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Printable electrolytes for highly efficient quasi-solid-state dye-sensitized solar cells
Electrochimica Acta 91 (2013) 302–306
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Printable electrolytes for highly efficient quasi-solid-state dye-sensitized solar cells Chaolei Wang, Liang Wang, Yantao Shi, Hong Zhang, Tingli Ma ∗ State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, China
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Article history: Received 17 November 2012 Received in revised form 20 December 2012 Accepted 22 December 2012 Available online 31 December 2012 Keywords: Printable Polymer gel electrolyte P(VA-co-MMA) Quasi-solid-state dye-sensitized solar cells
a b s t r a c t Novel polymer gel electrolytes (PGEs) with high ionic conductivity based on polyvinyl (acetate-co-methyl methacrylate) [P(VA-co-MMA)] were prepared by soaking porous copolymers in an organic electrolyte solution [acetonitrile (ACN) or 3-Methoxypropionitrile (MPN)] that contained an I3 − /I− as redox couple. Quasi-solid-state dye-sensitized solar cells (QS-DSSCs) were fabricated with the PGEs, and the best PGE was selected and optimized. Using the best PGE and under 100 mW cm−2 light illumination (AM1.5), the QS-DSSC achieved a high photovoltaic conversion efficiency of 9.10%, nearly the same as that for the DSSC based on the original liquid electrolyte. Introduction of TiO2 nanoparticles into the PGEs further enhanced PGEs ionic conductivity and the conversion efficiency to 9.40%. Subsequent results revealed that our QS-DSSC had a better stability because it could maintain 96.7% of its initial efficiency after long-time (1000 h) exposure to simulative sunlight. Besides, for the first time, large-area QS-DSSCs were fabricated by screen printing of PGE, other than the traditional vacuum injection that was infeasible for the viscous gel electrolyte. Finally, our 5 cm × 7 cm QS-DSSC sub-module exhibited a conversion efficiency higher than 4%. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction In the past two decades, dye-sensitized solar cells (DSSCs) have attracted considerable attention due to its high photovoltaic conversion efficiency (PCE), simple fabrication processes, and cost effectiveness [1]. So far, PCEs of 11% and 12.3% were obtained based on liquid electrolytes containing I− /I3 − and cobalt redox couples, respectively [2,3]. However, as we know, organic solvents (e.g., acetonitrile (ACN) or 3-methoxypropionitrile (MPN)) based liquid electrolytes always suffer from leakage and evaporation after long-term outdoor use [4]. To solve this problem, many efforts have been made by replacing the liquid electrolytes with ionic liquids [5,6], p-type semiconductors [7], organic hole transport materials [8], or polymer gel electrolytes (PGEs) [9–15]. PGEs, a potential alternative to liquid and solid electrolytes, have high ionic conductivity and good stability. This is because the polymer matrix in PGEs can effectively trap the liquid solvent to inhibit evaporation and meanwhile provide channels for fast transport of redox couples. Therefore, photovoltaic performances of the DSSCs fabricated with some specialized PGEs were comparable to that based on liquid electrolytes. For PGEs, polymer is one of the key components determining the
∗ Corresponding author. Tel.: +86 411 84986237; fax: +86 411 84986230. E-mail address: [email protected] (T. Ma). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.12.096
conductivity of electrolyte and the final efficiency of DSSC. According to previous reports, PGEs can be fabricated by gelating liquid electrolytes using polyacrylonitrile (PAN) [9], polyvinyidene fluoride–co-hexafluoro propylene (PVDF-HFP) [10], polyethylene oxide (PEO) [11], polyacrylonitrile–co-methyl methacrylate (PANMMA) [12,13], polymethyl methacrylate (PMMA) [14] and so on. However, polymers have high molecular weights and require elevated temperatures for casting or injection into cells, leading to the poor electrolyte/TiO2 interfacial contact and inconvenience in further production of large-area DSSC modules. To solve these problems, in situ polymerization by thermal or photo-polymerization has been developed [15,16]. Up to now, the efficiencies of DSSCs based on PGEs remain relatively low. PVA has strong adhesion and good solubility in polar solvents. These properties may enhance the interfacial contact between the PGE and photoanodes. MMA can be used as a co-monomer due to its ability to be gelled easily by polar solvents. The network structure of a polymeric gelling agent significantly influences the ionic conductivity of PGEs and the photovoltaic performance of quasi-solid-state dye-sensitized solar cells (QS-DSSCs). Therefore, in this study, a novel polyvinyl acetate-co-methyl methacrylate [P(VA-co-MMA)] copolymer was prepared and a series of PGEs was prepared by adjusting the addition. For our QS-DSSCs, high PCEs of 9.10% and 8.61% were achieved using ACN and MPN based PGEs containing 20 wt% P(VA-co-MMA) (versus liquid electrolyte). Moreover, by enhancing the ionic conductivity, introduction of
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TiO2 nanoparticles (5 wt%, versus liquid electrolyte) into the PGEs could promote the conversion efficiencies of our QS-DSSCs to 9.40% and 8.92%, respectively, for ACN and MPN based system. Long-term stability tests revealed that in our QS-DSSCs the PGE exhibited better stability after 1000 h exposure to simulative sunlight. By the screen printing of PGEs, we also fabricated 5 cm × 7 cm (ca. 16.2 cm2 in active area) parallel-connected QSDSSC sub-module and finally an efficiency higher than 4% was achieved. 2. Experimental 2.1. Materials Methyl methacrylate (MMA, 99%, AR), vinyl acetate (VA, 99%, AR), ammonium peroxydisulfate (AR), sodium dodecyl sulfate (AR), sodium bicarbonate (AR), aluminum sulfate (AR), acetonitrile (ACN, for HPLC, ≥99.9%), LiI (99%, GC), I2 (99.99% metals basis, Pharmaceutical Grade), 4-tert-butylpyridine (TBP, 98%, GC), guandine thiocyanate (GuSCN, ≥ 99.9%, Molecular Biology Grade) were purchased from Aladdin Chemical Reagent Co. Ltd., China. 3methoxypropionitrile (MPN, 98%, GC) was obtained from ACROS ORGANICS Co. Ltd., China. 1-Methyl-3-propylimidazolium iodide (PMII, 98%) was commercially available in China. All reagents except for the MMA and VA monomer were directly used without further purification. MMA and VA monomer were washed with 5 wt% NaOH solution and then washed with deionized water to neutral, finally distilled to remove the polymerization inhibitor before using. 2.2. Synthesis of P(VA-co-MMA) copolymer and preparation of PGEs P(VA-co-MMA) copolymers were prepared using the semicontinuous seed emulsion polymerization method [17]. Firstly 40 mL of deionized water containing 0.2 g of SDS, 70 mg of ammonium peroxydisulfate, and 70 mg of sodium bicarbonate was added to a 250 mL three-necked flask. The flask was heated to 70 ◦ C with stirring under a nitrogen atmosphere. After 20 min, 8 wt% of the mixed monomer (8 g MMA and 12 g VA) was added to the above aqueous solution. The seeds almost completed polymerization after 30 min, and the remaining mixed monomers were slowly dropped at about 0.6 mL/min. When the dropping was completed after 1 h, the reaction was stopped and cooled to room temperature. The solution was then poured into a 3 wt% aluminum sulfate aqueous solution to demulsify. After the solution was filtered, washed with deionized water several times, and dried at 60 ◦ C under a vacuum to constant weight. Using the same method, PMMA and PVA homo-polymers were synthesized. The PGEs for DSSCs were obtained by immersing varying weights of the as-prepared copolymers into the I3 − /I− -based liquid electrolyte with stirring for 24 h. The liquid electrolyte contained 0.1 M LiI, 0.6 M PMII, 0.05 M I2 , 0.1 M GuSCN, 0.5 M TBP, and ACN or MPN solvent. When 5 wt% (versus liquid electrolyte) TiO2 (P25) nanoparticles were added to the PGEs, PGEs with TiO2 were obtained. 2.3. Characterization Infrared absorption spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer (NEXUS). The PVA, PMMA and P(VA-co-MMA) copolymer samples were mixed with KBr powder respectively and pressed into films, and then were transferred to the FT-IR instrument. The ionic conductivity of liquid electrolytes
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and PGEs was measured with BEC-307 digitized conductivity meter (Bell, China) at room temperature. 2.4. Preparation of TiO2 photoanode The screen-printable TiO2 pastes were prepared according to a procedure developed by our group [18]. The 13 mthick nanocrystalline TiO2 films were fabricated on a F-doped conducting glass substrate (FTO, Asahi Glass Co., Ltd.; sheet resistance: 7 /sq) by screen printing, followed by sintering under airflow at 500 ◦ C for 30 min. After cooling to 80 ◦ C, the films were immersed in ACN and tert–butyl alcohol mixed solution (volume ratio, 1:1) containing ruthenium-535bis-TBA (0.5 mM) (N719, Solaronix) for 22 h at 25 ◦ C, followed by rinsing with ethanol. The active area of DSSCs was 0.16 cm2 . 2.5. Fabrication of DSSCs and photoelectrochemical measurements PGE was coated onto a sensitized electrode by doctor blading method. An open sandwich-type cell was assembled using two clips to clap a sputtered, platinized FTO counter electrode against the sensitized electrode coated with the PGE. The cells with liquid electrolyte were also tested for comparison. The photovoltaic performance of the DSSCs was evaluated at AM 1.5 illumination (100 mW cm−2 ; Peccell-L15, Peccell, Japan) using a Keithley digital source meter (Keithley 2601, USA). Dark currents were measured under the same conditions but in darkness. Electrochemical impedance spectroscopy (EIS) experiments were conducted using a computer-controlled potentiostat (ZeniumZahner, Germany). The measured frequency ranged from 100 mHz to 1 MHz while the AC amplitude was set to 10 mV. The bias of all EIS measurements was set to −0.75 V in darkness. The EIS analysis was fitted by Zview software. 2.6. Stability test of the DSSCs To study the stability of the P(VA-co-MMA)-based QS-DSSCs, the devices needed to be sealed. The sealed liquid electrolytebased cells were fabricated according to a previously reported procedure [19], whereas the QS-DSSCs were sealed as follows. The PGE was coated on a dye-coated TiO2 photoanode, around which a 60 m-thick Surlyn (1702 DuPont) was wrapped. Then, a “two-step dip-coated” Pt counter electrode was pressed on the photoanode [20]. The cell was placed at 125 ◦ C under a certain pressure until the sealing spacer completely melted. Excess PGE was extruded through the pre-drilled hole on the Pt counter electrode. Finally, the hole was sealed by a cover glass. To test the stabilities of the devices, the DSSCs based on liquid electrolyte and the PGE were exposed to simulative sunlight at 30 ◦ C. 2.7. Preparation of a 5 cm × 7 cm QS-DSSC sub-module by screen printing PGE The rectangular shapes surrounded by the current-collecting Ag grids were selected from a 5 cm × 7 cm DSSC sub-module structure [21]. This structure prevented the large resistivity increase of the FTO/glass with increased cell area. The size of one rectangle was 9 mm × 60 mm; the width of one Ag grid line covered by over-layers between two rectangles was 1 mm. Three rectangular shapes were included in a 5 cm × 7 cm DSSC sub-module. The active area of this sub-module was 16.2 cm2 . The submodel DSSC was sealed similarly as the small-sized cell based on PGE.
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Fig. 1. FT-IR spectra of PVA, PMMA and P(VA-co-MMA).
3. Results and discussion 3.1. FT-IR studies Fig. 1 shows the FT-IR spectra of PVA, PMMA, and P(VA-coMMA). The band at 1731 cm−1 corresponds to the C O stretching vibration, and the two doublet bands at 1150 and 1190 cm−1 as well as 1240 and 1265 cm−1 correspond to the C O stretching vibrations of the ester groups in PMMA [22–24]. The characteristic band at 1738 cm−1 corresponds to the C O stretching vibration, and the band at 1373 cm−1 corresponds to the CH3 symmetric bending vibrations in PVA [25]. The characteristic peaks in the PMMA and PVA spectra all appear in P(VA-co-MMA). All these bands in the infrared spectra prove that synthesis of P(VA-co-MMA) in our experiment was successful. 3.2. Performances of the DSSCs fabricated with PGEs with varying copolymer addition With increasing the addition of the polymer, PGE becomes viscous thus diffusion of the ionic redox couples will be slowed down, negatively affecting the performance of the DSSC. On the other hand, in addition to gelating the liquid electrolyte, polymers in the PGE have some other positive effects, e.g., facilitating electron transport and suppressing charge recombination. Therefore, the purpose of this experiment is to find out the optimized addition of our copolymer. Fig. 2 shows the J–V curves of the DSSCs fabricated with the MPN based PGEs with different additions of copolymer. Due to the competitive effects of the polymer in the electrolyte, finally, DSSCs performed best with 20 wt% P(VA-co-MMA) in the PGE. Unless stated, otherwise the addition of P(VA-co-MMA) is 20 wt% for our subsequent investigations. 3.3. Influences of the incorporation of TiO2 nanoparticles on the ionic conductivities of PGEs and the performance of QS-DSSCs Table 1 shows the measured ionic conductivities () of the liquid and gel-state electrolytes. For the liquid electrolytes, the ionic conductivity of the ACN system (8.79 mS cm−1 ) is more than two times higher than that of the MPN system (3.40 mS cm−1 ). After gelation with 20 wt% P(VA-co-MMA), ACN based system still remains high conductivity, 7.18 mS cm−1 , much higher than that of 2.60 mS cm−1 for MPN based. As stated previously, adding inorganic nanoparticles favors to enhance the conductivity through decreasing polymers
Fig. 2. J–V curves of DSSCs fabricated with PGEs based on MPN solvent with different copolymer additons.
crystallinity. In our experiment, 5 wt% TiO2 nanoparticles were added into our PGEs. It can be found from Table 1 that the conductivity of the ACN based PGE was further increased to 7.52 mS cm−1 , whereas no obvious improvement was realized for the MPN based PGE. Fig. 3 shows the current–voltage (J–V) curves of the DSSCs fabricated with liquid electrolyte and the PGEs based on ACN (a) and MPN (b) systems under AM 1.5, 100 mW cm−2 light illumination. For a better photovoltaic performance, we added a 4 m thicked light-scattering layer on the 13 m thicked nanocrystalline TiO2 . And the TiO2 films were further treated with a TiCl4 solution (40 mM) at 70 ◦ C for 30 min. The open circuit potential (Voc ), shortcircuit current density (Jsc ), fill factor (FF), and total PCE of these cells are also listed in Table 1 For the ACN system, the efficiency achieved by the QS-DSSCs is 9.10%, close to 9.59% for the DSSC fabricated with the original liquid electrolyte. Using the TiO2 contained PGE, a higher conversion efficiency of 9.40% was obtained. To the best of our knowledge, this efficiency can rank the tops of the QS-DSSCS. When MPN was used instead of ACN, the efficiency achieved by the cells fabricated with PGE is 8.61%, whereas 9.14% is achieved by the liquid-version cells at AM 1.5. Addition of 5 wt% TiO2 nanoparticles results in a similar effect as in the MPN system. Although Voc and FF decreased with the introduction of P(VA-co-MMA) to the liquid electrolyte, Jsc increased after 5 wt% TiO2 addition to the PGE. Consequently, the cells with PGE + TiO2 achieve an efficiency of 8.98%, which is 98.2% of the value based on liquid electrolyte. 3.4. EIS analysis of the DSSCs To investigate the impedance behavior of the DSSCs assembled with different types of electrolytes, the electrochemical impedance of the DSSCs was measured, and the results are shown in Fig. 4. Table 1 Ionic conductivity measured for the liquid electrolytes and PGEs as well as the photovoltaic parameters obtained from the J–V curves of DSSCs using these electrolytes. The performances of DSSCs were measured at AM 1.5 (100 mW cm−2 ) with an active area of 0.16 cm2 . Electrolyte
(mS cm−1 )
Voc (V)
Jsc (mA cm−2 )
FF
PCE (%)
ACN-liquid ACN-PGE ACN-PGE + TiO2 MPN-liquid MPN-PGE MPN-PGE + TiO2
8.79 7.18 7.52 3.40 2.60 2.62
0.75 0.74 0.75 0.74 0.71 0.73
17.27 16.66 16.93 17.40 17.32 17.58
0.74 0.74 0.74 0.71 0.70 0.70
9.59 9.10 9.40 9.14 8.61 8.98
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Table 2 The parameters of the equivalent circuits used to fit the EIS impedance data of the DSSCs in Fig.4.
Liquid PGE PGE + TiO2
Rs ()
Rct1 ()
ZN ()
13.17 13.39 11.32
41.01 36.31 38.09
29.02 62.32 47.74
frequency region), and Nernstian diffusion within the electrolyte (low frequency region) [26,27]. In our study, two semi-circles are observed because of overlaps happened between high and middle frequency circles. Therefore, they are labeled by Rct1 (charge transfer resistance at the TiO2 /dye/electrolyte interface) and ZN (Nernstian diffusion). The ohmic serial resistance (Rs ) (intersection of high-frequency circle and x-axis) is mainly associated with the sheet resistance of the FTO substrate. As shown in Fig. 4, it was seen that Rct1 decreased from 41.01 to 36.31 with the addition of our copolymer P(VA-co-MMA). However, it increased from 36.31 to 38.09 again with the addition of TiO2 nanoparticles to the PGE, suggesting that addition of TiO2 nanoparticles into the electrolyte could reduce the charge recombination at the TiO2 /dye/electrolyte interface. This result is consistent with the variations of the Voc and the dark currents in Fig. 3. On the other hand, ZN first increased from 29.02 to 62.32 with the addition of P(VA-co-MMA) and then it decreased to 47.74 with presence of TiO2 nanoparticles to the PGE, in good accordance with the data listed in Table 1. 3.5. Stability test of the DSSCs fabricated with PGE
Fig. 3. J–V curves of the DSSCs fabricated with liquid electrolytes as well as PGEs for ACN (a) and MPN (b) system at AM 1.5 (100 mW cm−2 ) and in dark. The PGEs contained 20 wt% P(VA-co-MMA) with or without 5 wt% TiO2 nanoparticles.
The parameters of the equivalent circuits in the inset of Fig. 4 that was used to fit the EIS impedance are summarized in Table 2. In general, there should be three semi-circles in a typical EIS spectra of the DSSC, corresponding to the charge transfer resistance at the Pt counter electrode (high frequency region), the charge transfer resistance at the TiO2 /dye/electrolyte interface (middle
Fig. 4. Nyquist plots of the DSSCs fabricated with liquid electrolyte and PGEs with and without 5 wt% TiO2 based on MPN solvent measured in dark under bias −0.75 V (inset is the equivalent circuit to fit the EIS impedance data of DSSCs).
Fig. 5 shows the long-term stability test of the DSSCs fabricated with liquid electrolyte and PGE, respectively. It can be found that DSSC based on the PGE containing 20 wt% P(VA-co-MMA) was more durable over a period of 1000 h since it could maintain 96.7% of its initial efficiency. By contrast, the other one based on liquid electrolyte just kept 65.0% of its initial efficiency. This finding indicates the potential of PGE in improving the stability of the DSSCs. 3.6. Performance of the 5 cm × 7 cm QS-DSSC sub-module by screen printing of PGE To accelerate the process of industrialization of DSSCs, the large-area module (5 cm × 7 cm, active area was 16.2 cm2 ) was constructed by screen printing of PGE (based on MPN system, with
Fig. 5. The normalized efficiency of the DSSC with liquid electrolyte and PGE at AM 1.5 versus soaking time.
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Fig. 6. J–V curve of the 5 cm × 7 cm QS-DSSC sub-module by screen printing PGE at AM 1.5 (insets are the photovoltaic parameters obtained from the J–V curves of QS-DSSC sub-module).
5 wt% TiO2 ). Fig. 6 shows the J–V curve of our large-area DSSC submodule and the obtained efficiency was 4.39%. To the best of our knowledge, this is the first time reporting the use of a PGE to construct a sub-module DSSC by screen printing method. 4. Conclusion Novel PGEs with high ionic conductivity based on copolymer polyvinyl (acetate-co-methyl methacrylate) were prepared in combination of organic electrolyte solution. QS-DSSCs were fabricated after the addition of our copolymer in PGEs was optimized to be 20 wt%. Under AM 1.5, 100 mW cm−2 light illumination, an efficiency of 9.10% for the QS-DSSCs was achieved and then was further enhanced to 9.40% through introducing TiO2 nanoparticles into the PGEs to increase the ionic conductivity. Long-term stability tests revealed that in our QS-DSSCs the PGE exhibited better stability after 1000 h exposure to simulative sunlight. Finally, we use the screen printing technique to paste PGE for fabrication of large-area QS-DSSC sub-module. With an active area of 16.2 cm2 , a high conversion of 4.39% was obtained. Our results provide a feasible way of screen printing of PGEs in future mass-production of QS-DSSC module. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant No. 51273032), the National High Technology Research and Development Program for Advanced Materials of China (Grant No. 2009AA03Z220), Specialized Research Fund for the Doctoral Program of Higher Education of China (20110041110003), Open project of the State Key Laboratory for Physical Chemistry of Solid Surfaces of Xiamen University (Grant No. 201210) and the State Key Laboratory of Fine Chemicals of China. References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Printable electrolytes for highly efficient quasi-solid-state dye-sensitized solar cells Chaolei Wang, Liang Wang, Yantao Shi, Hong Zhang, Tingli Ma ∗ State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, China
a r t i c l e
i n f o
Article history: Received 17 November 2012 Received in revised form 20 December 2012 Accepted 22 December 2012 Available online 31 December 2012 Keywords: Printable Polymer gel electrolyte P(VA-co-MMA) Quasi-solid-state dye-sensitized solar cells
a b s t r a c t Novel polymer gel electrolytes (PGEs) with high ionic conductivity based on polyvinyl (acetate-co-methyl methacrylate) [P(VA-co-MMA)] were prepared by soaking porous copolymers in an organic electrolyte solution [acetonitrile (ACN) or 3-Methoxypropionitrile (MPN)] that contained an I3 − /I− as redox couple. Quasi-solid-state dye-sensitized solar cells (QS-DSSCs) were fabricated with the PGEs, and the best PGE was selected and optimized. Using the best PGE and under 100 mW cm−2 light illumination (AM1.5), the QS-DSSC achieved a high photovoltaic conversion efficiency of 9.10%, nearly the same as that for the DSSC based on the original liquid electrolyte. Introduction of TiO2 nanoparticles into the PGEs further enhanced PGEs ionic conductivity and the conversion efficiency to 9.40%. Subsequent results revealed that our QS-DSSC had a better stability because it could maintain 96.7% of its initial efficiency after long-time (1000 h) exposure to simulative sunlight. Besides, for the first time, large-area QS-DSSCs were fabricated by screen printing of PGE, other than the traditional vacuum injection that was infeasible for the viscous gel electrolyte. Finally, our 5 cm × 7 cm QS-DSSC sub-module exhibited a conversion efficiency higher than 4%. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction In the past two decades, dye-sensitized solar cells (DSSCs) have attracted considerable attention due to its high photovoltaic conversion efficiency (PCE), simple fabrication processes, and cost effectiveness [1]. So far, PCEs of 11% and 12.3% were obtained based on liquid electrolytes containing I− /I3 − and cobalt redox couples, respectively [2,3]. However, as we know, organic solvents (e.g., acetonitrile (ACN) or 3-methoxypropionitrile (MPN)) based liquid electrolytes always suffer from leakage and evaporation after long-term outdoor use [4]. To solve this problem, many efforts have been made by replacing the liquid electrolytes with ionic liquids [5,6], p-type semiconductors [7], organic hole transport materials [8], or polymer gel electrolytes (PGEs) [9–15]. PGEs, a potential alternative to liquid and solid electrolytes, have high ionic conductivity and good stability. This is because the polymer matrix in PGEs can effectively trap the liquid solvent to inhibit evaporation and meanwhile provide channels for fast transport of redox couples. Therefore, photovoltaic performances of the DSSCs fabricated with some specialized PGEs were comparable to that based on liquid electrolytes. For PGEs, polymer is one of the key components determining the
∗ Corresponding author. Tel.: +86 411 84986237; fax: +86 411 84986230. E-mail address: [email protected] (T. Ma). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.12.096
conductivity of electrolyte and the final efficiency of DSSC. According to previous reports, PGEs can be fabricated by gelating liquid electrolytes using polyacrylonitrile (PAN) [9], polyvinyidene fluoride–co-hexafluoro propylene (PVDF-HFP) [10], polyethylene oxide (PEO) [11], polyacrylonitrile–co-methyl methacrylate (PANMMA) [12,13], polymethyl methacrylate (PMMA) [14] and so on. However, polymers have high molecular weights and require elevated temperatures for casting or injection into cells, leading to the poor electrolyte/TiO2 interfacial contact and inconvenience in further production of large-area DSSC modules. To solve these problems, in situ polymerization by thermal or photo-polymerization has been developed [15,16]. Up to now, the efficiencies of DSSCs based on PGEs remain relatively low. PVA has strong adhesion and good solubility in polar solvents. These properties may enhance the interfacial contact between the PGE and photoanodes. MMA can be used as a co-monomer due to its ability to be gelled easily by polar solvents. The network structure of a polymeric gelling agent significantly influences the ionic conductivity of PGEs and the photovoltaic performance of quasi-solid-state dye-sensitized solar cells (QS-DSSCs). Therefore, in this study, a novel polyvinyl acetate-co-methyl methacrylate [P(VA-co-MMA)] copolymer was prepared and a series of PGEs was prepared by adjusting the addition. For our QS-DSSCs, high PCEs of 9.10% and 8.61% were achieved using ACN and MPN based PGEs containing 20 wt% P(VA-co-MMA) (versus liquid electrolyte). Moreover, by enhancing the ionic conductivity, introduction of
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TiO2 nanoparticles (5 wt%, versus liquid electrolyte) into the PGEs could promote the conversion efficiencies of our QS-DSSCs to 9.40% and 8.92%, respectively, for ACN and MPN based system. Long-term stability tests revealed that in our QS-DSSCs the PGE exhibited better stability after 1000 h exposure to simulative sunlight. By the screen printing of PGEs, we also fabricated 5 cm × 7 cm (ca. 16.2 cm2 in active area) parallel-connected QSDSSC sub-module and finally an efficiency higher than 4% was achieved. 2. Experimental 2.1. Materials Methyl methacrylate (MMA, 99%, AR), vinyl acetate (VA, 99%, AR), ammonium peroxydisulfate (AR), sodium dodecyl sulfate (AR), sodium bicarbonate (AR), aluminum sulfate (AR), acetonitrile (ACN, for HPLC, ≥99.9%), LiI (99%, GC), I2 (99.99% metals basis, Pharmaceutical Grade), 4-tert-butylpyridine (TBP, 98%, GC), guandine thiocyanate (GuSCN, ≥ 99.9%, Molecular Biology Grade) were purchased from Aladdin Chemical Reagent Co. Ltd., China. 3methoxypropionitrile (MPN, 98%, GC) was obtained from ACROS ORGANICS Co. Ltd., China. 1-Methyl-3-propylimidazolium iodide (PMII, 98%) was commercially available in China. All reagents except for the MMA and VA monomer were directly used without further purification. MMA and VA monomer were washed with 5 wt% NaOH solution and then washed with deionized water to neutral, finally distilled to remove the polymerization inhibitor before using. 2.2. Synthesis of P(VA-co-MMA) copolymer and preparation of PGEs P(VA-co-MMA) copolymers were prepared using the semicontinuous seed emulsion polymerization method [17]. Firstly 40 mL of deionized water containing 0.2 g of SDS, 70 mg of ammonium peroxydisulfate, and 70 mg of sodium bicarbonate was added to a 250 mL three-necked flask. The flask was heated to 70 ◦ C with stirring under a nitrogen atmosphere. After 20 min, 8 wt% of the mixed monomer (8 g MMA and 12 g VA) was added to the above aqueous solution. The seeds almost completed polymerization after 30 min, and the remaining mixed monomers were slowly dropped at about 0.6 mL/min. When the dropping was completed after 1 h, the reaction was stopped and cooled to room temperature. The solution was then poured into a 3 wt% aluminum sulfate aqueous solution to demulsify. After the solution was filtered, washed with deionized water several times, and dried at 60 ◦ C under a vacuum to constant weight. Using the same method, PMMA and PVA homo-polymers were synthesized. The PGEs for DSSCs were obtained by immersing varying weights of the as-prepared copolymers into the I3 − /I− -based liquid electrolyte with stirring for 24 h. The liquid electrolyte contained 0.1 M LiI, 0.6 M PMII, 0.05 M I2 , 0.1 M GuSCN, 0.5 M TBP, and ACN or MPN solvent. When 5 wt% (versus liquid electrolyte) TiO2 (P25) nanoparticles were added to the PGEs, PGEs with TiO2 were obtained. 2.3. Characterization Infrared absorption spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer (NEXUS). The PVA, PMMA and P(VA-co-MMA) copolymer samples were mixed with KBr powder respectively and pressed into films, and then were transferred to the FT-IR instrument. The ionic conductivity of liquid electrolytes
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and PGEs was measured with BEC-307 digitized conductivity meter (Bell, China) at room temperature. 2.4. Preparation of TiO2 photoanode The screen-printable TiO2 pastes were prepared according to a procedure developed by our group [18]. The 13 mthick nanocrystalline TiO2 films were fabricated on a F-doped conducting glass substrate (FTO, Asahi Glass Co., Ltd.; sheet resistance: 7 /sq) by screen printing, followed by sintering under airflow at 500 ◦ C for 30 min. After cooling to 80 ◦ C, the films were immersed in ACN and tert–butyl alcohol mixed solution (volume ratio, 1:1) containing ruthenium-535bis-TBA (0.5 mM) (N719, Solaronix) for 22 h at 25 ◦ C, followed by rinsing with ethanol. The active area of DSSCs was 0.16 cm2 . 2.5. Fabrication of DSSCs and photoelectrochemical measurements PGE was coated onto a sensitized electrode by doctor blading method. An open sandwich-type cell was assembled using two clips to clap a sputtered, platinized FTO counter electrode against the sensitized electrode coated with the PGE. The cells with liquid electrolyte were also tested for comparison. The photovoltaic performance of the DSSCs was evaluated at AM 1.5 illumination (100 mW cm−2 ; Peccell-L15, Peccell, Japan) using a Keithley digital source meter (Keithley 2601, USA). Dark currents were measured under the same conditions but in darkness. Electrochemical impedance spectroscopy (EIS) experiments were conducted using a computer-controlled potentiostat (ZeniumZahner, Germany). The measured frequency ranged from 100 mHz to 1 MHz while the AC amplitude was set to 10 mV. The bias of all EIS measurements was set to −0.75 V in darkness. The EIS analysis was fitted by Zview software. 2.6. Stability test of the DSSCs To study the stability of the P(VA-co-MMA)-based QS-DSSCs, the devices needed to be sealed. The sealed liquid electrolytebased cells were fabricated according to a previously reported procedure [19], whereas the QS-DSSCs were sealed as follows. The PGE was coated on a dye-coated TiO2 photoanode, around which a 60 m-thick Surlyn (1702 DuPont) was wrapped. Then, a “two-step dip-coated” Pt counter electrode was pressed on the photoanode [20]. The cell was placed at 125 ◦ C under a certain pressure until the sealing spacer completely melted. Excess PGE was extruded through the pre-drilled hole on the Pt counter electrode. Finally, the hole was sealed by a cover glass. To test the stabilities of the devices, the DSSCs based on liquid electrolyte and the PGE were exposed to simulative sunlight at 30 ◦ C. 2.7. Preparation of a 5 cm × 7 cm QS-DSSC sub-module by screen printing PGE The rectangular shapes surrounded by the current-collecting Ag grids were selected from a 5 cm × 7 cm DSSC sub-module structure [21]. This structure prevented the large resistivity increase of the FTO/glass with increased cell area. The size of one rectangle was 9 mm × 60 mm; the width of one Ag grid line covered by over-layers between two rectangles was 1 mm. Three rectangular shapes were included in a 5 cm × 7 cm DSSC sub-module. The active area of this sub-module was 16.2 cm2 . The submodel DSSC was sealed similarly as the small-sized cell based on PGE.
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Fig. 1. FT-IR spectra of PVA, PMMA and P(VA-co-MMA).
3. Results and discussion 3.1. FT-IR studies Fig. 1 shows the FT-IR spectra of PVA, PMMA, and P(VA-coMMA). The band at 1731 cm−1 corresponds to the C O stretching vibration, and the two doublet bands at 1150 and 1190 cm−1 as well as 1240 and 1265 cm−1 correspond to the C O stretching vibrations of the ester groups in PMMA [22–24]. The characteristic band at 1738 cm−1 corresponds to the C O stretching vibration, and the band at 1373 cm−1 corresponds to the CH3 symmetric bending vibrations in PVA [25]. The characteristic peaks in the PMMA and PVA spectra all appear in P(VA-co-MMA). All these bands in the infrared spectra prove that synthesis of P(VA-co-MMA) in our experiment was successful. 3.2. Performances of the DSSCs fabricated with PGEs with varying copolymer addition With increasing the addition of the polymer, PGE becomes viscous thus diffusion of the ionic redox couples will be slowed down, negatively affecting the performance of the DSSC. On the other hand, in addition to gelating the liquid electrolyte, polymers in the PGE have some other positive effects, e.g., facilitating electron transport and suppressing charge recombination. Therefore, the purpose of this experiment is to find out the optimized addition of our copolymer. Fig. 2 shows the J–V curves of the DSSCs fabricated with the MPN based PGEs with different additions of copolymer. Due to the competitive effects of the polymer in the electrolyte, finally, DSSCs performed best with 20 wt% P(VA-co-MMA) in the PGE. Unless stated, otherwise the addition of P(VA-co-MMA) is 20 wt% for our subsequent investigations. 3.3. Influences of the incorporation of TiO2 nanoparticles on the ionic conductivities of PGEs and the performance of QS-DSSCs Table 1 shows the measured ionic conductivities () of the liquid and gel-state electrolytes. For the liquid electrolytes, the ionic conductivity of the ACN system (8.79 mS cm−1 ) is more than two times higher than that of the MPN system (3.40 mS cm−1 ). After gelation with 20 wt% P(VA-co-MMA), ACN based system still remains high conductivity, 7.18 mS cm−1 , much higher than that of 2.60 mS cm−1 for MPN based. As stated previously, adding inorganic nanoparticles favors to enhance the conductivity through decreasing polymers
Fig. 2. J–V curves of DSSCs fabricated with PGEs based on MPN solvent with different copolymer additons.
crystallinity. In our experiment, 5 wt% TiO2 nanoparticles were added into our PGEs. It can be found from Table 1 that the conductivity of the ACN based PGE was further increased to 7.52 mS cm−1 , whereas no obvious improvement was realized for the MPN based PGE. Fig. 3 shows the current–voltage (J–V) curves of the DSSCs fabricated with liquid electrolyte and the PGEs based on ACN (a) and MPN (b) systems under AM 1.5, 100 mW cm−2 light illumination. For a better photovoltaic performance, we added a 4 m thicked light-scattering layer on the 13 m thicked nanocrystalline TiO2 . And the TiO2 films were further treated with a TiCl4 solution (40 mM) at 70 ◦ C for 30 min. The open circuit potential (Voc ), shortcircuit current density (Jsc ), fill factor (FF), and total PCE of these cells are also listed in Table 1 For the ACN system, the efficiency achieved by the QS-DSSCs is 9.10%, close to 9.59% for the DSSC fabricated with the original liquid electrolyte. Using the TiO2 contained PGE, a higher conversion efficiency of 9.40% was obtained. To the best of our knowledge, this efficiency can rank the tops of the QS-DSSCS. When MPN was used instead of ACN, the efficiency achieved by the cells fabricated with PGE is 8.61%, whereas 9.14% is achieved by the liquid-version cells at AM 1.5. Addition of 5 wt% TiO2 nanoparticles results in a similar effect as in the MPN system. Although Voc and FF decreased with the introduction of P(VA-co-MMA) to the liquid electrolyte, Jsc increased after 5 wt% TiO2 addition to the PGE. Consequently, the cells with PGE + TiO2 achieve an efficiency of 8.98%, which is 98.2% of the value based on liquid electrolyte. 3.4. EIS analysis of the DSSCs To investigate the impedance behavior of the DSSCs assembled with different types of electrolytes, the electrochemical impedance of the DSSCs was measured, and the results are shown in Fig. 4. Table 1 Ionic conductivity measured for the liquid electrolytes and PGEs as well as the photovoltaic parameters obtained from the J–V curves of DSSCs using these electrolytes. The performances of DSSCs were measured at AM 1.5 (100 mW cm−2 ) with an active area of 0.16 cm2 . Electrolyte
(mS cm−1 )
Voc (V)
Jsc (mA cm−2 )
FF
PCE (%)
ACN-liquid ACN-PGE ACN-PGE + TiO2 MPN-liquid MPN-PGE MPN-PGE + TiO2
8.79 7.18 7.52 3.40 2.60 2.62
0.75 0.74 0.75 0.74 0.71 0.73
17.27 16.66 16.93 17.40 17.32 17.58
0.74 0.74 0.74 0.71 0.70 0.70
9.59 9.10 9.40 9.14 8.61 8.98
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Table 2 The parameters of the equivalent circuits used to fit the EIS impedance data of the DSSCs in Fig.4.
Liquid PGE PGE + TiO2
Rs ()
Rct1 ()
ZN ()
13.17 13.39 11.32
41.01 36.31 38.09
29.02 62.32 47.74
frequency region), and Nernstian diffusion within the electrolyte (low frequency region) [26,27]. In our study, two semi-circles are observed because of overlaps happened between high and middle frequency circles. Therefore, they are labeled by Rct1 (charge transfer resistance at the TiO2 /dye/electrolyte interface) and ZN (Nernstian diffusion). The ohmic serial resistance (Rs ) (intersection of high-frequency circle and x-axis) is mainly associated with the sheet resistance of the FTO substrate. As shown in Fig. 4, it was seen that Rct1 decreased from 41.01 to 36.31 with the addition of our copolymer P(VA-co-MMA). However, it increased from 36.31 to 38.09 again with the addition of TiO2 nanoparticles to the PGE, suggesting that addition of TiO2 nanoparticles into the electrolyte could reduce the charge recombination at the TiO2 /dye/electrolyte interface. This result is consistent with the variations of the Voc and the dark currents in Fig. 3. On the other hand, ZN first increased from 29.02 to 62.32 with the addition of P(VA-co-MMA) and then it decreased to 47.74 with presence of TiO2 nanoparticles to the PGE, in good accordance with the data listed in Table 1. 3.5. Stability test of the DSSCs fabricated with PGE
Fig. 3. J–V curves of the DSSCs fabricated with liquid electrolytes as well as PGEs for ACN (a) and MPN (b) system at AM 1.5 (100 mW cm−2 ) and in dark. The PGEs contained 20 wt% P(VA-co-MMA) with or without 5 wt% TiO2 nanoparticles.
The parameters of the equivalent circuits in the inset of Fig. 4 that was used to fit the EIS impedance are summarized in Table 2. In general, there should be three semi-circles in a typical EIS spectra of the DSSC, corresponding to the charge transfer resistance at the Pt counter electrode (high frequency region), the charge transfer resistance at the TiO2 /dye/electrolyte interface (middle
Fig. 4. Nyquist plots of the DSSCs fabricated with liquid electrolyte and PGEs with and without 5 wt% TiO2 based on MPN solvent measured in dark under bias −0.75 V (inset is the equivalent circuit to fit the EIS impedance data of DSSCs).
Fig. 5 shows the long-term stability test of the DSSCs fabricated with liquid electrolyte and PGE, respectively. It can be found that DSSC based on the PGE containing 20 wt% P(VA-co-MMA) was more durable over a period of 1000 h since it could maintain 96.7% of its initial efficiency. By contrast, the other one based on liquid electrolyte just kept 65.0% of its initial efficiency. This finding indicates the potential of PGE in improving the stability of the DSSCs. 3.6. Performance of the 5 cm × 7 cm QS-DSSC sub-module by screen printing of PGE To accelerate the process of industrialization of DSSCs, the large-area module (5 cm × 7 cm, active area was 16.2 cm2 ) was constructed by screen printing of PGE (based on MPN system, with
Fig. 5. The normalized efficiency of the DSSC with liquid electrolyte and PGE at AM 1.5 versus soaking time.
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Fig. 6. J–V curve of the 5 cm × 7 cm QS-DSSC sub-module by screen printing PGE at AM 1.5 (insets are the photovoltaic parameters obtained from the J–V curves of QS-DSSC sub-module).
5 wt% TiO2 ). Fig. 6 shows the J–V curve of our large-area DSSC submodule and the obtained efficiency was 4.39%. To the best of our knowledge, this is the first time reporting the use of a PGE to construct a sub-module DSSC by screen printing method. 4. Conclusion Novel PGEs with high ionic conductivity based on copolymer polyvinyl (acetate-co-methyl methacrylate) were prepared in combination of organic electrolyte solution. QS-DSSCs were fabricated after the addition of our copolymer in PGEs was optimized to be 20 wt%. Under AM 1.5, 100 mW cm−2 light illumination, an efficiency of 9.10% for the QS-DSSCs was achieved and then was further enhanced to 9.40% through introducing TiO2 nanoparticles into the PGEs to increase the ionic conductivity. Long-term stability tests revealed that in our QS-DSSCs the PGE exhibited better stability after 1000 h exposure to simulative sunlight. Finally, we use the screen printing technique to paste PGE for fabrication of large-area QS-DSSC sub-module. With an active area of 16.2 cm2 , a high conversion of 4.39% was obtained. Our results provide a feasible way of screen printing of PGEs in future mass-production of QS-DSSC module. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant No. 51273032), the National High Technology Research and Development Program for Advanced Materials of China (Grant No. 2009AA03Z220), Specialized Research Fund for the Doctoral Program of Higher Education of China (20110041110003), Open project of the State Key Laboratory for Physical Chemistry of Solid Surfaces of Xiamen University (Grant No. 201210) and the State Key Laboratory of Fine Chemicals of China. References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737.
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