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Multifunctional Iodide-Free Polymeric Ionic Liquid for Quasi-SolidState Dye-Sensitized Solar Cells with a High Open-Circuit Voltage Yi-Feng Lin,†,∥ Chun-Ting Li,‡,∥ Chuan-Pei Lee,‡ Yow-An Leu,† Yamuna Ezhumalai,§ R. Vittal,‡ Ming-Chou Chen,*,§ Jiang-Jen Lin,*,† and Kuo-Chuan Ho*,†,‡ †

Institute of Polymer Science and Engineering and ‡Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan § Department of Chemistry, National Central University Chung-Li 32054, Taiwan S Supporting Information *

ABSTRACT: A polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (a) acts as a gelling agent for the electrolyte of the DSSC, (b) possesses a redox mediator of SeCN−, which is aimed to form a SeCN−/(SeCN)3− redox couple with a more positive redox potential than that of traditional I−/I3−, (c) chelates the potassium cations through the lone pair electrons of the oxygen atoms of its poly(oxyethylene)-imide-imidazolium (POEI-I) segments, and (d) obstructs the recombination of photoinjected electrons with (SeCN)3− ions in the electrolyte through its POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (VOC) to the QSS-DSSC due to its functions of b−d and prolongs the stability of the cell due to its function of a. The QSS-DSSC with the gel electrolyte containing 30 wt % of the POEI-IS in liquid selenocyanate electrolyte exhibited a high VOC of 825.50 ± 3.51 mV and a high power conversion efficiency (η) of 8.18 ± 0.02%. The QSS-DSSC with 30 wt % POEI-IS retained up to 95% of its initial η after an at-rest stability test with the period of more than 1,000 h. KEYWORDS: dye-sensitized solar cell, electrolyte, iodide-free, polymeric ionic liquid, quasi-solid-state, polyoxyethylene, selenocyanate (2) thermosetting polymers as the host matrixes,10 (3) functional ionic liquids, both for gelling and as redox species,11,12 and (4) inorganic material/polymer hybrids as the conducting gel electrolytes.13−15 Among them, functional ionic liquids caught a lot of attention due to their unique properties, such as low volatility, good chemical stability, high ionic conductivity, attractive ability to dissolve various solutes, and wide electrochemical window.16−18 Especially, polymeric ionic liquids (PILs) are intensively focused on due to their high flexibility to have multiple functions via tuning their molecular constructions.19−24 For instance, Fang et al.20 synthesized an acidic PIL, poly[((3-(4-vinylpyridine) propanesulfonic acid) iodide)-co-(acrylonitrile)] (denoted as P−HI), and used this P−HI in an electrolyte of a DSSC. The electrostatic force between the sulfonate anions of P−HI and the imidazole cations of the electrolyte enables a homogeneous and continuous framework in the electrolyte for rapid transportation of the redox couple. Wang et al.21 synthesized and employed poly(1-ethyl-3-(acryloyloxy)hexylimidazolium iodide) (PEAII) as an all-solid-state electrolyte for a DSSC. They reported that the π−π stacking of the imidazolium side

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have become attractive candidates among clean energy sources in the past 2 decades owing to their advantages, e.g., inexpensive, simple to fabricate, ecofriendly, and highly efficient under weak light intensity and variable incident light angles.1,2 In general, a DSSC is composed of three parts: a dye-adsorbed mesoporous TiO2 photoanode, a counter electrode, and an electrolyte. Among them, the electrolyte plays an important role in transporting the redox mediators (mostly containing I−/I3− redox couple) and thereby regenerating the dye molecules; therefore, it determines the power conversion efficiency (η) and long-term stability of a cell.3,4 However, some drawbacks of the traditional liquid iodide electrolyte limit the performance of the DSSC; they include (a) evaporation of the organic solvent, (b) leakage of the electrolyte, (c) severe charge recombination, (d) low redox potential, and (e) complex internal chemical reactions.5−7 To solve the above-mentioned problems a−c, many kinds of quasi-solid-state (QSS) electrolytes were developed, because of the fact that they simultaneously exhibit cohesive property of a solid and diffusive property of a liquid. The QSS electrolytes usually exhibit good ionic conductivity, good penetrative ability into the semiconductor, and strong interfacial contact. In general, there are four types of materials used to prepare QSS electrolytes:8 (1) thermoplastic polymers for gelling purpose,9 © 2016 American Chemical Society

Received: March 4, 2016 Accepted: June 1, 2016 Published: June 1, 2016 15267

DOI: 10.1021/acsami.6b02767 ACS Appl. Mater. Interfaces 2016, 8, 15267−15278

Research Article

ACS Applied Materials & Interfaces Table 1. Partial Literature Review of the DSSC Performance with Polymeric Ionic Liquid Based Electrolytes PILs as electrolyte a

P−HI PEAIIb PEBII−POEMc poly[BVIm][HIm][TFSI]d POEI-II/MWCNTse POEI-IS

η (%)

VOC (mV)

JSC (mA cm−2)

FF

durability (% to the initial η)

ref

6.95 5.29 7.00 5.92 7.65 8.18

643 838 690 676 784 825

15.10 9.75 17.80 12.92 14.50 13.85

0.72 0.65 0.57 0.68 0.67 0.71

N.A.f 85% (after 1,000 h) N.A. 96% (after 1,200 h) 100% (after 1,000 h) 95% (after 1,000 h)

20 21 22 23 24 this work

a

Poly[((3-(4-vinylpyridine)propanesulfonic acid)iodide)-co-(acrylonitrile)]. bPoly(1-ethyl-3-(acryloyloxy)hexylimidazolium iodide). cPoly(1-((4ethenylphenyl)methyl)-3-butylimidazolium iodide)-poly(oxyethylene methacrylate). dPoly(1-butyl-3-(1-vinylimidazolium-3-hexyl)imidazolium bis(trifluoromethanesulfonyl)imide). ePoly(oxyethylene)-imide-imidazolium iodide/multiwall carbon nanotubes. fN.A. = not available.

Scheme 1. (a) Pale Yellow Viscoelastic Gel of POEI-IS Composed of the Poly(oxyethylene)-Imide-Imidazolium (POEI-I) Polymeric Cation and Selenocyanate Anions; (b) DSSC Employing POEI-IS;a (c) DSSC without Employing POEI-IS;b (d) POEI-IS with a Redox Mediator of SeCN−/(SeCN)3−c

a

The POEI-I segment chelates the potassium cations through its lone pair electrons, obstructs the recombination of injected electrons with the (SeCN)3− in the electrolyte, and thereby enhances VOC of the DSSC. bThere are severe charge recombination reactions on the TiO2 surface. cThis gives a more positive redox potential than that of traditional I−/I3− and thereby increases the VOC and facilitates the dye regeneration of the DSSC.

SeCN−/(SeCN)3− and SCN−/(SCN)2),27 and organic redox couples (e.g., thiolate/disulfide,28 2,2,6,6-tetramethylpiperidinN-oxyl (TEMPO),29 and its derivatives).30,31 Among them, the redox couple of SeCN−/(SeCN)3− is a promising alternative to I−/I3− because of the following reasons: (1) its redox potential is more positive than that of I−/I3−,32−35 and (2) its simple internal redox kinetics results in faster electron transfer than that possible with I−/I3−.36 Owing to the fact that the redox potential of SeCN−/(SeCN)3− is more positive than that of I−/ I3−, the energy level difference between its redox potential and the quasi-Fermi level of TiO2 would be larger; this implies a larger open-circuit voltage (VOC) for its DSSC compared to that of the DSSC with I−/I3−. Moreover, its more positive redox potential will be closer to the HOMO level of the dye, compared to the redox potential of I−/I3−; this situation benefits from a faster regeneration of the dye and thereby may lead to a higher JSC for the corresponding DSSC.

chain in PEAII plays a key role in the holes transport from the photoanode to the counter electrode. Chi et al.22 employed in a QSS-DSSC a copolymer-based electrolyte containing a PIL, namely ,poly(1-((4-ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII), and an amorphous rubbery poly(oxyethylene methacrylate) (POEM). The PEBII provided high conductivity to the electrolyte due to π−π stacking of its benzene segment. The POEM provided good mechanical properties to the electrolyte due to its polystyrene segment. As summarized in Table 1, these functionalized PILs rendered their DSSCs good η in a range of 5−7%, and some of them gave good long-term stability to their cells. Thus, the PILs are considered as potential candidates to replace the traditional liquid electrolyte. To further solve the above-mentioned problems d and e, there exists a number of alternative redox couples for I−/I3−, including metal complexes (e.g., cobaltII/III complexes25 and ferrocene/ferrocenium),26 pseudohalogen redox couples (e.g., 15268

DOI: 10.1021/acsami.6b02767 ACS Appl. Mater. Interfaces 2016, 8, 15267−15278

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imidazolium ending groups on both ends of its polymer chain; this reaction was preceded by mixing the POEI and the 1butylimidazole in the presence of a bridging compound, i.e., epichlorohydrin. Thus, product B, i.e., poly(oxyethylene)imide-imidazolium chloride (POEI-IC), was obtained. Third, the final product, POEI-IS, was obtained as a pale yellow gel via an anion-exchange reaction using the POEI-IC (with chloride anion) and the potassium selenocyanate (KSeCN). The byproduct of potassium chloride was removed by filtration. The newly synthesized POEI-IS is composed of POEI-I polymeric cation and selenocyanate anions. 2.2. Fourier Transform Infrared Spectra of the Synthesized Polymers. Figure 1a shows Fourier transform infrared (FT-IR) spectra of the synthesized polymers, including POEI, POEI-IC, and POEI-IS. The configuration of POEI contains a number of functional groups, including the oxyethylene group (-CH2−CH2−O-) and the aromatic imide group ((RCO)2NR′); several characteristic absorption peaks for these functional groups can be observed by the FT-IR spectra of POEI (the top green line in Figure 1a). For the oxyethylene group (-CH2−CH2−O-) on POEI, the peak appearing near 1355 cm−1 corresponds to the stretching of its C−O bond, while other peaks at 2880, 1120, and 945 cm−1 correspond to the symmetric stretching of its C−H bond, the stretching of its C−C bond, and rocking of its C−H bond, respectively. For the aromatic imide group ((RCO)2NR′) on POEI, the peaks appearing at 1713 cm−1 (with higher peak intensity) and 1770 cm−1 (with lower peak intensity) are, respectively, associated with the asymmetric stretch and the symmetric stretch of its CO bond.45 Thus, a successful imidization reaction is established. Since the three synthesized polymers in this study (POEI, POEI-IC, and POEI-IS) all involved the POEI structure, the above-mentioned characteristic peaks for the oxyethylene group and the aromatic imide group are all obtained in the FT-IR spectra of both POEI-IC (the middle blue line in Figure 1a) and POEI-IS (the bottom red line in Figure 1a). Additionally, a minor peak at 1560 cm−1 shown in the cases of POEI-IC and POEI-IS is attributed to the CN stretching vibration of the imidazole ending group on the POEI-I segment.24 It is thereby proved that the imidazole functional groups are successfully attached to the POEI chain. Moreover, a sharp peak at 2067 cm−1 only appears in the FT-IR spectra of POEI-IS due to the fact that it indicates the CN stretching vibration of the selenocyanate.46 The newly synthesized polymer, POEI-IS, is thus well characterized by FT-IR analysis. 2.3. Thermogravimetric Analyses of the Synthesized Polymers. Figure 1b shows the curves for the thermogravimetric analysis (TGA) of POEI, POEI-IC, and POEI-IS. The decomposition temperature (Td) at 5% weight loss for a polymer is directly obtained from the cross-point between its TGA curve and the horizontal line at 95 wt %. The values of Td for POEI and POEI-IC are evaluated to be 220 and 360 °C, respectively. The larger Td of POEI-IC, compared to that of POEI, indicates that the thermal stability of POEI-IC is significantly better than that of POEI. This is apparently due to the attachment of the imidazole functional group to the POEI. With regard to the POEI-IS, it shows a Td at 340 °C. It can thus be understood that the anion exchange of POEI-IC did not affect its thermal property. Since the newly synthesized POEIIS shows excellent thermal stability under 300 °C, it can be confirmed that POEI-IS is very suitable for the fabrication of a durable DSSC.

In this study, we aimed to combine the advantages of both PIL and SeCN−/(SeCN)3− in preparing a quasi-solid-state electrolyte for a DSSC. A novel polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEIIS), was synthesized and used for a functional gel electrolyte of a QSS-DSSC. POEI-IS is composed of the poly(oxyethylene)-imide-imidazolium (POEI-I) polymeric cation and selenocyanate anions; the former acts as a gelling agent in the electrolyte, while the latter works as a SeCN−/(SeCN)3− redox mediator (Scheme 1a). Besides, the POEI-I segment of POEI-IS has two key functions: (1) it could chelate potassium cations (K+) through the lone pair electrons of its oxygen atoms and thus prevent K+ from approaching the TiO2 surface;37−40 (2) it could retard the possible reaction between the bared (dye-unabsorbed) TiO2 surface and the electron-deficient (SeCN)3− (Scheme 1b).41−44 These two key functions of POEI-IS play very important roles in retarding the unfavorable recombination reactions in the QSS-DSSCs and thereby giving the high VOC to the cells. On the contrary, the cell without employing POEI-IS in the electrolyte may lead to a low VOC due to the severe charge recombination of photoinjected electrons with K+ and (SeCN)3− ions in the electrolyte (Scheme 1c). Moreover, the SeCN−/(SeCN)3− pair has more positive redox potential than normal I−/I3−; this enables a higher VOC to the DSSC and also benefits the dye regeneration (Scheme 1d). The QSS-DSSC with 30 wt % POEI-IS in its electrolyte exhibited a high VOC of 826 ± 4 mV and a high η of 8.18 ± 0.02%. This multifunctional polymeric ionic liquid paves a promising way for developing highly efficient QSS-DSSCs.

2. RESULTS AND DISCUSSION 2.1. Synthesis of POEI-IS. Poly(oxyethylene)-imideimidazolium selenocyanate was synthesized by a three-step pathway as shown in Scheme 2. First, product A, namely, Scheme 2. Synthetic Pathway for Obtaining Poly(oxyethylene)-Imide-Imidazolium Selenocyanate (POEI-IS)

poly(oxyethylene)-segmented imide (POEI), was prepared via an imidization reaction using the starting materials of poly[(oxypropylene)a-(oxyethylene) b-(oxypropylene)c]-segmented bis(2-aminopropyl ether) (POE2000; Mw = 2000 g mol−1, a + c = 6, and b = 39) and 4,4′-oxidiphthalic anhydride (ODPA). Second, the POEI was functionalized to have the 15269

DOI: 10.1021/acsami.6b02767 ACS Appl. Mater. Interfaces 2016, 8, 15267−15278

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ACS Applied Materials & Interfaces

Figure 1. (a) Fourier transform infrared spectra of POEI, POEI-IC, and POEI-IS. (b) Thermogravimetric analysis curves of POEI, POEI-IC, and POEI-IS. Field-emission scanning electron microscopy images of (c) POEI, (d) POEI-IC, and (e) POEI-IS.

2.5. Electrochemical Properties of POEI-IS-Based Electrolytes. First, cyclic voltammetry (CV) was applied to investigate the redox characteristics of the newly synthesized POEI-IS, using potassium selenocyanate (KSeCN) as a standard redox species to provide selenocyanate redox couple (SeCN−/(SeCN)3−). Figure S1 (Supporting Information) shows CV curves obtained in the ACN-based electrolytes containing 10.0 mM KSeCN or 10.0 mM POEI-IS in 0.1 M LiClO4, at the scan rate of 50 mV s−1. Both CV curves show an anodic peak and a cathodic peak, corresponding to the oxidation of SeCN− ions and the reduction of (SeCN)3− ions, respectively.49 The redox reaction of SeCN−/(SeCN)3− is given in

2.4. Field-Emission Scanning Electron Microscopy Analysis of the Synthesized Polymers. Panels c−e of Figure 1 show field-emission scanning electron microscopy (FE-SEM) images of POEI, POEI-IC, and POEI-IS, respectively. All films appear to be amorphous. In accordance with Figure 1c,d, POEI-IC appears to be smoother than pristine POEI, suggesting that both the insertion of the imidazole ending groups on the POEI-I segment and the incorporation of the heterogeneous chloride (Cl−) anions may occur to reduce the self-aggregation of the pristine POEI polymer chain. In accordance with Figure 1d,e, POEI-IS appears to be smoother than POEI-IC; this is because of the anion exchange from the smaller Cl− ion to the larger selenocyanate ion (SeCN−). A comparison among the surface morphologies of POEI, POEIIC, and POEI-IS clearly reveals a decrease in the aggregation of the polymer with the incorporation of imidazole ending groups, chloride, or selenocyanate. The POEI shows plenty of large aggregated and randomly distributed grains with large voids among them; this can result in poor contacts between the electrolyte and the electrodes (both of anode and cathode). On the contrary, the newly synthesized POEI-IS shows a uniform and nonaggregated surface morphology, indicating a homogeneous distribution of the polymer molecules. The nonaggregated POEI-IS can have good contacts to the electrodes, and thus can facilitate a better electron transfer at the electrolyte/TiO2 interface and also at the electrolyte/counter electrode interface.47,48 This type of polymer, POEI-IS, with homogeneous and nonaggregated surface is suitable for fabrication of a highly efficient DSSC.

3SeCN− ⇌ (SeCN)3− + 2e−

(1) +

The anodic peak of POEI-IS is at 0.30 V vs Ag/Ag , which is near that of standard KSeCN (0.28 V vs Ag/Ag+). The cathodic peak of POEI-IS is at −0.20 V vs Ag/Ag+, which is the same as that of standard KSeCN (−0.20 V vs Ag/Ag+). Therefore, it is confirmed that the newly synthesized POEI-IS solely possesses a reversible redox couple of SeCN−/(SeCN)3− with a standard potential of 0.05 V vs Ag/Ag+ (0.54 vs normal hydrogen electrode (NHE)). Moreover, POEI-IS shows a higher current density than that of KSeCN, indicating that the POEI-IS provides more SeCN− ions than KSeCN at the same concentration. Second, linear sweep voltammetry (LSV) analysis was employed to obtain the apparent diffusion coefficient (Dapp) value of a redox species in the electrolyte. Dummy cells (FTO/ Pt/electrolyte/Pt/FTO) containing various electrolytes (0.2 M 15270

DOI: 10.1021/acsami.6b02767 ACS Appl. Mater. Interfaces 2016, 8, 15267−15278

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Table 2. Electrochemical Parameters of the POEI-IS-Based Electrolytes and Interfacial Properties of the Corresponding DSSCs electrochemical properties

interfacial properties

wt % POEI-IS in the electrolyte

Dapp (10−6 cm2 s−1)

σ (mS cm−1)

Rct1 (Ω)

Rct2 (Ω)

ZW (Ω)

τe (ms)

Rrec (Ω)

0 10 20 30 40

5.58 4.23 3.33 2.19 1.76

12.12 11.06 7.94 7.01 4.52

9.9 9.3 9.4 9.4 9.8

11.1 15.0 13.6 11.5 13.4

4.4 6.2 8.5 8.9 9.8

2.39 5.43 5.79 5.87 6.62

34.7 40.8 44.3 50.3 61.1

electrolytes containing 0, 10, 20, 30, and 40 wt % POEI-IS were found to be 12.12, 11.06, 7.94, 7.01, and 4.52 mS cm−1, respectively (Table 2). It is again clear that the σ value decreases steadily with the increase in the content of POEI-IS in the electrolyte; this should again be due to the increase in the content of the nonconductive POEI-I segment in the electrolyte. However, all σ are in the same order; this infers that all POEI-IS-based electrolytes may result in the same order of current densities to their DSSCs. 2.6. Interfacial Properties of POEI-IS-Based DSSCs. An EIS analysis using the DSSCs (FTO/photoanode/electrolyte/ Pt/FTO) containing various electrolytes (0.2 M KSeCN, 0.05 M (SeCN)2, 0−40 wt % POEI-IS in ACN) was employed to investigate the interfacial properties of the DSSCs. Under 100 mW cm−2 (AM 1.5G) light illumination, the Nyquist plots of the various DSSCs generally show three semicircles in the frequency range of 10 mHz to 65 kHz, as shown in Figure 2a. In accordance with the equivalent circuit model shown in the

KSeCN, 0.05 M (SeCN)2, and 0−40 wt % of POEI-IS in ACN) were used for the LSV analysis. With the presence of POEI-IS in the electrolyte, the viscoelastic gel type electrolyte was obtained, as shown in Scheme 1a. In Figure S2 (Supporting Information), several symmetric LSV curves for various electrolytes were obtained. In accordance with the plateau of the cathodic current densities of these LSV curves, the values of limiting current densities (Jlim) were evaluated. The Jlim values were further used to calculate the Dapp of (SeCN)3−, using50 Dapp =

d J 2nFC lim

(2)

where d is the cell gap of the dummy cell (25 μm), n is the number of the charges transferred (n = 2), F is the Faraday constant (96485.4 C mol−1), and C is the concentration of (SeCN)3− (0.05 M). The values of Dapp for (SeCN)3− in the electrolytes containing 0, 10, 20, 30, and 40 wt % POEI-IS were calculated to be 5.58 × 10−6, 4.23 × 10−6, 3.33 × 10−6, 2.19 × 10−6, and 1.76 × 10−6 cm2 s−1, respectively (Table 2). It is clear that the Dapp value decreases steadily with the increase in the content of POEI-IS in the electrolyte; this is apparently due to the increase in the viscosity of the electrolyte. Values of viscosity of various gel electrolytes containing 0−40 wt % of POEI-IS were measured by a rotational viscometer at 25 °C. The viscosity of the electrolyte with 0 wt % of POEI-IS should be the viscosity of the solvent itself, which is acetonitrile (viscosity = 0.3 cP).8 The viscosities of the electrolytes containing 10 and 20 wt % POEI-IS could not be measured due to the detection limit of the rotational viscometer (11.5 Ω) than that of the cell with liquid type electrolyte (0 wt %, 11.1 Ω). This is due to the addition of a nonconductive POEI-I segment in the electrolytes which causes the decreases in the ionic conductivities and viscosities of the electrolytes, and thereby an obvious increase in their Rct2 values. Among these gel type DSSCs (10−40 wt %), the increase in the weight percent of POEI-IS in the electrolyte causes the Rct2 value to be increased first and then decreased; this result can be influenced by two key factors: (1) the increased concentration of the SeCN− and (2) the increased amount of the nonconductive POEI-I segment in the electrolytes. When the weight percent of POEI-IS in the electrolyte is increased from 10 to 30 wt %, the increased concentration of the SeCN− is expected to multiply the dye regeneration and thus increase the amount of charge transferring through the photoanode/electrolyte interfaces; then a decrease in the Rct2 values (15.0 to 11.5 Ω) can be observed. At this stage, the amount of nonconductive POEI-I segment also increases with the increase in the weight percent of POEI-IS in the electrolyte; however, it does not significantly increase the Rct2 values; therefore, it may infer that the addition of 10−30 wt % POEI-IS in the electrolyte is suitable for the application in DSSCs. When the weight percent of POEI-IS in the electrolyte is further increased from 30 to 40 wt %, despite the increased concentration of the SeCN−, the overdose nonconductive POEI-I segment in the electrolyte causes a severe decrease in the ionic conductivity (7.01 to 4.52 Ω) and thus an increase in the Rct2 value (11.5 to 13.4 Ω). In brief, an order on the Rct2 values corresponds to 10 > 20 > 40 > 30 > 0 wt % POEI-IS; this result suggests an opposite order with respect to that of the short-circuit current densities (JSC) of the pertinent DSSCs, since a lower Rct2 value generally would result in a higher JSC for the DSSC.54 Moreover, it is clear from Table 2 that the values of ZW increase steadily (from 4.4 to 9.8 Ω) with the increase in the weight percent of POEI-IS in the electrolyte; this is attributed to the increase in the viscosity of the electrolyte and thus the decrease Dapp values of the redox species in the electrolyte. Therefore, the tendency of ZW values of the DSSCs is consistent with that of Dapp values of the redox species. Figure 2b shows Bode phase plots of the cells using electrolytes containing different amounts of POEI-IS, obtained

τe = (2πfmax )−1

(4)

In an illuminated DSSC, there is energy loss owing to unfavorable recombination reactions, i.e., the reactions between the photoinjected electrons of its semiconductor and the oxidized redox species (i.e., (SeCN)3− in this study).55 A DSSC with longer electron lifetime (larger τe) indicates fewer recombination reactions at its photoanode/electrolyte interface. In Table 2, the τe values of the TiO2 associated with the electrolytes containing different concentrations of POEI-IS steadily increase from 2.39 to 6.62 ms, with the increase in the content of POEI-IS. This indicates that a higher content of POEI-IS in the electrolyte enables a better retardation for the recombination reactions. This better retardation happens owing to the following two key functions of the synthesized POEI-IS. (1) The POEI-IS could chelate K+ potassium cations through the lone pair electrons of oxygen atoms of its poly(oxyethylene) segment and thus prevent K+ from approaching the TiO2 surface.37−40 (2) The imidazole ending group could fill the vacancies on the TiO2 surface and thus forbid the reactions between these TiO2 vacancies and the electrondeficient (SeCN)3− (Scheme 1b).41−44 On the contrary, the cell with liquid type electrolyte (without employing POEI-IS) possesses the shortest τe owing to the severe charge recombination of the photoinjected electrons with K+ and (SeCN)3− ions (Scheme 1c). The tendency of the τe values (40 > 30 > 20 > 10 > 0 wt % POEI-IS) agrees with that of the VOC values. To verify the above-mentioned two key functions of POEIIS, X-ray photoelectron spectroscopy (XPS) was performed to evaluate the chemical bond between the K+ and the poly(oxyethylene) segment (function 1) and the chemical bond between the imidazole and the TiO2 (function 2). For function 1, the best electrolyte, containing 0.2 M KSeCN, 0.05 M (SeCN)2, and 30 wt % POEI-IS in dehydrated ACN, was used to obtain XPS of potassium 2p orbital, shown in Figure S3 (Supporting Information). Figure S3 shows two peaks at 290.8 and 293.9 eV: the former refers to the K−Se bond of KSeCN; the latter refers to the K−O bond between the K+ and the oxygen of the poly(oxyethylene) segment of POEI-IS. From this, it is verified that the poly(oxyethylene) segment of POEIIS can chelate with the K+ in the electrolyte. Function 1 of POEI-IS is thus established. There exist reports that poly(oxyethylene) segments chelate with cations (e.g., Li+, Na+, and K+) via the lone pair electrons of their oxygen atoms,37−40 For function 2, the TiO2 thin film was soaked in the best POEI-ISbased electrolyte (0.2 M KSeCN, 0.05 M (SeCN)2, and 30 wt % POEI-IS in ACN) and dried at 60 °C; XPS of this TiO2 was obtained for the titanium 2p orbital and is shown in Figure S4 (Supporting Information). Figure S4 depicts two peaks at 458.0 and 463.8 eV; the former refers to the Ti−O bond of the TiO2 and the latter to the Ti−N bond between the TiO2 and the nitrogen of the imidazole ending group of POEI-IS. Thus, it is shown that the imidazole ending groups of POEI-IS can attach to the vacancies on the TiO2 surface. Nitrogen-containing heterocyclic cations (e.g., imidazolium and pyridine) in the electrolyte of a DSSC were reported to adsorb on the vacancies of its TiO2 surface (i.e., dye-free TiO2 surface) and thereby 15272

DOI: 10.1021/acsami.6b02767 ACS Appl. Mater. Interfaces 2016, 8, 15267−15278

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ACS Applied Materials & Interfaces retard the charge recombination at the photoanode/electrolyte interface of the DSSC.41−44 Under dark conditions, the Nyquist plots measured at −0.85 V generally give two semicircles, as shown in Figure 3. The

Figure 3. EIS spectra of the DSSCs with various electrolytes, Nyquist plots measured in dark at −0.85 V bias.

charge recombination resistance (Rrec) values at the photoanode/electrolyte interfaces of the DSSCs containing various POEI-IS-based electrolytes were estimated by measuring the diameter of the second semicircle in the figure. The Rrec values listed in Table 2 show a decreasing tendency of 40 > 30 > 20 > 10 > 0 wt % POEI-IS. A larger Rrec value represents a lesser recombination reaction occurring at the photoanode/electrolyte interface, and thereby a larger VOC of the cell can be expected, and vice versa. It is noted that the results obtained from the τe values (measured at 100 mW cm−2) and the Rrec values (measured in dark), irrespective of their measurement techniques, are highly consistent. It is mentioned above that the values of both τe and Rrec increase gradually with the increase in the content of POEI-IS in the electrolyte from 0 to 40 wt %. We also obtained these values for a DSSC containing 50 wt % POEI-IS in its electrolyte, using a Bode phase plot obtained at 100 mW cm−2 and a Nyquist plot obtained in the dark (Figure S6a and Figure S6b (Supporting Information), respectively). In this case, the electrolyte became an extremely bulky and viscoelastic gel, with a viscosity value of 491.4 cP; such an electrolyte could hardly permeate into the semiconductor of a photoanode. A poor cell efficiency is expected in such a case.8 As expected, with reference to those of the cells with 0−40 wt % POEI-IS, the DSSC with 50 wt % POEI-IS in its electrolyte showed the worst τe of 2.01 ms (Figure S6a) and the irrational high Rrec of 5,530 Ω (Figure S6b), because the ineffective electrolyte with 50 wt % POEI-IS damages the forward charge transfer mechanisms in its DSSC. 2.7. Photovoltaic Performance of POEI-IS-Based DSSCs. 2.7.1. Cell Performance with the Liquid Type Selenocyanate and Iodide Electrolytes. Figure 4a shows the photocurrent density−voltage (J−V) curves of the DSSCs with various POEI-IS-based electrolytes (i.e., 0.2 M KSeCN, 0.05 M (SeCN)2, and 0−40 wt % of POEI-IS in ACN), obtained at the illumination of 100 mW cm−2 (AM 1.5G). The corresponding photovoltaic parameters, including the open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and cell efficiency (η), are listed in Table 3. Here, the POEI-ISbased electrolyte containing 0 wt % POEI-IS is commonly considered as the normal liquid type selenocyanate electrolyte;

Figure 4. Current density−voltage curves of the DSSCs with various electrolytes, measured (a) at 100 mW cm−2 (AM 1.5G) and (b) in the dark.

thus, the J−V curves of the DSSCs with this liquid selenocyanate electrolyte (i.e., 0.2 M KSeCN and 0.05 M (SeCN)2 in ACN) and the liquid iodide electrolyte (i.e., 0.2 M KI and 0.05 M I2 in ACN) were also measured for comparison, as shown in Figure S5 (Supporting Information). In accordance with the pertinent photovoltaic parameters listed in Table S1 (Supporting Information), the DSSC with liquid selenocyanate electrolyte shows a higher VOC (712 mV), a higher FF (0.65), but a lower JSC (14.20 mA cm−2) than those of the cell with the liquid iodide electrolyte (VOC = 691 mV, FF = 0.62, and JSC = 14.71 mA cm−2). The larger VOC is thus due to the fact that the redox couple of SeCN−/(SeCN)3− provides a higher standard redox potential than that of I−/I3− (Scheme 1d);34 the larger FF may be attributed to the better redox kinetics of SeCN−/ (SeCN)3−, and thereby to the faster electron transfer, compared to these of I−/I3−.33 In our previous report,36 we established that the selenocyanate redox couple (SeCN−/ (SeCN)3−) possesses a better redox kinetics than those of the iodide redox couple (I−/I3−). The heterogeneous reaction rate constants (k0) values of I−/I3− and SeCN−/(SeCN)3− were found to be 1.18 × 10−3 and 1.45 × 10−3 cm s−1, respectively; therefore, the electron transfer in the case of SeCN−/(SeCN)3− should be faster than that in the case of I−/I3−; i.e., the charge transfer resistance at the counter electrode/electrolyte interface in the case of SeCN−/(SeCN)3− is lower than that in the case of I−/I3−. Lower charge transfer resistance generally leads to a higher FF. Thus, we believe that the larger FF in the case of SeCN−/(SeCN)3− couple, compared to that in the case of I−/ I3− couple, can be attributed to the better redox kinetics of SeCN−/(SeCN)3−. In brief, the liquid selenocyanate electrolyte rendered higher VOC, FF, and thus higher η to its DSSC, compared to the liquid 15273

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ACS Applied Materials & Interfaces Table 3. Photovoltaic Parameters of the DSSCs with Various Electrolytesa wt % POEI-IS in the electrolyte 0 10 20 30 40 a

η (%) 6.58 5.90 7.16 8.18 7.04

± ± ± ± ±

0.13 0.08 0.05 0.02 0.05

JSC (mA cm−2)

VOC (mV) 712 785 811 826 841

± ± ± ± ±

9 9 5 4 5

14.20 10.70 12.47 13.85 12.82

± ± ± ± ±

0.03 0.02 0.01 0.05 0.03

Vonset (mV)

FF 0.65 0.70 0.71 0.72 0.65

± ± ± ± ±

0.01 0.01 0.00 0.00 0.00

616 730 751 753 776

The standard deviation data for each DSSC are obtained based on three cells.

iodide electrolyte; this result indicates that the SeCN−/ (SeCN)3− pair is a promising replacement of the traditional iodide redox couple. 2.7.2. Variation in VOC for the DSSCs with the Gel Type POEI-IS-Based Electrolytes. By increasing the weight percent (0−40 wt %) of POEI-IS in the liquid selenocyanate electrolyte, the VOC values of the pertinent DSSCs increase steadily from 712 to 841 mV (Table 3). This result is associated with the fact that an excellent retardation toward the unfavorable recombination reactions is provided by the POEI-I segment of POEIIS; this can be explained as follows (Scheme 1b): (1) The lone pair electrons on the oxygen atoms of the POEI-I segment could chelate with the K+ cations37−40 and thereby suppress the recombination of photoinjected electrons from the conduction band of the TiO2 with the K+ cations in the electrolyte. It is known that when the unfavorable recombination reactions are successfully suppressed, the Fermi level (EF) of TiO2 shifts to negative potentials56 and thus the VOC of the cell increases due to the increase in the difference between the Fermi level of TiO2 and the redox potential of the electrolyte.57 (2) The imidazole ending group on the POEI-I segment could fill the vacancies on the TiO2 surface and thus obstructs the recombination of photoinjected electrons from the conduction band of the TiO2 with the electron-deficient (SeCN)3− ions in the electrolyte (Scheme 1b).41−44 On the contrary, when the unfavorable recombination reactions occur in the DSSC with the bare liquid selenocyanate electrolyte containing 0 wt % POEI-IS, the lower Fermi level of its TiO2 causes the smaller VOC of the cell (Scheme 1c), compared to those of the cells containing POEI-IS. In brief, if the weight percent of POEI-IS increases in the electrolyte, the recombination reactions decrease and, thereby, the VOCs of the pertinent cells increase. When the weight percent of POEI-IS in the electrolyte was further increased to 50 wt %, the pertinent DSSC exhibited a worst cell efficiency of 0.99%, with a VOC of 845 mV, JSC of 2.31 mA cm−2, and an FF of 0.51 (Figure S7, in the Supporting Information)). As mentioned above, the electrolyte in this case was extremely bulky and viscous; its penetration into the semiconductor should have been worst. The worst τe of 2.01 ms and the worst Rrec of 5,530 Ω of the cell with this electrolyte (50 wt % POEI-IS) are consistent with its worst power conversion efficiency (0.99%). Therefore, this electrolyte with 50 wt % of POEI-IS is excluded for further investigation. The above-mentioned results obtained from the VOC values are once again verified by the dark current density−voltage curves of the DSSCs using various POEI-IS-based electrolytes shown in Figure 4b. From which, the values of the onset bias (Vonset) can be obtained from the intersection points of two lines: one is the tangent to the current density curve (starting from the voltage axis), and the other is the zero-current line. The higher Vonset indicates the better suppression of charge recombination reactions in a DSSC. In Table 3, the Vonset values show a tendency corresponding to the weight percent of POEI-

IS being 40 > 30 > 20 > 10 > 0 wt %; this reveals a good consistency with the VOC values of the corresponding DSSCs. It is important to note that the results obtained from the VOC values (obtained from illuminated J−V curves), Vonset values (obtained from dark J−V curves), τe values (obtained from illuminated EIS spectra), and Rrec values (obtained from dark EIS spectra), irrespective of their measurement techniques, are highly consistent. Moreover, compared to the cell with traditional liquid iodide electrolyte, our newly synthesized POEI-IS successfully improves the VOC of the pertinent gel type DSSCs via two promising approaches simultaneously: (1) the POEI-I segment (cation) in POEI-IS retards the recombination reactions and thus keeps the Fermi level of TiO2 at the more negative potential (Scheme 1b); (2) selenocyanate anions in POEI-IS provide a more positive standard redox potential (Scheme 1d). 2.7.3. Variation in JSC for the DSSCs with the Gel Type POEI-IS-Based Electrolytes. In the case of the POEI-IS-based DSSCs, the variation of JSC depends on several abovementioned parameters, e.g., the ionic conductivity (σ) of the electrolyte, the diffusion coefficient (Dapp) of the redox species in the electrolyte, the charge transfer resistance at the photoanode/electrolyte interface (Rct2) of the cell, and the Warburg diffusion resistance (Zw) of the cell, as listed in Table 2. When increasing the amount of POEI-IS from 0 to 10 wt % in the electrolyte, the JSC of the QSS-DSSC decreases drastically (from 14.20 to 10.70 mA cm−2); this is apparently due to the insertion of the nonconductive POEI-I segment causing a drastic increase in the viscosity of the electrolyte, resulting in a decrease of σ (from 12.12 to 11.06 mS cm−1), a decrease of Dapp (from 5.58 × 10−6 to 4.23 × 10−6 cm2 s−1), and thus increases in the values of Rct2 (from 11.1 to 15.0 Ω) and Zw (from 4.4 to 6.2 Ω). With further increase in the amount of POEI-IS from 10 to 30 wt % in the electrolyte, the JSCs show gradual increases (from 10.70 to 13.85 mA cm−2) due to the increased concentration of the SeCN−, causing the Rct2 decrease (from 15.0 to 11.5 Ω), despite relative increases in Zw (from 6.2 to 8.9 Ω), decrease in Dapp (from 4.23 × 10−6 to 2.19 × 10−6 cm2 s−1), and decrease in σ (from 11.06 to 7.01 mS cm−1). With an increase in POEI-IS from 30 to 40 wt % in the electrolyte, the JSC does not increase further, but rather decreases due to the overdose nonconductive POEI-I segment in the electrolyte causing severe decreases in σ and Dapp, and thus the increase in Rct2 and Zw (Table 2); these values justify the decrease of JSC from 13.85 to 12.82 mA cm−2. The lower JSC value of the DSSC with 40 wt % POEI-IS (12.82 mA cm−2) than that of the DSSC with 30 wt % POEI-IS (13.85 mA cm−2) is consistent with their values of viscosity (141.8 and 47.9 cP) and Dapp (1.76 × 10−6 and 2.19 × 10−6 cm2 s−1); that is to say that the increase in the viscosity of the electrolyte with 40 wt % of POEI-IS has caused a decrease in the JSC of the pertinent DSSC. The variation of JSC seems to be dependent on all of the mentioned parameters. 15274

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ACS Applied Materials & Interfaces 2.7.4. Variation in FF for the DSSCs with the Gel Type POEI-IS-Based Electrolytes. With regard to FF, it increases first with the addition of POEI-IS up to 30 wt % and then decreases with the addition of 40 wt % POEI-IS. Since FF indicates inversely proportional to the energy loss in a cell, the increase in FF in the first place is owing to the fact that the increased nonconductive POEI-I segment in the electrolyte provides a successful retardation effect for the recombination reactions, while the decrease in FF later is due to the overdose nonconductive POEI-I segment which causes a severe decrease in σ and Dapp. The lower FF value of the DSSC with 40 wt % POEI-IS (0.65) than that of the DSSC with 30 wt % POEI-IS (0.72) is consistent with their values of viscosity (141.8 cP for 40 wt % and 47.9 cP for 30 wt %); that is to say that the increase in the viscosity of the electrolyte with 40 wt % POEI-IS has caused a decrease in FF of the pertinent DSSC. Ultimately, decreases both in JSC and FF values of the DSSC with 40 wt % POEI-IS have caused its lower performance than that of the cell with 30 wt % POEI-IS. Thus, the best DSSC reaches an η of 8.18%, VOC of 826 mV, JSC of 13.85 mA cm−2, and FF of 0.72 via employing an electrolyte containing 30 wt % POEI-IS; as compared to the cell with an electrolyte containing 0 wt % POEI-IS (6.58%), a much higher η is obtained. 2.7.5. Long-Term Stability of POEI-IS-based DSSCs. Figure 5 shows the long-term durability data of the DSSCs employing

(POEI-I) polymeric cation and selenocyanate anions; these groups are characterized by FT-IR to reveal a successful synthesis of POEI-IS. Several functions of POEI-IS are verified via different techniques as follows. (1) The POEI-I segment not only works as a gelling agent but also provides a good retardation effect for the recombination reactions; this function is verified by the values of VOC (illuminated J−V curves), Vonset (dark J−V curves), τe (illuminated EIS spectra), and Rrec (dark EIS spectra). (2) The selenocyanate anion not only benefits the interfacial contacts between the POEI-IS-based electrolytes and the electrodes without the self-aggregation of the polymer (FESEM images) but also possesses a reversible redox couple of SeCN−/(SeCN)3− with a more positive standard potential than that of iodide species (CV curves). (3) POEI-IS shows only a 5% weight loss at 340 °C, revealing its high thermal stability (TGA curves), and thereby renders the best POEI-IS-based DSSC an excellent long-term durability; i.e., the cell efficiency maintains 95% of its initial value after more than 1,000 h (longterm J−V curves). Although the insertion of the nonconductive POEI-I segment in the electrolyte causes the decreases in σ and Dapp (LSV curves), and thus an increase in ZW (illuminated EIS spectra), it is found that the addition of 10−30 wt % POEI-IS in the electrolyte is tolerable for the application in DSSCs. Finally, the best DSSC reaches an η of 8.18%, VOC of 826 mV, JSC of 13.85 mA cm−2, and FF of 0.72 via employing an electrolyte containing 30 wt % POEI-IS; as compared to the cell with an electrolyte containing 0 wt % POEI-IS (6.58%), a much higher η is obtained. This multifunctional polymeric ionic liquid, POEI-IS, paves a promising way for developing highly efficient QSS-DSSCs.

4. EXPERIMENTAL SECTION 4.1. Materials. Poly[(oxypropylene)a-(oxyethylene)b-(oxypropylene)c]-segmented bis(2-aminopropyl ether) (POE2000, Mw = 2,000 g mol−1; waxy solid; a + c = 6; b = 39) was purchased from JeffamineHuntsman. 4,4′-Oxydiphthalic anhydride (ODPA, 97%), titanium(IV) tetraisoproproxide (TTIP, >98%), acetonitrile (ACN, 99.99%), ethanol (EtOH, 99.5%), isopropyl alcohol (IPA, 99.5%), poly(ethylene glycol) (PEG, Mw = 20,000 g mol−1), epichlorohydrin (99%), tertbutanol (tBA, 99.8%), 1-butylimidazole (98%), potassium selenocyanate (KSeCN, 99%), dichloromethane (DCM, 99.99%), 2-methoxyethanol (99.95%), bromine (Br2, >99.99%), potassium iodide (KI, ≥99.99%), iodine (I2, ≥99.8%), and chenodeoxycholic acid (CDCA, ≥ 97%) were obtained from Sigma-Aldrich. Tetrahydrofuran (THF, 95%) and ethyl alcohol (99.5%) were procured from Teida. Surlyn (SX1170-25, 25 μm) film was supplied by Solaronix (S.A., Aubonne, Switzerland). Fluorine-doped SnO2 (FTO, TEC-7, 7 Ω sq−1) conducting glasses were imported from NSG America, Inc., Clifton, NJ, USA. The commercial light scattering TiO2 particles (ST-41; average particle size = 200 nm) were acquired from Ishihara Sangyo, Ltd. The organic dye, 2-cyano-3-(5-(6-(4-(diphenylamino)phenyl)3,7-dipentadecylthieno[2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]thiophen2-yl)thiophen-2-yl)acrylic acid (TA, shown in Figure S8 in the Supporting Information),58 was synthesized and provided by M.-C.C.’s group in National Central University, Taiwan. 4.2. Synthesis of Poly(oxyethylene)-Imide-Imidazolium Selenocyanate. The novel polymeric ionic liquid, POEI-IS, was synthesized via a simple and three-step process, as shown in Scheme 2. (1) A solution of ODPA (1.264 g, 2.5 mmol) in THF was dropwise added into POE2000 (16.30 g, 5 mmol) for 1 h, under continuous vigorous stirring in a nitrogen atmosphere; then the temperature of this polymer mixture was raised to 150 °C, and maintained at that level for 3 h to obtain a product A (poly(oxyethylene)-segmented imide, POEI). (2) Epichlorohydrin (0.407 g, 4 mmol) was added dropwise into the above-mentioned product A (solution) at 60 °C for 24 h, and then a solution of 1-butylimidazole (0.497 g, 4 mmol) in ACN was

Figure 5. Long-term durability of the DSSCs employing the electrolytes containing 0 and 30 wt % POEI-IS.

the electrolytes containing 0 and 30 wt % POEI-IS. In this experiment, these two cells were first sealed by a 25 μm thick Surlyn film and then by an epoxy glue. The cell efficiency was measured once per day for the first 14 days and then once per 2 days for the following days. It is obvious from Figure 5 that the QSS-DSSC with 30 wt % POEI-IS shows much higher stability, compared to the cell with 0 wt % POEI-IS. The η of the QSSDSSC with 30 wt % POEI-IS remains at about 95% of its initial value, after more than 1,000 h. On the contrary, the η of the DSSC without POEI-IS remains about only 65% of its initial value after the same period. It is concluded that our newly synthesized POEI-IS renders its DSSC not only a high VOC but also an excellent long-term durability, indicating POEI-IS is a promising substitution of the traditional liquid iodide electrolyte.

3. CONCLUSIONS A thermally stable novel polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized as a functional gel electrolyte for quasi-solid-state dye-sensitized solar cells (QSS-DSSCs). POEIIS is composed of the poly(oxyethylene)-imide imidazolium 15275

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ACS Applied Materials & Interfaces added into the mixture at 90 °C for 24 h to get product B (poly(oxyethylene)-imide-imidazolium chloride, POEI-IC; Mw ≈ 7000 g mol−1).24 (3) Product B was then added into an ACN-based solution containing double-normal KSeCN, and thereby the anion-exchange reaction was triggered to produce the final product, (poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS). POEI-IS was further separated from the byproduct (KCl) by filtration, washed by DCM for several times, and dried in a vacuum oven for 12 h. 4.3. Preparation of POEI-IS Gel Electrolyte. The liquid electrolyte containing 0.2 M KSeCN and 0.05 M (SeCN)2 in ACN was prepared by the following procedure. A solution of KSeCN (3 mmol) in 5 mL of ACN was prepared under stirring in the dark. In the meantime, a bromine solution was prepared by dissolving Br2 (0.5 mmol) in 5 mL of ACN. After complete dissolution of the solutes in both solutions, the second solution was added to the first one dropwise, under the same conditions. The resulting suspension was filtered to remove precipitated KBr; a yellow solution was obtained as the filtrate. An additional filtration step was necessary in order to have a clear solution. Finally, gel electrolytes containing different weight percents of POEI-IS were prepared by adding 10, 20, 30, and 40 wt % POEI-IS into the liquid electrolytes individually. For the comparison, a liquid iodide electrolyte containing 0.2 M KI and 0.05 M I2 in ACN was prepared. 4.4. Cell Assembly. Fluorine-doped tin oxide conducting glasses (FTO, TEC-7, 7 Ω sq.−1; NSG America) were first cleaned with a neutral cleaner and then washed with deionized water, acetone, and isopropanol sequentially. A TiO2 colloid was prepared as follows. A 0.5 M amount of aqueous TTIP was added to 0.1 M nitric acid under stirring at 88 °C for 8 h; the obtained solution was then heated to 240 °C for 12 h in an autoclave (PARR 4540, Moline, IL, USA). The autoclaved solution was concentrated to contain 8 wt % crystalline TiO2 nanoparticles (NPs, ca. 20 nm) in the TiO2 slurry. A TiO2 paste for the light transparent layer (TL paste) was prepared by adding 25 wt % PEG (with respect to TiO2 NPs) to the TiO2 slurry obtained, while another TiO2 paste for the light scattering layer (SL paste) was prepared by adding 25 wt % PEG and 100 wt % ST-41 (with respect to TiO2 NPs) to the obtained TiO2 slurry. The PEG was used for preventing the TiO2 paste from being cracked during its casting on a FTO glass and also to control the pore size of the pertinent TiO2 film. A cleaned FTO glass was coated with a thin compact layer of TiO2 (100 nm), by using a solution of TTIP in 2-methoxyethanol (weight ratio of 1:3). A TiO2 film, containing a first layer of TL (4 μm) and a second layer of SL (4 μm) with a geometric area of 0.20 cm2 was coated on the treated FTO glass by a doctor blade technique. Each layer was separately sintered at 500 °C for 30 min in an air atmosphere.59 After the sintering process, the TiO2 film was immersed for 10 h in a 3.0 × 10−4 M TA dye and 0.01 M CDCA solution (in a mixed solvent of ACN/tBA/DCM with a volume ratio of 1:1:8) at room temperature. Finally, the TA-adsorbed TiO2 photoanode was coupled with a sputtered platinum counter electrode (CE) with a cell gap of 25 μm by using a 25 μm thick Surlyn film as the spacer. The electrolytes in this study were composed of 0.2 M potassium selenocyanate (KSeCN), 0.05 M selenocyanogen ((SeCN)2), and different amounts of POEI-IS (0−40 wt %) in ACN. These electrolytes were injected into the cell gap by capillarity. 4.5. Characterization of POEI-IS and the DSSCs. A Fourier transform infrared spectrometer (FT-IR, PerkinElmer Spectrum 100 FT-IR) was used to determine the functional groups of the synthesized polymers (i.e., POEI, POEI-IC, and POEI-IS) over the range of 800− 4000 cm−1, using a calcium fluoride (CaF2) plate as the blank. A thermogravimetric analyzer (TGA, PerkinElmer) was used to obtain the thermal stabilities of the synthesized polymers, while a fieldemission scanning electron microscope (FE-SEM, Nova NanoSEM 230, FEI) was used to observe the surface morphologies of the synthesized polymers. CV was employed to analyze the redox kinetics of the newly synthesized POEI-IS; the CV data were recorded by a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, Utrecht, The Netherlands) using a three-electrode electrochemical system. A platinum foil with a specific area of 1 cm2 was used as the working electrode, while another platinum foil and an Ag/Ag+ electrode were

used as the counter and reference electrodes, respectively. One of the two ACN-based electrolytes was used for the CV, namely, 10 mM KSeCN or 10 mM POEI-IS in 0.1 M LiClO4. Linear sweep voltammetry (LSV) was used to obtain apparent diffusion coefficients (Dapp) of the redox species in the electrolyte; the data were recorded by the same potentiostat/galvanostat. A dummy cell, with the configuration of FTO/Pt/electrolyte/Pt/FTO was scanned from −1.0 to 1.0 V at a low scan rate of 10 mV s−1. The prepared electrolytes (0.2 M KSeCN, 0.05 M (SeCN)2, 0−40 wt % POEI-IS in ACN) were used in the dummy cells. The viscosities (μ) of various electrolytes were measured at 25 °C by a rotational viscometer (Smart series, V200003, Fungilab), equipped with a smart L special spindle (TL5). Electrochemical impedance spectroscopy (EIS) was used to obtain the ionic conductivity (σ) of an electrolyte and the interfacial resistances of a DSSC; the EIS data were recorded by the same potentiostat/galvanostat, however equipped with a FRA2 module, between 10 mHz to 65 kHz with an AC amplitude of ±10 mV. While measuring σ of an electrolyte, a dummy cell structure (FTO/Pt/electrolyte/Pt/FTO) was used; the cell constant (d/A) was calibrated using a standard sodium chloride (NaCl) solution (σ = 12.9 mS cm−1, Model 011006, Thermo Orion, East Grinstead). Surface chemical analyses were performed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific Theta Probe, U.K.). The interfacial resistances of the DSSCs were measured under 100 mW cm−2 (at their open-circuit voltages) and in dark (at an applied bias of −0.85 V). Photovoltaic parameters of the DSSCs were measured by the same potentiostat/galvanostat under 100 mW cm−2, which was generated by a class A quality solar simulator (XES-301S, AM1.5G, San-Ei Electric Co. Ltd., Osaka, Japan). The incident light intensity (100 mW cm−2) was calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc.).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02767. Cyclic voltammograms of POEI-IS and KSeCN, linear sweep voltammetry curves of different electrolytes, X-ray photoelectron spectroscopy analysis of the POEI-IS electrolyte and the POEI-IS electrolyte embedded on TiO2film, photovoltaic properties of the DSSCs with liquid type SeCN-based electrolyte, interfacial and photovoltaic properties of the DSSCs with 50 wt % POEI-IS in electrolyte, and the molecular structure of the organic dye (TA) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.-C.H.). *E-mail: [email protected] (J.-J.L.). *E-mail: [email protected] (M.-C.C.). Author Contributions ∥

Y.-F.L. and C.-T.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under Grant Nos. 102-2221E-002-186-MY3 and 103-2119-M-007-012.

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REFERENCES

(1) O'Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740.

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