In Situ Gelation of Poly(vinylidene fluoride) Nanospheres for Dye

Jul 27, 2016 - Su-Jin Ha†, Sang Goo Lee‡, Jong-Wook Ha‡, and Jun Hyuk Moon†. † Department of Chemical and Biomolecular Engineering, Sogang ...
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In Situ Gelation of Poly(vinylidene fluoride) Nanospheres for DyeSensitized Solar Cells: The Analysis on the Efficiency Enhancement upon Gelation Su-Jin Ha,† Sang Goo Lee,‡ Jong-Wook Ha,‡ and Jun Hyuk Moon*,† †

Department of Chemical and Biomolecular Engineering, Sogang University, 1 Sinsu-dong, Mapo-gu, Seoul 121-742, Republic of Korea ‡ Center for Interface Materials and Chemical Engineering, Korea Research Institute of Chemistry Technology, 141 Gajeong-Ro, Daejeon 305-600, Republic of Korea S Supporting Information *

ABSTRACT: The in situ gelation that utilizes the dissolution of polymers inside the cell is allowed high concentration polymer gel without concerns regarding high viscous electrolyte incorporation into the cell as in the conventional approach. We demonstrate the in situ gelation of polymer composite electrolytes using poly(vinylidene fluoride) nanospheres (PVdF NSs). The PVdF NSs were synthesized by high pressure emulsion polymerization using gaseous vinylidene fluoride monomers. Compared to the liquid electrolyte (LE) DSCs without PVdF gelation, the PVdF polymer gel electrolyte (PGE) DSCs displayed higher η than the LE DSCs; specifically, the 10 wt % PVdF PGE DSCs display 8.1% of the η, while the LE DSCs only display 6.5%. We characterized the effect of PVdF PGE on the photovoltaic parameters in detail. We also compared the long-term stability of DSCs containing LE and PVdF PGE. The DSCs with PVdF PGE exhibited high stability compared to the LE DSCs, similar to a conventional PGE system. We believe that this facile in situ gelation approach could be utilized for not only the practical application of polymer gel electrolytes DSCs but also for various energy-storage devices.

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INTRODUCTION Dye-sensitized solar cells (DSCs), which were first reported by O’Regan and Gratzel in 1991, have been found to be promising photovoltaic devices because of their relatively high photon-toelectron conversion efficiency and low cost.1,2 Considerable efforts have been devoted to improving components of DSCs such as sensitizers, porous TiO2 anodes, cathodes, and liquid electrolytes (LEs) to achieve higher efficiency and greater longterm device stability. The use of volatile LEs and the resulting stability problems limit the practical application of DSCs.3,4 Many efforts have therefore been focused on replacing the electrolyte with solid-state or quasi-solid-state electrolyte such as p-type semiconductor, polymer membrane, hole-transport material, or conductive organic material.3,5−8 Organic holetransport materials (e.g., spiro-MeOTAD) have recently been found to have high efficiency when combined with a perovskite sensitizer.9−11 Among the solid-state electrolyte materials, polymer gel electrolytes (PGEs) have advantages over other materials in terms of cost and chemical stability. Importantly, PGEs are well suited for constructing flexible devices. Poly(ethylene oxide) (PEO), poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF), and polyacrylonitrile (PAN) have been used for DSCs in previous studies.12−15 These studies prepared the © XXXX American Chemical Society

polymer electrolytes ex situ and then incorporated them into the assembled DSCs. This approach often requires careful control over the viscosity of the PGE for facile incorporation into the cell and porous anode.16 A high polymer concentration is required for the long-term stable operation of the DSCs. However, the polymer concentration in the electrolyte solution is limited; many studies have reported achieving concentrations at low weight percentage of the polymer in the electrolyte solution, which may be due the solution viscosity. Since the leakage and high volatility of LE impairs cell performance, the addition of polymers that are compatible with the LE to high concentration improves stability. Lee et al. reported that DSCs containing 7 wt % PAN-based PGEs exhibit a gradual decrease in efficiency after 100 h of use, while DSCs with 15 wt % PANbased PGEs retained 90% of the initial efficiency. Recent studies have demonstrated the in situ gelation approach,9 which involves the dissolution of polymer spheres predeposited on the counter electrode upon injection of the electrolyte solution in fully assembled DSCs. In situ gelation leads to high polymer content in the gel electrolyte without Received: April 15, 2016 Revised: July 12, 2016

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(Aldrich), 55 mM I2 (Yakuri), 0.6 M dimethyl-propyl imidazole iodide (DMPII, Solaronix), and guanidinium thiocyanate (GuSCN) in an 8.5:1.5 (v/v) ratio with a mixture of acetonitrile (Aldrich) and valeronitrile (Aldrich), was injected into the cell to fill the gap. Characterization. The morphologies of the photoanodes and of the counter electrode were characterized using scanning electron microscopy (FE-SEM, AURIGA, Carl Zeiss) and energy-dispersive spectroscopy (EDS, Carl Zeiss). The thermal properties of PVdF were evaluated by thermogravimetric analysis (TGA, TA, TGA Q50) and differential scanning calorimetry (DSC, TA, DSC Q20) under N2 atmosphere. The J−V characteristics of the DSCs were measured using a source meter (Keithley Instruments) under 1 sun illumination. The solar light was simulated using a xenon lamp (300 W, Oriel) and an AM 1.5 filter. The intensity of the simulated solar light was calibrated using Si standard solar cells (Bunko-Keiki BS-520). The ion conductivities and diffusivities of the electrolytes were measured using cyclic voltammetry (Versastat, Ametek), which was performed using two parallel Pt-coated FTO electrodes with a 60-μm gap at a scan rate of 10 mV/s. The electrical impedance spectra (EIS) were measured using cyclic voltammetry with a scan rate of 10 mV/s over the frequency range of 105 Hz to 0.1 Hz and were analyzed using ZView software (Scribner Associates, Inc.).

incorporation of the electrolyte into the cell. However, the approach using polystyrene (PS) cannot achieve a high PGE concentration because PS has intrinsically low ion conductivity and low gelation ability.9 Therefore, there is a need for in situ gelation using the polymers such as PEO, PVdF, and PAN. In the present study, we demonstrated in situ gelation using poly(vinylidene fluoride) nanospheres (PVdF NSs). We prepared PVdF NSs through emulsion polymerization using gaseous vinylidene fluoride monomers, which we had liquefied under high-pressure conditions. PVdF is a polymer that is most favorable for use in DSCs because of its strongly electron withdrawing groups (−C−F) and high dielectric constant. These characteristics lead to a high tendency for ion dissociation and support a high concentration of charge carriers.17,18 Using in situ gelation, we achieved a high PVdF content in the PGE (up to 10 wt %). We observed that DSCs containing PVdF PGE are more efficient than are DSCs using conventional LEs. Specifically, the efficiency of DSCs with PVdF PGE was approximately 8.1%, which is 23% higher than that of DSCs with conventional LE. Previous studies have reported low performance of cells in which PGE had been incorporated due to impairment of ion diffusivity by the highly viscous PGE.19−21 Therefore, we examined in detail the effect of PVdF gelation on the photovoltaic parameters. We also compared the long-term stability of DSCs with PVdF PGE against that of DSCs with LE.

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RESULTS AND DISCUSSION In situ electrolyte gelation using PVdF NSs is depicted in Scheme 1. A dispersion of PVdF NSs was deposited on a PtScheme 1. In Situ Gelation of Gel Electrolytes Based on PVdF NSs

EXPERIMENTAL METHODS

Synthesis of Poly(vinylidene fluoride) Nanospheres. An aqueous dispersion of PVdF was prepared using a conventional emulsion polymerization method. Ultrapure water (290 g) produced using a Milli-Q system and 5 g of ammonium perfluorooctanoate (C 8F17COONH4, Tokyo Chemical Industry Co., Ltd.) were introduced into a 1 L jacketed stainless steel autoclave at ambient temperature. After closing and deaerating the reactor, magnetic stirring was initiated, and the reactor was heated to 82 °C. Then, the reactor was pressurized to 23 bar with vinylidene fluoride (Apollo Scientific Co.). The polymerization was started by injecting an initiator solution that consisted of 0.55 g of sodium persulfate (Na2S2O8, Sigma-Aldrich Co.) and 10 g of water, and the pressure was maintained at 23 bar by continuously feeding vinylidene fluoride. After introducing a total of 100 g of vinylidene fluoride over the course of 300 min, the monomer feed was stopped and the pressure was allowed to decrease to 12 bar. Cell Assembly for in Situ Gelation. The PVdF NSs were deposited on a Pt-coated counter electrode by doctor-blading method. The counter electrode was prepared by coating a FTO substrate with a 0.5 mM H2PtCl6 solution in anhydrous ethanol followed by heat treatment at 450 °C for 30 min. We utilized the mesoscale orderedpore TiO2 anode for fabricating PS colloidal crystal templates. The mesoscale ordered-pore TiO2 structures (or mesoscale inverse opal structure) have enhanced charge and ion transport as compared with those of conventional nanoparticulate TiO2 film. They also facilitate the infiltration of PGEs. Ordered-pore TiO2 films were fabricated by coating PS particles (99 nm in diameter, 10 wt % dispersion in water; Bangs Laboratories, Inc.) onto an FTO substrate. The deposition of TiO2 into the colloidal crystal templates was achieved by chemical vapor deposition using TiCl4 precursor. The removal of the templates by calcination at 500 °C for 2 h was followed, resulting in the formation of mesoscale ordered-pore structure. Scanning electron microscopy (SEM) images of the PS template and its inverse structure are shown in Figure S1. A scattering layer (CCIC, Japan) was coated on the TiO2 IO film followed by heat treatment at 450 °C for 15 min. The TiO2 film was sensitized by immersion in a dye solution (D205, 0.5 mM) for over 12 h at room temperature. The PVdF NSs coated counter electrode and the TiO2 photoanode were sandwiched with a 60-μm gap using a spacer film (Surlyn, Dupont). Finally, the electrolyte solution, which was prepared by dissolving 25 mM Lil

coated counter electrode. The amount of PVdF NSs deposited was varied to control the polymer concentration in the PGE. Upon injection of LE into the cell, the PVdF NSs were dissolved in the electrolyte to allow gelation of the electrolyte solution. Specifically, we injected a heated electrolyte solution to facilitate the dissolution of PVdF NSs. The PVdF NS layer became transparent within 1 min after injection, indicating complete dissolution. (see Figure S2) The thermal properties of PVdF NSs were characterized by TGA and DSC, as shown in Figure S3. The TGA thermogram suggests a large weight loss of PVdF NSs at about 500 °C via thermal decomposition. DSC curves show an endothermic peak at approximately 160 °C, which corresponds to the melting temperature of PVdF. An SEM image of the PVdF NS packed film is shown in Figure 1a. The diameter of PVdF NSs is approximately 50 nm. Element mapping of the material (inset of Figure 1a) shows the F atom content of PVdF. After the injection of LE, we observed that no particles remained, as shown in Figure 1b. In this study, we used 1.5, 3, and 8 μm thick coatings of the PVdF NS layer, which produced PGE with approximately 3, 5, and 10 wt % PVdF content, respectively, based on approximately 20 μL volume of LE in the cell. We also prepared a 15 μm thick PVdF NS layer but often observed delamination of the PVdF NS coating, which was thicker than 10 μm. Gelation of LE upon dissolution of the PVdF NSs can be observed in Figure S4. The rheological properties of the B

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with PGE containing 5−10 wt % PVdF. This increase in efficiency is due to the increase in JSC, even though VOC and FF remained constant. Figure S6 and Table S1 show the photovoltaic performance of DSCs containing 20 wt % and the decrease in JSC with respect to that of DSCs containing 10 wt %. Previous studies have reported a lower efficiency of DSCs containing PGE compared with that of DSCs containing LE.19−21,24 This difference is explained by the lower ion diffusivity due to the highly viscous polymer matrix in PGE, which increases the charge recombination rate at the TiO2/ electrolyte interface and decreases JSC, or, more often, VOC. On the other hand, a study has disputed the lower efficiency resulting from the use of PGEs.25 Thus, we investigated the factors that maintain VOC and increase JSC upon PVdF gelation at higher PVdF content of the PGE. First, we discuss the effect of in situ PVdF gelation on VOC. VOC is determined by the energy level of the TiO2 conduction band (CB) edge (EC) and free-electron density in the TiO2 conduction band (nc), as described by the following equation:

Figure 1. SEM images of (a) the deposited layer of PVdF NSs and (b) the electrode after dissolution of PVdF NSs in the electrolyte solution. The inset displays the element mapping result for F atoms from PVdF.

PVdF PGE is characterized, as shown in Figure S5. The response of storage and loss modulus as a function of frequency exhibits a typical behavior of polymer gel. The ion conductivity of the PVdF PGEs is measured by using a cyclic voltammetry.22 The ion conductivities of 3, 5, 10 wt % PVdF PGE are 6.9 × 10−4, 6.3 × 10−4, and 6.2 × 10−4, respectively. The magnitude of these ion conductivities is similar to those of reported in the literature.22,23 The photovoltaic performance of DSCs containing PVdF PGE was evaluated. For comparison, we also prepared a DSC containing LE only. Figure 2 presents the photocurrent−

VOC =

n ⎞ kT ⎛ Ec − Eredox + ln c ⎟ ⎜ q ⎝ kT Nc ⎠

where kT is the thermal energy, q is the elementary charge, Eredox is the potential energy of redox couples, and NC is the density of the accessible state in the CB.24,25 Previous studies have often reported that polymer gelation can change the energy level of the CB.19,24,26−28 In the case of PEO-based PGE, an EC upward shift was observed. This is due to the favorable coordination of lithium cations in the electrolyte solution with the PEO chain, which decreases the amount of lithium cations adsorbed on the TiO2 electrode surface.24 On the other hand, the use of PAN-based polymers led to a downshift in EC because of the greater amount of dissociated lithium cations upon gelation and the consequent increase in the amount of cations adsorbed on the TiO2 surface.26,29 In the present study, we carried out EIS measurements to evaluate the change in EC with increasing PVdF content of the PGE (results

Figure 2. Photocurrent−voltage curves for the DSCs with LEs and DSCs with various PVdF PGEs.

Table 1. Photovoltaic Parameters for the DSCs with LEs and DSCs with Various PVdF PGEs liquid 3 wt %-PGE 5 wt %-PGE 10 wt %-PGE

JSC [mA/cm2]

VOC [V]

FF

efficiency [%]

14.4 15.8 17.0 17.9

0.73 0.73 0.73 0.73

0.62 0.62 0.62 0.61

6.5 7.2 7.7 8.1

Figure 3. Nyquist plots obtained from DSCs with LE and DSCs with various PVdF PGEs. Solid lines represent the fitted results obtained from the equivalent circuit shown in Figure S7.

voltage (J−V) plots of these DSCs, and Table 1 lists the parameters obtained from these curves, including the photocurrent density (JSC), open-circuit voltage (VOC), and fill factor (FF). The photon-to-electric conversion efficiency (η) was calculated as JSC × VOC × FF (100 mW/cm2). Comparison with the DSC with LE only (i.e., containing no PVdF) shows that increasing the PVdF content increases the efficiency: η was 6.5% for the DSCs with LE, increasing to 7.2−8.1% for DSCs

are shown in Figure 3). We used the following equation that relates the EC and chemical capacitance (Cμ): Cμ = L(1 − p)α C

⎛ α(Eredox − Ec) ⎞ q2NL exp⎜ ⎟ ⎝ ⎠ kT kT DOI: 10.1021/acs.langmuir.6b01460 Langmuir XXXX, XXX, XXX−XXX

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Langmuir where L is the electrode thickness, p is the porosity, and NL is the trap state density.24 Inspection of the above equation shows that EC is inversely proportional to Cμ. Nyquist plots obtained from the EIS measurement are shown in Figure 3. Cμ values obtained by fitting the plots with an equivalent circuit model (R−RC−W−RC) are listed in Table 2.22,30,31 We found that Cμ increased with increasing PVdF content. Thus, we also expect EC to decrease with increasing PVdF content.

the tendency for coordination because of its acidic hydrogen, which can increase the concentrations of redox ion species and of cations adsorbed on the TiO2 surface. We also studied the effect of PVdF gelation on JSC. As observed in the above analysis, nc increased upon incorporation of PVdF, leading to higher JSC upon PVdF gelation. The recombination lifetime (τr) is a relevant parameter that determines JSC.31,33 A greater τr results in higher JSC. We obtained τr from EIS measurements by using the equation τr = CμRct.34,35 Table 2 lists τr data for DSCs with LE and for DSCs with various PVdF PGEs. As the PVdF content in PGE increased, τr also increased. Thus, JSC is expected to increase with increasing PVdF content. Previous studies have reported that polymer gelation changes Cμ and Rct.19,26,36 PAN-based PGE DSCs have been shown to have higher Cμ and τr as compared with those of bare DSCs with LE.26,36 The use of PEO-based PGE has yielded an Rct value higher than that attained with LE, resulting in a highly improved τr.19,37 The greater τr is attributed to the adsorption of the polymers on the TiO2 surface, which results in formation of a high potential barrier and/or a physical barrier for the recombination reaction. Similarly, the presence of PVdF reduces recombination, thus increasing the JSC.19,38 Finally, the long-term stabilities of DSCs containing LE and PVdF PGE were compared. Changes in the J−V parameters JSC, VOC, FF, and η during the aging period (Figure 4) were normalized according to the initial values of the as-prepared DSCs. The DSCs heated at 60 °C in the dark. Among the photovoltaic parameters, JSC for the DSCs with LE decreased considerably during the aging period, whereas that of DSCs with PVdF PGE remained constant. η of DSCs with LE decreased to 40% over 500 h, while η of DSCs with PVdF PGE stayed at 60%. Many studies have shown that leakage and evaporation of the electrolyte solution can reduce JSC. Thus, PVdF PGE formed by in situ gelation effectively prevented loss due to leakage, similar to a conventional PGE system.9,39,40

Table 2. Chemical Capacitances, Charge-Transfer Resistances, and Recombination Lifetimes of the DSCs with LE and DSCs with Various PVdF PGEs Rct [Ω] liquid 3 wt %-PGE 5 wt %-PGE 10 wt %-PGE

47.5 52.3 52.7 60.1

Cμ [F] 1.1 1.4 1.6 1.6

× × × ×

10−4 10−4 10−4 10−4

τr [s] 4.9 7.1 8.7 9.7

× × × ×

10−3 10−3 10−3 10−3

Meanwhile, we evaluated the change in nc during PVdF gelation. nc decreased because of charge recombination at the TiO2/electrolyte interface, the rate of which was determined by charge-transfer resistance at the TiO2/electrolyte interfaces (Rct). Rct values of DSCs with LE and those of DSCs with PVdF PGE (Table 2) were obtained by fitting EIS plots. As the PVdF content increased, the Rct values increased. In other words, the PVdF PGE retards charge recombination at the TiO2/electrolyte interface. Thus, nc is expected to decrease with increasing PVdF content. In summary, analysis of EC and nc showed that PVdF gelation of electrolytes decreases EC, whereas it increases nc. Thus, the lack of VOC variation in our experiment is explained by the fact that the change in VOC is offset by each effect. PVdF is known to be highly polar because of the strong surface dipoles generated by alternating CH2 and CF2 units. PVdF also has highly acidic hydrogen atoms.32 Thus, PVdF improves the dissociation of lithium cations but reduces

Figure 4. Normalized photovoltaic parameters for DSCs with LE and DSCs with PVdF PGE during the thermal aging period. D

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Preparation of Polymer Electrolytes for Dye-Sensitized Solar Cells. J. Photochem. Photobiol., C 2013, 16, 1. (5) Odobel, F.; Pellegrin, Y.; Gibson, E. A.; Hagfeldt, A.; Smeigh, A. L.; Hammarström, L. Recent Advances and Future Directions to Optimize the Performances of p-type Dye-Sensitized Solar Cells. Coord. Chem. Rev. 2012, 256, 2414. (6) Bella, F.; Pugliese, D.; Nair, J. R.; Sacco, A.; Bianco, S.; Gerbaldi, D.; Barolo, C.; Bongiovanni, R. A UV-Crosslinked Polymer Electrolyte Membrane for Quasi-Solid Dye-Sensitized Solar Cells with Excellent Efficiency and Durability. Phys. Chem. Chem. Phys. 2013, 15, 3706. (7) Ding, I.-K.; Tétreault, N.; Brillet, J.; Hardin, B. E.; Smith, E. H.; Rosenthal, S. J.; Sauvage, F.; Grätzel, M.; McGehee, M. D. Pore-Filling of Spiro-OMeTAD in Solid-State Dye Sensitized Solar Cells: Quantification, Mechanism, and Consequences for Device Performance. Adv. Funct. Mater. 2009, 19, 2431. (8) Huang, Y.; Zhou, X.; Fang, S.; Lin, Y. Molecular Organic Conductors with Triiodide/Hole Dual Channels as Efficient Electrolytes for Solid-State Dye Sensitized Solar Cells. RSC Adv. 2012, 2, 5550. (9) Lee, K. S.; Jun, Y.; Park, J. H. Controlled Dissolution of Polystyrene Nanobeads: Transition from Liquid Electrolyte to Gel Electrolyte. Nano Lett. 2012, 12, 2233. (10) Im, J. H.; Luo, J.; Franckevicius, M.; Pellet, N.; Gao, P.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, M.; Park, N. G. Nanowire Perovskite Solar Cell. Nano Lett. 2015, 15, 2120. (11) Giordano, F.; Abate, A.; Correa Baena, J. P.; Saliba, M.; Matsui, T.; Im, S. H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Hagfeldt, A.; Graetzel, M. Enhanced Electronic Properties in Mesoporous TiO2 via Lithium Doping for High-Efficiency Perovskite Solar Cells. Nat. Commun. 2016, 7, 10379. (12) Yang, H.; Liu, J.; Lin, Y.; Zhang, J.; Zhou, X. PEO-Imidazole Ionic Iquid-Based Electrolyte and the Influence of NMBI on DyeSensitized Solar Cells. Electrochim. Acta 2011, 56, 6271. (13) Shalu; Chaurasia, S. K.; Singh, R. K.; Chandra, S. Thermal Stability, Complexing Behavior, and Ionic Transport of Polymeric Gel Membranes Based on Polymer PVdF-HFP and Ionic Liquid, [BMIM][BF4]. J. Phys. Chem. B 2013, 117, 897. (14) Wu, M.; Wang, Y.; Lin, X.; Guo, W.; Wu, K.; Lin, Y.-n.; Guo, H.; Ma, T. TiC/Pt Composite Catalyst as Counter Electrode for DyeSensitized Solar Cells with Long-Term Stability and High Efficiency. J. Mater. Chem. A 2013, 1, 9672. (15) Dkhissi, Y.; Huang, F. Z.; Cheng, Y. B.; Caruso, R. A. QuasiSolid-State Dye-Sensitized Solar Cells on Plastic Substrates. J. Phys. Chem. C 2014, 118, 16366. (16) Wang, B.; Chang, S.; Lee, L. T. L.; Zheng, S.; Wong, K. Y.; Li, Q.; Xiao, X.; Chen, T. Improving Pore Filling of Gel Electrolyte and Charge Transport in Photoanode for High-Efficiency Quasi-SolidState Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 8289. (17) Song, J. Y.; Wang, Y. Y.; Wan, C. C. Review of Gel-Type Polymer Electrolytes for Lithium-Ion Batteries. J. Power Sources 1999, 77, 183. (18) Zhang, H.; Ma, X.; Lin, C.; Zhu, B. Gel Polymer ElectrolyteBased on PVDF/Fluorinated Amphiphilic Copolymer Blends for High Performance Lithium-Ion Batteries. RSC Adv. 2014, 4, 33713. (19) Cho, W.; Lim, J.; Kim, T.-Y.; Kim, Y. R.; Song, D.; Park, T.; Fabregat-Santiago, F.; Bisquert, J.; Kang, Y. S. Electron-Transfer Kinetics Through Interfaces Between Electron-Transport and IonTransport Layers in Solid-State Dye-Sensitized Solar Cells Utilizing Solid Polymer Electrolyte. J. Phys. Chem. C 2016, 120, 2494. (20) Wu, J.; Lan, Z.; Lin, J.; Huang, M.; Huang, Y.; Fan, L.; Luo, G. Electrolytes in Dye-Sensitized Solar Cells. Chem. Rev. 2015, 115, 2136. (21) Seidalilir, Z.; Malekfar, R.; Wu, H. P.; Shiu, J. W.; Diau, E. W. High-Performance and Stable Gel-State Dye-Sensitized Solar Cells Using Anodic TiO2 Nanotube Arrays and Polymer-Based Gel Electrolytes. ACS Appl. Mater. Interfaces 2015, 7, 12731. (22) Ha, S.-J.; Park, J. H.; Moon, J. H. Quasi-Solid-State DyeSensitized Solar Cells with Macropore-Containing Hierarchical Electrodes. Electrochim. Acta 2014, 135, 192.

CONCLUSION We demonstrated in situ gelation using PVdF NSs for DSCs with quasi-solid-state PGE. In situ gelation was achieved by depositing the PVdF NSs on the counter electrode and then dissolving them in electrolyte solution after cell assembly. This approach is facile and allows the formation of viscous PGEs without incorporation of electrolyte into the cell. Compared with the DSCs with LE, DSCs with PVdF PGE displayed higher η. DSCs with 10 wt % PVdF in the PGE and DSCs with LE displayed η values of 8.1% and 6.5%, respectively. These differing values are due to the increase in JSC with increasing PVdF content at constant VOC and FF values. We found that the lack of variation in VOC with increasing PVdF content is due to the compromising effects of decreasing EC and increasing nc. The increase in JSC is caused by the decrease in the recombination rate. PVdF adsorbed on the electrode may form a physical or potential barrier for the recombination reaction. Finally, we compared the long-term stability of DSCs containing LE and PVdF PGE. η values of the DSCs with PVdF PGE reached 60% of their initial values, whereas η of DSCs LE rapidly decayed because of the decrease in JSC. Thus, the PVdF PGE formed by in situ gelation improves the long-term stability, similar to a conventional PGE system. We believe that our facile in situ gelation approach can be utilized in practical applications of PGE DSCs and various energy-storage devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01460. Cross-sectional SEM images; digital images; TGA and DSC thermograms; storage modulus/loss modulus vs frequency curves; photocurrent vs voltage curves; photovoltaic parameters for DSCs; equivalent circuit for fitting of Nyquist plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Research Foundation of Korea (NRF) (Grant No. 2011-0030253). The Korea Basic Science Institute is also acknowledged for the SEM measurement.

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REFERENCES

(1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Film. Nature 1991, 353, 737. (2) Wu, C.; Jia, L.; Guo, S.; Han, S.; Chi, B.; Pu, J.; Jian, L. OpenCircuit Voltage Enhancement on the Basis of Polymer Gel Electrolyte for a Highly Stable Dye-Sensitized Solar Cell. ACS Appl. Mater. Interfaces 2013, 5, 7886. (3) Bella, F.; Nair, J. R.; Gerbaldi, C. Towards Green, Efficient and Durable Quasi-Solid Dye-Sensitized Solar Cells Integrated with a Cellulose-Based Gel-Polymer Electrolyte Optimized by a Chemometric DoE Approach. RSC Adv. 2013, 3, 15993. (4) Bella, F.; Bongiovanni, R. Photoinduced Polymerization: An Innovative, Powerful and Environmentally Friendly Technique for the E

DOI: 10.1021/acs.langmuir.6b01460 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b01460 Langmuir XXXX, XXX, XXX−XXX