Cross Stacking of Nanopatterned PEDOT Films for Use as Soft

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Cross Stacking of Nanopatterned PEDOT Films for Use as Soft Electrodes Chihyun Park, Jongbeom Na, and Eunkyoung Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07799 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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ACS Applied Materials & Interfaces

Cross Stacking of Nanopatterned PEDOT Films for Use as Soft Electrodes Chihyun Park, Jongbeom Na, and Eunkyoung Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea

KEYWORDS: nanopatterning, PEDOT, DSSC, light harvesting, TCO-free

ABSTRACT: Cross stacking of nanopatterned conductive polymer film was explored using a sacrificial soft template made of nanopatterned polystyrene (PS) film as a guide for nanopatterned conductive polymer film. For use as a conductive film, the PS pattern was filled with poly(3,4-ethylenedioxythiophene) (PEDOT), and then completely removed, to generate single-patterned PEDOT (SPDOT) film having a conductivity of 1079 S/cm, which was comparable to the pristine unpatterned PEDOT (UPDOT) film on a glass slide. SPDOT layers were stacked across each other to form double-layered (DPDOT) and multiple-layered patterned PEDOT film on a glass slide or polymeric substrate. The patterned PEDOT film showed enhanced optical and electrochemical activity; specifically compared with the UPDOT film on a glass slide, the DPDOT film showed an increase in reflectance and an enhanced electrochemically active surface by 23.4% and 32.8%, respectively. The patterned PEDOT film on a polymer substrate showed high bendability up to being complete folded and maintained its conductivity for over 10,000 cycles of bending. The patterned PEDOT 1 ACS Paragon Plus Environment


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layers were applied to dye-sensitized solar cells (DSSCs) as a transparent conductive oxide (TCO)-free counter electrode. An N719-based DSSC with a DPDOT film recorded a photoconversion efficiency of 7.54%, which is one of the highest values among the TCO-free DSSCs based on a PEDOT counter electrode.

Introduction Conjugated polymers (CPs) have attracted strong attention due to their high application potential as a transparent electrode as well as light modulating devices, which originates from their flexibility plus unique optical and electronic properties.1-11 Especially, poly(3,4ethylenedioxythiophene) (PEDOT) has been intensely researched for use in photovoltaic, photothermal, and thermoelectric applications.6, 12-13 As the charge transport and optical properties of CPs are inherited from their multi-directional transport properties, a multilayered nanopatterned conductive film could be of great interest because of several aspects including structural colors,5 light harvesting,14-15 and enhanced electrical and optical properties.16 Thus, various methods for producing nanopatterned structures of highly conductive PEDOTs have been attempted. However, their rigid molecular structure prevents them from being dissolved in any solvent or softened at elevated temperatures, causing intractability in the formation of multilayered structures. Some of the efforts to prepare nanopatterned PEDOT film include grafting of PEDOT onto a pre-patterned polymer surface by chemical vapor deposition,17-18 or using poly(3,4ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS), which has a relatively soft nature that makes it suitable for patterning.19-21 However, these methods have some problems in comparison to pristine PEDOT films such as poor conductivity, low thickness of the 2 ACS Paragon Plus Environment

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fabricated nanostructures, and/or complexity in procedures regarding their productivity. Hence, there has been significant efforts to find new routes for fabricating nanostructured PEDOT films cheaply. Nanosphere lithography (NSL) is a promising method which partially meets these requirements, and as a consequence, combining NSL and electropolymerization has been the subject of several studies. Sumida et al.22 reported the electrochemical preparation of macroporous polypyrrole (PPy) films using NSL on F-doped SnO2 (FTO)coated

glass. Recently, NSL followed by electropolymerization of 3,4-

ethylenedioxythiophene (EDOT) produced a PEDOT pattern on FTO-coated glass.19-21, 23 Since this method used electropolymerization for producing the patterned PEDOT film, it required a conductive substrate for the PEDOT pattern. A nanopatterned thin PEDOT film has been fabricated via spin coating PEDOT oligomer solution 18 on top of the pre-patterned polystyrene (PS), followed by thermal polymerization; the patterned PEDOT showed enhanced electrocatalytic properties compared to un-patterned PEDOT and was used to replace expensive TCO and Pt counter-electrodes in a dye-sensitized solar cell (DSSC).24 However, this method did not provide transferrable or stackable nanostructured PEDOT film. Herein, we report a simple method for transferrable and stackable nanopatterned PEDOT film using a sacrificial PS pattern layer on a Pt- or TCO-free substrate. Taking advantage of the highly conductive and light harvesting capabilities of nanostructured PEDOT film, we applied it as a counter electrode (CE) in DSSCs to afford Pt- and TCO-free DSSCs. Compared to other TCO-free DSSCs, this method provides a stackable solution with enhanced photoconversion efficiency (PCE).

Experimental Details 3 ACS Paragon Plus Environment


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Materials. Polystyrene (PS, weight average molar weight ~ 192,000), tetrahydrofuran (THF, purity > 99.9%), Iron(III) chloride hexahydrate (purity 97%), p-toluenesulfonic acid monohydrate (purity > 98.5%), anhydrous ethanol (purity > 99.5%), poly(ethylene glycol)block-poly(propylene glycol)-block-poly(ethylene glycol) (PEPG, weight average molecular weight 2800), EDOT (purity 97%), tetrabuthylammonium perchlorate (purity > 99%), and propylene carbonate (purity 99.7%) were purchased from Sigma Aldrich. The prepolymer and curing agent (Sylgard-184) for molding a polydimethylsiloxane (PDMS) stamp were purchased from Dow Corning. TiO2 paste (18NR-T) was purchased from Dyesol, and ruthenium dye N719 was purchased from Solaronix. Indium tin oxide (ITO) film was purchased from Sigma Aldrich (80 S/□) and used as a reference for the bending experiments. Polyethylene terephthalate (PET) film (thickness 0.175 mm) was purchased from Sigma Aldrich. Fabrication of the PDMS stamp. To prevent adhesion between the PDMS and the silicon mold (25 mm x 25 mm, period: 833.3 nm, groove depth: 200 nm; SNS-C12-2525-200-P, LightSmyth Technologies Inc., USA) by depositing a hydrophobic monolayer on the surface, the silicon master was placed in a vacuum chamber with a few drops of tridecafluoro-1,1,2,2tetrahydrooctyl-1-trichlorosilane (TFPCS), then a vacuum state was created and maintained for 1 h. The prepolymer and curing agent were mixed (10:1 weight ratio) and degassed for 30 min to remove residual air bubbles in the solution. Next, the solution was poured into a Petri dish with the silicon master in it and the Petri dish was placed in an oven at 70 °C for 1 h. The cured PDMS was peeled off the dish and master, then trimmed to leave only the nanopatterned area. Preparation of the nanopatterned PS template. PS in THF (1 wt% solution) was used to prepare a PS thin film by the drop-casting method. 1 ml of solution was dropped onto a glass 4 ACS Paragon Plus Environment

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slide (2.5 cm x 7.5 cm) and dried for 1 h at room temperature; the prepared PS film was patterned by a thermal imprinting method (Figure 1a). To soften the PS film, it was annealed at 150 °C and then the PDMS stamp was placed on the film. The PDMS stamp and the PS film were pressed under a weight of 1 kg for 20 min. The nanopatterned PS film prepared by this method was modified by oxygen plasma treatment for 20 min to enhance its affinity with the ethanol-based oxidative solution. Preparation of the oxidative solution. Iron(III) tris-p-sulfonate (Fe-Tos) was synthesized by a previously reported method.25 PEPG (200 mg) was added into 1 g of the oxidative solution containing 40 wt% Fe-Tos in ethanol. After stirring for 6 h, it was sonicated for 10 min and cooled down. EDOT (monomer, 44.1 mg) was added shortly before spin coating of the oxidative solution onto the PS template. Preparation and characterization of the nanopatterned PEDOT film. PEDOT films on a flat and patterned PS patterned surface were prepared from the above oxidative solution containing EDOT through the solution casting polymerization (SCP) method25. Thus the above oxidative solution containing EDOT was spin coated onto a slide glass and PS patterned substrate, to prepare flat and patterned PEDOT films. Then the EDOT coated layer was heated to 65 °C-70 °C for 1 h under 35% humidity, to polymerize EDOT. After been cooled down to room temperature, the samples were washed with ethanol to remove residual oxidative solution. The thickness of films was measured by a surface profiler (Alpha-Step IQ, KLA Tencor, USA). The thickness of the PEDOT film was controlled by the spin rate during the solution coating, as described in the literature.6 The spin rate of the EDOT solution coating was 2,500 rpm for the preparation of a single layer sample (SPDOT and UPDOT) with a 5 s acceleration and 30 s coating time to give 490 nm, 530 nm thick PEDOT film for UPDOT and SPDOT, 5 ACS Paragon Plus Environment


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respectively. For the double layered PEDOT film, the spin rate of the EDOT solution coating was 5,000 rpm with a 5 s acceleration and 30 s coating time for each PEDOT layer of the DPDOT. The thickness of the first coated layer (bottom) of the DPDOT was 270 nm and that after double layer formation was 510 nm, as determined by the above surface profiler. In this way the thickness of PEDOT in UPDOT, SPDOT, and DPDOT were made similar to each other. After this, the PEDOT film with the PS template film was detached from the substrate and attached to a glass slide PEDOT side first, which means that the PEDOT film surface with the PS template contacted first with the new substrate (Figure 1d, e). Next, the PS template was removed by washing in toluene for 2 h (Figure 1e). The sample was washed with acetone to remove residual PS and other impurities. In the case of fabricating a cross-stacked patterned PEDOT bilayer, these steps were repeated twice. Thus, the patterned PEDOT film was positioned in reciprocal direction on a glass slide without a coating of PEDOT film before the transferring step (Figure 1g-i). To fabricate patterned PEDOT films for the bending experiment, identical procedures were also carried out using a flexible polyethylene terephthalate (PET) substrate. Nanostructural measurements were performed to determine the thickness of the patterns using scanning electron microscope (SEM) and atomic force microscopy (AFM). SEM was performed using with JSM-7001F Schottky emission scanning electron microscope (JEOL Ltd.) with Pt coating (< 10 nm). AFM measurements were carried out with a Multimode atomic force microscope (Veeco Instruments) in tapping mode using an Si cantilever tip. Electrochemical measurements including during flexibility testing were performed using a CHI 624B universal potentiostat (CH Instruments, Inc.), and the produced patterns` reflectance spectra depending on the angles spectrum were obtained with a PerkinElmer Lambda 750 spectrophotometer. 6 ACS Paragon Plus Environment

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Preparation of the dye sensitized solar cell with the PEDOT CE. The compact 200 nm thick TiO2 layer was prepared by spin coating a solution containing 2% titanium bis(ethyl acetoacetate) in butanol onto FTO-coated (Pilkingston, 8 Ω-1) glass at 2,000 rpm for 30 s, followed by calcination at 450 °C and 500 °C, both for 15 min. Next, the TiO2 paste was coated onto the compact layer using the doctor-blade technique and dried at 70 °C for 1 h. After sintering at 500 °C for 15 min, the electrode was cooled down at 30 °C for 8 h, followed by being immersed in 0.3 mM N719 solution in ethanol for 24 h at room temperature to allow the dye to adsorb onto the surface of the TiO2. This photoanode was attached to the PEDOT CE using a hot-melt film (Surlyn, 25 µm) to fabricate a sandwichtype cell. The fabricated DSSC was filled with electrolyte solution consisting of 0.6 M 1,2dimethyl-3-propylimidazolium iodide, 0.1 M iodine, 0.5 M tert-butylpyridine, and 0.1 M lithium iodide in ethanol.14 Characterization of photoelectric performance. The PCE of the cell was measured using an electrochemical workstation (Keithley Model 2400) and a solar simulator (1000 W xenon lamp, Oriel, 91193) calibrated to sunlight intensity with a Si solar cell (Fraunhofer Institute for Solar Energy Systems, Mono-Si + KG filter, Certificate No. C-ISE269), and confirmed with an NREL-calibrated Si solar cell (PV Measurements Inc.)

Results and Discussion Patterning and characterization of PEDOT layer PEDOT film was nanopatterned with nanostructured PS templates, as described in Figure 1. First, PS films were coated onto a glass slide by drop-casting a 1 wt% solution of PS in THF to obtain ~3 µm thick films. The PS layer was pressed with a PDMS mold consisting of 7 ACS Paragon Plus Environment


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periodic 416 nm wide lines separated by 417 nm wide gaps. After removal of the PDMS mold, PEDOT was coated onto the PS patterned surface. PEDOT films on a flat and patterned PS patterned surface were prepared from the solution containing PEPG, Fe-Tos, and EDOT, through solution casting polymerization (SCP) method.25 Thus the above solution containing EDOT was spin coated onto a slide glass and PS patterned substrate, to prepare unpatterned (UPDOT) and single layered patterned PEDOT film (SPDOT), respectively. Then the EDOT coated layer was heated to 65 °C-70 °C for 1 h under 35% humidity, to polymerize EDOT (Figure 1b, c). The thickness of the PEDOT film was controlled by the spin rate during the solution coating, as described in the literature6. The transmittance of the PEDOT layer become lower as the film was thicker, and above 600 nm the transmittance was < 50 %. On the other hands, when the film was thin (< 150 nm) the pattern was not formed well. Thus we made ~ 500 nm thick PEDOT film for each samples. The thickness of PEDOT in UPDOT, SPDOT, and DPDOT were made similar to each other, in order to avoid thickness effects. The spin rate of the EDOT solution coating was 2,500 rpm for the preparation of a single layer sample (UPDOT and SPDOT) to give 490 nm, 530 nm thick PEDOT film, respectively, as determined by a surface profiler (Alpha-Step IQ). The PEDOT-coated PS layer was detached from the glass substrate, turned upside down, and placed onto a new glass substrate (Figure 1e). Following this, the top PS layer was washed off with toluene (Figure 1e) and the PEDOT surface was dried to afford SPDOT (Figure 1f). The DPDOT film was prepared by stacking the detached PEDOT on PS layer (from Figure 1g) onto an SPDOT layer in a crosswise direction with the PEDOT surface facing toward the SPDOT film surface (Figure 1g-i). In order to make the total thickness of the of PEDOT layer in the DPDOT equal to that of SPDOT, each layer was prepared at a high spin rate 8 ACS Paragon Plus Environment

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(5,000 rpm). The thickness of the first coated layer (bottom) of the DPDOT was 270 nm and that after double layer formation was 510 nm. In this way the thickness of PEDOT in UPDOT, SPDOT, and DPDOT were made similar to each other.

This process was repeated

to prepare multi-layered patterned PEDOT film on various surfaces including a TCO-free glass slide and a flexible transparent PET film. However, as the number of layers was increased, the color of the patterned film became intense because of the increased thickness and the surface conductivity was smaller than the UPDOT film. Thus, a quadruple-layered patterned PEDOT (QPDOT) film showed an intense dark blue color (Figure S2), and very low transmittance (5 ~ 15%). The electrical conductivity (σ) of the SPDOT film was 1072 S/cm, which was comparable to the pristine film (1089 S/cm) of a similar thickness (Table 1), as determined by the 4-probe method. The cross-stacked patterned PEDOT bilayer showed relatively low conductivity (658 S/cm), possibly due to the low fill factor and contact resistance26 between the 1st and 2nd PEDOT layers. Nonetheless, it was still high enough to function as an electrode. The SEM images of the SPDOT film on a glass slide showed uniform parallel lines with a period of 869 nm (Figure 2a), and the depth of the pattern was determined as 67 nm from the cross-cut SEM image of the SPDOT film (Figure 2b). Furthermore, the atomic force microscopy (AFM) image on the patterned layer (Figure 2c) showed the same patterns as that of the SEM. In the case of the cross-stacked DPDOT film (Figure 2d), the line patterns of the top layer were clearly resolved with a similar period to the SPDOT film, but those of the bottom layer show as blurry lines, which is normal for the top line patterns in an SEM image. The bottom layer of the DPDOT film was more clearly resolved in the AFM (Figure 2f) than the SEM image. The 45o-tilted cross-section SEM image for the DPDOT film (Figure 2e) showed a clear cross-patterned bilayer structure. The thermal stability of the patterned films 9 ACS Paragon Plus Environment


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was determined at 100 ℃, The reflectance spectra for the SPDOT and DPDOT films after heat treatment for 3 h at 100 ℃ were almost the same as the pristine samples as compared in Figure S3. This result confirmed that the nanopatterned structure of PEDOT films was thermally stable under this condition. Reflection and harvesting of light from patterned PEDOT When light is incident to periodic line grating, it can be diffracted in a backward direction at a certain angle which can be predicted by following equation:27 mλ = + ,

(1)

where m and λ are the order of diffraction and the wavelength of the incident light, respectively, is the effective refractive index of the active layer (PEDOT in this case), p is the period of the grating, and and are the angles of incidence and diffracted light, respectively. The refractive index of PEDOT in the visible spectrum region is known to be 1.63 ~ 1.04 28. Diffracted light from the patterned PEDOT surface with p of 869 nm can be observed within 250 nm ≤ λ < 270 nm for m values of 0 ~ ±5. Other diffracted light from the lower order (|m|

Cross Stacking of Nanopatterned PEDOT Films for Use as Soft

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