decreased from 12.11 and 8.23 Ω to 11.79 and 7.82 Ω, respectively, when EAMNM (0.3 wt %) is incorporated into the PBMI-gel electrolyte system. Moreover, the diffusivity of I3-, calculated from the EIS spectra shown in Figure 6, increases from 4.53 × 10-6 to 5.67 × 10-6 cm2/s in the presence of EAMNM in the gel electrolyte. However, excess of EAMNM (>0.5 wt %) does not provide this beneficial effect, instead, EAMNM blocks the diffusion of I- and I3- in the PBMI geltype electrolyte, as evidenced by the cell performance displays in Figure 5. Now it is clear that the exfoliated EAMNM nanoplatelets can facilitate the diffusion of I3-. One possible reason for the

(1)

where k is the Boltzmann constant, T is the absolute temperature, e is the electric charge, Iinj is the incident photo flux, ncb is the concentration of electrons at the TiO2 surface, and ket is the rate constant for the back electron transfer reaction. The photovoltage of a DSSC is kinetically limited by the dark reaction occurring in the photoelectrochemical system, where the electrons from the conduction band of TiO2 recombine with the oxidizing species (I3-) in the electrolyte.36 An increase in the I2 concentration produces a higher concentration of I3- and increases the rate of the back electron recombination. Thus, increases the dark current, and decreases the open-circuit voltage (Voc) of the DSSC. Electrochemical impedance spectroscopy (EIS) has been successfully used to characterize the charge recombination kinetics of DSSCs by analyzing the variation in impedances associated with the device configurations.37 Rs, the ohmic serial resistance, is associated with the series resistance of the electrolytes. Rct1 is the charge transfer resistance at the Pt/ electrolyte interface in the DSSCs. According to Lagemaat, Rct2 is associated with the charge transfer resistance at the TiO2/ dye/electrolyte interface.38 However Rdiff is associated with Nernstian diffusion within the electrolyte.38 The EIS data analysis in this study focuses on the Nyquist plots of DSSCs with PBMI-gel electrolyte consisting of 94 wt % of 0.6 M BMImI/0.1 M GuSCN/0.5 M TBP with various I2 concentrations (0.01-0.4 M) in ACN/PC (1:1 in vol. ratio) and 6 wt % of BMI monomer, under the open-circuit voltage and 100 mW/ cm2 illumination. The impedance spectra display in Figure 4 show three semicircles in the measuring frequency from 65 kHz to 10 mHz and the inset is the Rdiff and Rct2 values vs I2 concentration. It is clearly seen that DSSC with PBMI-gel electrolyte containing 0.1 M I2 possesses the lowest Rct2 and Rdiff. Figure 5 shows the photocurrent density-voltage curves of DSSCs fabricated with the PBMI-gel electrolyte consisting of 94 wt % of 0.6 M BMImI/0.1 M GuSCN/0.1 M I2/0.5 M TBP with various amounts of EAMNM (0 to 3 wt % vs BMI) in ACN/PC (1:1 in vol. ratio) and 6 wt % of BMI monomer under 100 mW/cm2 illumination. EAMNM effectively enhances the

Figure 1. (a) The cross-sectional view of the highly porous TiO2 electrode; (b) Molecular structure of ruthenium supersensitizer CYCB6S.

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Figure 2. Reaction steps for the formation of BMI based cross-linked resin.

Figure 3. The photocurrent density-voltage curves of the DSSCs fabricated with the PBMI-gel electrolyte consisted of 94 wt % of 0.6 M BMImI/0.1 M GuSCN/0.5 M TBP with various I2 concentration (0.01-0.4 M) in ACN/PC (1:1 in vol. ratio) and 6 wt % of BMI monomer under AM 1.5 100 mW/cm2 illumination.

TABLE 1: Photovoltaic Performance of DSSCs and the Room-Temperature Ionic Conductivities of the BMI Based Gel-Type Electrolyte Containing Various I2 Concentrations under AM 1.5 Simulated Sun Light (100 mW/cm2) Illuminationa I2 concentration

Voc

Jsc

(M)

(mV)

(mA · cm-2)

0.01 0.03 0.05 0.1 0.2 0.3 0.4

0.601 0.628 0.631 0.645 0.612 0.601 0.573

8.23 13.23 14.50 15.32 13.63 13.06 9.25

FF 0.63 0.64 0.66 0.65 0.67 0.67 0.69

η

σ

(%)

(10-3 S · cm-1)

3.29 5.34 5.99 6.41 5.55 5.26 3.63

9.23 ( 0.28 10.12 ( 0.31 11.34 ( 0.35 13.88 ( 0.24 12.97 ( 0.41 12.07 ( 0.33 10.28 ( 0.23

a

The gel-type electrolyte consisted of 0.6 M BMImI, 0.1 M GuSCN, 0.5 M TBP, and different I2 amounts in a mixture of ACN and PC (1:1, v/v) and 6 wt % of BMI monomer. Jsc: short-circuit photocurrent; Voc: open-circuit photovoltage; FF: fill factor; η: energy conversion efficiency; σ: ionic conductivity at room temperature (25 °C).

enhancement in diffusivity can be explained by the Grotthusstype charge-transfer mechanism,39 in which electron hopping and polyiodide bond exchange were coupled, thus increasing the effective ionic conductivity of the gel electrolyte when an appropriate amount of the exfoliated EAMNM nanoplatelets are incorporated. Another possibility in enhancing the diffusivity or ionic conductivity is associated with the two-dimensional nature of the exfoliated EAMNM nanoplatelets in the gel-type

Figure 4. The impedance spectra of the DSSCs fabricated with the PBMI-gel electrolyte consisted of 94 wt % of 0.6 M BMImI/0.1 M GuSCN/0.5 M TBP with various I2 concentrations (0.01-0.4 M) in ACN/PC (1:1 in vol. ratio) and 6 wt % of BMI monomer under AM 1.5, 100 mW/cm2 illumination. The inset shows the dependence of Rdiff on the I2 concentration.

electrolyte. It is noticed that the layer dimension and lateral dimension of EAMNM nanoplatelets used in this work are around 1 nm and 300-600 nm, respectively. Therefore, it is reasonable to assume that EAMNM nanoplatelets are not able to penetrate through the TiO2 nanopores. In fact, our recent work40 revealed that the incorporation of EAMNM in the polyvinyidene fluoride-co-hexafluoro propylene (PVDF-HFP) gel electrolytes caused the reduction of crystallization of PVDFHFP, which was confirmed by X-ray diffraction (XRD) analysis, leading to the enhancement of ionic conductivity. Similarly, reducing the crystallization of PBMI would decrease the ionic diffusion resistance if a proper amount of EAMNM were incorporated in the PBMI gel-type electrolyte. Incorporating 0.3 wt % EAMNM into a PBMI-gel type electrolyte results in remarkable enhancement in the device stability under continuous light soaking of one sun (100 mW/ cm2) at 55 °C. Figure 7(a) shows that after 500 h of light-soaking at 55 °C, the Voc and FF of DSSCs with liquid-type electrolyte (without PBMI and EAMNM), PBMI-gel electrolyte and PBMI/ 0.3 wt % EAMNM-gel electrolyte drop less than 1%. Figure 7(b) shows that after 500 h of light-soaking at 55 °C, the efficiencies (η) of DSSCs with liquid-type electrolyte (without PBMI and EAMNM), PBMI-gel electrolyte and PBMI/0.3 wt % EAMNM-gel electrolyte decrease by 9.2%, 4.2%, and 1.7%, respectively. Although initially the Jsc (17.08 mA/cm2) and η (7.02%) of DSSC with PBMI/0.3 wt % EAMNM-gel electrolyte are lower than those of liquid-type electrolyte (Jsc ) 17.52 mA/

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Figure 5. The photocurrent density-voltage curves of the DSSCs fabricated with the PBMI-gel electrolyte consisted of 94 wt % of 0.6 M BMImI/0.1 M GuSCN/0.1 M I2/0.5 M TBP and various amounts of EAMNM (0 to 3 wt %) in ACN/PC (1:1 in vol. ratio) and 6 wt % of BMI monomer under AM 1.5, 100 mW/cm2 illumination.

TABLE 2: Photovoltaic Performance of DSSCs Using BMI Based Gel-Type Electrolyte with Different EAMNM Contents under AM 1.5 Simulated Sun Light (100 mW/cm2) Illuminationa EAMNM content in gel-type electrolyte

Voc

Jsc

FF 2

(wt%)

(mV)

(mA/cm )

0.0 0.1 0.3 0.5 1.0 3.0

0.645 0.628 0.650 0.651 0.623 0.584

15.32 14.56 17.14 16.41 12.12 9.48

η (%)

0.65 0.64 0.63 0.64 0.60 0.65

6.41 5.85 7.02 6.85 4.49 3.58

σ -3

(10

S · cm-1)

13.88 ( 0.24 13.92 ( 0.23 15.36 ( 0.17 14.52 ( 0.13 14.21 ( 0.11 6.21 ( 0.43

a The gel-type electrolyte consisted of 0.6 M BMImI, 0.1 M GuSCN, 0.5 M TBP, and 0.1 M I2 in a mixture of ACN and PC (1:1, v/v) and 6 wt % of BMI monomer.

Figure 7. The cell performance of DSSCs with the PBMI gel-type electrolyte containing 0.3 wt % EAMNM, neat PBMI gel-type electrolyte, and liquid-type electrolyte (without BMI and EAMNM) under continuous light soaking of one sun (100 mW/cm2) at 55 °C.

only improves the Jsc and η, but also increases the long-term stability of DSSCs under continuous light soaking at 55 °C. 4. Conclusions This work reports the fabrication of gel-type dye-sensitized solar cells (DSSCs) by in situ low temperature polymerization of 1,1′-(methylenedi-4,1-phenylene)bismaleimide polymerized in liquid electrolyte without an initiator. The incorporation of 0.3 wt % of the exfoliated alkyl-modified nanomica (EAMNM) in the gel electrolytes leads to an increase in the short-circuit current density (from 15.34 to 17.08 mA/cm2) and efficiency (from 6.24 to 7.05%). Furthermore, DSSC with PBMI-gel type electrolyte containing 0.3 wt % EAMNM possesses remarkable stability under continuous light soaking of one sun (100 mW/ cm2) at 55 °C. The efficiency of DSSC with PBMI/0.3 wt % EAMNM-gel electrolyte decreases only 1.7% (from 7.05% to 6.93%) after 500 h of continuous light soaking under one sun at 55 °C.

Figure 6. The impedance spectra of DSSCs with BMI-based gel electrolytes, with and without 0.3 wt % EAMNM under AM 1.5 100 mW/cm2 illumination. The inset contains the Rct2, Rdiff, and conductivity values calculated from the EIS data.

cm2, η ) 7.39%), nevertheless long-term stability of DSSCs with PBMI/0.3 wt % EAMNM-gel electrolyte is superior to that with liquid-type electrolyte. This result shows that PBMI-gel electrolytes possess better stability than liquid-type electrolytes and blending 0.3 wt % EAMNM into PBMI-gel electrolyte not

Acknowledgment. We gratefully acknowledge the financial support received from the National Science Council of Taiwan. Supporting Information Available: Structures of mica [Figure S1(a)] and EAMNM [Figure S1(b)]. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gra¨tzel, M. J. Photochem. Photobiol. A:Chem. 2004, 164, 3. (2) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737.

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