Multifunctional Iodide-Free Polymeric Ionic Liquid for Quasi-Solid

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A 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, Ramamurthy Vittal, Ming-Chou Chen, Jiang-Jen Lin, and Kuo-Chuan Ho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02767 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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ACS Applied Materials & Interfaces

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A Multifunctional Iodide-Free Polymeric Ionic

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Liquid for Quasi-Solid-State Dye-Sensitized Solar

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Cells with a High Open-Circuit Voltage

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Yi-Feng Lin,†,# Chun-Ting Li,‡,# Chuan-Pei Lee,‡ Yow-An Leu,† Yamuna Ezhumalai,§ R. Vittal,‡

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Ming-Chou Chen,*,§ Jiang-Jen Lin, *,† and Kuo-Chuan Ho*,†,‡

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†

Institute of Polymer Science and Engineering and ‡Department of Chemical Engineering,

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National Taiwan University, Taipei 10617, Taiwan

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§

Department of Chemistry, National Central University Chung-Li 32054, Taiwan

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Keywords: dye-sensitized solar cell; electrolyte; iodide-free; polymeric ionic liquid; quasi-solid-

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state

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ACS Paragon Plus Environment

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ABSTRACT: A polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate

2

(POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-

3

state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (a) acts as a gelling

4

agent for the electrolyte of the DSSC, (b) possesses a redox mediator of SeCN−, which is aimed

5

to form a SeCN−/(SeCN)3− redox couple with a more positive redox potential than that of

6

traditional I−/I3−, (c) chelates the potassium cations through the lone pair electrons of the oxygen

7

atoms of its poly(oxyethylene)-imide-imidazolium (POEI-I) segments, and (d) obstructs the

8

recombination of photo-injected electrons with (SeCN)3− ions in the electrolyte through its

9

POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (VOC) to the QSS-DSSC

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due to its functions of (b), (c), and (d), and prolongs the stability of the cell due to its function of

11

(a). The QSS-DSSC with the gel electrolyte containing 30 wt% of the POEI-IS in liquid

12

selenocyanate electrolyte exhibited a high VOC of 825.50±3.51 mV and a high power conversion

13

efficiency (η) of 8.18±0.02%. The QSS-DSSC with 30 wt% of POEI-IS retained up to 95% of its

14

initial η after an at-rest stability test with the period of more than 1,000 h.

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1.

Introduction

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Dye-sensitized solar cells (DSSCs) have become attractive candidates among clean energy

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sources in the past two decades owing to their advantages, e.g., inexpensive, simple to be

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fabricated, eco-friendly, and highly efficient under weak light intensity and variable incident

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light angles.1-2 In general, a DSSC is composed of three parts: a dye-adsorbed mesoporous TiO2

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photoanode, a counter electrode, and an electrolyte. Among them, the electrolyte plays an

22

important role in transporting the redox mediators (mostly containing I−/I3− redox couple) and

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thereby regenerating the dye molecules; therefore, it determines the power conversion efficiency

24

(η) and long-term stability of a cell.3-4 However, some drawbacks of the traditional liquid iodide

25

electrolyte limit the performance of the DSSC; they include (a) evaporation of the organic

26

solvent, (b) leakage of the electrolyte, (c) severe charge recombination, (d) low redox potential,

27

and (e) complex internal chemical reactions.5-7

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To solve the above-mentioned problems (a)-(c), many kinds of quasi-solid-state (QSS)

29

electrolytes were developed, because of the fact that they simultaneously exhibit cohesive

30

property of a solid and diffusive property of a liquid. The QSS electrolytes usually exhibit good


2

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ionic conductivity, good penetrative ability into the semiconductor, and strong interfacial

2

contact. In general, there are four types of materials used to prepare QSS electrolytes:8 (1)

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thermoplastic polymers for gelling purpose,9 (2) thermosetting polymers as the host matrixes,10

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(3) functional ionic liquids, both for gelling and as redox species,11-12 and (4) inorganic

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material/polymer hybrids as the conducting gel electrolytes.13-15 Among them, functional ionic

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liquids caught a lot of attention due to their unique properties, such as low volatility, good

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chemical stability, high ionic conductivity, attractive ability to dissolve various solutes, and wide

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electrochemical window.16-18 Especially, polymeric ionic liquids (PILs) is intensively focused

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due to their high flexibility to have multiple functions via tuning their molecular constructions.19-

10

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For instance, Fang et al.20 synthesized an acidic PIL, P[((3-(4-vinylpyridine) propanesulfonic

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acid) iodide)-co-(acrylonitrile)] (denoted as P-HI), and used this P-HI in an electrolyte of a

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DSSC. The electrostatic force between the sulfonate anions of P-HI and the imidazole cations of

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the electrolyte enables a homogeneous and continuous framework in the electrolyte for rapid

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transportation of the redox couple. Wang et al.21 synthesized and employed poly(1-ethyl-3-

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(acryloyloxy)hexylimidazolium iodide) (PEAII) as an all-solid-state electrolyte for a DSSC.

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They reported that the π-π stacking of the imidazolium side chain in PEAII plays a key role in

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the holes transport from the photoanode to the counter electrode. Chi et al.22 employed in a QSS-

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DSSC

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ethenylphenyl)methyl)-3-butyl-imidazolium iodide) (PEBII), and an amorphous rubbery

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poly(oxyethylene methacrylate) (POEM). The PEBII provided high conductivity to the

21

electrolyte due to π-π stacking of its benzene segment. The POEM provided good mechanical

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properties to the electrolyte due to its polystyrene segment. As summarized in Table 1, these

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functionalized PILs rendered their DSSCs good η in a range of 5~7%, and some of them gave

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good long-term stability to their cells. Thus, the PILs are considered as potential candidates to

25

replace the traditional liquid electrolyte.

a

copolymer-based

electrolyte

containing

a

PIL,

namely

poly(1-((4-

26

To further solve the above-mentioned problems (d)-(e), there exists a number of alternative

27

redox couples for I−/I3−, including metal complexes (e.g., cobaltII/III complexes25 and

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ferrocene/ferrocenium),26

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SCN−/(SCN)2),27

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tetramethylpiperidin-N-oxyl (TEMPO),29 and its derivatives).30-31 Among them, the redox couple

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of SeCN−/(SeCN)3− is a promising alternative to I−/I3− because of the following reasons: (1) its

and

pseudohalogen organic

redox

redox couples

couples (e.g.,

(e.g.,

SeCN−/(SeCN)3−

thiolate/disulfide,28

and

2,2,6,6-


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redox potential is more positive than that of I−/I3−,32-35 and (2) its simple internal redox kinetics

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results in faster electron transfer than that possible with I−/I3−.36 Owing to the fact that the redox

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potential of SeCN−/(SeCN)3− is more positive than that of I−/I3−, the energy level difference

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between its redox potential and the quasi-Fermi level of TiO2 would be larger; this implies a

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larger open-circuit voltage (VOC) for its DSSC compared to that of the DSSC with I−/I3−.

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Moreover, its more positive redox potential will be closer to the HOMO level of the dye,

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compared to the redox potential of I−/I3−; this situation benefits a faster regeneration of the dye,

8

thereby may lead to a higher JSC to the corresponding DSSC.

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In this study, we aimed to combine the advantages of both PIL and SeCN−/(SeCN)3− in

10

preparing a quasi-solid-state electrolyte for a DSSC. A novel polymeric ionic liquid,

11

poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was synthesized and used for a

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functional gel electrolyte of a QSS-DSSC. The POEI-IS is composed of the poly(oxyethylene)-

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imide imidazolium (POEI-I) polymeric cation and selenocyanate anions; the former acts as a

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gelling agent in the electrolyte, while the later works as a SeCN−/(SeCN)3− redox mediator

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(Scheme 1a). Besides, the POEI-I segment of POEI-IS has two key functions: (1) it could

16

chelate potassium cations (K+) through the lone pair electrons of its oxygen atoms and thus

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prevent K+ from approaching the TiO2 surface;37-40 (2) it could retard the possible reaction

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between the bared (dye-unabsorbed) TiO2 surface and the electron-deficient (SeCN)3− (Scheme

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1b).41-44 These two key functions of POEI-IS play very important roles in retarding the

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unfavorable recombination reactions in the QSS-DSSCs and thereby giving the high VOC to the

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cells. On the contrary, the cell without employing POEI-IS in the electrolyte may lead to a low

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VOC due to the severe charge recombination of photo-injected electrons with K+ and (SeCN)3−

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ions in the electrolyte (Scheme 1c). Moreover, the SeCN−/(SeCN)3− pair has more positive redox

24

potential than normal I−/I3−; this enables a higher VOC to the DSSC and also benefits the dye

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regeneration (Scheme 1d). The QSS-DSSC with 30 wt% of the POEI-IS in its electrolyte

26

exhibited a high VOC of 826±4 mV and a high η of 8.18±0.02%. This multifunctional polymeric

27

ionic liquid paves a promising way for developing highly efficient QSS-DSSCs.

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2.

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2.1. Synthesis of POEI-IS

Results and Discussion

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Poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS) was synthesized by a three-

4

step pathway as shown in Scheme 2. First, the product A, namely poly(oxyethylene)-segmented

5

imide (POEI), was prepared via an imidization reaction using the starting materials of

6

poly[(oxypropylene)a-(oxyethylene)b-(oxypropylene)c]

7

(POE2000; Mw=2000 g mol−1, a+c=6, b=39) and 4,4’-oxydiphthalic anhydride (ODPA). Second,

8

the POEI was functionalized to have the imidazolium ending groups on both ends of its polymer

9

chain; this reaction was preceded by mixing the POEI and the 1-butylimidazole in the presence

10

of a bridging compound, i.e., epichlorohydrin. Thus, the product B, i.e., poly(oxyethylene)-imide

11

imidazolium chloride (POEI-IC), was obtained. Third, the final product, poly(oxyethylene)-

12

imide imidazolium selenocyanate (POEI-IS), was obtained as a pale yellow gel via an anion

13

exchange reaction using the POEI-IC (with chloride anion) and the potassium selenocyanate

14

(KSeCN). The by-product of potassium chloride was removed by the filtration. The newly

15

synthesized POEI-IS is composed of poly(oxyethylene)-imide imidazolium (POEI-I) polymeric

16

cation and selenocyanate anions.

segmented

bis(2-aminopropyl

ether)

17 18

2.2. Fourier transform infrared spectra of the synthesized polymers

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Figure 1a shows Fourier transform infrared (FT-IR) spectra of the synthesized polymers,

20

including POEI, POEI-IC, and POEI-IS. The configuration of POEI contains a number of

21

functional groups, including the oxyethylene group (-CH2-CH2-O-) and the aromatic imide group

22

((RCO)2NR’); several characteristic absorption peaks for these functional groups can be

23

observed by the FT-IR spectra of POEI (the top green line in Figure 1a). For the oxyethylene

24

group (-CH2-CH2-O-) on POEI, the peak appearing near 1355 cm−1 corresponds to the stretching

25

of its C–O bond, while other peaks at 2880, 1120, and 945 cm−1 correspond to the symmetric

26

stretching of its C–H bond, the stretching of its C–C bond, and rocking of its C–H bond,

27

respectively. For the aromatic imide group ((RCO)2NR’) on POEI, the peaks appearing at 1713

28

cm−1 (with higher peak intensity) and 1770 cm−1 (with lower peak intensity) are, respectively,

29

associated with the asymmetric stretch and the symmetric stretch of its C=O bond.45 Thus, a

30

successful imidization reaction is established. Since the three synthesized polymers in this study

31

(POEI, POEI-IC, and POEI-IS) all involved the POEI structure, the above-mentioned


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characteristic peaks for the oxyethylene group and the aromatic imide group are all obtained in

2

the FT-IR spectra of both POEI-IC (the middle blue line in Figure 1a) and POEI-IS (the bottom

3

red line in Figure 1a). Additionally, a minor peak at 1560 cm−1 shown in the cases of POEI-IC

4

and POEI-IS is attributed to the C=N stretching vibration of the imidazole ending group on the

5

POEI-I segment.24 It is thereby proved that the imidazole functional groups are successfully

6

attached to POEI chain. Moreover, a sharp peak at 2067 cm−1 only appears in the FT-IR spectra

7

of POEI-IS due to the fact that it indicates the C≡N stretching vibration of the selenocyanate.46

8

The newly synthesized polymer, POEI-IS, is thus well characterized by FT-IR analysis.

9 10

2.3. Thermogravimetric analyses of the synthesized polymers

11

Figure 1b shows the curves for the thermogravimetric analysis (TGA) of POEI, POEI-IC, and

12

POEI-IS. The decomposition temperature (Td) at 5% weight loss for a polymer is directly

13

obtained from the cross point between its TGA curve and the horizontal line at 95 weight percent

14

(wt%). The values of Td for POEI and POEI-IC are evaluated to be 220 and 360 oC, respectively.

15

The larger Td of POEI-IC, compared to that of POEI, indicates that the thermal stability of POEI-

16

IC is significantly better than that of POEI. This is apparently due to the attachment of the

17

imidazole functional group to the POEI. As regards to the POEI-IS, it shows a Td at 340 oC. It

18

can thus be understood that the anion exchange of POEI-IC did not affect its thermal property.

19

Since the newly synthesized POEI-IS shows an excellent thermal stability under 300 oC, it can be

20

confirmed that POEI-IS is very suitable for the fabrication of a durable DSSC.

21 22

2.4. Field-emission scanning electron microscopy analysis of the synthesized polymers

23

Figure 1c, d, and e show field-emission scanning electron microscopy (FE-SEM) images of

24

POEI, POEI-IC, and POEI-IS, respectively. All films appear to be amorphous. In accordance

25

with Figure 1c and d, POEI-IC appears to be smoother than pristine POEI, suggesting that both

26

the insertion of the imidazole ending groups on the POEI-I segment and the incorporation of the

27

heterogeneous chloride (Cl–) anions may occur to reduce the self-aggregation of the pristine

28

POEI polymer chain. In accordance with Figures 1d and e, POEI-IS appears to be smoother than

29

POEI-IC; this is because of the anion exchange from the smaller Cl– ion to the larger

30

selenocyanate ion (SeCN–). A comparison among the surface morphologies of POEI, POEI-IC,

31

and POEI-IS clearly reveals a decrease in the aggregation of the polymer with the incorporation


6

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of imidazole ending groups, chloride, or selenocyanate. The POEI shows plenty of large

2

aggregated and randomly distributed grains with large voids among them; this can result in poor

3

contacts between the electrolyte and the electrodes (both of anode and cathode). On the contrary,

4

the newly synthesized POEI-IS shows a uniform and non-aggregated surface morphology,

5

indicating a homogeneous distribution of the polymer molecules. The non-aggregated POEI-IS

6

can have good contacts to the electrodes, and thus can facilitate a better electron transfer at the

7

electrolyte/TiO2 interface and also at the electrolyte/counter electrode interface.47-48 This type of

8

polymer, POEI-IS, with homogeneous and non-aggregated surface is suitable for fabrication of a

9

highly efficient DSSC.

10 11

2.5. Electrochemical properties of POEI-IS-based electrolytes

12

First, cyclic voltammetry (CV) was applied to investigate the redox characteristics of the

13

newly synthesized POEI-IS, using potassium selenocyanate (KSeCN) as a standard redox species

14

to provide selenocyanate redox couple (SeCN−/(SeCN)3−). Figure S1 (Supporting Information)

15

shows CV curves obtained in the ACN-based electrolytes containing 10.0 mM KSeCN or 10.0

16

mM POEI-IS in 0.1 M LiClO4, at the scan rate of 50 mV s−1. Both CV curves show an anodic

17

peak and a cathodic peak, corresponding to the oxidation of SeCN− ions and the reduction of

18

(SeCN)3− ions, respectively.49 The redox reaction of SeCN−/(SeCN)3− is given in Equation (1).

19

3SeCN− ⇌ (SeCN)3− + 2e−

(1)

20

The anodic peak of POEI-IS is at 0.30 V vs. Ag/Ag+, which is near that of standard KSeCN (0.28

21

V vs. Ag/Ag+). The cathodic peak of POEI-IS is at −0.20 V vs. Ag/Ag+, which is the same as that

22

of standard KSeCN (−0.20 V vs. Ag/Ag+). Therefore, it is confirmed that the newly synthesized

23

POEI-IS solely possesses a reversible redox couple of SeCN−/(SeCN)3− with a standard potential

24

of 0.05 V vs. Ag/Ag+ (0.54 vs. normal hydrogen electrode (NHE)). Moreover, POEI-IS shows a

25

higher current density than that of KSeCN, indicating that the POEI-IS provides more SeCN−

26

ions than KSeCN at the same concentration.

27

Second, linear sweep voltammetry (LSV) analysis was employed to obtain the apparent

28

diffusion coefficient (Dapp) value of a redox species in the electrolyte. Dummy cells

29

(FTO/Pt/electrolyte/Pt/FTO) containing various electrolytes (0.2 M KSeCN, 0.05 M (SeCN)2,

30

and 0~40 wt% of POEI-IS in ACN) were used for the LSV analysis. With the present of the

31

POEI-IS in the electrolyte, the viscoelastic gel type electrolyte was obtained, as shown in


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Scheme 1a. In Figure S2 (Supporting Information), several symmetric LSV curves for various

2

electrolytes were obtained. In accordance with the plateau of the cathodic current densities of

3

these LSV curves, the values of limiting current densities (Jlim) were evaluated. The Jlim values

4

were further used to calculate the Dapp of (SeCN)3−, using Equation (2)50:

Dapp =

5

d J lim 2nFC

(2)

6

where d is the cell gap of the dummy cell (25 µm), n is the number of the charges transferred

7

(n=2), F is the Faraday constant (96485.4 C mol−1), and C is the concentration of (SeCN)3– (0.05

8

M). The values of Dapp for (SeCN)3– in the electrolytes containing 0, 10, 20, 30, and 40 wt% of

9

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–

10

1

11

content of POEI-IS in the electrolyte; this is apparently due to the increase in the viscosity of the

12

electrolyte. Values of viscosity of various gel electrolytes containing 0~40 wt% of POEI-IS were

13

measured by a rotational viscometer at 25 oC. The viscosity of the electrolyte with 0 wt% of

14

POEI-IS should be the viscosity of the solvent itself, which is acetonitrile (viscosity=0.3 cP).8

15

The viscosities of the electrolytes containing 10 and 20 wt% of POEI-IS could not be measured

16

due to the detection limit of the rotational viscometer (< 30 cP). The viscosities of the

17

electrolytes containing 30 and 40 wt% of POEI-IS were found to be 47.9 and 141.8 cP,

18

respectively. The viscosity of the electrolyte with 40 wt% of POEI-IS has apparently increased

19

with the increase in the content of POEI-IS in the electrolyte (to 40 wt% of POEI-IS). The higher

20

viscosity of the electrolyte with 40% of the POEI-IS is thus consistent with its lowest Dapp (1.76

21

×10-6 cm2 s-1). However, all Dapp are in the same order; this is assumed to be due to the addition

22

of POEI-IS in the electrolyte may not largely influence the mobility of the redox species24, i.e.,

23

SeCN– and (SeCN)3– in this study.

, respectively (Table 2). It is clear that the Dapp value decreases steadily with the increase in the

24

Third, electrochemical impendence spectroscopy (EIS) analysis was used to evaluate the ionic

25

conductivity (σ) of an electrolyte using the same dummy cells mentioned above. For measuring

26

the σ of an electrolyte, a value of serial resistance (Rs) was obtained by extrapolating the onset of

27

its EIS spectra (not shown). The σ of an electrolyte was calculated from its Rs, according to

28

Equation (3)51:

d

σ= 29

A Rs

(3)


8

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1

where the cell constant d/A was calibrated by using a standard NaCl electrolyte (σ = 12.9 mS

2

cm–1). The values of σ of the electrolytes containing 0, 10, 20, 30, and 40 wt% POEI-IS were

3

found to be 12.12, 11.06, 7.94, 7.01, and 4.52 mS cm–1, respectively (Table 2). It is again clear

4

that the σ value decreases steadily with the increase in the content of POEI-IS in the electrolyte;

5

this should again be due to the increase in the content of the non-conductive POEI-I segment in

6

the electrolyte. However, all σ are in the same order; this infers that all the POEI-IS based

7

electrolytes may result in the same order of current densities to their DSSCs.

8 9

2.6. Interfacial properties of POEI-IS based DSSCs

10

An EIS analysis using the DSSCs (FTO/photoanode/electrolyte/Pt/FTO) containing various

11

electrolytes (0.2 M KSeCN, 0.05 M (SeCN)2, 0~40 wt% of POEI-IS in ACN) was employed to

12

investigate the interfacial properties of the DSSCs. Under 100 mW cm−2 (AM 1.5G) light

13

illumination, the Nyquist plots of the various DSSCs generally show three semicircles in the

14

frequency range of 10 mHz to 65 kHz, as shown in Figure 2a. In accordance with the equivalent

15

circuit model shown in the inset of Figure 2a,52 the ohmic series resistance (Rs) is determined in

16

the high frequency region where the phase is zero. The first and second semicircles in the middle

17

frequency range represent the heterogeneous electron transfer resistances at the Pt/electrolyte

18

interface (Rct1) and photoanode/electrolyte interface (Rct2), respectively. The third semicircle in

19

the high frequency range refers to the Warburg diffusion resistance (ZW) in the electrolyte.53 The

20

values of Rct1, Rct2, and ZW for each DSSC are obtained from the diameters of the first, second,

21

and third semicircle, respectively, as summarized in Table 2. The DSSCs containing 10~40 wt%

22

of POEI-IS in the electrolytes (i.e., the gel type electrolyte) all possess similar Rct1 values about 9

23

Ω to the cell with 0 wt% of POEI-IS in the electrolyte (i.e., the liquid type electrolyte); this

24

indicates that the amount of charge transferring through the Pt/electrolyte interfaces in all the

25

DSSCs are comparable. From which, it is attributed to the uniform and non-aggregated surface

26

contacts between the POEI-IS based electrolytes and their Pt counter electrodes.

27

On the other hand, a lower Rct2 value implies a higher amount of charge transferring through

28

the photoanode/electrolyte interface. The DSSCs with gel type electrolytes containing 10~40

29

wt% of POEI-IS all possess larger Rct2 values (> 11.5 Ω) than that of the cell with liquid type

30

electrolyte (0 wt%, 11.1 Ω). This is due to the addition of non-conductive POEI-I segment in the

31

electrolytes which causes the decreases in the ionic conductivities and viscosities of the


9


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Page 10 of 38

1

electrolytes, and thereby an obvious increase in their Rct2 values. Among these gel type DSSCs

2

(10~40 wt%), the increase in the wt% of POEI-IS in the electrolyte causes the Rct2 value being

3

increased first and then decreased; this result can be influenced by two key factors: (1) the

4

increased concentration of the SeCN− and (2) the increased amount of the non-conductive POEI-

5

I segment in the electrolytes. When the wt% of POEI-IS in the electrolyte is increased from 10 to

6

30 wt%, the increased concentration of the SeCN− is expected to multiply the dye regeneration

7

and thus increasing the amount of charge transferring through the photoanode/electrolyte

8

interfaces; then a decrease in the Rct2 values (15.0 Ω to 11.5 Ω) can be observed. At this stage,

9

the amount of non-conductive POEI-I segment also increases with the increase in the wt% of

10

POEI-IS in the electrolyte; however, it does not significantly increase the Rct2 values; therefore, it

11

may infer that the addition of 10 to 30 wt% of POEI-IS in the electrolyte is suitable for the

12

application in DSSCs. When the wt% of POEI-IS in the electrolyte is further increased from 30

13

to 40 wt%, despite the increased concentration of the SeCN−, the overdose non-conductive

14

POEI-I segment in the electrolyte causes a severe decrease in the ionic conductivity (7.01 Ω to

15

4.52 Ω) and thus an increase in the Rct2 value (11.5 Ω to 13.4 Ω). In brief, an order on the Rct2

16

values corresponds to 10 > 20 > 40 > 30 > 0 wt% of POEI-IS; this result suggests an opposite

17

order with respect to that of the short-circuit current densities (JSC) of the pertinent DSSCs, since

18

a lower Rct2 value generally would result in a higher JSC for the DSSC.54 Moreover, it is clear

19

from Table 2 that the values of ZW increase steadily (from 4.4 to 9.8 Ω) with the increase in the

20

wt% of POEI-IS in the electrolyte; this is attributed to the increase in the viscosity of the

21

electrolyte and thus the decrease in the apparent diffusion coefficient (Dapp) values of the redox

22

species in the electrolyte. Therefore, the tendency of the ZW values of the DSSCs is consistence

23

with that of the Dapp values of the redox species.

24

Figure 2b shows Bode phase plots of the cells using electrolytes containing different amounts

25

of POEI-IS, obtained at 100 mW cm-2. The electron lifetime (τe) in a photoanode of a DSSC can

26

be calculated using the frequency value at the maxima phase (fmax), as shown in Equation (4).54

27

τ e = (2πf max ) −1

28

In an illuminated DSSC, there is energy loss owing to unfavorable recombination reactions,

29

i.e., the reactions between the photo-injected electrons of its semiconductor and the oxidized

30

redox species (i.e., (SeCN)3− in this study).55 A DSSC with longer electron lifetime (larger τe)

31

indicates fewer recombination reactions at its photoanode/electrolyte interface. In Table 2, the τe

(4)


10

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1

values of the TiO2 associated with the electrolytes containing different concentrations of POEI-

2

IS steadily increase from 2.39 to 6.62 ms, with the increase in the content of POEI-IS. This

3

indicates that a higher content of POEI-IS in the electrolyte enables a better retardation for the

4

recombination reactions. This better retardation happens owing to the following two key

5

functions of the synthesized POEI-IS. (1) The POEI-IS could chelate K+ potassium cations

6

through the lone pair electrons of oxygen atoms of its poly(oxyethylene)-segment and thus

7

prevent K+ from approaching the TiO2 surface.37-40 (2) The imidazole ending group could fill the

8

vacancies on the TiO2 surface and thus forbid the reactions between these TiO2 vacancies and the

9

electron-deficient (SeCN)3− (Scheme 1b).41-44 On the contrary, the cell with liquid type

10

electrolyte (without employing POEI-IS) possesses the shortest τe owing to the severe charge

11

recombination of the photo-injected electrons with K+ and (SeCN)3− ions (Scheme 1c). The

12

tendency of the τe values (40 > 30 > 20 > 10 > 0 wt% of POEI-IS) agrees with that of the VOC

13

values.

14

To verify the above-mentioned two key functions of POEI-IS, X-ray photoelectron

15

spectroscopy (XPS) was performed to evaluate the chemical bond between the K+ and the

16

poly(oxyethylene)-segment (function (1)) and the chemical bond between the imidazole and the

17

TiO2 (function (2)). For function (1), the best electrolyte, containing 0.2 M KSeCN, 0.05 M

18

(SeCN)2, and 30 wt% of POEI-IS in dehydrated ACN, was used to obtain XPS of potassium 2p

19

orbital, shown in Figure S3 (Supporting Information). Figure S3 shows two peaks at 290.8 and

20

293.9 eV; the former refers to the K-Se bond of KSeCN, the latter refers to the K-O bond

21

between the K+ and the oxygen of the poly(oxyethylene) segment of POEI-IS. From this, it is

22

verified that the poly(oxyethylene) segment of POEI-IS can chelate with the K+ in the

23

electrolyte. The function (1) of POEI-IS is thus established. There exists reports that

24

poly(oxyethylene)-segments chelate with cations (e.g., Li+, Na+, and K+) via the lone pair

25

electrons of their oxygen atoms,37-40 For function (2), the TiO2 thin film was soaked in the best

26

POEI-IS based electrolyte (0.2 M KSeCN, 0.05 M (SeCN)2, and 30 wt% of POEI-IS in ACN)

27

and dried at 60 oC; XPS of this TiO2 was obtained for titanium 2p orbital, and is shown in Figure

28

S4 (Supporting Information). Figure S4 depicts two peaks at 458.0 and 463.8 eV; the former

29

refers to Ti-O bond of the TiO2 and the latter to Ti-N bond between the TiO2 and the nitrogen of

30

the imidazole ending group of POEI-IS. Thus it is shown that the imidazole ending groups of

31

POEI-IS can attach to the vacancies on the TiO2 surface. Nitrogen-containing heterocyclic


11


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Page 12 of 38

1

cations (e.g., imidazolium and pyridine) in the electrolyte of a DSSC were reported to adsorb on

2

the vacancies of its TiO2 surface (i.e., dye-free TiO2 surface), and thereby retard the charge

3

recombination at the photoanode/electrolyte interface of the DSSC.41-44

4

Under dark conditions, the Nyquist plots measured at -0.85 V generally give two semicircles,

5

as shown in Figure 3. The charge recombination resistance (Rrec) values at the

6

photoanode/electrolyte interfaces of the DSSCs containing various POEI-IS based electrolytes

7

were estimated by measuring the diameter of the second semicircle in the figure. The Rrec values

8

listed in Table 2 show a decreasing tendency of 40 > 30 > 20 > 10 > 0 wt% of POEI-IS. A larger

9

Rrec value represents a lesser recombination reaction occurring at the photoanode/electrolyte

10

interface, and thereby a larger VOC of the cell can be expected, and vice versa. It is noted that the

11

results obtained from the τe values (measured at 100 mW cm-2) and the Rrec values (measured in

12

dark), irrespective of their measurement techniques, are highly consistent. It is mentioned above

13

that both the values of τe and Rrec increase gradually with the increase in the content of POEI-IS

14

in the electrolyte from 0 to 40 wt%. We also obtained these values for a DSSC containing 50

15

wt% of POEI-IS in its electrolyte, using Bode phase plot obtained at 100 mW cm-2 and Nyquist

16

plot obtained in dark (Figure S6a and Figure S6b (Supporting Information), respectively). In

17

this case, the electrolyte became an extremely bulky and viscoelastic gel, with a viscosity value

18

of 491.4 cP; such electrolyte could hardly permeate into the semiconductor of a photoanode. A

19

poor cell efficiency is expected in such a case.8 As expected, with reference to those of the cells

20

with 0-40 wt% of POEI-IS, the DSSC with 50 wt% of POEI-IS in its electrolyte showed the

21

worst τe of 2.01 ms (Figure S6a) and the irrational high Rrec of 5,530 Ω (Figure S6b), because

22

the ineffective electrolyte with 50 wt% of POEI-IS damages the forward charge transfer

23

mechanisms in its DSSC.

24 25

2.7. Photovoltaic performance of POEI-IS based DSSCs

26

2.7.1. Cell performance with the liquid type selenocyanate and iodide electrolytes

27

Figure 4a shows the photocurrent density-voltage (J-V) curves of the DSSCs with various POEI-

28

IS-based electrolytes (i.e., 0.2 M KSeCN, 0.05 M (SeCN)2, and 0~40 wt% of POEI-IS in ACN),

29

obtained at the illumination of 100 mW cm-2 (AM 1.5G). The corresponding photovoltaic

30

parameters, including the open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF),

31

and cell efficiency (η) are listed in Table 3. Here, the POEI-IS based electrolyte containing 0


12

Page 13 of 38

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1

wt% of POEI-IS is commonly considered as the normal liquid type selenocyanate electrolyte;

2

thus, the J-V curves of the DSSCs with this liquid selenocyanate electrolyte (i.e., 0.2 M KSeCN

3

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

4

ACN) were also measured for comparison, as shown in Figure S5 (Supporting Information). In

5

accordance with the pertinent photovoltaic parameters listed in Table S1 (Supporting

6

Information), the DSSC with liquid selenocyanate electrolyte shows a higher VOC (712 mV), a

7

higher FF (0.65), but a lower JSC (14.20 mA cm-2) than those of the cell with the liquid iodide

8

electrolyte (VOC=691 mV, FF=0.62, and JSC=14.71 mA cm-2). The larger VOC is thus due to the

9

fact that the redox couple of SeCN−/(SeCN)3− provides a higher standard redox potential than

10

that of I−/I3− (Scheme 1d)34; the lower JSC is because of the higher diffusion coefficient of

11

SeCN−/(SeCN)3− than that of I−/I3− 36; the larger FF may be attributed to the better redox kinetics

12

of SeCN−/(SeCN)3−, and thereby to the faster electron transfer, compared to these of I−/I3−.33 In

13

our previous report,36 we established that selenocyanate redox couple (SeCN−/(SeCN)3−) possess

14

a better redox kinetics than those of iodide redox couple (I-/I3-). The heterogeneous reaction rate

15

constants (k0) values of I−/I3− and SeCN−/(SeCN)3− were found to be 1.18×10−3 cm s−1 and

16

1.45×10−3 cm s−1, respectively; therefore, the electron transfer in the case of SeCN−/(SeCN)3−

17

should be faster than that in the case of I−/I3−, i.e., the charge transfer resistance at the counter

18

electrode/electrolyte interface in the case of SeCN−/(SeCN)3− is lower than that in the case of

19

I−/I3−. Lower charge transfer resistance generally leads to a higher FF. Thus, we believe that the

20

larger FF in the case of SeCN−/(SeCN)3− couple, compared to that in the case of I−/I3− couple,

21

can be attributed to the better redox kinetics of SeCN−/(SeCN)3−.

22

In brief, the liquid selenocyanate electrolyte rendered higher VOC, FF, and thus higher η to its

23

DSSC, compared to the liquid iodide electrolyte; this result indicates that the SeCN−/(SeCN)3−

24

pair is a promising replacement of the traditional iodide redox couple.

25 26

2.7.2. Variation in VOC for the DSSCs with the gel type POEI-IS based electrolytes

27

By increasing the wt% (0 to 40 wt%) of POEI-IS in the liquid selenocyanate electrolyte, the VOC

28

values of the pertinent DSSCs increase steadily from 712 to 841 mV (Table 3). This result is

29

associated with the fact that an excellent retardation toward the unfavorable recombination

30

reactions is provided by the POEI-I segment of POEI-IS; this can be explained as follows

31

(Scheme 1b): (1) The lone pair electrons on the oxygen atoms of the POEI-I segment could


13


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Page 14 of 38

1

chelate with the K+ cations,37-40 and thereby suppress the recombination of photo-injected

2

electrons from the conduction band of the TiO2 with the K+ cations in the electrolyte. It is known

3

that when the unfavorable recombination reactions are successfully suppressed, the Fermi level

4

(EF) of TiO2 shifts to negative potentials56 and thus the VOC of the cell increases due to the

5

increase in the difference between the Fermi level of TiO2 and the redox potential of the

6

electrolyte.57 (2) The imidazole ending group on the POEI-I segment could fill the vacancies on

7

the TiO2 surface, and thus obstructs the recombination of photo-injected electrons from the

8

conduction band of the TiO2 with the electron-deficient (SeCN)3− ions in the electrolyte (Scheme

9

1b).41-44 On the contrary, when the unfavorable recombination reactions occur in the DSSC with

10

the bare liquid selenocyanate electrolyte containing 0 wt% of POEI-IS, the lower Fermi level of

11

its TiO2 causes the smaller VOC of the cell (Scheme 1c), compared to those of the cells

12

containing POEI-IS. In brief, if the wt% of POEI-IS increases in the electrolyte, the

13

recombination reactions decrease and, thereby, the VOC’s of the pertinent cells increase. When

14

the wt% of POEI-IS in the electrolyte was further increased to 50 wt%, the pertinent DSSC

15

exhibited a worst cell efficiency of 0.99%, with a VOC of 845 mV, JSC of 2.31 mA cm-2, and an

16

FF of 0.51 (Figure S7, in the Supporting Information)). As mentioned above, the electrolyte in

17

this case was extremely bulky and viscous; its penetration into the semiconductor should have

18

been worst. The worst τe of 2.01 ms and the worst Rrec of 5,530 Ω of the cell with this electrolyte

19

(50 wt% of POEI-IS) are consistent with its worst power conversion efficiency (0.99%).

20

Therefore, this electrolyte with 50 wt% of POEI-IS is excluded for further investigation.

21

The above-mentioned results obtained from the VOC values are once again verified by the dark

22

current density–voltage curves of the DSSCs using various POEI-IS based electrolytes shown in

23

Figure 4b. From which, the values of the onset bias (Vonset) can be obtained from the intersection

24

points of two lines: one is the tangent to the current density curve (starting from the voltage

25

axis), and the other is the zero-current line. The higher Vonset indicates the better suppression of

26

charge recombination reactions in a DSSC. In Table 3, the Vonset values show a tendency

27

corresponding to the wt% of POEI-IS being 40 > 30 > 20 > 10 > 0 wt%; this reveals a good

28

consistency with the VOC values of the corresponding DSSCs. It is important to be noted that the

29

results obtained from the VOC values (obtained from illuminated J-V curves), Vonset values

30

(obtained from dark J-V curves), τe values (obtained from illuminated EIS spectra), and Rrec

31

values (obtained from dark EIS spectra), irrespective of their measurement techniques, are highly


14

Page 15 of 38

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1

consistent. Moreover, compared to the cell with traditional liquid iodide electrolyte, our newly

2

synthesized POEI-IS successfully improves the VOC of the pertinent gel type DSSCs via two

3

promising approaches simultaneously: (1) the POEI-I segment (cation) in POEI-IS retards the

4

recombination reactions and thus keeps the Fermi level of TiO2 at the more negative potential

5

(Scheme 1b); (2) selenocyanate anions in POEI-IS provide a more positive standard redox

6

potential (Scheme 1d).

7 8

2.7.3. Variation in JSC for the DSSCs with the gel type POEI-IS based electrolytes

9

In the case of the POEI-IS based DSSCs, the variation of JSC depends on several above-

10

mentioned parameters, e.g., the ionic conductivity (σ) of the electrolyte, the diffusion coefficient

11

(Dapp) of the redox species in the electrolyte, the charge transfer resistance at the

12

photoanode/electrolyte interface (Rct2) of the cell, and the Warburg diffusion resistance (Zw) of

13

the cell, as listed in Table 2. When increasing the amount of POEI-IS from 0 to 10 wt% in the

14

electrolyte, the JSC of the QSS-DSSC decreases drastically (from 14.20 mA cm-2 to 10.70 mA

15

cm-2); this is apparently due to that the insertion of the non-conductive POEI-I segment causes a

16

drastic increase in the viscosity of the electrolyte, resulting in a decrease of σ (from 12.12 mS

17

cm–1 to 11.06 mS cm–1), a decrease of Dapp (from 5.58×10–6 cm2 s–1 to 4.23×10–6 cm2 s–1), and

18

thus the increases in the values of Rct2 (from 11.1 Ω to 15.0 Ω) and Zw (from 4.4 Ω to 6.2 Ω).

19

With further increasing in the amount of POEI-IS from 10 wt% to 30 wt% in the electrolyte, the

20

JSC’s show gradual increases (from 10.70 mA cm–2 to 13.85 mA cm–2) due to the increased

21

concentration of the SeCN−, causing the Rct2 decrease (from 15.0 Ω to 11.5 Ω), despite relative

22

increases in Zw (from 6,2 Ω to 8.9 Ω), decrease in Dapp (from 4.23×10–6 cm2 s–1 to 2.19×10–6 cm2

23

s–1), and decrease in σ (from 11.06 mS cm–1 to 7.01 mS cm–1). With increasing in the POEI-IS

24

from 30 wt% to 40 wt% in the electrolyte, the JSC does not increase further, but rather decrease

25

due to that the overdose non-conductive POEI-I segment in the electrolyte causes the severe

26

decreases in σ and Dapp, and thus the increased in Rct2 and Zw (Table 2); these values justify the

27

decrease of JSC from 13.85 mA cm-2 to 12.82 mA cm-2. The lower JSC value of the DSSC with 40

28

wt% of POEI-IS (12.82 mA cm–2) than that of the DSSC with 30 wt% of POEI-IS (13.85 mA

29

cm–2) is consistent with their values of viscosity (141.8 and 47.9 cP) and Dapp (1.76×10−6 and

30

2.19×10−6 cm2 s–1); that is to say that the increase in the viscosity of the electrolyte with 40 wt%


15


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Page 16 of 38

1

of POEI-IS has caused a decrease in the JSC of the pertinent DSSC. The variation of JSC seems to

2

be dependent on all the mentioned parameters.

3 4

2.7.4. Variation in FF for the DSSCs with the gel type POEI-IS based electrolytes

5

As regards to the FF, it increases first with the addition of POEI-IS up to 30 wt%, and then

6

decreases with the addition of 40 wt% of the POEI-IS. Since the FF indicates inversely

7

proportional to the energy loss in a cell, the increase in FF in the first place is owing to the fact

8

that the increased non-conductive POEI-I segment in the electrolyte provides a successful

9

retardation effect for the recombination reactions, while the decrease in FF later is due to the

10

overdose non-conductive POEI-I segment which causes a severe decrease in the σ and Dapp. The

11

lower FF value of the DSSC with 40 wt% of POEI-IS (0.65) than that of the DSSC with 30 wt%

12

of POEI-IS (0.72) is consistent with their values of viscosity (141.8 cP for 40 wt% and 47.9 cP

13

for 30 wt%); that is to say that the increase in the viscosity of the electrolyte with 40 wt% of

14

POEI-IS has caused a decrease in the FF of the pertinent DSSC. Ultimately, the decreases both

15

in the JSC and FF values of the DSSC with 40 wt% of POEI-IS have caused its lower

16

performance than that of the cell with 30 wt% of POEI-IS. Thus, the best DSSC reaches an η of

17

8.18%, VOC of 826 mV, JSC of 13.85 mA cm-2, and FF of 0.72 via employing an electrolyte

18

containing 30 wt% POEI-IS; as compared to the cell with an electrolyte containing 0 wt% POEI-

19

IS (6.58%), a much higher η is obtained.

20 21

2.7.5. Long-term stability of POEI-IS based DSSCs

22

Figure 5 shows the long-term durability data of the DSSCs employing the electrolytes

23

containing 0 wt% and 30 wt% of the POEI-IS. In this experiment, these two cells were first

24

sealed by a 25 µm-thick Surlyn® film and then by an epoxy glue. The cell efficiency was

25

measured once per day for the first 14 days and then once per two days for the following days. It

26

is obvious from Figure 5 that the QSS-DSSC with 30 wt% of POEI-IS shows much higher

27

stability, compared to the cell with 0 wt% of POEI-IS. The η of the QSS-DSSC with 30 wt% of

28

POEI-IS remains to be about 95% of its initial value, after more than 1,000 h. On the contrary,

29

the η of the DSSC without the POEI-IS remains about only 65% of its initial value after the same

30

period. It is concluded that our newly synthesized POEI-IS renders its DSSC not only a high VOC


16

Page 17 of 38

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1

but also an excellent long-term durability, indicating POEI-IS is a promising substitution of the

2

traditional liquid iodide electrolyte.

3 4

3.

Conclusions

5

A thermally stable novel polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium

6

selenocyanate (POEI-IS), was newly synthesized as a functional gel electrolyte for quasi-solid-

7

state dye-sensitized solar cells (QSS-DSSCs). The POEI-IS

8

poly(oxyethylene)-imide imidazolium (POEI-I) polymeric cation and selenocyanate anions;

9

these groups are characterized by FT-IR to reveal a successful synthesis of POEI-IS. Several

10

functions of POEI-IS are verified via different techniques as follows. (1) The POEI-I segment

11

not only works as a gelling agent but also provides a good retardation effect for the

12

recombination reactions; this function is verified by the values of VOC (illuminated J-V curves),

13

Vonset (dark J-V curves), τe (illuminated EIS spectra), and Rrec (dark EIS spectra). (2) The

14

selenocyanate anion not only benefits the interfacial contacts between the POEI-IS based

15

electrolytes and the electrodes without the self-aggregation of the polymer (FE-SEM images),

16

but also possesses a reversible redox couple of SeCN−/(SeCN)3− with a more positive standard

17

potential than that of iodide species (CV curves). (3) The POEI-IS shows only a 5% weight loss

18

at 340 oC, revealing its high thermal stability (TGA curves), and thereby renders the best POEI-

19

IS based DSSC an excellent long-term durability, i.e., the cell efficiency maintains 95% of its

20

initial value after more than 1,000 h (long-term J-V curves). Although the insertion of the non-

21

conductive POEI-I segment in the electrolyte causes the decreases in σ and Dapp (LSV curves),

22

and thus an increase in ZW (illuminated EIS spectra), it is found that the addition of 10~30 wt%

23

of POEI-IS in the electrolyte is tolerable for the application in DSSCs. Finally, the best DSSC

24

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

25

electrolyte containing 30 wt% POEI-IS; as compared to the cell with an electrolyte containing 0

26

wt% POEI-IS (6.58%), a much higher η is obtained. This multifunctional polymeric ionic liquid,

27

POEI-IS, paves a promising way for developing highly efficient QSS-DSSCs.

is composed of the

28 29

4.

30

4.1. Materials

Experimental Section


17


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Page 18 of 38

1

Poly [(oxypropylene)a-(oxyethylene)b-(oxypropylene)c] segmented bis(2-aminopropyl ether)

2

(POE2000, Mw=2,000 g mol-1, waxy solid, a+c=6, b=39) was purchased from Jeffamine®-

3

Huntsman. 4,4’-Oxydiphthalic anhydride (ODPA, 97%), titanium (IV) tetraisoproproxide (TTIP,

4

>98%), acetonitrile (ACN, 99.99%), ethanol (EtOH, 99.5%), isopropyl alcohol (IPA, 99.5%),

5

poly(ethylene glycol) (PEG, Mw=20,000 g mol-1), epichlorohydrin (99%), tert-butanol (tBA,

6

99.8%), 1-butylimidazole (98%), potassium selenocyanate (KSeCN, 99%), dichloromethane

7

(DCM, 99.99%), 2-methoxyethanol (99.95%), bromine (Br2, >99.99%), potassium iodide (KI,

8

≥99.99%), iodine (I2, ≥99.8%), and chenodeoxycholic acid (CDCA, ≥97%) were obtained from

9

Sigma Aldrich. Tetrahydrofuran (THF, 95%) and ethyl alcohol (99.5%) were procured from

10

Teida. Surlyn® (SX1170–25, 25 µm) film was supplied by Solaronix (S.A., Aubonne,

11

Switzerland). Fluorine–doped SnO2 (FTO, TEC–7, 7 Ω sq.-1) conducting glasses were imported

12

from NSG America, Inc., New Jersey, USA. The commercial light scattering TiO2 particles (ST–

13

41, average particle size = 200 nm) were acquired from Ishihara Sangyo, Ltd. The organic dye,

14

2-cyano-3-(5-(6-(4-(diphenylamino)phenyl)-3,7-dipentadecylthieno[2',3':4,5]thieno[3,2-

15

b]thieno[2,3-d]thiophen-2-yl)thiophen-2-yl)acrylic acid (TA, shown in Figure S8 in the

16

Supporting Information),58 was synthesized and provided by Prof. Ming-Chou Chen’s group in

17

National Central University, Taiwan.

18 19

4.2. Synthesis of poly(oxyethylene)-imide imidazolium selenocyanate (POEI-IS)

20

The novel polymeric ionic liquid, POEI-IS, was synthesized via a simple and three-step

21

process, as shown in Scheme 2. (1) A solution of ODPA (1.264 g, 2.5 mmol) in THF was drop-

22

wise added into POE2000 (16.30 g, 5 mmol) for 1 h, under continuous vigorous stirring in a

23

nitrogen atmosphere; then the temperature of this polymer mixture was raised to 150 oC, and

24

maintained at that level for 3 h to obtain a product A (poly(oxyethylene)-segmented imide,

25

POEI). (2) Epichlorohydrin (0.407 g, 4 mmol) was added drop-wise into the above-mentioned

26

product A (solution) at 60 oC for 24 h, and then a solution of 1-butylimidazole (0.497 g, 4 mmol)

27

in ACN was added into the mixture at 90 oC for 24 h to get the product B (poly(oxyethylene)-

28

imide imidazolium chloride, POEI-IC, Mw ≈ 7000 g mol-1).24 (3) Product B was then added into

29

an ACN-based solution containing double-normal KSeCN, and thereby the anion-exchange

30

reaction was triggered to produce the final product, (poly(oxyethylene)-imide imidazolium


18

Page 19 of 38

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1

selenocyanate (POEI-IS). POEI-IS was further separated from the by-product (KCl) by filtration,

2

washed by DCM for several times, and dried in a vacuum oven for 12 h.

3 4

4.3. Preparation of POEI-IS gel electrolyte

5

The liquid electrolyte containing 0.2 M KSeCN and 0.05 M (SeCN)2 in ACN was prepared by

6

the following procedure. A solution of KSeCN (3 mmol) in 5 mL of ACN was prepared under

7

stirring in the dark. In the meantime, a bromine solution was prepared by dissolving Br2 (0.5

8

mmol) in 5 mL of ACN. After complete dissolution of the solutes in both the solutions, the

9

second solution was added to the first one dropwise, under the same conditions. The resulting

10

suspension was filtered to remove precipitated KBr; an yellow solution was obtained as the

11

filtrate. An additional filtration step was necessary in order to have a clear solution. Finally, gel

12

electrolytes containing different wt% of POEI-IS were prepared by adding 10 wt%, 20 wt%, 30

13

wt%, and 40 wt% of POEI-IS into the liquid electrolytes individually. For the comparison, a

14

liquid iodide electrolyte containing 0.2 M KI and 0.05 M I2 in ACN was prepared.

15 16

4.4. Cell assembly

17

Fluorine–doped tin oxide conducting glasses (FTO, TEC-7, 7 Ω sq.-1, NSG America, Inc., New

18

Jersey, USA) were first cleaned with a neutral cleaner and then washed with de-ionized water,

19

acetone, and isopropanol sequentially. A TiO2 colloid was prepared as follows. 0.5 M aqueous

20

TTIP was added to 0.1 M nitric acid under stirring at 88 oC for 8 h; the obtained solution was

21

then heated to 240 oC for 12 h in an autoclave (PARR 4540, USA). The autoclaved solution was

22

concentrated to contain 8 wt% crystalline TiO2 nanoparticles (NPs, ca. 20 nm) in the TiO2 slurry.

23

A TiO2 paste for the light transparent layer (TL paste) was prepared by adding 25 wt% PEG

24

(with respect to TiO2 NPs) to the TiO2 slurry obtained, while another TiO2 paste for the light

25

scattering layer (SL paste) was prepared by adding 25 wt% PEG and 100 wt% ST–41 (with

26

respect to TiO2 NPs) to the obtained TiO2 slurry. The PEG was used for preventing the TiO2

27

paste from being cracked during its casting on a FTO glass and also to control the pore size of

28

the pertinent TiO2 film.

29

A cleaned FTO glass was coated with a thin compact layer of TiO2 (100 nm), by using a

30

solution of TTIP in 2-methoxyethanol (weight ratio of 1:3). A TiO2 film, containing a first layer

31

of TL (4 µm) and a second layer of SL (4 µm) with a geometric area of 0.20 cm2 was coated on


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

1

the treated FTO glass by a doctor blade technique. Each layer was separately sintered at 500 oC

2

for 30 min in an air atmosphere.59 After the sintering process, the TiO2 film was immersed for 10

3

h in a 3.0×10−4 M TA dye and 0.01 M CDCA solution (in a mixed solvent of ACN/tBA/DCM

4

with a volume ratio of 1:1:8) at room temperature. Finally, the TA-adsorbed TiO2 photoanode

5

was coupled with a sputtered platinum counter electrode (CE) with a cell gap of 25 µm by using

6

a 25 µm-thick Surlyn® film as the spacer. The electrolytes in this study were composed of 0.2 M

7

potassium selenocyanate (KSeCN), 0.05 M selenocyanogen ((SeCN)2), and different amounts of

8

POEI-IS (0 to 40 wt%) in ACN. These electrolytes were injected into the cell gap by capillarity.

9 10

4.5. Characterization of POEI-IS and the DSSCs

11

A Fourier transform infrared spectrometer (FT-IR, Perkin Elmer Spectrum 100 FT-IR) was

12

used to determine the functional groups of the synthesized polymers (i.e., POEI, POEI-IC, POEI-

13

IS) over the range of 800-4000 cm-1, using a Calcium Fluoride (CaF2) plate as the blank. A

14

thermogravimetric analyzer (TGA, Perkin-Elmer) was used to obtain the thermal stabilities of

15

the synthesized polymers, while a field-emission scanning electron microscope (FE-SEM, Nova

16

NanoSEM 230, FEI, Oregon, USA) was used to observe the surface morphologies of the

17

synthesized polymers. Cyclic voltammetry (CV) was employed to analyze the redox kinetics of

18

the newly synthesized POEI-IS; the CV data were recorded by a potentiostat/galvanostat

19

(PGSTAT 30, Autolab, Eco-Chemie, Utrecht, the Netherlands) using a three-electrode

20

electrochemical system. A platinum foil with a specific area of 1 cm2 was used as the working

21

electrode, while another platinum foil and an Ag/Ag+ electrode were used as the counter and

22

reference electrodes, respectively. One of the two ACN-based electrolytes was used for the CV,

23

namely 10 mM KSeCN or 10 mM POEI-IS in 0.1 M LiClO4.

24

Linear sweep voltammetry (LSV) was used to obtain apparent diffusion coefficients (Dapp) of

25

the redox species in the electrolyte; the data were recorded by the same potentiostat/galvanostat.

26

A dummy cell, with the configuration of FTO/Pt/electrolyte/Pt/FTO was scanned from -1.0 to

27

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,

28

0~40 wt% POEI-IS in ACN) were used in the dummy cells. The viscosities (µ) of various

29

electrolytes were measured at 25 oC by a rotational viscometer (Smart series, V200003,

30

Fungilab), equipped with a smart L special spindle (TL5). Electrochemical impedance

31

spectroscopy (EIS) was used to obtain the ionic conductivity (σ) of an electrolyte and the


20

Page 21 of 38

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1

interfacial

resistances

of

a

DSSC;

the

EIS

data

were

recorded

by

the

same

2

potentiostat/galvanostat, however equipped with a FRA2 module, between 10 mHz to 65kHz

3

with an AC amplitude of ±10 mV. While measuring σ of an electrolyte, a dummy cell structure

4

(FTO/Pt/electrolyte/Pt/FTO) was used; the cell constant (d/A) was calibrated using a standard

5

sodium chloride (NaCl) solution (σ=12.9 mS cm-1, Model 011006, Thermo Orion). Surface

6

chemical analyses were performed by X-ray photoelectron spectroscopy (XPS, Thermo

7

Scientific Theta Probe, UK). The interfacial resistances of the DSSCs were measured under 100

8

mW cm-2 (at their open-circuit voltages) and in dark (at an applied bias of -0.85 V). Photovoltaic

9

parameters of the DSSCs were measured by the same potentiostat/galvanostat under 100 mW

10

cm-2, which was generated by a class A quality solar simulator (XES–301S, AM1.5G, San–Ei

11

Electric Co. Ltd., Osaka, Japan). The incident light intensity (100 mW cm–2) was calibrated with

12

a standard Si cell (PECSI01, Peccell Technologies, Inc.).

13


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

1 2

Scheme 1. (a) The pale yellow viscoelastic gel of POEI-IS is composed of the

3

poly(oxyethylene)-imide imidazolium (POEI-I) polymeric cation and selenocyanate anion. (b)

4

The DSSC employing with POEI-IS: the POEI-I segment chelates the potassium cations through

5

its lone pair electrons, obstructs the recombination of injected electrons with the (SeCN)3− in the

6

electrolyte, and thereby enhances VOC of the DSSC. (c) The DSSC without employing POEI-IS:

7

there are severe charge recombination reactions on the TiO2 surface. (d) The POEI-IS possesses

8

a redox mediator of SeCN−/(SeCN)3− giving a more positive redox potential than that of

9

traditional I−/I3−, and thereby increases the VOC and facilitates the dye regeneration of the DSSC.

10


22

Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Scheme 2. Synthetic pathway for obtaining poly(oxyethylene)-imide-imidazolium selenocyanate

3

(POEI-IS).

4 5 6 7 8 9 10 11


23


(b)

100

POEI POEI-IC POEI-IS

80

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

60 40 20 0

200

300

400

500

600

700

800

o

Temperature ( C)

1

2 3

Figure 1. (a) Fourier transform infrared spectra of POEI, POEI-IC, and POEI-IS. (b)

4

Thermogravimetric analysis curves of POEI, POEI-IC, and POEI-IS. Field-emission scanning

5

electron microscopy images of (c) POEI, (d) POEI-IC, and (e) POEI-IS.

6


24

Page 25 of 38

(a)

-Z'' (ohm)

1

Under 1 Sun wt% of POEI-IS in the electrolyte 0 wt% 10 wt% 20 wt% 30 wt% 40 wt%

15

10

Equivalent circuit model Rct2 Zw Rct1 ct ct RS CPE1

CPE2

5

0

15

20

25

30

35

40

45

50

55

60

Z' (ohm) 2

(b) 15

Phase (-Theta)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

Under 1 Sun wt% of POEI-IS in the electrolyte 0 wt% 10 wt% 20 wt% 30 wt% 40 wt%

5

0

-1

10 3

0

10

1

10

2

10

3

10

4

10

5

10

Frequency (Hz)

4

Figure 2. EIS spectra of the DSSCs with various electrolytes: (a) Nyquist plots, and (b) Bode

5

plots measured at 100 mWcm-2 (AM 1.5G).


25


25 Under dark wt% of POEI-IS in the electrolyte 0 wt% 10 wt% 20 wt% 30 wt% 40 wt%

20

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

15

10

5

0

0

10

20

30

40

50

60

70

80

90

100

Z' (ohm)

1 2

Figure 3. EIS spectra of the DSSCs with various electrolytes, Nyquist plots measured in dark at -

3

0.85 V bias.

4 5


26

Page 27 of 38

-2

Photocurrent density (mA cm )

(a) 15

10 Under 1 Sun wt% of POEI-IS in the electrolyte

0 wt% 10 wt% 20 wt% 30 wt% 40 wt%

5

0

0

200

400

600

800

Voltage (mV)

1

(b) -2

Dark current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

0 -1 -2

Under dark wt% of POEI-IS in the electrolyte 0 wt% 10 wt% 20 wt% 30 wt% 40 wt%

-3 -4 -5

0

200

400

600

800

Voltage (mV)

3

Figure 4. Current density–voltage curves of the DSSCs with various electrolytes, measured (a)

4

at 100 mWcm-2 (AM 1.5G) and (b) in dark.


27


1

Normalized efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 38

1.0 0.8 0.6 wt% of POEI-IS in the electrolyte 0 wt% 30 wt%

0.4 0.2 0.0

0

200

400

600

800

1000

Time (h)

2 3

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

4

30 wt% of POEI-IS.

5 6


28

Page 29 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Table 1. A partial literature review of the DSSC performance with polymeric ionic liquid (PIL)-

2

based electrolytes. PILs as electrolyte

η

VOC

JSC

Durability

Ref.

(%)

(mV)

(mA cm–2)

P-HI(a)

6.95

643

15.10

0.72

N. A.

20

PEAII(b)

5.29

838

9.75

0.65

85% (after 1,000 h)

21

PEBII-POEM(c)

7.00

690

17.80

0.57

N. A.

22

Poly[BVIm][HIm][TFSI](d)

5.92

676

12.92

0.68

96% (after 1,200 h)

23

POEI-II/MWCNTs(e)

7.65

784

14.50

0.67

100% (after 1,000 h)

24

POEI-IS

8.18

825

13.85

0.71

95% (after 1,000 h)

FF

3

(a)

(% to the initial η)

P[((3-(4-vinylpyridine) propanesulfonic acid) iodide)-co-(acrylonitrile)]; iodide);

(c)

4

(acryloyloxy)hexylimidazolium

5

imidazolium iodide)-poly(oxyethylene methacrylate);

6

hexyl)-imidazolium

7

imidazolium iodide/multi-wall carbon nanotubes

(b)

This work

poly(1-ethyl-3-

poly(1-((4-ethenylphenyl)methyl)-3-butyl(d)

poly(1-butyl-3-(1-vinylimidazolium-3-

bis(triﬂuoromethanesulfonyl)imide);

(e)

poly(oxy-ethylene)-imide

8


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

1

Table 2. Electrochemical parameters of the POEI-IS based electrolytes and interfacial properties

2

of the corresponding DSSCs. Electrochemical properties wt% of POEI-IS

Interfacial properties

Dapp

σ

Rct1

Rct2

ZW

τe

Rrec

(10−6 cm2 s–1)

(mS cm–1)

(Ω)

(Ω)

(Ω)

(ms)

(Ω)

0

5.58

12.12

9.9

11.1

4.4

2.39

34.7

10

4.23

11.06

9.3

15.0

6.2

5.43

40.8

20

3.33

7.94

9.4

13.6

8.5

5.79

44.3

30

2.19

7.01

9.4

11.5

8.9

5.87

50.3

40

1.76

4.52

9.8

13.4

9.8

6.62

61.1

in the electrolyte

3 4

Table 3. Photovoltaic parameters of the DSSCs with various electrolytes. The standard deviation

5

data for each DSSC are obtained based on three cells. wt% of POEI-IS η

VOC

JSC

Vonset FF

in the electrolyte

(%)

(mV)

(mA cm )

0

6.58±0.13

712±9

14.20±0.03

0.65±0.01

616

10

5.90±0.08

785±9

10.70±0.02

0.70±0.01

730

20

7.16±0.05

811±5

12.47±0.01

0.71±0.00

751

30

8.18±0.02

826±4

13.85±0.05

0.72±0.00

753

40

7.04±0.05

841±5

12.82±0.03

0.65±0.00

776

–2

(mV)

6


30

Page 31 of 38

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1

ASSOCIATED CONTENT

2

Supporting Information.

3

“This material is available free of charge via the Internet at http://pubs.acs.org.”

4

cyclic voltammograms of POEI-IS and KSeCN, linear sweep voltammetry curves of different

5

electrolytes, X-ray photoelectron spectroscopy analysis of the POEI-IS electrolyte and the POEI-

6

IS electrolyte embed on TiO2 film, photovoltaic properties of the DSSCs with liquid type SeCN-

7

based electrolyte, interfacial and photovoltaic properties of the DSSCs with 50 wt% POEI-IS in

8

electrolyte, and the molecular structure of the organic dye (TA)

9 10

AUTHOR INFORMATION

11

Corresponding Author

12

* E-mail: [email protected] (Kuo-Chuan Ho)

13

* E-mail: [email protected] (Jiang-Jen Lin)

14

* E-mail: [email protected] (Ming-Chou Chen)

15 16

Author Contributions

17

#These authors contributed equally.

18 19

ACKNOWLEDGMENT

20

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under

21

grant numbers 102-2221-E-002-186-MY3 and 103-2119-M-007-012.

22


31


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1

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Page 38 of 38

Table of Contents

2 3

Poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS) is newly synthesized and used

4

for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC).

5

The QSS-DSSC containing 30 wt% of the POEI-IS exhibits a high cell efficiency (η) of

6

8.18±0.02% with a high open-circuit voltage of 826 mV and retains up to 95% of its initial η

7

after 1,000 h.


38

Multifunctional Iodide-Free Polymeric Ionic Liquid for Quasi-Solid

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