Amphiphilic block-graft copolymer templates for organized mesoporous TiO2 films in dye-sensitized solar cells

Journal of Power Sources 301 (2016) 18e28

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Amphiphilic block-graft copolymer templates for organized mesoporous TiO2 films in dye-sensitized solar cells Jung Yup Lim a, Chang Soo Lee a, Jung Min Lee b, Joonmo Ahn b, Hyung Hee Cho c, Jong Hak Kim a, * a b c

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 120-749, South Korea The 4th R&D Institute, Agency for Defense Development, Yuseong-gu, Daejeon, 305-152, South Korea Department of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 120-749, South Korea

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


SBS-g-POEM block-graft copolymers are synthesized via free radical polymerization.
Crack free, 6-mm-thick, OM-TiO2 films are prepared via one-step doctorblading. exhibited
DSSC with OM-TiO2 improved efficiency.
High efficiency is due to the higher dye loading and reduced charge recombination.
Upon using PEBII and mesoporous TiO2 spheres, efficiency increased up to 7.5%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 August 2015 Received in revised form 25 September 2015 Accepted 28 September 2015 Available online xxx

Amphiphilic block-graft copolymers composed of poly(styrene-b-butadiene-b-styrene) (SBS) backbone and poly(oxyethylene methacrylate) (POEM) side chains are synthesized and combined with hydrophilically preformed TiO2 (Pre-TiO2), which works as a structural binder as well as titania source. This results in the formation of crack free, 6-mm-thick, organized mesoporous TiO2 (OM-TiO2) films via onestep doctor-blading based on self-assembly of SBS-g-POEM as well as preferential interaction of POEM chains with Pre-TiO2. SBS-g-POEM with different numbers of ethylene oxide repeating units, SBS-gPOEM(500) and SBS-g-POEM(950), are used to form OM-TiO2(500) and OM-TiO2(950), respectively. The efficiencies of dye-sensitized solar cells (DSSCs) with a quasi-solid-state polymer electrolyte reach 5.7% and 5.8% at 100 mW/cm2 for OM-TiO2(500) and OM-TiO2(950), respectively. The surface area of OMTiO2(950) was greater than that of OM-TiO2(500) but the light reflectance was lower in the former, which is responsible for similar efficiency. Both DSSCs exhibit much higher efficiency than one (4.8%) with randomly-organized particulate TiO2 (Ran-TiO2), which is attributed to the higher dye loading, reduced charge recombination and improved pore infiltration of OM-TiO2. When utilizing poly((1-(4ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII) and mesoporous TiO2 spheres as the solid electrolyte and the scattering layer, the efficiency increases up to 7.5%, one of the highest values for N719based solid-state DSSCs. © 2015 Elsevier B.V. All rights reserved.

Keywords: Block-graft copolymer Dye-sensitized solar cell Preformed TiO2 Polymer electrolyte Free radical polymerization

* Corresponding author. E-mail address: [email protected] (J.H. Kim). http://dx.doi.org/10.1016/j.jpowsour.2015.09.109 0378-7753/© 2015 Elsevier B.V. All rights reserved.

J.Y. Lim et al. / Journal of Power Sources 301 (2016) 18e28

1. Introduction Over the past decade, environmental concerns have increased in light of climate change caused by the extensive use of fossil fuels. Scientists and researchers have focused on sustainable energy sources to solve these problems. Many types of renewable devices and systems have been developed to achieve this purpose. Among these, solar cells have received significant attention for their great potential to produce abundant solar energy. A new photovoltaic device, the dye-sensitized solar cell (DSSC) was introduced by the Gratzel group in the early 1990s [1]. Since then, DSSCs have become the center of global research due to their low cost, green production process, ease of fabrication, and high efficiency. Conventional DSSCs are composed of a titanium dioxide (TiO2) photoanode sensitized by Ru dye, a platinum (Pt) -coated counter electrode, and an electrolyte containing I =I3  redox couple. Because each component contributes to the overall energy conversion efficiency, tremendous efforts have been focused on developing and modifying 1) dye that absorbs more light over a wide wavelength range [2], 2) electrolytes with long-term stability that are compatible with the electrode [3,4], 3) counter electrodes as alternative materials for expensive Pt [5,6], and 4) nanostructured mesoporous TiO2 photoanodes [7e12]. Above all, the photoanode plays a pivotal role as a scaffold. It requires a large specific surface area to allow sufficient sensitizer loading and high porosity for facile electrolyte infiltration into pores. The photovoltaic properties of DSSCs are directly related to photoanode properties such as particle size, interconnectivity, pore morphology, phase, crystallinity, and specific surface area. Many research groups have tried to prepare photoanodes with various nanostructures such as nanowires [13], nanosheets [14,15], nanodisks [16], nanotubes [17], and nanospheres [18]. Despite the high energy conversion efficiency (~11%) of DSSCs with a conventional I =I3  liquid electrolyte, there has been much interest in fabricating solid-state or quasi-solid-state DSSCs due to the potential to decrease the overall weights of cells and provide long-term durability [15e20]. However, there have been only a few reported efficiencies greater than 7.0% at one sun condition [17,18]. One of the key considerations in fabricating such DSSCs is to obtain deep penetration of large molecular volume solid electrolytes into the TiO2 photoanode, which largely depends on the porosity, pore size and pore connectivity. It has been reported that TiO2 photoanodes with well-organized morphology are more efficient than those with randomly organized structures [20e25]. Well-organized TiO2 films have primarily been prepared using an amphiphilic block copolymer as a structure-directing agent [20e22]. This method is more effective than using a homopolymer to prepare organized metal oxides due to the well-defined microphase-separated structure of amphiphilic block copolymer. Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) triblock copolymer has been used most commonly because it is commercially available at a reasonable cost under the trade name Pluronic (P123, F127) [21]. Pluronic-templated TiO2 films typically have a small pore size less than 10 nm due to their low molecular weight. Large molecular weight solid polymer electrolytes infiltrate such small pores less effectively and Pluronic impedes complete crystallization while maintaining structural integrity. An alternative block copolymer is poly(isoprene-blockethylene oxide), which has been primarily used by the Snaith group [22]. However, this block copolymer is typically synthesized via living anionic polymerization, which is very sensitive to impurities such as water and oxygen. Our group reported the effective use of graft copolymer poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) as a structure-directing agent to form wellorganized mesoporous TiO2 films for DSSC applications [25,26].

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However, such organized structure with high porosity and good interconnectivity has been obtained only at low film thickness, typically below 1 mm [20e26] and thus the preparation of organized mesoporous TiO2 film with high thickness is needed. Poly(styrene-b-butadiene-b-styrene) (SBS) block copolymer is relatively cheap and widely used in industry and daily life due to important properties including flexibility, high traction, and sealing abilities, with increased resistance to heat, weathering, and chemicals. This polymer is made up of three segments as follows: sequence polystyrene (PS), polybutadiene (PB), and PS. The PB segments are rubbery with low glass transition temperature (Tg ¼ 100  C) to provide a rubber-like property while the PS segments are glassy with high Tg (¼100  C) to provide hard and rigid mechanical properties [27]. Because of well-defined morphology and unique physicochemical properties, SBS block copolymer was used in polymer membranes to separate gas molecules [27,28] and polymer electrolyte to deliver proton ions [29]. However, the use of SBS block copolymer as a template to form metal oxide has not been reported. This is likely because both PS and PB segments are hydrophobic and their polarities are not significantly different from each other. Here, we report a high efficiency DSSCs based on crack free, micron-thick, organized mesoporous TiO2 (OM-TiO2) films templated by novel amphiphilic block-graft copolymers. Hydrophilic poly(oxyethylene methacrylate) (POEM) side chains from the hydrophobic SBS backbone were grafted via free radical polymerization to form amphiphilic SBS-g-POEM block-graft copolymers. Fourier transform infrared spectroscopy (FT-IR) and 1H nuclear magnetic resonance (1H NMR) were used to confirm the polymer synthesis. SBS-g-POEM block-graft copolymers were combined with hydrophilically preformed TiO2 (Pre-TiO2), which worked as a structural binder and titania source, to prepare organized mesoporous TiO2 (OM-TiO2) films. SBS-g-POEM with different numbers of ethylene oxide repeating units, SBS-g-POEM(500) and SBS-gPOEM(950), were used to form OM-TiO2(500) and OM-TiO2(950), respectively. The effect of polymer side chain length on OM-TiO2 properties was investigated in detail using field-emission scanning electron microscope (FE-SEM), ultraviolet (UV)-visible reflectance spectroscopy, and N2 adsorption-desorption measurements. Quasisolid-state and solid-state polymer electrolyte based DSSCs were fabricated with OM-TiO2 film as a photoanode, and their photovoltaic properties were characterized by current densityevoltage (JeV) curves, electrochemical impedance spectroscopy (EIS), and intensity modulated photocurrent spectroscopy (IMPS)/intensity modulated photovoltage spectroscopy (IMVS) measurements. 2. Experimental section 2.1. Materials Polystyrene-block-polybutadiene-block-polystyrene (SBS, styrene 30 wt%, Mw ¼ 140,000 g/mol), poly(oxyethylene methacrylate) (POEM, poly(ethylene glycol) methyl ether methacrylate, Mn ¼ 500 g/mol, 950 g/mol), dicumyl peroxide (DCP, 98%), titanium (IV) bis(ethyl acetoacetato) diisopropoxide (75 wt % in isopropanol), titanium(IV) chloride (TiCl4, 99.9%), benzyl alcohol (99.8%), hydrochloric acid (HCl, 37%), poly(ethylene glycol) (PEG 10 k, Mn ¼ 10,000 g/mol), 1-methyl-3-propylimidazolium iodide (MPII,  98%), iodine (I2, 99.99%), and lithium iodide (LiI, 99.9%) were purchased from Sigma Aldrich. Toluene, tetrahydrofuran (THF), methanol, and acetonitrile were purchased from J.T. Baker. Fluorine-doped tin oxide (FTO) conducting glass substrate (TEC8, 8 U/square, 2.3 mm thick) was purchased from Pilkington, France. Ruthenium dye (535-bisTBA, N719) was purchased from Solaronix, Switzerland. All chemical materials and solvents were obtained

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from commercial sources and used as received without any modification or purification. 2.2. Synthesis of SBS-g-POEM block-graft copolymers First, 8 g of SBS was dissolved in 100 ml of toluene by vigorous stirring at room temperature. After completely dissolving, 8 ml of POEM (Mn ¼ 500 g/mol or 950 g/mol) and 0.3 g of DCP were added to each solution. The transparent mixture solution was placed in a reactor and stirred until it became homogeneous. This was followed by purging with nitrogen gas for 30 min. The grafting reaction was carried out at 100  C for 16 h. The resultant solution mixture was precipitated in methanol to obtain the products, and then further washed with methanol several times to remove unreactive POEM. The obtained product was placed in a vacuum oven at 50  C overnight to evaporate remaining solvent. Two kinds of SBS-g-POEM block-graft copolymers, SBS-g-POEM(950) and SBSg-POEM(500), were obtained as a solid powder. 2.3. Preparation of OM-TiO2 films The SBS-g-POEM solution was prepared by dissolving 0.2 g of SBS-g-POEM in 1.8 ml of THF. Separately, Pre-TiO2 was prepared according to our previous reports [9,23]. In brief, 1.5 ml of TiCl4 was slowly dropped into 10 ml of toluene with vigorous stirring at room temperature for 10 min. The orange colored solution was added to 50 ml of benzyl alcohol drop-by-drop and further stirred for 30 min. The mixture was placed in an oven at 70  C for 18 h. The resultant white solution was centrifuged at 12,000 rpm for 30 min. Then, 0.3 g of as-prepared Pre-TiO2 was added to the SBS-g-POEM solution. Viscous pastes were prepared by adding 0.1 ml of HCl under mild stirring for 2 days. As-prepared viscous pastes containing SBS-g-POEM and Pre-TiO2 were deposited on FTO glasses with a blocking layer via the doctor-blading technique, and placed in sealed plastic dishes for 2 days at room temperature to induce structure self-assembly. Films were then calcined at 500  C for 30 min to eliminate SBS-g-POEM structure templates and organic compounds. Crack-free OM-TiO2 films were approximately 6 mm thick. 2.4. Fabrication of DSSC DSSCs with an active area of 0.16 cm2 were prepared by dropcasting the polymer electrolyte solution onto photoelectrodes and covering with counter electrodes, according to our previous reports [15e18]. First, FTO glasses employed as a transparent conductive oxide (TCO) glass substrate were immersed in large amounts of ethanol by sonication for 30 min. This was followed by performing the same procedure with acetone. Blocking layers were prepared by spin-coating titanium (IV) bis(ethyl acetoacetato) diisopropoxide solution (2 wt % in butanol) on FTO glasses at 1500 rpm for 20 s, and then calcining at 450  C for 30 min. The OM-TiO2 films were sensitized in ruthenium (N719) dye solution (13 mg dissolved in 50 g distilled ethanol) at 50  C for 3 h. Reference DSSC was also fabricated by the same procedure using commercial TiO2 pastes (Dyesol 18NR-T). The resulting TiO2 film was named randomly organized TiO2 (Ran-TiO2). To prepare counter electrodes, 1 wt% H2PtCl6 solution (dissolved in isopropanol) was spin-coated on FTO glasses and then calcined at 450  C for 30 min. The quasi-solid-state polymer electrolyte was prepared by dissolving a crystalline solid polymer (PEG 10 k), ionic liquid (MPII), metal salt (LiI), and iodine (I2) in acetonitrile [24e26]. Poly((1-(4-ethenylphenyl)methyl)-3butyl-imidazolium iodide) (PEBII) without any additives was used as the single component solid electrolyte [16,17]. The resulting polymer electrolyte solution was directly cast onto the TiO2

photoanodes. Both electrodes were superimposed and pressed between two glass plates to slowly evaporate the solvent and to produce a thin electrolyte layer. The cells were placed in a vacuum oven for 24 h to allow complete evaporation of the solvent, and were then sealed with an epoxy resin. Six identical DSSCs were fabricated and characterized. The average estimated error of efficiency was approximately ±0.2%. 2.5. Characterizations FT-IR spectra of amphiphilic triblock copolymer SBS-g-POEM were characterized using an Excalibur Series FT-IR (DIGLAB Co.) instrument with a frequency range of 4000e500 cm1. 1H NMR measurements were collected using a 400-MHz, solid/microimaging high resolution NMR spectrometer (AVANCE 400 WB, Bruker, Germany). The morphologies and structures of OM-TiO2 were observed by a field-emission scanning electron microscope (FE-SEM) (JSM-7001F, JEOL Ltd., Japan). The reflectance spectra of OM-TiO2 and Ran-TiO2, and the concentration of adsorbed dye were measured by UVevisible spectrophotometer (PerkinElmer, USA). The photoelectrochemical properties of DSSCs were measured using a Keithley Model 2400 and a solar simulator (1000 W xenon lamp, Oriel, 91193). The light intensity was equivalent over an 8  8 in2 area and was calibrated with a Si solar cell (Fraunhofer Institute for Solar Energy System, Mono-Si þ KG filter, Certificate No. CISE269) to obtain the sunlight intensity of AM 1.5 (100 mW cm2). The intensity was backed up with an NREL-calibrated Si solar cell (PV Measurements Inc.). The photo-electrochemical performances of DSSCs were characterized using a short-circuit current density (Jsc, mA/cm2), an open-circuit voltage (Voc, V), fill factor (FF) and an energy conversion efficiency (h, %). Incident photon to conversion efficiency (IPCE) was measured in the wavelength range of 300 nme800 nm (McScience, K3100). Electrochemical impedance spectroscopy (EIS) analysis was performed in the frequency range from 100 kHz to 0.05 Hz with potential modulation of 0.02 V under AM 1.5 (100 mW/cm2) using a compact stat electrochemistry analyzer (IVIUM Technologies). Intensity modulated photocurrent spectroscopy (IMPS)/intensity modulated photovoltage spectroscopy (IMVS) were performed with frequencies ranging from 10 kHz to 0.1 Hz under 635 nm modulated LED illumination (ModuLightmodule, IVIUM). The surface areas and pore diameters of TiO2 films were measured using BET surface area and a pore size analyzer (BELSORP-mini II, BEL Japan, Inc., Japan). The specific surface area of TiO2 films was calculated using BET methods, while average pore diameter and size distribution were measured by BarretteJoynereHalenda (BJH) methods. TiO2 films were degassed at 70  C under a dynamic vacuum (102 Torr) for 1 h before measurement. The amount of dye adsorption was analyzed by adsorptionedesorption of the sensitizer of dye-sensitized TiO2 films. Dye desorption occurred when TiO2 films were immersed in a NaOH (102 M) solution ethanol/water (1:1 v/v) mixture. The amount of desorbed dye can be calculated using solution concentration and a UVevisible spectrophotometer (HewlettePackard, Hayward, USA). 3. Result and discussion 3.1. Synthesis of SBS-g-POEM block-graft copolymer Scheme 1a shows the synthesis of amphiphilic SBS-g-POEM block-graft copolymer. SBS-g-POEM was synthesized by grafting hydrophilic POEM macromonomer via free radical polymerization, a cheap and easy method for copolymer synthesis [30,31]. Two POEM macromonomers with different numbers of ethylene oxide repeating units were used to synthesize SBS-g-POEM(500) and

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SBS-g-POEM(950). The selection of solvent and initiator is the most important factor for free radical polymerization. In our study, toluene was used as the solvent due to its good solubility for both SBS and POEM macromonomers. The solubility parameter (d) indicates how strongly different materials interact to be able to be miscible. As a rough rule-of-thumb (or heuristic principle), a polymer tends to dissolve when the solubility parameter difference between solvent and polymer is below 1 [32]. The d of toluene is 8.9 cal1/2 cm3/2, while those of SBS and POEM are 8.6 and 9.7 cal1/

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cm3/2, respectively [33,34]. Thus, toluene is likely to dissolve both SBS and POEM to form the homogeneous single phase necessary for free radical polymerization. It should be noted that POEM macromonomer is a liquid, which is entropically favorable for solubility. Polymer solubility is affected by entropy as well as enthalpy, but the solubility parameter is only a measure of enthalpy because it is determined by change in internal energy. Another advantage of toluene as solvent is due to 1) its slightly higher boiling point (111  C) than the thermal decomposition temperature

2

Scheme 1. (a) Synthesis of SBS-g-POEM block-graft copolymer via free radical polymerization, (b) schematic illustration of OM-TiO2 films templated by SBS-g-POEM block-graft copolymers depending on side chain length.

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of the radical initiator (100  C for DCP) and 2) the ease of synthesized copolymer precipitation in a nonsolvent (methanol) due to the miscibility between toluene and methanol. The grafting reaction mechanism may occur as follows: 1) When the solution mixture reaches the decomposition temperature of DCP (~100  C), DCP decomposes to generate free radicals; 2) The radicals attack pep bonds of the PB regions of SBS and POEM; 3) Broken bonds interact with each other to form more stable bonds; 4) Initiated chains propagate until there are no more monomers to react; 5) The reaction is terminated by decreasing temperature [35]. DCP was chosen as the initiator is due to its longer half-life than other initiators, such as benzoyl peroxide (BPO) and azobisisobutyronitrile (AIBN). Because the radical decay of DCP is slower than that of other initiators, it prevents the chain scissions that are induced by further formed free radicals. Also, the reactive sites of SBS and POEM are maintained for grafting until they are mixed well [36]. The FT-IR spectra of pristine SBS, POEM macromonomers, and SBS-g-POEM block-graft copolymers with different numbers of ethylene oxide repeating units are shown in Fig. 1a. Pristine SBS block copolymer exhibited aromatic C]C stretching vibrations of

the PS blocks at 1602, 1493, and 1450 cm1. POEM macromonomers (both 500 and 950) exhibited sharp peaks at 1717 and 1637 cm1 caused by stretching vibrations of the carbonyl group (eC]O) in the ester and the unsaturated double bond (eC]C), respectively. These two peaks were less intense for POEM(950) than for POEM(500) due to a lower concentration of terminal methacrylate groups in the former. The asymmetric ether (eOe) stretching peak of ethylene oxide repeating units was observed at 1099 cm1 for both POEM macromonomers. Upon graft copolymerization, a weak peak at 1637 cm1 was hardly observable. This indicates that carbon double bonds were cleaved and single bonds covalently linked POEM and SBS. New broad peaks appeared at 3480 and 1646 cm1 due to the stretching and bending mode, respectively, of eOH groups strongly bound to hygroscopic POEM chains. Synthesized SBS-g-POEM copolymers exhibited the characteristic peaks of carbonyl (eC]O) and ether (eOe) groups at 1727 and 1106 cm1, respectively. There were slight peak shifts from 1717 to 1727 cm1 and from 1099 to 1106 cm1 upon graft copolymerization. Carbonyl and ether bonds are stronger in SBS-g-POEM than POEM macromonomers because SBS main chains sterically interfere and reduce secondary bonding interactions among POEM interchains/intrachains [31]. The successful synthesis of SBS-g-PEOM block-graft copolymer was also confirmed by 1H NMR spectra, as shown in Fig. 1b. The benzene groups of PS blocks correspond to peaks at 7.0 and 6.5 ppm while the peak at 3.6 ppm is attributed to the ethylene oxide repeating units of POEM [30,37]. The actual content of POEM side chains grafted from SBS main chains was calculated by comparing the integral area of PS peaks at 7.0 ppm (or 6.5 ppm) with that of the POEM peak at 3.6 ppm (or 3.4 ppm). As a result, POEM content was determined to be approximately 18 wt% and 16 wt% for SBS-gPOEM(500) and SBS-g-POEM(950), respectively. The slightly lower POEM content in SBS-g-POEM(950) is because POEM(950) is more difficult to graft onto SBS than POEM(500) due to the steric hindrance of its longer side chain and its lower mobility. The relative peak intensity of the methyl group (eCH3) at 3.4 ppm (denoted ‘d’) located at the end of POEM side chains was stronger in SBS-gPOEM(500) than in SBS-g-POEM(950). This is because the side chain length of SBS-g-POEM(950) is longer than that of SBS-gPOEM(500) due to the presence of more repeating units in POEM(950) than in POEM(500). 3.2. Preparation and characterization of OM-TiO2 films

Fig. 1. (a) FT-IR spectra of neat SBS, neat POEM monomers, and SBS-g-POEM blockgraft copolymers. (b) 1H NMR spectra of SBS-g-POEM block-graft copolymers with different side chain lengths.

An effective one-step method is suggested to prepare crack-free, 6-mm-thick, OM-TiO2 photoanodes with high porosity and good interconnectivity using the doctor-blading technique via selfassembly of SBS-g-POEM block-graft copolymer and Pre-TiO2 nanoparticles, as illustrated in Scheme 1b. Such an organized structure is effective for polymer electrolyte DSSCs because it allows facile penetration of large molecular weight polymer into photoanode pores. Photographs of the solegel solution and their films on FTO glass after calcination are shown in Fig. 2a and b, respectively. Solution containing SBS-g-POEM and Pre-TiO2 formed a homogeneous mixture with high viscosity regardless of the length of ethylene oxide units. After one-step doctor-blading and calcination at 500  C, the resulting OM-TiO2 films were crack-free and translucent. The solution containing neat SBS and Pre-TiO2 underwent macrophase separation, and the resulting TiO2 film was extremely thin due to low viscosity and heterogeneity. Neat SBS/ Pre-TiO2 resulted in the formation of a large number of macrocracks on the TiO2 surface together with aggregated morphology and lower porosity, as shown in the FE-SEM images (Fig. 2c,d). SBSg-POEM plays a pivotal role not only as a macromolecular template on the mesoscopic scale, but also as a structural binder in

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Fig. 2. Photograph of (a) solutions containing SBS/Pre-TiO2, SBS-g-POEM(500)/Pre-TiO2, and SBS-g-POEM(950)/Pre-TiO2. (b) TiO2 films templated by SBS block copolymer and SBSg-POEM block-graft copolymers. (c, d) SEM image of TiO2 films templated by SBS block copolymer.

macroscopic TiO2 crystal growth. This originates from its amphiphilic microphase-separated nanostructure, which interacts with hydrophobic SBS back bones and hydrophilic POEM side chains. Benzyl alcohol capping provides hydrophilicity on the particle surface, so Pre-TiO2 nanoparticles have strong and preferential interaction with hydrophilic POEM side chains. Pre-TiO2 nanoparticles were confined to the POEM domains of SBS-g-POEM, resulting in good solution miscibility and homogeneity. Hydrophilic domains of the copolymer interact with hydrophilic precursor to form a continuous metal oxide phase, while the hydrophobic domains generate mesopores after calcination at high temperature. Thus, the hydrophobic domains provide a sacrificing template without collapsing the final structure. However, neat SBS is so hydrophobic that there is no specific site to interact with Pre-TiO2 nanoparticles. This is responsible for the heterogeneous macrophase-separation of solegel solution and the formation of

macro-cracks on the film. It should be also noted that the use of SBS-g-POEM with high POEM content (i.e. SBS:POEM ¼ 1:3) resulted in TiO2 film with macro-cracks, small pores and aggregated particles, indicating the importance of the ratio of hydrophobic SBS to hydrophilic POEM chains for the film morphology. Fig. 3 shows the FE-SEM surface images of OM-TiO2 films templated by SBS-g-POEM block-graft copolymers with different side chain lengths. In contrast to the TiO2 films prepared by commercial TiO2 paste (Fig. S1), and neat SBS (Fig. 2c,d), the OM-TiO2 films were crack-free, homogeneous, and well-organized, with worm-like mesoporous structures, high porosity, and good interconnectivity. The side chain length of SBS-g-POEM greatly affected morphology. The use of SBS-g-POEM(500) with shorter side chain length resulted in larger 100e200 nm pores at the film surface compared to the use of SBS-g-POEM(950), which resulted in smaller 15e30 nm pores. This is because SBS-g-POEM(950) with enriched POEM side

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Fig. 3. FE-SEM images of OM-TiO2 films templated by SBS-g-POEM block-graft copolymers. (a, c, e) OM-TiO2(500) and (b, d, f) OM-TiO2(950).

chains has better miscibility with hydrophilic Pre-TiO2 nanoparticles. Thus, Pre-TiO2 can tightly pack in the expanded POEM domains, leading to a closely packed TiO2 structure with smaller pores. It should be noted that pores are predominantly generated from hydrophobic domains that cannot confine the TiO2 precursor. The solegel mixture may undergo microphase separation into hydrophilic POEM/Pre-TiO2 regions and hydrophobic SBS regions. The former generates the matrix while the latter generates pores upon calcination at 500  C, as described in Scheme 1b. Major factors affecting the Jsc value include pore structure, specific surface area, dye loading, and light scattering properties of TiO2 films. The N2 adsorption-desorption measurement was employed to characterize specific surface area, pore size, and distribution using the BET and BJH methods, as shown in Fig. S2 and Fig. 4a. The results are summarized in Table 1. Both OM-TiO2 exhibited IV type N2 adsorption-desorption curves, indicating a mesoporous structure. The overall pore diameter and average pore size of OM-TiO2(950) were smaller than those of OM-TiO2(500). This is because SBS-g-POEM(950) has a larger hydrophilic domain that interacts with hydrophilic Pre-TiO2, and a smaller hydrophobic domain that forms the pores. The surface area of OM-TiO2(950) reached 107.9 m2/g, larger than that of OM-TiO2(500) (92.1 m2/g), indicating that pore size and surface area are inversely related. The dye loading of OM-TiO2(950) (96.3 nmol/cm2) was greater than that of OM-TiO2(500) (84 nmol/cm2) due to a direct relation between surface area and dye loading. As seen in Fig. 4a, OM-TiO2(950)

exhibited relatively uniform pore size distribution and a highly packed structure compared to OM-TiO2(950), consistent with the above SEM results. Both OM-TiO2 films exhibited higher dye loading and surface area than those of the Ran-TiO2 film prepared by commercial TiO2 paste, indicating the effectiveness of wellorganized mesoporous structure. The average pore sizes of OMTiO2 films determined by BET/BJH methods were smaller than those determined by the above SEM analysis. This is because the SEM method characterizes only surface properties. However, the general trends are consistent with each other. Since DSSC efficiency is also strongly dependent on photoanode light scattering ability, the reflectance spectra of TiO2 films with different structures were characterized to investigate the effect of film structure on optical properties, as shown in Fig. 4b. The reflectance values of Ran-TiO2 and OM-TiO2(950) films were approximately 10e22% in the range of 400e800 nm due to their high transparency. The OM-TiO2(500) film exhibited higher reflectance up to 30%, suggesting that a significant fraction of incident light was scattered and redirected. This property could lead to enhanced utilization of solar light in DSSCs. 3.3. Photovoltaic performances of DSSCs Quasi-solid-state and solid-state DSSCs based on the solid polymer with high molecular volume were fabricated due to the need for long-term stability, flexible design, and lightweight cells.

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Fig. 4. (a) Pore size distributions determined by BJH method and (b) UVevisible reflectance spectra of OM-TiO2 films templated by SBS-g-POEM block-graft copolymer and Ran-TiO2 film with randomly organized structure.

First, the DSSCs were fabricated with different TiO2 photoanode structures and a quasi-solid-state polymer electrolyte with a 6 mm TiO2 layer, as confirmed by cross-sectional SEM images (Fig. S3). The corresponding JeV curves of each DSSC were determined under simulated sunlight radiation of 100 mW/cm2, as shown in Fig. 5a. The detailed photovoltaic performances of these DSSCs are summarized in Table 1. The quasi-solid-state polymer electrolyte consisted of PEG10 k, MPII, I2, and LiI, which has previously been shown to generate DSSCs with high performance and stability [14]. The efficiency of DSSCs based on OM-TiO2(950) was similar to that of OM-TiO2(500), but both efficiencies (5.7e5.8%) were greater than that of the less organized Ran-TiO2 cell (4.8%) due to improved Jsc,

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Voc, and FF values. This indicates the importance of an organized TiO2 photoanode structure with large specific surface area, high porosity, and interconnected morphology. Deep penetration of the polymer electrolyte with large molecular volume is highly important. Thus, the effect of an organized structure for DSSCs with polymer electrolyte is obvious. The improved Jsc values of OM-TiO2 films result from greater dye loading and higher light scattering ability, as described in the previous section. Better electron transport and reduced recombination also increase Voc values, which will be characterized using IMPS/IMVS measurements. Improved FF values are attributed to the improved interfacial electrode/electrolyte properties of OM-TiO2 films due to good interconnectivity and higher porosity, which also will be characterized using EIS analysis. The surface area (dye loading) of OM-TiO2(950) was greater than that of OM-TiO2(500) but the light reflectance was lower in the former, which is responsible for similar efficiency. The light harvesting efficiency of DSSCs was also investigated by IPCE values, as shown in Fig. 5b. DSSCs fabricated with OM-TiO2 films showed enhanced IPCE values compared to the Ran-TiO2 cell, ranging from 400 nm to 750 nm. Similar to Jsc values, the enhanced IPCE value is due to higher dye loading and greater light scattering with OM-TiO2 films. OM-TiO2(500) has a lower specific surface area than OM-TiO2(950), but its conversion efficiency reached a similar level. This results from greater reflectance of OM-TiO2(500) due to wide pore size distribution, as revealed by SEM and N2 adsorptiondesorption measurements. DSSCs fabricated with OM-TiO2 films were also analyzed using the EIS measurement, which is an excellent method for examining electron transport parameters. EIS spectra of DSSCs were measured under sun conditions (at 100 mW/cm2) with an applied bias of Voc and 0.02 V of potential modulation (Fig. S4a). Nyquist curves with three semicircles were obtained in the frequency range of 100 kHze0.05 Hz under AM 1.5. The first semicircle observed in the kHz frequency range corresponds to charge transfer at the counter electrode while the third semicircle obtained in mHz shows I3  mass diffusion in the electrolyte. The second semicircle in the frequency range reflects charge transfer impedance across TiO2 electrode/dye/electrolyte interfaces under illumination conditions while it reflects resistance for the recombination reaction under dark conditions [38]. Here, we focused on the second semicircle due to use of the same electrolyte and counter electrode for all DSSCs. The size of the second semicircle was similar between OMTiO2(950) and OM-TiO2(500), but their values are much smaller than that of Ran-TiO2. This indicates that OM-TiO2 based DSSCs have better charge transfer at the TiO2/electrolyte interface due to the high porosity and well-organized structure of the OM-TiO2 film. Preferable interactions of the OM-TiO2 film with electrolyte leads to facile infiltration of the polymer electrolyte with large molecular volumes, resulting in the improved Voc and FF values observed with JeV curve measurements. Under dark conditions, DSSC Nyquist plots were also obtained to compare charge recombination resistances by the size of the second semicircle, which was measured at Hz frequencies as shown in Fig. S4b. As mentioned above, the second curves reflect the recombination resistance of TiO2 films

Table 1 Pore size, specific surface area, and dye loading of OM-TiO2(500), OM-TiO2(950), and Ran-TiO2 with performances of DSSCs fabricated using quasi-solid-state polymer electrolyte at 100 mW/cm2. Electrode

Pore sizea (nm)

Surface areaa (m2/g)

Dye loading (nmol/cm2)

Jsc (mA/cm2)

Voc (V)

FF

h (%)

OM-TiO2(500) OM-TiO2(950) Ran-TiO2b

22 16 37

92.1 107.9 68.9

84.3 96.3 68.9

12.9 13.1 11.8

0.70 0.70 0.67

0.63 0.63 0.61

5.7 5.8 4.8

a b

Determined by BET/BJH methods using N2 adsorptionedesorption measurement. Ran-TiO2: randomly organized TiO2 film prepared using commercial TiO2 paste (Dyesol 18NR-T).

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under dark conditions. OM-TiO2 showed greater recombination resistance than Ran-TiO2 despite the larger surface area of the former, which is again explained by the interconnectivity and organized structure of OM-TiO2. DSSC impedance data were supported by IMPS/IMVS measurements, which characterize electron diffusion coefficients, lifetime, and diffusion length (Fig. 6). The OM-TiO2(950) cell showed the highest electron diffusion coefficient by providing a direct electron transport path along the TiO2 photoanode. This was followed by the OM-TiO2(500) cell and Ran-TiO2 cell. The electron lifetime of OMTiO2(950) was also the greatest among the DSSCs. Due to larger specific surface area, OM-TiO2(950) could allow more opportunity for recombination, leading to increased electron trap sites than Ran-TiO2. However, this negative factor could be overcome by enhanced interconnectivity [16]. Electron diffusion lengths calculated using electron diffusion coefficients and electron lifetimes were plotted in Fig. 6c. In conclusion, the electron transport and recombination ability of OM-TiO2(950) were greater than those of OM-TiO2(500) and Ran-TiO2 cells, resulting in improved Voc values. We also investigated the functionality of OM-TiO2 as a photoanode layer by fabricating solid-state DSSCs using a single component solid electrolyte and mesoporous TiO2 spheres (MTS)

Fig. 6. (a) Diffusion coefficients, (b) electron lifetime, and (c) electron diffusion length of DSSC fabricated with various TiO2 photoanodes and polymer electrolyte.

Fig. 5. (a) JeV curves and (b) IPCE spectra of DSSC fabricated with various TiO2 photoanodes and polymer electrolyte at 100 mW/cm2 (AM 1.5).

scattering layer. The MTS layers possess bifunctionality, i.e. the ability to reflect light back into the dye as well as the large surface area that allows high dye loading [39]. A PEBII worked as a solid electrolyte even without the incorporation of additive such as

J.Y. Lim et al. / Journal of Power Sources 301 (2016) 18e28

iodine (I2) and iodide salt [15e18]. PEBII solidifies by means of radical polymerization of an ionic liquid, and shows high ionic conductivity as well as good mechanical stability. As shown in Fig. 7, the efficiency of solid-state DSSCs based on OM-TiO2(950) and OMTiO2(500) photoanode reached 7.5 and 6.9%, respectively, at 100 mW/cm2, which is one of the highest values for N719-based solid-state DSSCs [39e48]. The improved efficiency of PEBII-based DSSC is attributed to higher ionic concentration, pep stacking interactions and lower glass transition temperature of PEBII, resulting in well-organized microstructure and great chain mobility.

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Acknowledgments This work was supported by Agency for Defense Development as a part of basic research program (UD130049GD), the Center for Advanced Meta-Materials (CAMM) (2014M3A6B3063716) and the Korea Center for Artificial Photosynthesis (KCAP) (2009-0093883). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.09.109.

4. Conclusions References OM-TiO2 films with larger specific surface area and good interconnectivity were prepared using amphiphilic SBS-g-POEM blockgraft copolymer as a structure-directing agent. SBS-g-POEM was synthesized by grafting a hydrophilic POEM macromonomer from hydrophobic SBS back bones via free radical polymerization, as confirmed by FT-IR and 1H NMR spectroscopy. The neat SBS/PreTiO2 mixture solution underwent macrophase separation to cause a large number of macro-cracks on the TiO2 film with aggregated morphology and lower porosity. However, SBS-g-POEM/Pre-TiO2 led to good miscibility and homogeneity in solution due to the preferential interaction of hydrophilic POEM side chains with hydrophilic Pre-TiO2 nanoparticles. This resulted in the formation of crack-free OM-TiO2 films with well-organized, worm-like mesoporous structure, high porosity, and good interconnectivity, as confirmed by SEM analysis. The use of SBS-g-POEM(950) with longer side chain length generated smaller pores than SBS-gPOEM(500) due to better miscibility with Pre-TiO2 nanoparticles. Quasi-solid-state polymer electrolyte DSSCs fabricated with OMTiO2 films reached 5.7e5.8%, which is greater than that of the less organized Ran-TiO2 cell (4.8%), indicating the importance of organized structure. The enhanced efficiency of OM-TiO2 resulted from improved Jsc, Voc, and FF values, which were due to greater surface area (or dye loading), better light scattering ability, better electron transport, and reduced recombination. These properties were characterized by N2 adsorption-desorption, reflectance spectroscopy, EIS, and IMPS/IMVS measurements. Furthermore, the efficiency of solid-state DSSCs fabricated using a single component PEBII electrolyte and MTS scattering layer reached up to 7.5%, one of the highest values for N719-based solid-state DSSCs cells.

Fig. 7. JeV curve of the solid-state DSSC fabricated with a solid PEBII electrolyte and OM-TiO2/MTS photoanode at 100 mW/cm2.

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