Efficient Gel-Type Electrolyte with Bismaleimide via In Situ Low Temperature Polymerization in Dye-Sensitized Solar Cells JIAN-GING CHEN,1 KEN-YEN LIU,2 CHIA-YUAN CHEN,3 CHIA-YU LIN,1 KUAN-CHIEH HUANG,1 YI-HSUAN LAI,1 CHUN-GUEY WU,3 KING-FU LIN,2,4 KUO-CHUAN HO1,4 1
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617
2
Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 10617
3
Department of Chemistry, National Central University, Chung-Li, Taiwan 32001
4
Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan 10617
Received 17 March 2010; accepted 12 August 2010 DOI: 10.1002/pola.24290 Published online 20 September 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: This study reports the characteristics of gel-type
dye-sensitized solar cells (DSSCs), fabricated with gel-type electrolyte containing poly-1,10 -(methylenedi-4,1-phenylene)bismaleimide (PBMI), or poly-1,10 -(3,30 -dimethyl-1,10 -biphenyl-4,40 diyl)bismaleimide (PDBBMI), or poly-N,N0 -(4-methyl-1,3-phenylene)bismaleimide (PMPBMI), prepared by in situ polymerization of the corresponding monomer without an initiator at 30 C. Incorporating 0.3 wt % content of exfoliated alkyl-modified nanomica (EAMNM) into PBMI-gelled electrolyte leads to higher short-circuit current density (Jsc ¼ 17.14 mA cm2) and efficiency (g ¼ 7.02%) than that of neat PBMI-gel electrolyte (Jsc ¼
INTRODUCTION Dye-sensitized solar cells (DSSCs) perform the best when liquid electrolytes are used.1,2 However, the commercialization of these cells has been impeded by technological problems related to hermitic sealing, precipitation of salts at low temperature, evaporation of liquids at high temperature, corrosion, the lack of long-term stability of the cells caused by volatility of the liquid electrolytes, and other problems. To address these drawbacks, researchers have made many attempts to supersede the liquid electrolytes using solid or quasi-solid state electrolytes,3,4 including plastic crystal electrolytes,5,6 organic hole-transfer conducting polymers,7 polymer gel electrolytes,8,9 ionic liquid (IL)-based electrolytes,10,11 and liquid electrolytes solidified with physically crosslinked gelators.12
In practice, quasi-solid electrolytes have difficulty in penetrating the pores of TiO2 electrodes due to their high viscosity.13,14 However, this problem can be solved by thermal polymerization after penetration of the electrolyte incorporated with monomer into the pores of the TiO2 electrode. For instance, a photochemically stable fluorine polymer, poly(vinylidenefluoride-co-hexafluoropropylene, can be used to solidify a 3-methoxypropionitrile-based liquid electrolytes,
15.32 mA cm2, g ¼ 6.41%). Incorporating 0.3 wt % EAMNM into PBMI-gelled electrolyte results in remarkably stable device performance under continuous light soaking under one sun (100 mW cm2) at 55 C. The efficiency of DSSCs based on PBMI/0.3 wt % EAMNM-gelled electrolyte drops by only 1.7% (g ¼ 6.93%) C 2010 Wiley Periodicals, after 500 h of continuous light soaking. V Inc. J Polym Sci Part A: Polym Chem 48: 4950–4957, 2010 KEYWORDS: bismaleimide; CYC-B6S dye; dye-sensitized solar
cell; exfoliated alkyl-modified nanomica; gel-type electrolyte; nanocomposites; nanoparticles; nanotechnology
producing a conversion efficiency of 6% for DSSCs based on quasi-solid-state gel electrolytes.15 Additionally, chemically cross-linked gelators are adequate for in situ polymerization of DSSC electrolytes because the quaternization reaction occurs at high temperatures even though the electrolytes contain iodine.16–19 Nevertheless, these gelators containing high molecular-weight constituent with high viscosity may have difficulty in penetrating the pores of TiO2, causing the low photocurrents of DSSCs. On the other hand, Wang et al.12 reported a 7.72% efficient DSSC based on gel-type electrolytes containing chemically cross-linked gel electrolyte precursors, such as polypyridyl-pendant poly(amidoamine) dendritic derivatives. Recently, Winther-Jensen et al.14 reported for the first time an in situ photopolymerization of model co-monomers, 2-hydroxyethyl methacrylate, and tetra (ethylene glycol) diacrylate, in an IL electrolyte containing I2 for DSSCs. TiO2 nanoparticles were used as the photo-initiator and cogelator in a charge transfer polymerization reaction and DSSCs using the gel-IL electrolyte showed energy conversion efficiency of 3.9% at 1 sun (AM1.5) and 5.0% at 0.39 sun illumination.14 Poly-1,10 -(methylenedi-4,1-phenylene)bismaleimide, commonly known as bismaleimide (hereafter referred to as
Additional Supporting Information may be found in the online version of this article. Correspondence to: K.-C. Ho (E-mail:
[email protected]) or K.-F. Lin (E-mail:
[email protected]) C 2010 Wiley Periodicals, Inc. Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4950–4957 (2010) V
4950
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA
ARTICLE
SCHEME 1 Thermal polymerization of 1,10 -(methylenedi-4,1-phenylene)bismaleimide (BMI) upon heating.
PBMI), polymerizes on heating by the condensation reaction, shown in Scheme 1.20–22 BMI compounds are capable of homopolymerizing without the addition of a free-radical initiator at high temperature.22 Previous studies have shown that homopolymerization involves free radicals, as polymerization is affected by freeradical inhibitors.22 In addition, bismaleimide-based network polymers possess high glass transition temperatures and high thermal stability and flexibility.23 The electron deficit of the maleimide double bond arises from the electron attracting effect of the adjacent carbonyl groups. As a result, maleimide double bonds are more reactive toward nucleophilic or anionic reactants than classical ethylene bonds.21 Published results24 have shown that this double bond can undergo Diels-Alder or back Diels-Alder reactions, since it is an excellent diene attractor. It can also homopolymerize and copolymerize in solution or the molten state after different types of priming at high temperature (>150 C) even though by radicals or anions.22–25 This study reports the incorporation of bismaleimide-based monomers in gel-type electrolytes by in situ low temperature polymerization of the corresponding monomer for DSSCs. Interestingly, it can be homopolymerized to form a gel even at a low temperature (30 C). This study also explores other derivatives, including poly-1,10 -(3,30 -dimethyl-1,10 -biphenyl4,40 -diyl)bismaleimide (PDBBMI) and poly-N,N0 -(4-methyl1,3-phenylene)bismaleimide (PMPBMI). Furthermore, our previous work successfully demonstrated that a gel electrolyte prepared by poly(n-isopropylacrylamide) incorporated with the exfoliated montmorillonite (MMT) nanocomposite can improve the photovoltaic performance of a DSSC.26 It shows that the photocurrent and power efficiency of DSSCs made with gel-type electrolyte incorporating MMT were significantly enhanced compared with those of neat gel-type DSSCs.26 This study attempts to incorporate other exfoliated silica nano-plates, called exfoliated alkyl-modified nanomica (EAMNM), into bismaleimide-based electrolytes to form nanocomposite gel electrolytes. Mica is a natural clay and belongs to a structural family known as the 2:1 phyllosilicates [Supporting Information Fig. S1(a)]. Compared with nanoparticles, such as TiO2 and SiO2, nanomicas have high aspect ratio due to their thin platelet structure, resulted from exfoliated process. Their crystal lattice consists of twodimensional 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.27 The layer thickness
EFFICIENT GEL-TYPE ELECTROLYTE WITH PBMI, CHEN ET AL.
and lateral dimension of nanomica used here are around 1 nm and 300–600 nm, respectively, shown in Supporting Information Figure S1(b). Stacking of layer silicate platelets create a regular van der Waals gap between the platelets, which called the interlayer. Even though nature mica is hydrophilic, it would become hydrophobic and has good compatibility with organic electrolytes, if modified with organogroups. The effect of the EAMNM content on the cell performance was investigated, and the EAMNM content was optimized in this study. Besides, to further improve the cell performance, a porous TiO2 photoanode was prepared. EXPERIMENTAL
Anhydrous LiI (þ98%), I2, poly(ethylene glycol), 4-tert-butylpyridine (TBP) (96%), 1-butyl-3-methyl-imidazolium iodide (BMImI), propylene carbonate (PC) and acetonitrile (ACN) were obtained from Merck. Titanium (IV) isopropoxide, 1,10 (methylenedi-4,1-phenylene)bismaleimide (BMI, 95%) 1,10 (3,30 -dimethyl-1,10 -biphenyl-4,40 -diyl)bismaleimide (DBBMI, 99%) and N,N0 -(4-methyl-1,3-phenylene)bismaleimide (MPBMI, 99%) were acquired from Aldrich. Guanidinium thiocyanate (GuSCN) was purchased from Acros. In addition, fluorine-doped tin oxide (FTO) conducting glass plates (15 X/sq.) and ionomer resin (SurlynV, SX1170-25) were obtained from Solaronix S.A., Aubonne, Switzerland. EAMNM (type: NM-933) was acquired from NanoMica Technology Co., Taiwan. R
A platinum-sputtered FTO conducting glass was used as the counter electrode. The composition of liquid electrolyte is as follows: 0.6 M BMImI, 0.1 M I2, 0.5 M TBP, 0.1 M GuSCN in PC and ACN (1:1 in v/v). PBMI, PDBBMI, or PMPBMI geltype electrolytes, was prepared by adding 6 wt % of BMI, DBBMI or MPBMI (vs. liquid electrolyte), respectively, into the liquid electrolyte. The PBMI/EAMNM composite gel electrolyte was prepared by adding 6 wt % of BMI and 0.3 wt % of EAMNM into the liquid electrolyte. A highly porous TiO2 electrode 15 lm thick was prepared by depositing TiO2 colloidal paste containing monodispersed PMMA spherical particles (weight ratio of PMMA/TiO2 ¼ 3.75) on a FTO glass by a doctor-blade method, and then followed by sintering at 500 C for 30 min in air. A PMMA suspension was prepared by dispersing 10 wt % PMMA microspheres (ca., 350 nm) and 0.5 wt % sodium dodecyl sulfate (SDS) in a solution of H2O and ethanol (volume ratio ¼ 9:1). Sintering removed the monodispersed PMMA particles, producing larger holes in the TiO2 electrode. Forming larger pores in the nanocrystalline TiO2 electrode increases the 4951
JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 (a) The top view of the standard TiO2 electrode (VPMMA/TiO2 ¼ 0) and (b) the prepared high porous TiO2 electrode (VPMMA/TiO2 ¼ 3.75).
penetration of the monomer and electrolyte, thus improving the contact between electrolytes and TiO2. The porosity of the photoanode was controlled by adjusting weight ratio of PMMA microsphere suspension to TiO2 paste (denoted as VPMMA/TiO2). Figure 1(a,b) present a top-view of SEM images of the standard TiO2 electrode (VPMMA/TiO2 ¼ 0) and the prepared high porous TiO2 electrode (VPMMA/TiO2 ¼ 3.75), respectively. It could be seen from the micrograph that the TiO2 electrodes with VPMMA/TiO2 ¼ 3.75 has many micropores, and the diameter is around 300 nm. The prepared highly porous TiO2 film electrode with a 0.4 0.4 cm2 geometric area was immersed into ACN/TBP mixture (volume ratio ¼ 1:1) containing 2 104 M CYC-B6S dye sensitizer28 overnight. Instead of using the commercially available N3 or N719 dye, the experiments in this study used a ruthenium super-sensitizer coded CYC-B6S, in which one of the
bipyridine ligands was functionalized with a thienyl-carbazole moiety, as the photon-to-current conversion center.28 Furthermore, it was demonstrated that a cell based on CYCB6S dye achieves a higher conversion efficiency than that containing N3 dye under the same cell fabrication and measuring procedures. Subsequently, the dye-sensitized TiO2 photoanode was rinsed with ACN, air dried, and then sandwiched with a Pt-sputtered FTO glass as a counter electrode using a thin transparent hot-melt ionomer resin of 25 lm thick (SurlynV 1702, DuPont). After filling the electrolyte through one of the two small holes drilled in the counter electrode, the holes were covered and sealed with small squares of SurlynV and sealed completely with Torr SealV cement (Varian, MA). Afterward, the cell was placed in a thermostatic chamber at a set temperature of 30 C for 3 h to make a gel-type electrolyte DSSC. R
R
R
FIGURE 2 Reaction steps of BMI based cross-linked resin formation and chemical structures of MPBMI, DBBMI, and BMI monomer.
4952
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA
ARTICLE
brated with aqueous KCl standard solution (12.9 mS cm1) at a temperature of 25 C before the experiments. The frequency-dependence of impedance was measured via a frequency response analyzer (FRA) on 10 mV of amplitude modulation. Fourier transform infrared spectra were recorded with a Bruker IFS 28 Equinox Fourier Transform spectrophotometer. The crystalline structure of the dried electrolyte was determined by X-ray diffraction (XRD, X’Pert, PANalytical, the Netherlands). Glassy transition temperature was measured by using a differential scanning calorimeter (DSC, PerkinElmer Pyris 6) at a heating rate of 10 C min1. Additionally, in continuous light-soaking tests, the hermetically sealed cells were covered with a 2 mm-thick ultraviolet cut-off filter (item no. UVCUT 400, Rocoes, Taiwan) and irradiated at the open-circuit condition at 55 C in a light soaking chamber (1 sun, LSC, Dyesol, Australia). RESULTS AND DISCUSSION
FIGURE 3 (a) The representative infrared spectra of BMI monomer (before gelation) and cured BMI polymer (after gelation). (b) The X-ray diffraction spectra of BMI monomer (before gelation) and cured BMI polymer (after 3 h of gelation at 30 C). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com].
The cell was illuminated by a Peccell solar simulator (PECL11). The photoelectrochemical characteristics of the DSSC were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, the Netherlands). Electrochemical impedance spectra (EIS) were obtained by the same potentiostat/galvanostat equipped with a FRA2 module under constant light illumination of 100 mW cm2. To interpret the characteristics of the DSSC, the impedance spectra were analyzed by an equivalent circuit model.29 Impedance parameters were determined by fitting the impedance spectrum using Z-view software. The photocurrent action spectra of the DSSCs were measured with a monochromator (Oriel Instruments, model 74100). Additionally, this study measured the ionic conductivities of the gel-type electrolytes using EIS analysis with a symmetric cell consisting of two Pt-sputtered FTO conducting glasses and a spacer (effective electrode area ¼ 0.25 cm2).30,31 The cell constant was caliEFFICIENT GEL-TYPE ELECTROLYTE WITH PBMI, CHEN ET AL.
Wan and Huang25 reported that the anionic homopolymerization of N-phenylmaleimide can be carried out via the transfer of hydrogen protons in the presence of pyridine even at a low temperature of 0 C. Figure 2 shows the proposed mechanism of bismaleimide-based cross-linked resin formation in the investigated system, as well as the chemical structures of BMI, DBBMI, and MPBMI. In the anionic polymerization of BMI in the presence of TBP, a stronger electron donor, TBP cations, and BMI anions are formed via the transfer of protons as shown in Figure 2. The resulting BMI anions then initiate BMI polymerization. The PDBBMI and PMPBMI resins might be formed by the same reaction mechanism.25 The three polymer gels are not soft-sticky pastes, but cross-linking gels similar to high-elastic sponge with some mechanical strength. In addition, we did not find any
FIGURE 4 Plots of conductivity-temperature data in the VTF coordinates for the liquid-type, PMPBMI, PDBBMI, and PBMIgelled electrolytes and PBMI/0.3 wt % EAMNM-gelled electrolyte. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com].
4953
JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
sign of phase separation between the gel and the liquid electrolyte after polymerization. Figure 3(a) shows the representative infrared spectra of BMI monomer (before gelation) and cured BMI polymer (after gelation). The intensity of the peaks at 1702 cm1 (C¼ ¼O stretching) and 1391 cm1, contributed by the imido groups, slightly decreased; while a new peak at 1645 cm1, presumably contributed by the amido groups, gradually appeared. These results suggest that some of the imido rings of BMI have been opened and become the amido groups.32 In addition, the corresponding CANAC (maleimide ring) stretch is present as a strong band at 1149 cm1 in the spectrum of uncured BMI.33 After polymerization, the characteristic strong broad band centered at 1168 cm1 was assigned to the CANAC (succinimide ring) stretch appeared.33 Further, the characteristic band at 3105 cm1 shifted to 2952 cm1, representing a C¼ ¼C double bond of maleimide ring that becomes a CAC single bond after BMI resin polymerization. Besides, the band at 947 cm1 contributed by the C¼ ¼C stretching diminished. Accordingly, the polymerization through the ‘‘ene’’ reaction is almost complete. Further, BMI monomer is known to be strongly crystalline and shows relatively strong X-ray powder diffraction peaks, as revealed in Figure 3(a). However, interestingly, Figure 3(b) shows no XRD peaks for PBMI (after gelation). This implies that the BMI incorporating iodide-contained IL is amorphous.
Tg ¼
tp Tg;p ðal ag Þ þ ts Tg;s as tp ðal ag Þ þ ts as
(2)
where Tg,p and Tg,s are the glass transition temperatures of polymers and solvents, tp and ts are the volume fractions of polymers and solvents, al ag is the difference between the thermal expansion coefficients of liquid state and glassy state of the polymers, and as is the thermal expansion coefficient
Wang et al.15 showed that the conductivity–temperature data of polymer-gelled electrolytes were better fitted by the Vogel–Tammann–Fulcher (VTF) equation (eq 1)34 rðTÞ ¼ AT 1=2 exp½B=ðT T0 Þ
(1)
where A and B are constants and T0 is the temperature at which the diffusion of ions ceases to exist, which may be considered as the glass transition temperature (Tg) of the polymer-gelled system. Figure 4 shows PBMI-gelled electrolyte incorporated with 0.3 wt % of EAMNM has higher ionic conductivities than neat PBMI-gelled electrolytes, and similar to that of the liquid-type electrolyte (without BMI). In addition, the ionic conductivities of PDBBMI- and PMPBMI-gelled electrolytes are almost the same at various temperatures. For example, at a specific temperature of 30 C, the ionic conductivities of liquid-type electrolyte, PMPBMI-gelled, PDBBMI-gelled, PBMI-gelled, and PBMI/0.3 wt % EAMNMgelled electrolyte are 17.21, 15.72, 15.69, 16.02, and 17.03 mS cm1, respectively. Figure 5 shows the DSC thermograms of PMPBMI, PDBBMI, and PBMI polymers measured at a scanning rate of 10 C min1. Their Tg estimated from the step change of the curves was inserted in Figure 5. PMPBMI and PDBBMI possess the lowest Tg than that of PBMI, are 262, 240, and 283 C, respectively. As PMPBMI, PDBBMI, and PBMI polymers were employed to gel the 0.6 M BMImI/0.1 M I2/0.5 M TBP electrolyte system in PC and ACN (1:1 in v/v). This process significantly reduced their glass transition temperature according to the Kelly and Bueche equation,35
4954
FIGURE 5 DSC thermograms of: (a) PMPBMI, (b) PDBBMI, and (c) PBMI.
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA
ARTICLE
FIGURE 6 The photocurrent-voltage curves of the DSSC fabricated with the liquid-type, PMPBMI, PDBBMI, and PBMI-gelled electrolytes and PBMI/0.3 wt % EAMNM-gelled electrolyte under 100 mW cm2 illumination. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com].
of the solvents. If 120 K is adopted as the solvent Tg,36,37and al ag ¼ 3.0 104 K1 and as ¼ 3.0 104 K1,38,39 then the Tg of the PMPBMI-gelled, PDBBMI-gelled, and PBMIgelled systems can be estimated as 120.3, 118.3, and 128.2 K, respectively on the basis of eq 2. Figure 6 shows the photocurrent-voltage curve of DSSCs fabricated with the liquid-type electrolyte, PMPBMI-gelled, PDBBMI-gelled, PBMI-gelled, or PBMI/0.3 wt % EAMNMgelled electrolyte under 100 mW cm2 illumination. The composition of liquid electrolyte is consisted of 0.6 M BMImI, 0.1 M I2, 0.5 M TBP, 0.1 M GuSCN in PC and ACN (1:1 in v/v). The gel-type electrolytes were prepared by adding 6 wt % of MPBMI, DBBMI, or BMI monomers (vs. liquid electrolyte) into the liquid electrolyte. The PBMI/EAMNM composite gel electrolytes were prepared by adding 6 wt % BMI monomer and 0.3 wt % of EAMNM into the liquid electrolyte. Figure 6 and Table 1 show the photovoltaic parameters of Jsc, Voc, the light-to-electricity conversion efficiency (g) for five electrolytes studied. It shows that a proper
FIGURE 7 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 values of Rct1, Rct2, and Rdiff, all calculated from the EIS data. While the diffusivity (D) of I3 was calculated from Rdiff.
amount of EAMNM can effectively enhance the cell efficiency of gel-type DSSCs, and an efficiency up to 7.02% can be acquired by adding 0.3 wt % EAMNM to BMI gel-type electrolyte, as indicated in Table 1. Besides, the efficiencies of DSSCs prepared with PMPBMI-gelled, PDBBMI-gelled, or PBMI-gelled electrolytes are 5.83, 5.42, and 6.41%, respectively. Although this study shows that the addition of EAMNM can significantly enhance the efficiency of DSSC, the reason for this behavior remains unclear. It may be associated with a Grotthuss type transport mechanism40 or the space charge layer model in the electrolytes with nanocomposites41 of the exfoliated mica nanoplatelets that facilitate the diffusion of the ions. However, excess (>0.5 wt %) of EAMNM not only negate this beneficial effect but block the diffusion of I and I 3 in the BMI gel-type electrolyte, as evidenced by the cell performance in the Supporting Information Figure S2. Moreover, EIS technique is also used to realize the effect of adding EAMNM (0.3 wt %) on the PBMI geltype electrolyte. The EIS spectra of the DSSCs (Fig. 7) show three semicircles in the measured frequency range of 10 mHz–65 kHz. The ohmic serial resistance, Rs, is associated
TABLE 1 The Photovoltaic Performance of DSSCs with Liquid-Type Electrolyte, PMPBMI-Gelled, PDBBMI-Gelled, or PBMI-Gelled Electrolytes Under AM 1.5 Simulated Sunlight (100 mW cm22) Illumination Electrolytes
Voc (mV)
Jsc (mA cm2)
Liquid-type
0.650
MPBMI-gelled DBBMI-gelled
FF
g (%)
17.52
0.63
7.39
0.627
14.54
0.64
5.83
0.616
13.73
0.64
5.42
BMI-gelled
0.645
15.32
0.65
6.41
BMI/0.3 wt % EAMNM-gelled
0.650
17.14
0.63
7.02
The liquid-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); the gel-type electrolyte is consisted of 94 wt % of electrolyte containing 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.
EFFICIENT GEL-TYPE ELECTROLYTE WITH PBMI, CHEN ET AL.
Jsc: short-circuit photocurrent; Voc: open circuit photovoltage; FF: fill factor; g: energy conversion efficiency.
4955
JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
increased in the beginning stage (the first 10–20 h) because of the increase of Jsc, which was caused by penetration of the electrolyte through the dye-coated nanoporous TiO2 electrode. After 500 h of light-soaking at 55 C, the efficiencies (g) of DSSCs with liquid-type electrolytes (without BMI and EAMNM), PBMI-gel electrolytes, and PBMI/0.3 wt % EAMNM-gel electrolytes dropped by 9.2, 4.2, and 1.7%, respectively. Although initially the Jsc and g of DSSC with PBMI/0.3 wt % EAMNM-gel electrolyte (Jsc ¼ 17.14 mA cm2 and g ¼ 7.02%) are lower than that of liquid-type electrolyte (Jsc ¼ 17.52 mA cm2 and g ¼ 7.39%), the 500 h long-term stability of a DSSC made from PBMI/0.3 wt % EAMNM-gel electrolyte is superior to that of a DSSC based on liquid-type electrolyte. This reveals that PBMI-gel electrolyte possesses higher stability. Further, incorporating 0.3 wt % EAMNM into BMI-gel electrolytes not only improves the Jsc and g of DSSCs but increases the long-term stability of DSSCs under continuous light soaking at 55 C. CONCLUSIONS
FIGURE 8 The cell performance of DSSCs with the liquid-type, PMPBMI, PDBBMI, and PBMI-gelled electrolytes and PBMI/0.3 wt % EAMNM-gelled electrolyte under continuous light soaking of one sun (100 mW cm2) at 55 C.
with the series resistance of the electrolytes and electric contacts in the DSSCs. And Rct1, Rct2, and Rdiff correspond to the charge transfer process occurring at the Pt counter electrode (corresponding the first arc), the TiO2/dye/electrolyte interface (corresponding the second arc) and the Warburg diffusion process of I/I 3 in the electrolyte (corresponding the third arc), respectively. The EIS spectra show that Rct2 and Rdiff decreased from 12.11 and 8.23 X to 11.79 and 7.82 X, respectively, when EAMNM (0.3 wt %) is incorporated into the PBMI-gel electrolyte system. Further, the diffusivity of I 3 (D), which was calculated from Rdiff, as shown in the inset of Figure 7, increases from 4.53 106 to 5.67 106 cm2 s1 in the presence of EAMNM in the gel electrolyte. Incorporating 0.3 wt % EAMNM into a BMI-gel type electrolyte results in some enhancement in the device stability under continuous light soaking of one sun (100 mW cm2) at 55 C, as revealed in Figure 8. Figure 8(a) shows that after 500 h of light-soaking at 55 C, the Voc and FF of DSSCs based on liquid-type electrolytes (without BMI or EAMNM), BMI-gel electrolytes, and BMI/0.3 wt % EAMNM-gel electrolytes dropped less than 1%. Figure 8(b) shows even more impressive results. As expected, the conversion efficiency
4956
The characteristics of gel-type DSSCs fabricated with PMPBMI-gelled, PDBBMI-gelled, or PBMI-gelled electrolytes were explored in this work. The incorporation of 0.3 wt % of the EAMNM in the BMI-gelled electrolyte leads to higher short-circuit current density (Jsc ¼ 17.14 mA cm2) and efficiency (g ¼ 7.02%) of the cell than that of neat BMI-gel electrolyte (Jsc ¼ 15.32 mA cm2, g ¼ 6.41%). Encouragingly, the use of the 0.3 wt % EAMNM incorporated in PBMI-gelled electrolyte was found to result in some enhancement in the device stability under continuous light soaking with one sun (100 mW cm2) at 55 C. The efficiency (g ¼ 7.02%) of DSSCs with PBMI/0.3 wt % EAMNM-gel electrolyte had only dropped by 1.7% (g ¼ 6.93%) after 500 h of continuous light soaking measurement. Consequently, 0.3 wt % EAMNM blended in PBMI-gel electrolyte indeed not only improve the Jsc and g of DSSCs, but increase the long-term stability of DSSCs. The authors acknowledge the financial support received from the National Science Council of Taiwan. REFERENCES AND NOTES 1 Gra¨tzel, M. J Photochem Photobiol A Chem 2004, 164, 3–14. 2 O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740. 3 Hara, K.; Nishikawa, T.; Sayama, K.; Aika, K.; Arakawa, H. Chem Lett 2003, 32, 1014–1015. 4 Wang, H. X.; Li, H.; Xue, B. F.; Wang, Z. X.; Meng, Q. B.; Chen, L. Q. J Am Chem Soc 2005, 127, 6394–6401. 5 Wang, P.; Dai, Q.; Zakeeruddin, S. M.; Forsyth, M.; MacFarlane, D. R.; Gra¨tzel, M. J Am Chem Soc 2004, 126, 13590–13591. 6 Dai, Q.; MacFarlane, D. R.; Howlett, P. C.; Forsyth, M. Angew Chem Int Ed Engl 2005, 44, 313–316. 7 Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissorter, F.; Salbeck, J.; Speritzer, H.; Gra¨tzel, M. Nature 1998, 395, 583–585.
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA
ARTICLE 8 Kim, D. W.; Jeong, Y. B.; Kim, S. H.; Lee, D. Y.; Song, J. S. J Power Sources 2005, 149, 112–116.
25 Wan, D.; Huang, J. J Polym Sci A Polym Chem 1999, 37, 2755–2761.
9 Zhang, X.; Yang, H.; Xiong, H. M.; Li, F. Y.; Xia, Y. Y. J Power Sources 2006, 160, 1451–1455.
26 Tu, C. W.; Liu, K. Y.; Chien, A. T.; Yen, M. H.; Weng, T. H.; Ho, K. C.; Lin, K. F. J Polym Sci A Polym Chem 2008, 46, 47–53.
10 Wang, M.; Yin, X.; Xiao, X. R.; Zhou, X. W.; Yang, Z. Z.; Li, X. P.; Lin, Y. J Photochem Photobiol A Chem 2008, 194, 20–26.
27 Ray, S. S.; Okamoto, M. Prog Polym Sci 2003, 28, 1539–1641.
11 Gorlov, M.; Kloo, L. Dalton Trans 2008, 2655–2666.
28 Chen, C. Y.; Chen, J. G.; Wu, S. J.; Li, J. Y.; Wu, C.G.; Ho, K. C. Angew Chem Int Ed Engl 2008, 47, 7342–7345.
12 Wang, L.; Fang, S.; Lin, Y.; Zhou, X.; Li, M. Chem Commun 2005, 5687–5689. 13 Suzuki, K.; Yamaguchi, M.; Hotta, S.; Tanabe, N.; Yanagida, S. J Photochem Photobiol A Chem 2004, 164, 81–85. 14 Winther-Jensen, O.; Armel, V.; Forsyth, M.; MacFarlane, D. R. Macromol Rapid Commun 2010, 31, 479–483. 15 Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Gra¨tzel, M. Nat Mater 2003, 2, 402–407. 16 Murai, S.; Mikoshiba, S.; Sumino, H.; Hayase, S. J Photochem Photobiol A Chem 2002, 148, 33–39. 17 Murai, S.; Mikoshiba, S.; Sumino, H.; Kato, T.; Hayase, S. Chem Commun 2003, 1534–1536. 18 Suzuki, K.; Yamaguchi, M.; Hotta, S.; Tanabe, N.; Yanagida, S. J Photochem Photobiol A Chem 2004, 164, 81–85. 19 Kato, T.; Okazaki, A.; Hayase, S. Chem Commun 2005, 363–365. 20 Tripathi, V. S.; Lal, D.; Aggarwal, S. K.; Sen, A. K. J Appl Polym Sci 1997, 66, 1613–1619. 21 Oremce, M. F.; OT, G. L.; Cunha, L. D. Eur Polym J 1998, 34, 95–102. 22 Hopewell, J. L.; Hill, D. J. T.; Pomery, P. J. Polymer 1998, 39, 5601–5607. 23 Rozenberg, B. A.; Dzhavadyan, E. A.; Morgan, R.; Shin, E. Polym Adv Technol 2002, 13, 837–844. 24 Seris, A.; Feve, M.; Mechin, F.; Pascault, J. P. J Appl Polym Sci 1993, 48, 257–269.
EFFICIENT GEL-TYPE ELECTROLYTE WITH PBMI, CHEN ET AL.
29 Longo, C.; Freitas, J.; De Paoli, M. A. J Photochem Photobiol A Chem 2003, 159, 33–39. 30 Han, L.; Koide, N.; Chiba, Y.; Mitate, T. Appl Phys Lett 2004, 84, 2433–2435. 31 Hoshikawa, T.; Kikuchi, R.; Eguchi, K. J Electroanal Chem 2006, 588, 59–67. 32 Lin, K. F.; Lin, J. S.; Cheng, C. H. J Polym Sci A Polym Chem 1997, 35, 2469–2478. 33 Baek, J. B.; Ferguson, J. B.; Tan, L. S. Macromolecules 2003, 36, 4385–4396. 34 Huang, S. Y.; Schlichtho¨rl, G.; Nozik, A. J.; Gra¨tzel, M.; Fank, A. J. J Phys Chem B 1997, 101, 2576–2582. 35 Kelley, F. N.; Bueche, F. J Polym Sci 1961, 50, 549–556. 36 Ghoral, P. K.; Matyushov, D. V. J Chem Phys 2006, 124, 144510. 37 Kim, J. Y.; Hong, S. U.; Won, J.; Kang, Y. S. Macromolecules 2000, 33, 3161–3165. 38 Scho¨nhals, A.; Kremer, F.; Hofmann, A.; Fischer, E. W.; Schlosser, E. Phys Rev Lett 1993, 70, 3459–3462. 39 Donnellan, T. M.; Roylance, D. Polym Eng Sci 1992, 32, 415–420. 40 Yanagida, S. C. R. Chimie 2006, 9, 597–604. 41 Jiang, S.; Wagner, J. B. J Phys Chem Solids 1995, 56, 1113–1124.
4957
