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Imidazolium Iodide-doped PEDOT Nanofibers as Conductive Catalysts for Highly Efficient Solid-state Dyesensitized Solar Cells Employing Polymer Electrolyte Tea-Yon Kim, Wei Wei, Tae Kyung Lee, Byung Su Kim, Seul Chan Park, Sungjin Lee, Eui Hyun Suh, Jaeyoung Jang, Juan Bisquert, and Yong Soo Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16017 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Materials & Interfaces

Imidazolium Iodide-doped PEDOT Nanofibers as Conductive Catalysts for Highly Efficient Solid-state Dye-sensitized Solar Cells Employing Polymer Electrolyte

Tea-Yon Kim,‡a Wei Wei,‡a Tae Kyung Lee,a Byung Su Kim,a Seul Chan Park,a Sungjin Lee,a Eui Hyun Suh,b Jaeyoung Jang*b, Juan Bisquertc,d, Yong Soo Kang*a

a

Department of Energy Engineering and Center for Next Generation Dye-Sensitized Solar Cells,

Hanyang University, Seoul 04763, Korea. b

c

Department of Energy Engineering, Hanyang University, Seoul 04763, Korea.

Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló, Spain

d

Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

Keywords PEDOT; Doping; Ionic liquids; Metal-free redox reduction catalysts; Solid-state dye solar cells

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ABSTRACT The

electrical

conductivity

and

catalytic

activity

of

nanofibrous

poly(3,4-

ethylenedioxythiophene)s (PEDOT NFs) was improved by re-doping with dimethyl imidazolium iodide (DMII) as a charge transfer facilitator. Addition of the new DMII dopant into the PEDOT NFs reduced the concentration of dodecyl sulfate anions (DS-) pre-doped during the polymerization process and concomitantly enhanced the doping concentration of I- by ion exchange. Re-doping with DMII increased the mobility of the PEDOT NFs by up to 18-fold and improved the conductivity due to the enhanced linearization, suppressed aggregation, and improved crystallinity of the PEDOT chains. The catalytic activity was also improved, primarily due to the increase in the compatibility and the effective surface area upon replacement of sticky DS- with the more basic and smaller I- of DMII on the surface of the PEDOT NFs. The charge transfer resistance across the interface between the poly(ethylene oxide)-based solid polymer electrolyte and PEDOT NF counter electrode (CE) was thus reduced to a large extent, giving an energy conversion efficiency (ECE) of 8.52% for solid-state dye-sensitized solar cells (DSCs), which is even better than that achieved with Pt CE (8.25%). This is the highest ECE reported for solid-state DSCs with conductive polymer CEs under 1 sun conditions.

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1. Introduction

The proliferating demand for solid-state electrochemical devices (ECDs) is driving the need for materials with advanced solid-solid interfacial properties to achieve improved efficiency and stability.1-3 To achieve good interfacial properties, enhancing the charge transfer ability, which is a primary determinant of the catalytic activity, is of prime importance, along with enhancement of the compatibility between the two solid-state materials and the interfacial area, as well as the electrical conductivity of the electrodes.4-6 Conducting polymers (CPs) are attractive for solidstate ECDs due to their high electrocatalytic and conducting properties, along with their low-cost and ease of processability and integration into solid-state devices.7-9 Among the known CPs, poly(3,4-ethylenedioxythiophene) (PEDOT) has been widely used due to its high conductivity, remarkable durability, and catalytic properties.10,11 In particular, its conductivity, and possibly, its catalytic properties can be tuned by simple doping.12 A variety of dopants such as organic solvents, polymers, ionic liquids, inorganic salts, zwitterions, organic acids, and bases have been used to increase the conductivity of PEDOT by enhancing the charge carrier concentration and mobility by changing its chain linearization.13-19 Among these dopants, imidazolium-based ionic liquids are noted for improving the conductivity of PEDOT. However, such dopants have generally been designed to increase the conductivity between the solid-state electrolyte and the electrode,48 but not the catalytic properties of polymers for ECDs. In this regard, maximizing the performance of ECDs with PEDOT electrodes is a very attractive undertaking. Dye-sensitized solar cells (DSCs), as representative ECDs, have been highlighted as nextgeneration energy conversion devices, where various interfaces between different materials have been used for such devices.20,21 In particular, solid-state DSCs employing solid polymer 3



electrolytes (SPEs) have received considerable attention due to their advantages of prolonged stability, where a reasonably high energy conversion efficiency (ECE, beyond 9% under 1 sun conditions) can be maintained for a long time and flexible-device applications are possible.22-25 The common configuration of DSCs with SPEs has two interfaces with a photoanode and a counter-electrode (CE).24 The CE acts as a catalyst to reduce the oxidized redox mediator (I3- + 2e- 3I- in the case of the I3-/I- redox mediator) in an electrolyte.25 Even though platinum (Pt) is widely used as a CE material due to its notable catalytic activity and conductivity toward the reduction of many redox mediators in liquid electrolytes, it shows serious limitations for highly efficient solid-state DSCs due to its poor contact with the SPE, resulting in poor charge transfer ability.25-27 Among the various CEs studied as replacements for Pt in DSCs employing a SPE, PEDOT CEs have shown outstanding potential, out-performing the Pt CE when a PEDOT-bPEG block copolymer was used to improve the interfacial performance with the SPE.25 Moreover, a nanofibrous PEDOT web (PEDOT NF) has been proposed as a prime candidate for CE materials to maximize the interfacial interaction between the CE and SPE due to its high surface area derived from its highly porous network structure, which provides desirable catalytic activity with liquid electrolytes. However, the ECE achieved with PEDOT NF CEs is still much lower than that achieved with Pt CEs in solid-state DSCs employing a SPE.25,28 Therefore, new methods should be developed for improving both the catalytic activity and electrical conductivity of PEDOT NFs to enhance the charge transfer ability, and consequently, to increase the ECE. In this study, to maximize the interfacial performance of PEDOT NF CEs with a SPE for highly efficient solid-state DSCs, an ionic liquid dopant, 1,3-dimethylimidazolium iodide (DMII), is introduced into the PEDOT NFs as a charge transfer facilitator. The I- ions from DMII replace a large amount of the dodecyl sulfate anions (DS-) initially doped into the polymer during the 4


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formation of the PEDOT NFs, because DMI+ may spread out the active doping sites of the PEDOT NFs by mitigating the electrostatic interactions between PEDOT and DS-. Therefore, ion-exchange and additional doping of I- may occur in the neat PEDOT NFs containing DS-. As demonstrated herein, after re-doping with DMII, the carrier mobility of the PEDOT NFs is dramatically enhanced (by an order of magnitude), plausibly because the high doping with Ileads to greatly enhanced linearization, reduced aggregation, and increased crystallinity of the PEDOT chains with decreased d-spacings. Accordingly, the conductivity of the PEDOT NFs was increased to 577.9 S cm-1, which is ca. 5 times higher than that of the neat PEDOT NFs without DMII. The catalytic activity with the SPE could also be improved as the re-doping with DMII makes the surface of the PEDOT NFs more compatible with SPE and enlarges their surface area. As a result, the charge transfer ability of the PEDOT NF CEs with SPE is drastically improved, leading to an ECE of 8.52% when employed in a solid-state DSC, which is greater than that achieved with the PEDOT NF (7.85%) and even better than that obtained with the Pt CE (8.25%). To the best of our knowledge, this is the highest ECE reported for solid-state DSCs with CP CEs under 1 sun conditions (AM 1.5G, 100 mW cm-2).

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2. Experimental

Materials: PEO (Mw = 1,000,000 g mol-1), poly(ethylene glycol) dimethyl ether (PEGDME; Mw = 500 g mol-1), potassium iodide (KI), iodine (I2), 1-methyl-3-propylimidazolium iodide (MPII), 4-tert-butylpyridine (tBP), guanidinium thiocyanate (GuSCN), deionized water (DIwater), 3,4-ethylenedioxythiophene (EDOT), sodium dodecyl sulfate (SDS), iron(ІІІ) chloride, acetonitrile (ACN), methanol (MeOH), and lithium iodide (LiI) were purchased from Sigma Aldrich. 1,3-Dimethlylimidazole iodide (DMII) was purchased from TCI (Tokyo Chemical Industry). Transparent, conductive, FTO glass substrates (TEC-7) were purchased from Pilkington. Titanium dioxide (TiO2) nanoparticulate paste (30NR-T and PST-400C) was purchased from Dyesol (30NR-T) and CCIC (PST-400C). C106 Ru dye and chenodexoycholic acid (CDCA) were purchased from Dyesol. All chemicals were used as received without further purification. Preparation and doping of PEDOT nanofiber CE: The details of the synthesis of the PEDOT nanofibers (NFs) using a cylindrical micellar template method have been described in our previous study.13 Solution-sequential processing was used for the PEDOT NF doping process. First, the PEDOT NFs, which were well dispersed in MeOH, were drop-casted onto a cleaned FTO glass substrate, and then sintered on a hot plate (80 °C, 1 h). Subsequently, doping was carried out by spin-coating 0.21 M of LiI or DMII dissolved in MeOH on the PEDOT NF film at 2000 rpm for 40 s. Finally, the films were dried in a vacuum oven at 50 °C for 30 min. The thickness of the PEDOT NF layers was maintained at 2–3 ㎛ (Figure S1, Supporting Information). 6


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Preparation of the solid-state polymer electrolyte (SPE): To prepare the PEO-based polymer electrolytes, a blend of PEO and PEGDME was dissolved in ACN at a molar ratio of [−O−]:[MPII]:[KI]:[I2] = 10:1:0.05:0.1, where the weight ratio of PEO to PEGDME was fixed at 4:6. Thereafter, 0.5 M tBP, 0.1 M KI, and 0.1 M GuSCN were added to the electrolyte. A detailed description of the process is provided in our previous report.23 Preparation of electrodes: FTO glass substrates with an area of 0.20 cm2 were cleaned in an ultrasonic bath with a detergent solution. The cleaned FTO substrate was used for both the working and counter electrodes. To prepare the working electrode, a blocking layer of TiO2 film was formed by immersing the cleaned FTO glasses in a 40 mM TiCl4 aqueous solution at 70 °C for 30 min, followed by sintering at 450 °C for 30 min. Two layers of TiO2 nanoparticles (~30 nm, Dyesol) and one layer of TiO2 light scattering particles (~400 nm, CCIC) were then screenprinted onto the blocking layer-coated FTO glass, followed by sintering at 450 °C for 30 min. The thickness of the completed electrode was 12 µm. After cooling to ambient temperature, the substrate was subjected to the same TiCl4 treatment described above. For single sensitization, the prepared TiO2 film was then dipped into a 0.5 mM C106 dye solution for 12 h or into 0.5 mM CDCA. The dye solution consisted of a mixture of ACN and tert-butanol (1:1, v/v). For preparation of the common Pt counter electrode (CE), 0.01 M H2PtCl6 in isopropanol was spincoated onto FTO glass and then sintered at 400 °C for 30 min. To prepare the complete device, a 25-µm-thick polymer (Surlyn) layer, used as a spacer, was thermally attached to the working electrode, and the as-prepared polymer electrolyte was cast and dried. In the final stage, the device was enclosed in various CEs using a two-step method. Characterization: XPS data were acquired using the Theta Probe base system (Thermo Fisher Scientific Co.) at the Hanyang Center for Research Facilities (Seoul). The system was equipped 7



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with an X-ray source (Monochromated Al-Kα rays) operated at 15 kV and 6.7 mA. The XPS spectra were calibrated relative to the C 1s peak (284.5 eV). The samples were etched using Ar ions with an energy of 2 kV using an emission current of 15 mA. Raman spectra were acquired using a 533 nm laser with a Uni-G2D (Uninanotech Co., Ltd., Republic of Korea) instrument and SAXS data were acquired with a SAXSess (Anton Paar, Austria) apparatus at 40 kV and 50 mA (2 KW) output using a Cu-Kα X-ray source. GIWAXS measurements were performed at the 3CSAXSl beam line of the Pohang Accelerator Laboratory (PAL) using a high-resolution synchrotron X-ray beam source (9.6086 eV) with a two-dimensional (2-D) charge-coupled device (CCD) detector (Mar165 CCD). For 2D-GIWAXS analysis, the incidence angle of the Xray beam was set at 0.16°, which is between the critical angles of the films and the silicon substrate.

A

Tristar

3020

(Micromeritics,

USA)

instrument

was

used

for

the

Barrett−Joyner−Halenda (BJH) pore size analysis with N2 as the physisorption agent; measurements were performed at the Hanyang Center for Research Facilities (Seoul). The samples were preprocessed at 200 °C for 2 h prior to the pore size analysis to obtain more accurate results. The AFM images were acquired with an XE-100 (Parks Systems Co., Ltd., Republic of Korea) instrument at the Hanyang Center for Research Facilities (Seoul). SEM images were captured using an FE-SEM (NOVA NANO SEM 450, FEI) instrument at an acceleration voltage of 10.0 kV in field-free lens mode at the Hanyang Center for Research Facilities (Seoul). The images for the contact angle analysis were captured by using a Phoenix 300 (SEO Co., Ltd., Republic of Korea) instrument. The carrier mobility was characterized by Hall Effect measurements using a HL5500PC Hall effect measurement system (Accent Optical Technologies). The devices used for the Hall measurements were fabricated in van der Pauw (VDP) geometry. To prepare the devices, the PEDOT NF film deposited on a glass substrate was 8


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manually patterned using a razor blade, leading to a square pattern having an approximate area of < 0.5 × 0.5 cm2. All samples were analyzed in air under a dark environment at room temperature. The conductivities of the PEDOT NFs were calculated based on the sheet resistance, measured with a four-point probe system (CMT-100MP, Advanced Instrument Technology) that was also used to analyze the film thickness. The J-V characteristics of the DSCs were measured by using a Keithley Model 2400 source meter and a solar simulator, with a 300 W Xenon arc-lamp (Newport) under 1-sun illumination (AM 1.5G, 100 mW cm-2). A light shading mask, placed on the residual area of the front side of the FTO substrate, excluding the 0.20 cm2 titanium oxide active area, was employed to prevent overvaluation of the ECE. The interfacial properties were characterized by IS and from the Tafel plot acquired using an Autolab (Metrohm) instrument. IS measurements were fitted by using Z-View software to provide numerical values. The IPCE of the DSCs was measured with a QEX7 (PV Measurements, Inc.) instrument.

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3. Results and discussion

3.1. Conductivity improvement by re-doping with MPII Doping can lead to changes in the physico-chemical properties of PEDOT NFs, such as the strength of the chemical bonds and the conformation of the conjugated polymer chains, along with changes in the macroscopic contacts among the PEDOT NFs. These changes can affect the charge carrier mobility of the PEDOT NFs, ultimately resulting in higher electrical conductivity. The extent of re-doping of the PEDOT NFs induced by LiI and DMII was estimated from Xray photoelectron spectroscopy (XPS) analysis of the binding energy of sulfur. Figure 1a shows the XPS data for the PEDOT NFs doped with dodecyl sulfate (PEDOT:DS NF) during polymerization and subsequently re-doped with LiI (PEDOT:LiI NF) or DMII (PEDOT:DMII NF). The typical S (2p) XPS peaks for the PEDOT NFs were observed at 166−170 eV and 162−165 eV, corresponding to the sulfate groups in the dodecyl sulfate anion (DS-) and the thiophene ring of the PEDOT chain, respectively.13,29 In the case of PEDOT:PSS, successful re-doping with new organic materials or ions has frequently been manifested as a reduction in the intensity ratio of the sulfonate of PSS to the thiophene of PEDOT.12,30,31 Similarly, in this work, the intensity of the peak at ~169 eV corresponding to the sulfate of DS- was decreased significantly for PEDOT:DS vs. PEDOT:LiI and PEDOT:DMII NFs, while the intensity of the peak at ~163 eV, corresponding to the thiophene, remained nearly unchanged. Therefore, the intensity ratio of the S (2p) peak of DS/PEDOT to that of the thiophene group of PEDOT correspondingly decreased from 0.82 for the PEDOT:DS NFs to 0.75 and 0.53 for the PEDOT:LiI and PEDOT:DMII NFs, respectively (Figure 1b). Notably, the intensity ratio of 0.53 for the PEDOT:DMII NFs implies that a large 10


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amount of I- was effectively doped into PEDOT during the re-doping process. This may be possible because the imidazolium cation interacts more strongly with DS- than I-, and consequently the electrostatic interactions between PEDOT+ and DS- can be readily mitigated.3133

Figure 1b also presents the decrease in the S (2p) binding energy for the thiophene unit of PEDOT:DS NF (163.65 eV) relative to PEDOT:LiI NF (163.59 eV) and PEDOT:DMII NF (163.47 eV), due to the compositional change of the PEDOT NFs upon re-doping with LiI and DMII. The decrease in the binding energy, which follows a trend identical to that of the DS/PEDOT ratio, is due to the replacement of DS- with I- because I- is a stronger Lewis base than ROSO3- in DS-.32,33 Therefore, the reduction of the amount of DS- in the PEDOT NFs can lead to enhancement of the I- concentration, which lowers the binding energy.34 The difference in the amount of I- re-doped into the PEDOT NFs with the use of LiI and DMII can influence the properties of the PEDOT NFs such as expanding the chain linearization and relaxing the chain aggregation. Raman spectroscopy has been commonly used to investigate changes in the PEDOT chains from a benzoid to a quinoid structure.35 Generally the quinoid structure favors a more extended conformation due to the rotational restriction imposed by the double bonds between the repeating units of PEDOT, whereas the benzoid structure is the preferred structure for the random coil conformation in the PEDOT chain.13 Figure 2a shows the Raman spectra, indicating the molecular vibrational profiles of the doped-PEDOT NFs. The Raman shift between 1400−1550 cm-1 corresponds to the symmetric and asymmetric stretching vibrations of the Cα = Cβ bond in the thiophene unit of the PEDOT NFs,19 and the intensity of this peak was reduced by re-doping with both LiI and DMII, implying that the chemical structure of the thiophene in PEDOT underwent significant conversion from the 11



benzoid to quinoid structure.36 Small-angle X-ray scattering (SAXS) is a useful tool for characterizing the physical doping of conducting polymers, particularly for measuring the degree of polymer chain aggregation of PEDOT.37 Figure 2b shows the SAXS intensity of the PEDOT:DS NF and PEDOT NFs re-doped with LiI and DMII below q = 100 nm-1 (q is a function of the momentum transfer vector, q = 4πsinθ λ-1, where λ is the wavelength of the X-rays).13 The scattered intensity I(q) decreased in sequence for PEDOT:DS NF vs. PEDOT:LiI NF and PEDOT:DMII NF, indicating that the dopants help to relax the aggregation and to elongate the radius of gyration of the PEDOT chains, which can also enhance the chain linearization, consistent with the data in Figure 2a. Interestingly, compared to PEDOT:LiI NF, PEDOT:DMII NF showed greater conformational transformation from the benzoid to the quinoid structure and relaxed chain aggregation of the PEDOT chains. These features may originate from the additional I- doping effect, rather than from DS-. The enhanced linearization and reduced aggregation of the PEDOT chains induced by redoping can also affect the chain/crystallite orientation and the crystallinity of the PEDOT NFs. Synchrotron-based grazing-incidence wide angle X-ray scattering (GIWAXS) was used to scrutinize the transformation of the crystalline structures of the PEDOT NF films upon redoping.18,31,36,38-41 The 2D GIWAXS patterns of the PEDOT NF films and 1D scattering profiles along the qz axis extracted from the 2D patterns are shown in Figure 2d and 2c, respectively. In Figure 2d, the typical scattering patterns of PEDOT were clearly observed for all the films, with most of the scattered intensity distributed along the out-of-plane (qz) axis.36,38-41 The 1D profiles along the qz axis reveal four distinct peaks for the films (Figure 2c). The three peaks at qz = 0.43, 0.86, and 1.3 Å-1 are likely due to the (h00) diffraction of the PEDOT chains, whereas the broad 12


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hump on the high-angle side (near qz = 1.8 Å-1) is due to the π-π stacking of the PEDOT thiophene rings (i.e., the (020) plane).36 The (h00) diffraction peaks were the dominant peaks observed along the qz axis for all samples, indicating that most of the PEDOT chains adopt an edge-on orientation with respect to the substrate. Nevertheless, the π-π stacking was rather randomly oriented as the corresponding diffraction intensity exhibits a ring-shaped pattern. The overall scheme of the crystalline structure of the PEDOT NFs, based on the GIWAXS results, is presented in Figure S3 (Supporting Information). Notably, the intensities of all the peaks became stronger with re-doping, especially when DMII was employed. This result implies that the more linearized, and therefore less aggregated, PEDOT chains are more suitable for forming an ordered lamellar structure, resulting in increased crystallinity. To garner in-depth information on the crystallinity, we calculated the d-spacings of the lamellar packing in the a direction and the ππ stacking in the b direction by using Bragg’s law, 2dsinθ = nλ. Compared to the PEDOT:DS NF, the two re-doped films showed well-ordered cystallites as well as reduced lamellar packing distances (PEDOT:DS NF: 15.40 Å, PEDOT:LiI NF: 15.03 Å, and PEDOT:DMII NF: 14.37 Å). The π-π stacking distance of the PEDOT NFs in the b direction underwent a slight reduction as follows: PEDOT:DS NF (3.55 Å), PEDOT:LiI NF (3.50 Å), PEDOT:DMII NF (3.49 Å). These results suggest that the PEDOT chains are more compactly packed in both the a and b directions after re-doping, which can support the dominant effect of I- on the PEDOT chain structure, as mentioned in relation to Figure 2a and 2b.18,31 Compared to PEDOT:DS NF and PEDOT:LiI NF, PEDOT:DMII showed largely enhanced π-π stacking of the PEDOT thiophene rings without a preferential direction. This might be related to the substantial decline in the DS- concentration in the PEDOT NFs caused by imidazolium, DMI+, doping.31 It is surmised that charge transport in the PEDOT NF films may be facilitated by the 13



previously described physico-chemical changes of the PEDOT chains upon re-doping. The carrier mobility of the PEDOT NFs, characterized by Hall effect measurement, was an order of magnitude larger for PEDOT:DMII (2.43 cm2 V-1s-1) compared to PEDOT:DS NF (0.13 cm2 V1 -1

s ). This difference can be explained by the above-mentioned changes in the PEDOT chains

upon re-doping, such as the enhanced chain linearization, suppressed chain aggregation, increased crystallinity, and reduced d-spacings. Thus, the DC conductivity (σ) of the PEDOT NF films was demonstrably improved, as measured using the conventional four-point probe method. As shown in Table 1, the σ of PEDOT:DS NF (119.4 S cm-1) increased significantly upon redoping with DMII, with a value of 577.9 S cm-1 for PEDOT:DMII NF, which is almost a 5-fold increase. This conductivity of PEDOT:DMII NF is the highest recorded for PEDOT NFs prepared by the chemical oxidative polymerization method to the best of our knowledge. Interestingly, the conductivity of PEDOT:DS NF treated with only methanol (the solvent used for re-doping) was 120.2 S cm-1, implying that the effect of solvent swelling on the conductivity of the PEDOT NFs may be negligible.

3.2. Morphology & catalytic activity Re-doping with LiI or DMII not only influenced the conductivity, but also the catalytic activities of the PEDOT NFs by making the surface of the PEDOT NFs more compatible with SPE and providing a greater surface area for improved contact with SPE. Figure 3 and S4 (Supporting Information) show the AFM topographic and SEM images of (a) PEDOT:DS NFs, (b) PEDOT:LiI NFs, and (c) PEDOT:DMII NFs coated on fluorine-doped tin oxide (FTO) glass. Interestingly, the surface roughness (characterized by the root mean square roughness, Rq) increased in sequence for PEDOT:DS NF (247 nm), PEDOT:LiI NF (330 nm), and 14


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PEDOT:DMII NF (447 nm) based on the AFM topographs, and the fibrous features were also more distinctly manifested in the SEM images. These changes may originate from the detachment of sticky DS- from the PEDOT NFs, deagglomeration of the PEDOT NFs, and the consequent improvement of the chain linearization upon re-doping. The SEM images show that the re-doping process can also enlarge the pores in the PEDOT NF films. Compared to the PEDOT:DS NF film (Figure S4a, Supporting Information), the pores were sparsely distributed in the PEDOT:LiI NF film (Figure S4b, Supporting Information), whereas the PEDOT:DMII NF film showed enlarged pores (Figure S4c, Supporting Information). Figure S5 (Supporting Information) shows the Barrett−Joyner−Halenda (BJH) pore size distribution of the PEDOT NF films. Interestingly, the average pore size, determined from the BJH desorption analysis, increased in sequence for PEDOT:DS NF (15.97 nm), PEDOT:LiI NF (27.54 nm), and PEDOT:DMII NF (48.41 nm). It is proposed that the pore size becomes larger during the re-doping of the PEDOT NF films as a larger amount of DS- is removed. The larger pore sizes of the redoped PEDOT NF films may improve their compatibility with the poly(ethylene oxide) (PEO)-based SPE, and thereby extend the interfacial contact area between the SPE and CE. The contact angle of a polyethylene glycol (PEG, Mw: 400) droplet changed from 46.9° for PEDOT:DS NF to 36.7° for PEDOT:LiI NF and 15.8° for PEDOT:DMII NF. This change may reflect the morphological changes in the films and the development of macropores, as demonstrated, and may also be due to the chemical similarity between SPE containing I-/I3- and the DMII-doped PEDOT NFs containing I- on their surface. These changes may improve the catalytic activity of the PEDOT NFs and eventually facilitate enhancement of the charge transfer ability between the SPE and PEDOT NFs via a unique mechanism to achieve a high ECE in solid-state DSCs. 15



3.3. Photovoltaic properties Figure 4a shows the J-V characteristics of DSCs employing the SPE with the Pt, PEDOT:DS NF, PEDOT:LiI NF, and PEDOT:DMII NF CEs; the corresponding photovoltaic parameters as summarized in Table 1. Although the open-circuit voltages (Voc) were similar, the short-current density (Jsc) showed a larger enhancement upon I- re-doping of the materials, following the sequence: PEDOT NF CEs: PEDOT:DS NF (15.02 mA cm-2), PEDOT:LiI NF (15.67 mA cm-2), and PEDOT:DMII NF (17.33 mA cm-2) CE; the energy conversion efficiency (ECE) also increased in the same order. The incident photon-to-electron conversion efficiency (IPCE) spectra in Figure 4b are in accordance with the Jsc tendency. Interestingly, DSCs employing the SPE with the PEDOT:DMII NF CE exhibited outstanding photovoltaic performance (8.52%), exceeding that of the commonly employed Pt CE (8.25%). This is the highest ECE of solid-state DSCs with CP CEs under 1 sun conditions (AM 1.5G, 100 mW cm-2) to the best of our knowledge. The excellent performance of the DSC with the PEDOT:DMII NF CE is within reason based on the enhanced conductivity and electrocatalytic activity induced by re-doping, resulting in suppression of the charge transfer resistance to a large extent at the interface between the CE and SPE. Figure 4c shows the Nyquist plots at 0 V for the symmetric cells with different CEs (CE/SPE/CE) based on impedance spectroscopy (IS) measurement. According to the Randlestype circuit (see inset in Figure 4c), typical Nyquist plots of the symmetric cells are presented in Figure 4c, where the high-frequency intercept on the real axis represents the series resistance (Rs) and the successive semicircle is related to the charge transfer resistance (Rct) through the interface between the CEs and SPE, which changes inversely with the catalytic activity of the 16


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different CEs upon reduction of I3- and reflects the compatibility with the SPE.42,47 Because the Rs is similar for all the CEs, its effect on the ECE can be ignored. However, interestingly, the Rct declined with increasing concentration of I- in the re-doped PEDOT NFs as follows: PEDOT:DS NF (14.78 Ω) > PEDOT:LiI NF (11.76 Ω) > PEDOT:DMII NF (8.94 Ω). Notably, PEDOT:DMII NF exhibited better performance than Pt (9.54 Ω) in terms of the charge transfer ability, which is attributed to the enhanced conductivity of the PEDOT chains, as well as the improved catalytic activity of the PEDOT NFs, as described previously. The Tafel polarization curve is also a powerful tool for analyzing the charge transfer ability of CEs at the interface with the SPE by revealing the relationship between the overpotential and the electrochemical reaction rate in the CEs.25 The moderate potential region of the Tafel curve is directly related to the charge transfer ability of the CE, where the extrapolated intercept of the anodic or cathodic branch corresponds to the exchange current density (J0) of the CE at equilibrium potential.43 J0 can also be calculated from Rct, procured from the IS spectra in Figure 3c, by using the equation J0 = RT/nFRct, where R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the reduction of I3- at the electrode (here n = 2), and F is Faraday’s constant.44 Figure 4d and Table 1 show the data from the Tafel polarization curves for the symmetric cells employing Pt, PEDOT:DS NF, PEDOT:LiI NF, and PEDOT:DMII NF CEs with a SPE. J0 also follows the order of the I- concentration in the redoped PEDOT NFs, where J0 for PEDOT:DMII NF (5.17 mA cm-2) is higher than that obtained with Pt (4.85 mA cm-2), which is consistent with the Jsc and ECE results. Consequently, from Figure 4c and d, the PEDOT:DMII NF CE exhibits excellent interfacial charge transfer properties, where an epoch-making improvement of the conductivity and the electrocatalytic activity was achieved by re-doping, leading to an outstanding Jsc and ECE of the DSCs. 17



3.4 Relationship between Rct, Rd, and Rrec In solid-state DSCs, the diffusion of I3- in the electrolyte is normally slower than in liquid electrolytes. Therefore, I3- produced by the oxidation of I- at the photoanode moves slowly back to the CE side and thus its concentration at the photoanode would change very slowly, leading to a decrease in the electron recombination resistance, Rrec, at the photoanode, which may reduce the overall efficiency of the solar cell.22,43 Thus, improvement of the charge transfer kinetics at the solid electrolyte/CE interface can reduce the I3- concentration near the photoanode and thereby suppress electron recombination at the photoanode side. Figure 5a shows the Rct of the DSCs with Pt, PEDOT:DS NF, PEDOT:LiI NF, and PEDOT:DMII NF CEs, measured by IS under 1 sun illumination conditions at various bias voltages. The Rct increased in the order: PEDOT:DMII NF < Pt < PEDOT:LiI NF < PEDOT:DS NF, which is the inverse order of the charge transfer ability with the SPE, which is consistent with Figure 4c for the symmetric cells. This enhanced charge transfer reaction accelerates the reduction of I3- and can also increase the mass flux of I3- from the photoanode through the SPE.45,46 Figure 5b shows the diffusion resistance (Rd) through the SPE in DSCs with different CEs. The Rd of I3-, estimated from the Warburg impedance, was lower for the PEDOT:DMII NF than for the other samples, suggesting a lower concentration of I3- in the near photoanode and consequently leading to the expectation of suppressed electron recombination through the photoanode.45,46 In other words, Rd changes in the same order as Rct as follows: PEDOT:DMII NF < Pt < PEDOT:LiI NF < PEDOT:DS NF. This may suggest that the diffusion of I3- through the SPE is strongly affected by Rct and can also affect electron recombination at the photoanode/electrolyte interface. Figure 5c shows the longer electron lifetime of the DSCs with the PEDOT:DMII NF, 18


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compared to those with PEDOT:DS NF, PEDOT:LiI NF, and even the Pt CE, where the trend is the same as that of the Jsc and ECE of the DSCs. Thus, the superior photovoltaic performance of the DSCs with PEDOT:DMII NF CE is primarily attributed to the decrease in the Rct, along with the reduced Rd and suppressed electron recombination at the photoanode/electrolyte interface of the PEDOT NFs due to re-doping, eventually enhancing the conductivity and catalytic activity. Figure 5d also shows the effect of re-doping on the stability of the DSCs under 1 sun conditions at room temperature without any sealing process. Whereas all the DSCs with CEs retained over 80% of the ECE from the initial irradiation time (0 s to 600 h), the DSCs with the PEDOT:DMII NF and Pt CEs showed considerably increased stability, with retention of over 90% of the initial ECE. Surprisingly, as the concentration of I- in the PEDOT NFs increased, the stability also improved. It is thus proposed that the new dopant DMII can improve the device performance as well as the durability of solid-state DSCs employing a SPE.

19



4. Conclusions

Doping with the ionic liquid DMII enhanced the charge transfer ability of the PEDOT NF CE, leading to the achievement of highly efficient solid-state DSCs employing a SPE. I- from DMII replaces DS- in the PEDOT NFs to a large extent and may also additionally dope the PEDOT NFs, causing an order of magnitude increase of carrier mobility by converting the chain conformation from a benzoid to a quinoid structure, suppressing chain aggregation, and improving the crystallinity of the PEDOT NFs with reduced d-spacings. Accordingly, the conductivity of the PEDOT NF was increased by almost 5-fold, leading to a record-high value of 577.9 S cm-1. DMII also boosts the catalytic activity of the PEDOT NFs by making the surface properties of the PEDOT NFs more compatible with the SPE and also enlarges the surface area and pore size. Thus, the PEDOT NFs doped with DMII have a lower Rct for I3- reduction, which in turn reduces Rd from the diffusion of I3- through the SPE and increases Rrec for electron recombination, yielding superior performance and improved long-term stability: The solid-state DSC with the DMII-doped PEDOT NF CE gave rise to an ECE of 8.52%, which exceeds that of the PEDOT:DS NF (7.85%) and even the Pt CE (8.25%), and is the highest ECE for solid-state DSCs employing a SPE with a conducting polymer CE under 1 sun conditions (AM 1.5G, 100 mW cm-2). Therefore, the current method may pave the way for achieving enhanced performance of conducting catalysts in electrochemical devices.

20


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Table 1. Photovoltaic characteristics of DSCs: DC conductivity (σ) from the four-point probe measurement, carrier mobility from the Hall effect measurement, and exchange current density (J0) from Tafel polarization curves from the charge transfer resistance determined by IS measurements with Pt, PEDOT:DS NF, PEDOT:LiI NF, and PEDOT:DMII NF-based CEs. The numbers in parentheses indicate the average photovoltaic characteristics and standard deviations for the five different cells (see Table S1, Supporting Information) Voc (V)

Jsc (mA cm-2)

FF (%)

ECE (%)

Pt

0.69 (0.70)

17.21 (17.21)

0.69 (0.68)

8.25 (8.17 ± 0.08)

PEDOT:DS NF

0.70 (0.69)

15.02 (15.14)

0.70 (0.71)

7.39 (7.36 ± 0.03)

119.4

0.13

3.13

PEDOT:LiI NF

0.69 (0.69)

15.67 (15.66)

0.71 (0.70)

7.64 (7.60 ± 0.02)

391.8

1.22

3.93

PEDOT:DMII NF

0.70 (0.70)

17.33 (17.24)

0.71 (0.71)

8.52 (8.47 ± 0.04)

577.9

2.43

5.17

Counter electrode

21


Σ (S cm-1)

µ (cm V-1s-1) 2

J0 (mA cm-2) 4.85


Figure 1. (a) S (2p) XPS spectra and (b) change in DS/PEDOT concentration ratio and S (2p) binding energy for PEDOT. The key for the PEDOT NF films is as follows: PEDOT:DS NF (PEDOT:DS NF, red-triangles), PEDOT NF doped with LiI (PEDOT:LiI NF, green-stars), and PEDOT NF doped with DMII (PEDOT:DMII NF, blue-circles).

22


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Figure 2. (a) Normalized Raman spectra, (b) log-log plot of the scattered intensity as a function of the momentum transfer vector determined by small angle X-ray scattering (SAXS) measurements, and (c) 1D scattering profiles along the qz axis, extracted from the 2D GIWAXS patterns for the PEDOT NF films (PEDOT:DS NF, filled red-triangle; PEDOT:LiI NF, filled green-star; and PEDOT:DMII NF; filled blue-circle). Inset shows an enlarged plot of the intensity for qz region over 1.0 Å-1. (d) 2D GIWAXS patterns for the PEDOT NF films.

23



Figure 3. AFM topographic images and contact angles of liquid polyethylene glycol (PEG, Mw: 400) on (a) (d) PEDOT:DS NF, (b) (e) PEDOT:LiI NF, and (c) (f) PEDOT:DMII NF films coated on FTO substrates.

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Figure 4. (a) J-V characteristics, (b) incident photon-to-electron conversion efficiency (IPCE) of DSCs with Pt (empty black-squares), PEDOT:DS NF (filled red-triangles), PEDOT:LiI NF (filled green-stars), and PEDOT:DMII NF (filled blue-circles)-based CEs under 1 sun illumination conditions (AM 1.5G, 100 mW cm-2) with an active area of 0.20 cm2. (c) Nyquist plots at 0 V according to IS measurements and (d) Tafel polarization curves of symmetric dummy cells.

25



Figure 5. (a) Charge transfer resistance (Rct) between the CE and the solid polymer electrolyte, (b) diffusion resistance (Rd) of the SPE with respect to various bias voltages (Vbias), (c) electron lifetime as a function of the equivalent common conduction band (CB) voltages, Vecb (such that the energy level difference between the Fermi level and CB is the same in all cases), and (d) long-term stability over 600 h for DSCs assembled with Pt (empty black-squares), PEDOT:DS NF (filled red-triangles), PEDOT:LiI NF (filled green-stars), and PEDOT:DMII NF (filled bluecircles)-coated CEs without sealing. All data were obtained from the IS data under 1-sun conditions (AM 1.5, 100 mW cm-2).

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■

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images, Normalized S(2p) XPS spectra, Scheme of the crystalline structure, BET and BJH for average pore sizes, Nyquist plots, Impedance parameters such as chemical capacitance and Rrec, Individual and average photovoltaic parameters and standard deviations of DSCs. ■

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (J. Jang), [email protected] (Y.S Kang)

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interest. ■

Acknowledgments

This work was supported by the Korea Center for Artificial Photosynthesis (KCAP) located in Sogang University, funded by the Minister of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (Number 2009-0093883). S. D. G.

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