Toward Higher Energy Conversion Efficiency for Solid Polymer

Perspective pubs.acs.org/JPCL

Toward Higher Energy Conversion Efficiency for Solid Polymer Electrolyte Dye-Sensitized Solar Cells: Ionic Conductivity and TiO2 Pore-Filling Donghoon Song, Woohyung Cho, Jung Hyun Lee, and Yong Soo Kang* Center for Next Generation Dye-sensitized Solar Cells and Department of Energy Engineering, Hanyang University, Seoul 133-791, Korea ABSTRACT: Even though the solid polymer electrolyte has many intrinsic advantages over the liquid electrolyte, its ionic conductivity and mesopore-filling are much poorer than those of the liquid electrolyte, limiting its practical application to electrochemical devices such as dye-sensitized solar cells (DSCs). Two major shortcomings associated with utilizing solid polymer electrolytes in DSCs are first discussed, low ionic conductivity and poor pore-filling in mesoporous photoanodes for DSCs. In addition, future directions for the successful utilization of solid polymer electrolytes toward improving the performance of DSCs are proposed. For instance, the facilitated mass-transport concept could be applied to increase the ionic conductivity. Modified biphasic and triple-phasic structures for the photoanode are suggested to take advantage of both the liquid- and solid-state properties of electrolytes.

A

stress, which is quite advantageous for both the roll-to-roll process and flexible devices.8,9 In polymer electrolyte DSCs, a great deal of study has been aimed at improving their overall energy conversion efficiency by enhancing the ionic conductivity. For instance, the ionic conductivity of PEO-based polymer electrolytes was improved with the help of decreasing their crystallinity and adding plasticizers10−12 or inorganic fillers.13−20 In addition to PEO polymer derivatives,11,21−27 PEO-free polymers have also been utilized.12,28−31 The typical polymers used are polymeric ionic liquids (poly(ILs),27,32−38 poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP),28,39−42 poly(acryl onitrile) derivatives (PAN),12,43,44 poly(styrene) (PS) derivatives,29,45 and poly(methylmethacrylate) (PMMA) derivatives.30,46,47 In a DSC application, a polymer electrolyte may not readily penetrate into the mesopores of the dye-decorated photoanodes, resulting in poor contact with dyes and consequently low energy conversion efficiency. Note that the average size of the photoanode mesopore is around 15−20 nm depending on the TiO2 nanoparticles (NPs) used, whereas many polymers have a coil shape in solution with a diameter much bigger than the mesopore size. For instance, the PEO of its Mw = 1 million has a coil diameter of 100−120 nm depending on the solvent used.48 The poor contact between the dyes and the polymer electrolyte is mostly associated with poor penetration of the polymer electrolyte simply due to the size difference between the TiO2 mesopore and the polymer coil in a given solvent. Eliminating such problems associated with poor contact has been attempted

diversity of applications for polymer electrolytes has been created in the fields of secondary lithium batteries, fuel cells, and solar cells. Polymer electrolytes are defined as solid ionic conductors formed by the dissolution of salts in suitable high molecular weight polymers.1 Polymer electrolytes have advantages in commercialization due to their long-term and thermal stabilities, mechanical strength, synthesis from naturally abundant sources (C, N, O, etc.), and cost effectiveness. However, they suffer from poor ionic conductivity arising from the nonfluidic solid-state properties of the polymer solvent compared to those of the liquid one. For instance, the ionic conductivities in poly(ethylene oxide) (PEO) polymer electrolytes are scaled 10−8−10−4 S/cm, which is roughly 2−6 orders of magnitude lower than those of liquid electrolytes.2 Therefore, low ion diffusion strongly impacts electrochemical device performance. Dye-sensitized solar cells (DSCs) typically configure an electrolyte containing redox couples dissolved in a solvent, which is sandwiched between a dye-coated titania layer and a counter electrode.3 High efficiencies of over 11% are usually obtained by either cobalt (II/III)4,5 or iodide (I−)/(I3−)6,7 redox couples dissolved in liquid solvents like acetonitrile and 3-methoxy propionitrile, which are fluidic enough not to significantly impede a cobalt (II/III) or I−/I3− redox diffusion. However, solvent leakage or drying-out in liquid DSCs brings out poor long-term stability, which demands highly stable electrolytes such as polymer electrolytes. In addition, problems associated with dye detachment may not be serious compared to liquid electrolytes, and relatively high energy conversion efficiency can be observed under low light intensity. Furthermore, for device commercialization, polymer redox electrolytes provide the required mechanical strength and flexibility to endure bending © 2014 American Chemical Society

Received: February 9, 2014 Accepted: March 18, 2014 Published: March 18, 2014 1249

dx.doi.org/10.1021/jz5002727 | J. Phys. Chem. Lett. 2014, 5, 1249−1258

The Journal of Physical Chemistry Letters

Perspective

by first introducing a low viscous monomer and subsequently in situ polymerization photochemically49 and thermally.32,50 An initiated chemical vapor deposition (iCVD)46 method has also been attempted. The representative efficiencies are summarized in Figure 1, where the cell efficiency of state-of-the-art DSCs

The overall energy conversion efficiency (η) for DSCs is related to a short-circuit current (JSC), open-circuit voltage (VOC), fill factor (FF), and power of incident light (Pin). η(%) =

JSC VOCFF Pin

× 100

(1)

As described previously, the deteriorated photovoltaic performances of the DSCs with polymer electrolytes are mostly attributable to a slow dye regeneration rate by poor ion conduction and TiO2 pore-filling.51,52 In most cases, low JSC is the main factor in lowering the energy conversion efficiency, which can be evidenced by the large photocurrent decrease depicted in Figure 2. Furthermore, large decreases in VOC are occasionally observed in addition to the decrease in JSC for more serious cases. This can be understood from the following theoretical relation53 VOC = Figure 1. Trends of the energy conversion efficiency of DSCs based on polymer electrolytes.10,12,13,18,21,22,24−26,28,33,34,39,41−44,49,58,100−105 The data were obtained at 100 mW cm−2 illumination conditions except ∗ at 30, ∗∗ at 28, ∗∗∗ at 65.6, and ∗∗∗∗ at 60 mW cm−2. The efficiency marked as “+” was recently achieved in our laboratory (VOC: 0.68 V; JSC: 21.0 mA cm−2; FF: 0.62; and η: 8.9%).

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

(2)

where k is the Boltzmann constant, T is the absolute temperature, e is the electric charge, Iinj is the flux of the electron injection from the dyes, ket is the rate constant for the electron recombination, and ncb is the number of electrons at the TiO2 conduction band. This indicates that VOC is proportional to ln(Iinj); therefore, low VOC is recorded if less electrons as a result of significantly slow dye regeneration are injected into the TiO2 conduction band. The low ion conduction can increase the diffusion resistance to drop the FF value. However, the TiO2 pore-filling effects on FF have to be studied further. Accelerating Ionic Conduction. One simple way to improve the ionic conductivity of solid polymer electrolytes is to incorporate a solvent or plasticizer to make the swollen polymer electrolyte. When a small amount of solvent is added in a polymer, the polymeric chain mobility is increased to some extent, resulting in an increase in ionic conductivity and a decrease in the glass transition temperature (Tg). For instance, incorporation of ethylene carbonate and propylene carbonate into PEO caused improved energy conversion efficiency of 2.9%.10 PAN was plasticized with the same materials, giving 4.4% efficiency under 30 mW cm−2.12 DSCs assembled with poly(ethylene oxide-coepichlorohydrin) (P(EO-EPI)) electrolytes plasticized by γbutyrolactone (GBL) performed with cell efficiencies of 3.3 and 3.5% at 100 and 10 mW cm−2 illumination, respectively.11 Another approach is to make liquid solvent a nonfluidic gel with the help of a polymer matrix, in which the liquid solvent is only partially compatible with the polymer. PVdF-HFP, PMMA, PAN, or PS is commonly used for this purpose. In contrast to a coordinating polymer like PEO, such a polymer gel electrolyte has a binary phase of coexisting liquid and solid phases where ion conduction occurs primarily through the liquid phase, which ensures a high level of ion conductivity similar to that of a liquid electrolyte. Even though the mechanical properties of an electrolyte become poorer with increasing liquid, high efficiency comparable to a liquid one can be achieved. In some polymer gel electrolyte cases, the photovoltaic performances are comparable to those of liquid DSCs.39,40,43 Typical examples are PVdF-HFP electrolytes with a high boiling point or nonvolatile ionic liquid solvents.28,33 Noticeably, a cell efficiency of over 10% was reported under 1 sun illumination conditions using heteroleptic ruthenium DSCs based on a PAN derivative.43 Lately, a cobalt complex redox mediator along with a PVdF polymer gel electrolyte resulted in cell efficiency of over 10% under 0.1 sun illumination conditions. In addition, novel polymer gel electrolytes

using solid polymer electrolyte is included. Despite the great efforts described above, the overall energy conversion efficiencies with the polymer DSCs are still quite low in comparison to those (13%)5 with liquid electrolytes. This difference is primarily attributable to the low photocurrent, as evidenced by the serious photocurrent transient decay, especially under a high light intensity (Figure 2).12

Figure 2. Photocurrent maximum and steady-state photocurrent obtained from photocurrent transients versus monochromatic (524 nm) light intensity; (○) photocurrent maximum, (Δ) steady-state photocurrent. The inset shows the photocurrent transients at light intensities of (a) 5.4 mW cm−2 where iss = 0.93 mA cm−2 and (b) 90 mW cm−2 where imax = 12.3 mA cm−2 and iss = 8.0 mA cm−2. Reprinted from ref 12.

Therefore, in this Perspective, we concentrate on two issues for DSCs employing polymer electrolytes toward improving the energy conversion efficiency, (a) intrinsically low ion conduction and (b) poor interfacial contact between the dyes in the photoanode and the polymer electrolyte, and suggest feasible directions for improvement of the DSC performance. 1250



Perspective

Figure 3. Molecular structures of the poly(ILs), (a) poly(N-alkyl-4-vinylpyridine) iodide34 and (b) poly(1-butyl-3-(1-vinylimidazolium-3hexyl)imidazolium bis(trifluoromethanesulfonyl)imide),37 and iodine-free poly(ILs) (c) poly(1-alkyl-3-(acryloyloxy)hexylimidazolium iodide)38 and (d) poly[((3-(4-vinylpyridine) propanesulfonic acid) iodide)-co-(acrylonitrile)].36

Figure 4. Schematic drawing of the “Oligomer Approach”. Reproduced from ref 48.

was more than two times higher than that of the pioneering work (1.6%) by Nogueira et al.21 Since then, several researchers have employed metal oxide NPs14−17 to improve ionic conductivity. Furthermore, we gain further positive aspects, (1) light scattering effects of the NPs by increasing their size over 100 nm18 and (2) chemical effects of the NPs in controlling the local concentration of the ionic species.17 In addition, to reinforce stability as well as performance, IL-functionalized SiO2 NPs, which are much more compatible with polymer electrolyte than bare SiO2 NPs, were developed.20 The beneficial effects of the incorporation of NPs could be maximized by changing their type, content, size, and shape. Another interesting work has been recently reported describing the incorporation of graphene or a carbon nanotube, which is electrically conductive, to improve ion diffusion.56,57 Despite improving the ionic conductivity of the polymer electrolytes, these nanomaterials absorb visible light, and how they increase the ionic conductivity is still poorly understood. Poly(ILs) have also been intensively investigated because ILs have many beneficial properties such as high ionic conductivity and dielectric constant, tunable chemical structure, and electrochemical and thermal stability.32−35,37 Several examples of poly(ILs) are depicted in Figure 3. Poly(ILs) are only selectively permeable to I− and I3− ions, whereas IL cations are bound to a

have been intensively developed, such as gelation by PS nanobeads,29 poly(ethylene oxide) dimethyl ether (PEODME) with fumed silica,32 poly(β-hydroxyethyl methacrylate),21 or thixotropic xanthan gum.22 In an interesting work,29 PS nanobeads were deposited on Pt electrodes followed by injection of a liquid electrolyte, which dissolves the PS nanobeads to become gel-state electrolytes. Using this concept, high photovoltaic performance was recorded with a JSC of 15.3 mA/cm2, VOC of 0.77 V, FF of 0.64, and η of 7.54%. Although some polymer gel electrolytes show nearly comparable energy conversion efficiency with liquid electrolytes, not all of the problems associated with liquid solvents can be eliminated. Incorporating nanofillers into a polymer matrix is an interesting idea. For instance, incorporating titania nanofillers (Degussa P25) into highly crystalline PEO-based polymer exhibited a low crystallinity of ∼14.8%,13 resulting in enlargement of the amorphous phase and eventually enhancement of the mobility of the I−/I3− redox couple. In addition to the significant decrease in the PEO crystallinity, it has been accepted that free volume is generated at the interfaces between the NPs and the polymer matrix for additional ion transport pathways,55 resulting in increased ionic conductivity. This method enabled polymer DSCs to yield 4.19% of the energy conversion efficiency, which 1251



Perspective

in an energy conversion efficiency of 4.59 and 3.34% evaluated at 42.9 and 100 mW cm−2 illumination conditions, respectively.22 Oligo-PEGs were also incorporated with either NPs or solid polymer to become polymer electrolytes. In these approaches, the liquid oligo-PEGs were selectively penetrated into the mesoporous photoanode, whereas the solidified oligo-PEGs by either silica NPs54 or solid polymer58 stayed outside of the photoanode, bulk electrolyte, yielding cell efficiency up to 4.50 or 4.42% at 1 sun conditions, respectively. In addition to the enhanced pore-filling by oligomers, high cell efficiency was also contributed to by the increased ionic conductivity. For instance, the ionic conductivity of the oligo-PEG with a solid polymer observed was also beneficially increased up to 2.57 × 10−5 S/cm at room temperature, nearly 14-fold higher than that of a pristine PEO electrolyte.58 For better TiO2 pore penetration and interfacial contact between the polymer matrix and the electrolyte, applying oligomeric coadsorbents seems to be attractive. Recently, a PEG-based oligomeric coadsorbent was applied for passivation of the TiO2 photoanode in liquid electrolyte DSCs.60 In addition to the typical roles of coadsorbent inhibiting electron recombination reaction, it is inferred that the oligomeric coadsorbent could improve its compatibility with the electrolyte, consequently improving the interfacial contact between the dyes and electrolyte. Similarly, the enhanced compatibility of the photoanode and the PEO electrolyte could be successfully demonstrated from a K51 Ru dye61 having a short PEO chain. The most common way to improve pore-filling could be to fill the mesopores with monomers first and subsequently in situ polymerize them to become a solid-state polymer electrolyte. For instance, oligoethylene glycol methacrylate, which was photopolymerized in situ, was used as a solid polymer electrolyte.49 Polymer gel electrolytes via in situ alkylimidazolium polymerization32 and chemical cross-linking50 were also reported. The direct deposition of poly(2-hydroxyethyl methacrylate) to a TiO2 pore was realized via iCVD.46 Despite the beneficial effects of in situ polymerization, some negative effects are inevitable. For example, the unreacted monomers may give secondary problems, and therefore, the polymerization yield should nearly be unity. The redox reactions of the redox couples or I2 dissolved in electrolyte may hinder or disturb a well-controlled radical polymerization process. The dyes anchored to the TiO2 surface may also be degraded or detached under UV irradiation for photopolymerization or thermal polymerization. During polymerization, polymer volume shrinkage presumably occurs, leading to the formation of some voids at the electrolyte and the mesopore interface and consequently a reduction in the dyeregeneration rate. Poor penetration of the polymer electrolyte is mostly due to the size difference between the mesopore in the photoanode and the coil size of the polymeric chain. Therefore, if the pore size of the photoanode can be increased, the polymer electrolyte may penetrate into the mesopores deeply, resulting in good contact between the electrolyte and the dyes. Experimentally, the pore size has been enlarged by using 1D photoanode structures utilizing nanowire,62 nanorod,63 nanofiber,64 nanotube,65 and other materials. A TiO2 nanofiber electrode obtained from the electrospinning method yielded 4.6% energy conversion efficiency with a gel electrolyte.64 Polymer electrolyte DSCs based on TiO2 nanotubes showed an overall efficiency of 4.03% at 100 mW cm−2, higher than one based on TiO2 NPs.65 It was reported that well-organized mesoporous TiO2 films with excellent channel connectivity developed via the sol−gel process

polymer and are thus not movable. Their cation structure effects on ionic conductivity have been investigated.35 In 2008, an N-alkyl-4-vinylpyridine iodide/poly(N-alkyl-4-vinyl-pyridine) iodide composite polymer electrolyte combined with iodine retained a high ionic conductivity of 6.41 × 10−3 S/cm, referenced to the 10−8−10−4 S/cm of common PEOs, for allsolid-state polymer electrolyte DSCs.34 Consequently, the cell efficiency reached as high as 5.64%.34 More recently, bis(imidazolium)-based poly(ILs) also gave a cell efficiency of 5.94% with an iodide-based IL electrolyte.37 Iodine-free polymer electrolytes have also been an emerging research topic.27,36,38 In this iodine-free electrolyte system, similar to poly(IL) electrolytes previously discussed, IL cations tethered onto a polymer main chain are fixed; therefor, only iodide can diffuse. Poly(1-ethyl-3-(acryloyloxy)hexylimidazolium iodide) as an all-solid-state polymer electrolyte was successfully used to gain a cell efficiency of 5.29% under 1 sun condition.38 Poly[((3-(4-vinylpyridine) propanesulfonic acid) iodide)-co(acrylonitrile)] was also synthesized and applied to DSCs with ILs and additives, and its cell efficiency was 6.95% without the addition of iodine.36 Note that the π−π stacking interaction among the IL cations may provide a favorable channel for hole transport from the photoanode to the counter electrode.38 However, despite the great potential of iodine-free poly(IL) electrolytes toward higher efficiency, a deep understanding of the conduction mechanism is lacking.

However, despite the great potential of iodine-free poly(IL) electrolytes toward higher efficiency, a deep understanding of the conduction mechanism is lacking. Viable Routes to Better TiO2 Pore-Filling. In highly efficient DSCs, the mesoporous TiO2 layer has a typical pore size of about 20 nm. Fluidic liquid electrolytes can penetrate through the TiO2 pore and contact the dyes without difficulty, resulting in efficient dye regeneration and consequently leading to high cell performance. However, nonfluidic polymer electrolytes seriously suffer from poor TiO2 pore penetration. This can cause dye regeneration to deteriorate, resulting in a decrease of the photocurrent. Therefore, the importance of the pore-filling of the electrolyte in a photoanode was proposed based on supporting evidence.51 For instance, liquid oligomers of poly(ethylene glycol), oligoPEG, or poly(propylene glycol), oligo-PPG, with a coil size of less than 3 nm were successfully utilized in improving the interfacial contact between the electrolyte and the dye as well as the ionic conductivity using the “Oligomer Approach”.9,17,18,22,23,48,54,58,59 The molecular weight of the liquid oligomers used was typically around 250−1000 with a coil diameter in the range of 1−3 nm depending on the solvent used. Therefore, liquid oligomers might readily penetrate into the mesopores of the photoanode and contact with dyes, which were subsequently solidified to become a solid polymer electrolyte by either the formation of multiple hydrogen bonds22,23 or the incorporation of NPs or a polymer,17,18,54 as illustrated in Figure 4. Oligo-PEGs (Mw = 1000 g mol−1) having quadruple hydrogen bond sites at both ends were filled into the TiO2 mesopores first and then solidified via quadruple hydrogen bonds between the oligo-PEGs, resulting 1252



Perspective

using an organized (poly(vinyl chloride)-g-(oxyethylene methacrylate)) (PVC-POEM) graft copolymer were synthesized by the one-pot atom-transfer radical polymerization (ATRP) method.66 Furthermore, the pore size of the TiO2 films was controlled to be between 28 and 73 nm by varying the ratio of PVC to POEM. The solid-state DSCs performances based on the ordered structure were two times better than those of the randomly oriented TiO2 films with a small pore size. In general, though, enlarging the TiO2 pore size described above might be indicative that the TiO2 surface area for dye adsorption is significantly decreased, resulting in photocurrent loss. Therefore, a hierarchical photoanode with many small nanoprimary particles (∼9.1 nm) aggregated into bigger particles (∼ up to several μm) becomes very attractive because they have a larger pore size as well as a large surface area.67 Therefore, photoanodes with TiO2 hierarchical or mesoporous beads68 appear more effective in collecting incident light due to the larger amount of dye adsorbed on the increased surface and partly the deeper electrolyte penetration through the enlarged pores. In this case, additional light scattering effects can also be obtained.69 Furthermore, photoanode structures based on a mixture of small and large NPs,70 inverse opal,71 hollow sphere,72 scattering spherical void,73 and 3D photoanodes74 are strong candidates for improvement in the pore-filling of a polymer electrolyte into TiO2 pores. The representative structures are shown in Figure 5. Redox Energy Level Tuning for Higher Open-Circuit Voltage. Toward increasing η so far, we have discussed how to achieve enhanced ion conduction and TiO2 pore-filling, which may chiefly enhance JSC. Improvement of VOC is another approach to increase η. VOC is defined by the gap between the TiO2 pseudoFermi level and the redox energy level (Eredox) of the electrolyte, thus indicating that lower Eredox implies higher VOC. The Eredox of the common I−/I3− redox couple is 0.35 V (vs NHE), which is more than adequate for dye regeneration, representing much room for further energy level tuning to increase VOC.75 Therefore, alternative redox couples based on cobalt and iron complexes with their Eredox’s tuned to become lower than the I−/I3− redox couple may also be attractive toward higher VOC in solid-state DSCs.4,39,76−81 Future Challenges and Outlook. The energy conversion efficiency of DSCs using solid polymer electrolyte is still quite lower than the one using liquid electrolyte. This difference is mostly due to the intrinsic problems associated with slow diffusion through solid polymer electrolyte and its poor interfacial contact with dyes adsorbed on a photoanode surface, as described previously. Therefore, the energy conversion efficiency could be improved to a large extent by overcoming these two hurdles. Accelerating the ion conduction via Grotthus or an exchange reaction mechanism56 could provide a solution to the slow diffusion problem through solid polymer electrolyte. According to the Grotthus mechanism,82 additional mass transport may occur under highly packed ions in electrolyte via self-exchange redox reactions. Therefore, the transport of a specific solute is facilitated due to the contribution of the exchange reaction in addition to the normal physical diffusion, which is thus termed facilitated transport.83 The apparent diffusion coefficient D for facilitated transport can be expressed according to the Dahms− Ruff equation84−86 as D = Dpys + Dex = Dphys +

kexδ 2c 6

Figure 5. (a) Diagram for the electrolyte diffusion through the external (A) and internal (B) pores in the titania film fabricated from the nanoporous TiO2 spheres. (b) a) SEM and b) high-resolution (HR) TEM images of SnO2 MHSs. c) SEM and d) HRTEM images of TiO2− SnO2 MHSs. (c) Spherical voids are left behind in the film of TiO2 when the polystyrene spheres are burnt out during the heating or the sintering of the films at 450 °C. (d) a) A schematic illustration for the fabrication of a 3D TiO2 photoanode. b−e) SEM images of pillar-, prism-, pyramid-, and inverted-pyramid-patterned TiO2 photoanodes consisting of 20 nm TiO2 nanocrystals (scale bars in the SEM pictures: 10 μm), respectively. Adapted from refs 67 and 72−74.

where Dpys is the physical diffusion coefficient, Dex is the exchange-reaction-based diffusion coefficient, kex is the exchange reaction rate constant, δ is the center-to-center intersite distance for the exchange reaction, and c is the concentration. The apparent

(3) 1253



Perspective

efficiency still needs to be improved to be competitive with other energy resources. In this Perspective, two new approaches have been proposed to advance the “oligomer approach” by triplephase and modified bi-phase structures, as shown in Figure 7. In the triple-phase structure, the mesopore in the photoanode is filled with nonvolatile liquid such as ILs or oligomers, whereas the bulk electrolyte is a slightly swollen but apparently solid polymer electrolyte. The ionic conductivity is further increased by incorporating solid nanofillers, whose chemical effects may also play important roles in the efficiency.17 The triple-phase structure would also help the contact problem with the solid counter electrode. Therefore, the electrolyte comprises the liquid state inside of the mesopores of the photoanode and slightly swollen but still apparently solid electrolyte with solid nanofillers at bulk. In the case of the modified bi-phase structure, liquid-state oligomers or ILs fill the mesopores, as previously described. The ionic conductivity through the liquid state is around ∼1.02 × 10−3 S/cm, whereas that through the solid state is ∼2.15 × 10−4 S/cm, nearly 20 times slower.18 One way to enhance the ionic conductivity while maintaining the bulk electrolyte apparent solid state is to have a bicontinuous structure, continuous hydrophilic channels dispersed in a hydrophobic matrix. The hydrophilic channels are filled with liquid oligomer for ionic conductivity enhancement, whereas the hydrophobic matrix may provide the necessary mechanical strength for separation of the two electrodes. In these advanced oligomer approaches, the advantages of both the liquid and solid-state properties are utilized for improving ionic conductivity and pore-filling phenomena at the apparent solid state. However, further study of dye detachment is required, which may occur in both triplephase and bi-phase structures because the liquid oligomers or ILs mostly contact with dyes. For a given diffusion coefficient or conductivity of the ionic species, the ionic flux could be increased by reducing the distance for the ions to be diffused according to Fick’s first law. Considering the typical thickness of an electrolyte layer of about 25 μm, there is much room available to reduce the thickness of the electrolyte layer for the achievement of high ion flux and, consequently, fast, efficient dye regeneration. However, when the distance between two electrodes becomes shorter, the possibility of a short circuit by contacting two electrodes becomes greater. In this regard, electrode contact might be prevented by encapsulating the dye-sensitized TiO2 particles using a polymer gel electrolyte membrane layer.88 In addition, this direct contact issue between the two electrodes can be solved by introducing a bifunctional layer composed of insulating Al2O3 NPs onto the top of the TiO2 layer, which prevents it from contacting the two

diffusion coefficient is linearly dependent on the electrontransfer rate constant, kex, according to the Dahms−Ruff equation, indicating that fast ion conduction can be achieved by the fast exchange reaction rate constant. Interestingly, ferrocene/ferrocenium (Fc/Fc+) show a very high electron-transfer rate constant, kex, of ∼107 M−1 s−1, nearly 5 orders of magnitude higher than that of ∼5 × 102 M−1 s−1 of I−/I3−.87 Therefore, ferrocene-based polymer electrolytes would show faster ion diffusion. Schematic electron transport through a solid polymer electrolyte is illustrated in Figure 6. Therefore,

Figure 6. Schematic drawing of the electron exchange reaction occurring at the ferrocene redox-based PEO polymers.

facilitated transport phenomena could be observable to help increase the ionic conductivity through solid polymer electrolytes for DSCs. Another advantage over iodine-free redox couples is to improve VOC due to the tunable energy levels, as discussed before.

Facilitated transport phenomena could be observable to help increase the ionic conductivity through solid polymer electrolytes for DSCs. Oligomers have been successfully utilized in improving the penetration of polymer electrolyte without significantly sacrificing the ionic conductivity. However, the energy conversion

Figure 7. Schematic drawing of (a) triple-phase and (b) modified bi-phase DSCs structures based on the advanced Oligomer Approach. 1254



Perspective

electrodes and also acts as a scattering layer, yielding very high energy conversion efficiency, as shown in Figure 1. Recently, solar cells based on extremely high light absorbing perovskites have paved the way for high cell efficiency by as much as 15%.89,90 They are efficiently operated in mesoscopic solar cells based on a thin TiO2 or even Al2O3 film with common holetransfer materials such as 2,2,7,7-tetrakis-(N,N-di-p-methoxyphenylamine-9,9-bifluorene (spiro-OMeTAD). Of course, alternative hole conductors have been recently tried in perovskite-sensitized solar cells.91 This appears to also be positive in achieving high efficiency of the solar cells based on the solid polymer electrolyte. However, the chemical stability of perovskites, which are weak to water and common solvents, against polymeric solvent and redox couples has to first be studied and obtained.

University in 2014. Currently, her research interests focus on facilitated mass transport in the solid state using ionic carriers in applications for gas separation membrane and ionic conductive polymer electrolytes for dye-sensitized solar cells. Yong Soo Kang, Professor in the Department of Energy Engineering, Hanyang University, Seoul, Korea and the Director for Next Generation Dye-sensitized Solar Cells, received his Ph.D. from Tufts University, MA, USA in 1986. His research interests focus on facilitated transport phenomena in the solid state and their applications including mesoscopic solar cells and separation membranes. He has more than 280 original scientific publications and 50 patents in those fields. More details are available at http://www.eml.or.kr.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) from the Center for Next Generation Dye-sensitized Solar Cells (No. 2013004800) and by the Korea Center for Artificial Photosynthesis (KCAP) (No. 2009-0093883).

In this Perspective, two new approaches have been proposed to advance the “oligomer approach” by triple-phase and modified bi-phase structures.

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Utilizing high light absorbing sensitizers is also interesting because it can reduce the thickness of the photoanode and consequently solve the pore-filling issue of polymer electrolytes. For example, some organometallic,4,7,92,93 organic,94−96 or nearIR dyes97−99 have molar absorption capabilities as much as 1.5−9 times larger than the generally used N719 dye. Through this Perspective, it is anticipated that utilization of solid polymer electrolytes will be attractive and broad because of their intrinsic advantages. However, there are several hurdles to be solved for practical applications that could be overcome by improvement of both the ionic conductivity and the pore-filling issues. The solid polymer DSC efficiency started from 1.6 × 10−4% in 1999,100 and it is currently 8.9%. We believe therefore that the routes discussed can contribute to overwhelm state-ofthe-art DSCs (∼13%), thus being viable to trigger broad interest.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Donghoon Song received his B.S. at Hanyang University in 2009 and is currently a Ph.D. student at Hanyang University under the guidance of Professor Yong Soo Kang. His current research includes the characterization of interfacial phenomena between polymer electrolytes and photoanodes using various photoelectrochemical methods and also the development of nanomaterials compatible with polymer electrolytes in mesoscopic solar cells. Woohyung Cho obtained his B.S. at Hanyang University in 2010 and has been a Ph.D. student from Professor Yong Soo Kang’s research group since 2010. He has studied the electrochemistry of electrolytes, especially alternative redox mediators and solid-state polymer electrolytes in dye-sensitized solar cells. Jung Hyun Lee obtained her M.S. in Chemical Engineering in 2008 and her Ph.D. in the Department of Energy Engineering at Hanyang 1255



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