Controlled Dissolution of Polystyrene Nanobeads: Transition from

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Controlled Dissolution of Polystyrene Nanobeads: Transition from Liquid Electrolyte to Gel Electrolyte Kun Seok Lee,† Yongseok Jun,‡ and Jong Hyeok Park*,† †

School of Chemical Engineering and SAINT, Sungkyunkwan University, Suwon 440-746, Republic of Korea Interdisciplinary School of Green Energy, KIER-UNIST ACE, UNIST, Ulsan 689-798, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The widespread commercialization of dyesensitized solar cells (DSSCs) remains limited because of the use of highly volatile liquid electrolytes. Recently, gel-type quasi-solid electrolytes containing a polymer additive or inorganic nanomaterial have shown promising results in terms of the cell efficiency. However, most gel electrolytes have serious obstacles for pore-filling because of their high viscosity. Herein, we report the first observation of the transition from a liquid to a gel electrolyte after filling the cell with the liquid electrolyte using the controlled dissolution of polystyrene nanobeads on the counter electrode, suggesting that the pore-filling problem can be diminished in quasi-solid state DSSCs. The time-resolved solidification allows for the preparation of the gel electrolyte without interfering with the cell performance. The optimal DSSC composed of the gel electrolyte exhibits almost the same power conversion efficiency as the liquid electrolyte based DSSC when measured using an AM1.5G solar simulator at 100 mW/cm2 light illumination. Moreover, the long-term stability of the DSSC was greatly improved. KEYWORDS: Dye-sensitized solar cells, gel electrolyte, long-term stability, high efficiency, transition

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because the quasi-solid electrolyte is viscous and cannot penetrate easily into the mesopores of the TiO2 film. The mesoporous structure makes it especially difficult for the capillary force to transport these gel-type materials due to the viscous and steric effect.16 To solve these drawbacks of gel-type electrolytes, postpolymerization after the penetration of the monomers into the mesoporous TiO2 electrode has been proposed.17−21 However, to induce polymerization, high-temperature conditions or several additives, such as a free-radical initiator, are essential, which may influence the long-term stability of the DSSCs. Meanwhile, colloidal crystals of monodispersed polymeric particles with opaline structures have attracted attention as sacrificial templates for 3D nanostructures. The pore structure can be carefully controlled across a wide range of scales by varying the size of the colloidal particles, and the connectivity between the pores is well-defined due to the close-packing of the particles.22 In the present paper, we demonstrate some of the possibilities for preparing gel electrolytes which can easily penetrate into mesoporous TiO2 nanoparticles. Generally, the solvents used for the electrolytes in DSSCs are composed of acetonitrile and valeronitrile. Polystyrene beads are insoluble in acetonitrile (Figure S1) but are well dissolved in valeronitrile (Figure 1). This unique characteristic can make the PS beads

ye-sensitized solar cells (DSSCs) with a network of interconnected TiO2 nanoparticles are currently attracting widespread scientific and technological interest as a cheap and high-efficiency alternative to conventional inorganic photovoltaic devices.1 At present, however, various practical impediments such as the leakage of the liquid electrolyte remain serious obstacles to their application.2 In this regard, one of the major research subjects is the solidification of the electrolyte. Two different types of electrolyte solidification have been reported. First, all-solid-state DSSCs3−5 have been proposed without using any solvent. However, they suffer from the low ionic conductivity of the electrolytes, resulting in relatively poor cell performance. As an alternative approach, quasi-solid DSSCs6−8 filled with a gel electrolyte have been reported. Polymers, such as poly(ethylene oxide), poly(methyl methacrylate), and poly(vinyidene fluoride-co-hexafluoropropylene) (PVDF−HFP), are widely used as additives for the solidification of liquid electrolytes.9−11Among these polymers, PVDF−HFP shows superior mechanical strength and high ionic conductivity and can be prepared easily. As an alternative method, scientists have focused on quasi-solid-state electrolytes incorporating small molecular organogels 12−14 and SiO2 nanoparticles.15 The addition of nanoscale particles to such liquid electrolytes to form a thixotropic fluid results in increased viscosity, improved electrode−electrolyte interfacial contact, and reduced fluidity. However, the conversion efficiencies of solid-state DSSCs are in most cases about ∼8% while DSSCs based on liquid electrolytes have reached as high as 12% © 2012 American Chemical Society

Received: December 5, 2011 Revised: February 20, 2012 Published: April 2, 2012 2233

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Figure 1. (a) Schematic of the preparation of PS nanobeads on the Pt/FTO substrate. (b) Comparison schematic of the pore-filling during the dissolution of the PS nanobeads or PS film. (c) Top-view SEM images of the PS nanobeads on the Pt/FTO substrate before the dissolution process (left) and after the dissolution process (right). (d) Photographs of PS nanobeads on Pt/FTO substrate before (left) and after (right) the dropping of the electrolyte containing valeronitrile.

Figure 2. Schematic of the conversion of the liquid electrolyte into a gel electrolyte. By controlling the content of valeronitrile in the solvent, we can control the dissolvable fraction of PS from the PS nanobeads.

Pt/FTO substrate instead of the PS beads, it is not easy to dissolve and prevent the direct contact between the electrolyte and Pt on the FTO substrate (Figure 3 vs Figure S2). The scanning electron microscopy (SEM) image of the PS on the Pt/FTO substrate shows that the particles are well distributed and well packed (Figure 1c). Furthermore, when the film of PS spheres was contacted with the electrolyte, a color change was observed, resulting in the formation of a gel (Figure 1d). After the formation of the gel, the gel electrolyte was washed out and we observed the surface SEM image of the Pt/FTO substrate again. As can be seen in Figure 1c, some very small PS beads still remained, which means that the amount of PS beads is sufficient for the formation of the gel electrolyte. Figure 2 shows a schematic diagram of the DSSC fabrication procedure that is assembled from the PS nanobeads-coated Pt/ FTO counter electrode and the dye-coated TiO2 photoanode. After preparing the unit cell, the liquid electrolyte with a composition of 0.6 M butylmethylimidazolium iodide, 0.03 M I2, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tertbutylpyridine in a mixture of acetonitrile and valeronitrile (v/ v, 85:15) was added to the cell. It is believed that the liquid electrolyte can easily penetrate into not only the mesoporous TiO2 electrode but also the PS opaline counter electrode because of the high capillary force resulting from their meso- or macroporous structure. After the electrolyte filling process, the

enter the gelled state when they are directly contacted with the acetonitrile/valeronitrile mixture solvent. First, we synthesized PS beads with a mean size of ∼250 nm by the dispersion copolymerization of styrene.23 Then, the water solution of the PS beads was spin-coated on the Pt/FTO counter electrode (Figure 1a). Counter electrodes were prepared by coating an FTO glass with a drop of 100 mM H2PtCl6 solution in 2-propanol and heating it at 400 °C for 15 min. As would be expected from Figure 1b, when the liquid electrolyte with valeronitrile solvent is injected into the DSSC, the liquid electrolyte can easily reach the surface of the Pt/FTO substrate because of the capillary force. In this study, the electrolyte containing valeronitrile was used to dissolve the PS nanobeads to form gelled state after filling the liquid electrolyte in DSSC. Generally, the viscosity of a polymer solution depends on concentration of the dissolved polymer. So, the increased concentration of PS molecules in the liquid electrolyte is the origin of gelation. It is assumed that when a PS nanobead was contacted with the injected liquid electrolyte, a two-step process controls the dissolution. The first step involves the swelling of the polymer nanobead due to the used solvent, and the second step corresponds to the viscous yield of the gel electrolyte and is controlled by the cooperative diffusion coefficient of PS in the liquid electrolyte. On the other hand, even though the PS film can also be applied on the top of the 2234

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15.3 mA/cm2, 0.77 V, and 0.64, respectively, corresponding to an overall energy conversion efficiency of 7.54. As expected, the gel electrolyte-based DSSC shows a similar Jsc value. However, it is clearly evident that Voc is consistently higher for the gelbased DSSC compared to the liquid one for all of the DSSCs we tested. This phenomenon can be understood from the following theoretical consideration:24

PS beads can be dissolved in valeronitrile and well distributed into the liquid electrode, becoming a gel electrolyte. Figure 3 shows the typical current−voltage (J−V) characteristics of the DSSCs with liquid electrolyte or PS nanobead-

Voc =

⎞ Iinj ⎛ kT ⎞ ⎛ ⎜ ⎟ ln⎜ ⎟ − 1 ⎝ e ⎠ ⎝ ncbket[I3 ] ⎠

(1)

where k is the Boltzmann constant, T is the absolute temperature, e is the electric charge, Iinj is the incident photo flux, ncb is the concentration of electrons at the TiO2 surface, and ket is the rate constant for the back-electron-transfer reaction. The photovoltage of a DSSC is kinetically limited by the back-electron-transfer reaction, where the electrons from the conduction band of TiO2 recombine with the triiodide in the electrolyte.25 The higher Voc of the gel DSSCs is caused by the additional function of the PS matrix for passivating the active TiO2 surface and partly suppressing the dark reaction. It is reasonable to expect that the contact of the quasi-solid polymer TiO2 nanoparticle surface through the dissolution and diffusion of PS can alter the interfacial properties. As can be seen in Figure 3a, the gel type DSSC shows a relatively lower FF compared to that of the liquid DSSC. To find the reason for this, electrochemical impedance spectroscopy (EIS) was used to characterize the cell resistances. Ro in Figure 3c is the ohmic series resistance of the dye-sensitized solar cells, representing the electron transport processes with a very short time constant. The first semicircle (Rct1) is the resistances at the interface of the Pt/electrolyte solution. The second semicircle (Rct2) represents the resistance component at the TiO2/dye/electrolyte interfaces.26 However, neither of the DSSCs showed the additional third semicircle representing Nernstian diffusion, resulting from reduced ion mobility. The impedance spectra displayed in Figure 3c show two semicircles in the measuring frequency from 100 kHz to 100 mHz. In this study, the first semicircle in the impedance spectrum of the DSSC with the gel electrolyte was much larger than that for the liquid electrolyte-based cell, indicating that the former has an additional resistance at the Pt/electrolyte interface. The EIS spectra show that Rct1 increased from 4.4 to 9.9 Ω when PS was incorporated into the liquid electrolyte system. This might be due to the additional resistance, which originates from the PS molecules used for passivating Pt on the FTO substrate. In addition, Rct2 also increased from 22.6 to 54.6 Ω, which originates from the interference of the PS molecules with the charge transfer between the dye on TiO2 and the electrolyte. This increased cell resistance is the main reason for the decreased FF in the DSSC-based gel electrolyte. To investigate the long-term stability of the DSSCs with different electrolytes, light soaking−aging tests were carried out on the sealed DSSCs. The cells were stored in 1 sun illumination at room temperature, and their efficiencies were measured once every 7 days. Figure 4 shows the long-term stabilities of the liquid DSSC and the DSSC with the gel electrolyte in air at room temperature. Incorporating PS into the DSSC results in a remarkable enhancement of the device stability. The Voc and FF of the DSSCs with the liquid electrolyte and gel electrolyte drop by less than 5%. Figure 4d shows that after 21 days of the light-soaking test the cell

Figure 3. J−V curves (a), IPCE (b), and impedance spectra (c) for DSSCs with different electrolytes. The area of the photoelectrode was 0.15 cm2.

based gel electrolyte under the illumination of simulated AM1.5 solar light (100 mW/cm2). On the basis of the current−voltage measurements, the cell efficiency (η) of each cell was derived from Jsc, Voc, and FF. Since the power density of the illuminated light (P0) is known, η was calculated as η = (FF)JscVoc/P0. In the case of the liquid-based DSSCs, the Jsc, Voc, and FF values are 15.3 mA/cm2, 0.73 V, and 0.68, respectively, corresponding to an overall energy conversion efficiency of 7.59. In the case of the DSSC with the gel electrolyte, the Jsc, Voc, and FF values are 2235

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Table 1. Rct1, Rct2, and Rdiff Values Calculated from the EIS Data (Unit: Ω)

counterpart. We attribute this observed comparable efficiency to the efficient dye regeneration as a result of the exceptional pore-filling achieved by the in-situ transition from a liquid to gel electrolyte. This unique approach promises to be a valuable processing pathway which has the potential to solve the major drawback of the pore-filling problem in gel-type electrolytebased DSSCs.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

efficiencies of the devices with the liquid electrolyte and gel electrolyte decrease to approximately 4.52% and 6.95%, respectively. This provides good evidence for the positive effect of the gel electrolyte on the long-term stability. This might be due to the fact that the liquid electrolytes easily volatilize and leak during long-term operation, resulting in a decrease of the energy conversion efficiency of the DSSC. These results confirm that converting it into a gel electrolyte can suppress the leaking or evaporation of the liquid electrolyte, thereby improving the device durability. In summary, we successfully utilized the controlled dissolution of PS beads by the solvent used for the electrolyte to control its viscosity. By adding valeronitrile to the liquid electrolyte, the dissolution of the PS beads on the counter electrode was observed, resulting in the solidification of the liquid electrolyte. The efficiency of the quasi-solid-state electrolyte DSSC is similar to that of the liquid electrolyte

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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 an NRF grants funded by the Korea Ministry of Education, Science and Technology (MEST) (2011-0030254), the NCRC program (2011-0006268) and Future-based Technology Development program (20100029321).

Figure 4. Cell performance decay of the DSSCs with the liquid-type and gel-type electrolytes under light soaking of 1 sun illumination (100 mW/ cm2). 2236

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