multi-walled carbon nanotube composites as counter electrodes for high performance dye-sensitized solar cells

Accepted Manuscript Electropolymerized polyaniline/graphene nanoplatelet/multiwalled carbon nanotube composites as counter electrodes for high performance dye-sensitized solar cells

Yen-Chen Shih, Hsiao-Li Lin, King-Fu Lin PII: DOI: Reference:

S1572-6657(17)30249-7 doi: 10.1016/j.jelechem.2017.04.010 JEAC 3222

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

24 April 2016 18 February 2017 5 April 2017

Please cite this article as: Yen-Chen Shih, Hsiao-Li Lin, King-Fu Lin , Electropolymerized polyaniline/graphene nanoplatelet/multi-walled carbon nanotube composites as counter electrodes for high performance dye-sensitized solar cells. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jeac(2017), doi: 10.1016/j.jelechem.2017.04.010

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ACCEPTED MANUSCRIPT Electropolymerized Polyaniline/Graphene nanoplatelet/Multi-walled carbon nanotube Composites as Counter Electrodes for High Performance Dye-Sensitized Solar Cells Yen-Chen Shiha, Hsiao-Li Linb, King-Fu Lina,b* a

Department of Materials Science and Engineering, National Taiwan University, Taipei

Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617,

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b

PT

10617, Taiwan.

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Taiwan.

*

Corresponding author:

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Prof. King-Fu Lin

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CE

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[email protected]

ACCEPTED MANUSCRIPT ABSTRACT Polyaniline (PANI)/graphene nanoplatelet (GNP)/multi-walled carbon nanotube (MWCNT) composite films deposited on fluorine-doped tin oxide (FTO) substrates were successfully fabricated through electrochemical method without pre-treatment of GNP and MWCNT to

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provide a lower cost counter electrode (CE) for dye-sensitized solar cells (DSSCs). By varying the contents of GNP and MWCNT in the PANI composite CE, the short-circuit

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current density of DSSC was found to linearly relate to the reduction current density of

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I− /I3− redox couples measured by cyclic voltammetry. The open-circuit voltage (VOC) is also highly dependent on the reduction potential of redox couples. Besides, the charge transfer

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resistance at the electrolyte/CE interface measured by electrochemical impedance

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spectroscopy has an approximately linear relationship with the sheet resistance of PANI composite CE. Notably, the DSSC with the CE fabricated by the weight ratio of PANI/GNP/MWCNT at 1:0.003:0.0045 yielded the highest power conversion efficiency of

Keywords:

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7.67±0.05%, which is comparable with the conventional Pt cell (7.62±0.07%). Polyaniline;

graphene

nanoplatelet;

multi-walled

carbon

nanotube;

CE

dye-sensitized solar cells; electrochemical impedance spectroscopy

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1. Introduction

The development of dye-sensitized solar cells (DSSCs) has already reached the commercial stage and is well recognized as the third generation of solar cells in views of their easy processing, fairly high power conversion efficiency (PCE) and low cost for production [1-5]. Conversion of photons to electric current can be achieved through the light absorption by a dye-sensitized mesoporous titanium dioxide (TiO2) film as photoelectrode.

While the electrons generated from the photoexcited dye molecules

ACCEPTED MANUSCRIPT inject into the conduction band of TiO2, the oxidized dye can be recovered by the electrolyte solution inserted between photoelectrode and counter electrode (CE) through the oxidation reaction from I− to I3− ions. The significant role of CE is to transport the electrons from external circuit to the electrolyte and catalyze the reduction reaction of I3− / I− redox couples. Therefore, the selected material of CEs for

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high performance of DSSCs should possess low sheet resistance and high catalytic

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power for the reduction reactions of the redox couples.

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To date, Pt deposited fluorine-doped tin oxide (FTO) glass is the most commonly used CE in DSSCs. Although Pt exhibits the excellent catalytic activity for I− 3

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reduction and high conductivity, high cost and limited reserve obstruct its application

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in practice [6, 7]. Alternative catalysts for CEs such as carbon black [8, 9], carbon nanotube [10, 11], graphene [11-17], polyaromatic hydrocarbon [18], and conductive polymers such as polypyrrole [19, 20], poly(3,4-ethylenedioxythiophene) [21-24] and

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polyaniline (PANI) [25-31] have been attempted over the past decades. Among them, PANI has been demonstrated as a promising material for CE due to its desirable properties such as high conductivity, easy synthesis, environmental stability, and high

CE

catalytic activity for I− 3 reduction [6, 32]. To further improve its performance, PANI was blended with carbon additives such as carbon nanotube [33-36], graphene [37-41],

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and dual [42,43]. A few researchers even showed that such composite with appropriate amount of additives has better performance than Pt [33,37]. However, the fabrication of these composites were usually required the pre-treatment of carbon additives such as surface oxidation to attain better dispersion. In this work, we considered that the aromatic ring of aniline and its positive charge provided by sulfuric acid dopant are capable of adsorbing to the surface of graphene nanoplatelet (GNP) and multi-walled carbon nanotube (MWCNT) through π-π and cation-π

ACCEPTED MANUSCRIPT interactions [44, 45]. We polymerized PANI composites from aniline sulfate through electrochemical deposition (ED) method on FTO glass substrate with the addition of GNP and MWCNT without further oxidation. The performance of DSSCs could be optimized by choosing a proper polymerization condition as well as the contents of GNP and MWCNT. By varying the contents of GNP and MWCNT in the PANI composite CE, we observed the

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short-circuit current density of DSSC was linearly related to the reduction current density

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of I− /I3− redox couples measured by cyclic voltammetry (CV). We also found that the

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open-circuit voltage (VOC) was highly dependent on the reduction potential of redox couples. Besides, the charge transfer resistance at the electrolyte/CE interface measured by

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sheet resistance of PANI composite CE.

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electrochemical impedance spectroscopy (EIS) has a roughly linear relationship with the

2. Experimental

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2.1 Materials

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Aniline sulfate and sulfuric acid were obtained from Hayashi and Scharlau respectively. Hydrogen peroxide (35% solution in water), isopropanol (99%), titanium isopropoxide (TTIP,

CE

96%), acetonitrile, LiI (99%), I2 (99.8+%), 4-tert-butylpyridine (TBP, 99%), guanidinium thiocyanate (GuNCS), 3-methoxypropionitrile (MPN, 98%), tert-butanol and lithium

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perchlorate (LiClO4) were received from Acros. Hydrochloric acid solution (37%) and ammonium

hydroxide

(35%

solution

in

water)

were

obtained

from

Fisher.

3-Propyl-1-methyl-imidazolium iodide (PMII, 99.5%) and poly(ethylene glycol) (PEG, Mw -1

~20,000) were obtained from Merck. FTO (~15 Ω sq ) coated glass substrates and cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)

ruthenium(II) 2

(N3)

were

-1

acquired from Solaronix. GNP (specific surface area 450 m g , purity >99%, average thickness 3 nm, average particle size 0.5-40 μm) and MWCNT (O.D. 10-15 nm, I.D. 2-6 nm,

ACCEPTED MANUSCRIPT length 0.1-10 μm) were purchased from Uni-Region and Arkema respectively. All chemicals and solvents were used without further purification. 2.2 Preparation of PANI composite CEs All PANI composite films were deposited on FTO glass substrates by polymerization

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through ED method in an aqueous solution containing 0.5 M aniline sulfate, 1 M sulfuric acid,

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and various amounts of GNP and MWCNT (see Table 1) in a three-electrode system as described below. At first, FTO glass substrates were sequentially cleaned with an acid

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solution (volume ratio of H2O : H2O2 : HCl(aq) = 6 : 1 : 1), alkaline solution (volume ratio of

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H2O : H2O2 : NH3(aq) = 5 : 1 : 1) and isopropanol each for 20 min. The cleaned FTO glass, a Pt wire, and a saturated Ag/AgCl were used as the working electrode, counter electrode and

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reference electrode respectively. Carbon additives were pre-added into the solution through sonication for 2 h and subsequently stirring for 12 h. The CE sample codes and their

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corresponding component ratios were listed in Table 1. ED process was carried out by a

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computer-controlled potentiostat instrument (PGSTAT 302N, Autolab) with a constant potential of 0.8 V (vs. Ag/AgCl) and the electric charge densities set at 20, 30, and 40 mC -2

cm respectively. The as-formed PANI composite films were washed with 1 M sulfuric acid

CE

to remove the unreacted chemicals. The thickness of resulting PANI films was ~100 nm as

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measured by SEM.

2.3 Fabrication of DSSCs The photoelectrodes for DSSCs were fabricated as follows. TiO2 precursor paste was synthesized by sol-gel method according to the literature [4]. The TiO2 paste was then coated on FTO glass by using doctor-blade method and subsequently sintered at 450 °C for 30 min. Then, coating and sintering process were repeated to achieve a desired thickness of ~18 μm as measured by SEM. An active area of 0.25 cm2 selected from the sintered film was then immersed into a N3 dye (3×10-4 M) in a mixed solvent of acetonitrile and tert-butanol for 24

ACCEPTED MANUSCRIPT h. The dye-adsorbed photoelectrode was then rinsed with acetonitrile and dried in air. The liquid electrolyte containing 0.6 M PMII, 0.1 M LiI, 0.05 M I2, 0.1 M GuNCS, and 0.5 M TBP in MPN was injected into a gap between photoelectrode and PANI composite CE separated by a 60 μm spacer. A FTO glass sputtered with ~100 nm thick Pt as CE was also

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prepared for comparison.

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2.4 Measurements

Fourier-transform infrared (FTIR) spectra were recorded on a Thermo Nicolet

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NEXUS470 ATR-FTIR spectrometer and JASCO FT/IR-410. Thermogravimetric analysis -1

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(TGA) was conducted using a TA Instruments Q50 at a heating rate of 10 °C min under N2. A four-point probe with a Keithley 2400 sourcemeter was used to measure the sheet

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resistance of prepared PANI composite CEs. JEOL JSM-6300 SEM was used to investigate the morphology of PANI composites. CV measurements was carried out in the acetonitrile containing

0.01

M

LiI,

0.001M

I2

and

0.1M

LiClO4

by using

a

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solution

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potentiostat/galvanostat (PGSTAT 302N, Autolab) with PANI composite film as working electrode, a saturated Ag/AgCl as reference, and a Pt wire as the auxiliary electrode. The −1

potential range was from −0.5 V to 1.2 V at a scan rate of 50 mV s . The

CE

photocurrent-voltage characterizations of all the DSSCs were recorded on PGSTAT 302N -2

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under 100 mW cm with a 1000 W ozone-free Xenon lamp equipped with a water-based IR filter and AM 1.5 filter (Newport Corporation). EIS data were recorded by a potentiostat/galvanostat instrument equipped with FRA2 module under full sunlight with a 10 mV of AC amplitude. The frequency range employed was from 65 kHz to 10 mHz and the bias voltage was set at the open-circuit voltage. 3. Results and discussion 3.1 Characterization of electropolymerized PANI composite EDs

ACCEPTED MANUSCRIPT PANI polymerized from aniline sulfate through ED method was characterized by the FTIR spectra as shown in Fig. 1 with the peak assignments listed in Table 2. PANI depositing Table 1. Properties of PANI/GNP/MWCNT composite films prepared by ED method. Carbon additive Composite films

content in film

weight ratioa

(wt%) b

Film weight

Sheet resistance

(mg cm-2)c

(Ω sq-1)d

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ANI/GNP/MWCNT

NA

NA

NA

4.69±0.04

PANI

1 /0/0

0

0.38±0.04

PANI-G1

1/0.0030/0

10.36

0.43±0.04

11.08±0.10

PANI-G2

1/0.0045/0

14.85

PANI-G3

1/0.0060/0

11.96

PANI-C1

1/0/0.0030

PANI-C2

15.02±0.12

0.48±0.03

9.36±0.11

0.46±0.02

10.76±0.11

6.86

0.41±0.02

13.40±0.15

1/0/0.0045

12.62

0.47±0.04

12.10±0.10

PANI-C3

1/0/0.0060

9.25

0.46±0.05

12.92±0.11

PANI-G1C1

1/0.0030/0.0030

N/A

0.46±0.05

9.46±0.15

PANI-G1C2

1/0.0030/0.0045

N/A

0.45±0.02

7.65±0.11

PANI-G1C3

1/0.0030/0.0060

N/A

0.46±0.04

8.25±0.14

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Pt

Ratio of the major components in the solution for electropolymerization.

b

Weight percentage of carbon additives in the PANI composite estimated by TGA shown in Fig. 2.

c

Estimated by TGA data shown in Fig. 2.

d

Measured by four-point probe method.

CE

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a

-1

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on the surface of FTO exhibited the absorption peaks appearing at 1564 and 1482 cm , which is attributed to the stretching vibration of quinoid (N=Q=N) and benzenoid (N–B–N) respectively. Disappearance of the -1

peak at 1680 cm attributed to the NH2 of aromatic amines (Fig. 1a) and appearance of the peak at 3387 -1

cm attributed to the N–H (Fig. 1b) also confirm the formation of PANI.

Moreover, the PANI films

revealed green color, indicating that they were at the emeraldine salt state. As we varied the -2 electric charge density from 20 to 40 mC cm to synthesize the PANI films, that with 30 mC -1

cm-2 was found to have the minimum sheet resistance of 15.02±0.12 Ω sq . To further decrease the sheet resistance, various amounts of GNP and MWCNT were incorporated into

ACCEPTED MANUSCRIPT the aniline sulfate solution prior to polymerization. Sheet resistances of all the polymerized

(a) FTIR spectrum of aniline sulfate and (b) ATR-FTIR spectrum of PANI film

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Fig. 1.

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samples prepared at the electric charge density of 30 mC cm-2 are listed in Table 1.

polymerized by ED.

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Table 2. Peak assignments for the FTIR spectra shown in Fig. 1. Aniline sulfate

Wavenumber (cm-1)

Assignment

3051

υC-H

3387

δN-H

1680

δNH2

2975

υC-H

1555

υPh

1597

υPh

1461

υPh

1564

νC-N (N=Q=N)

νC-N

1482

νC-N (N-B-N)

νC-N

1289

νC-N

δC-H

1229

νC-N

δC-H

1173

δC-H

735

γC-H

1069

δC-H

687

γPh

1009/884/850/739

γC-H (substituted benzene)

532

δS=O2

576

δS=O

1135 1023

CE AC

1292

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Assignmenta

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Wavenumber (cm-1)

1339

a

PANI film

υ, stretching; δ, in-plane bending; γ, out-of-plane bending; Ph, phenyl.

Notably, we considered that aniline surfate is an efficient dispersing agent to debundle the MWCNTs and avoid GNP aggregation. GNP and MWCNT were directly

ACCEPTED MANUSCRIPT used without further surface modification. As the result, after polymerization through ED process, incorporating a trace amount of GNP or MWCNT to PANI exhibited an obvious decrease of sheet resistance (see Table 1). The carbon content in the synthesized PANI composites was estimated from their TGA data as shown in Fig. 2 according to the literature [45, 46], because it is too trivial to be measured by

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analytical balance. For example, at 750 °C the remaining weight of GNP is 74.15%,

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whereas that of PANI-G1 composite is 32.69% more than pure PANI (27.90%) on

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account of the remaining GNP (see Fig. 2a). If we assume the weight of GNP in PANI-G1 is x grams per 100g in composite, the remaining weight of PANI-G1 at 750

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°C would be 0.7415x+0.2790×(100-x) = 32.69 (g). Therefore, 100 g of PANI-G1 contains 10.36 g of GNP, which is much higher than the ratio we added in the

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monomer solution. It might be due to the fact that the aniline monomers have been physisorbed onto the carbon additives through the π-π and cation-π interactions [46, All the estimated carbon additive contents for the PANI composite samples are

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47].

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included in Table 1. Interestingly, the GNP and MWCNT contents increase with their amount of addition first, but decreased with further increasing the addition, probably

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due to the aggregation of carbon additives. The influence of GNP and MWCNT additives on the morphology of PANI was

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investigated by SEM. A top view of the electropolymerized PANI on FTO glass is shown in Fig. 3a, where an intertwined irregular ribbon-like structure with the width of ~200 nm is observed. As GNP is incorporated into the PANI film, the size of the ribbons reduces and the intertwined structure is more compact. As we looked more closely, we can observe some small particles (~100 nm) grown on top (Fig. 3b). Interestingly, as MWCNT is incorporated into the PANI film, the interconnected ribbon-like structure disappears and is replaced by the small particles in the size of

ACCEPTED MANUSCRIPT ~100 nm topping with sparse worm-like species (Fig. 3c). As both GNP and MWCNT are incorporated, the surface is crowded with small particles and worm-like species (Fig. 3d). To look more closely, the worm-like species are formed by the linkage of small particles. Notably, the PANI composite incorporating both GNP and MWCNT exhibits the maximum surface roughness, which is presumably beneficial to their

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CE

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electric conductivity and catalytic activity.

ACCEPTED MANUSCRIPT

(b)

(c) )

(d)

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(a)

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TGA plots of various indicated PANI composite films.

SEM images of PANI and its composite films prepared by ED process (30

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Fig. 3.

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Fig. 2.

mC cm-2); (a) Pure PANI, (b) PANI-G2, (c) PANI-C2, and (d) PANI-G1C2.

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Fig. 4 shows the CV plots for the reaction kinetics of I− /I3− redox couples with PANI composite films as working electrode. Besides, Pt was also included for comparison. Two pairs of redox peaks which signified the catalytic activity of the

CE

working electrode were observed and mainly contributed by the following redox

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reactions [31, 48],

I3− + 2𝑒 − ↔ 3I−

(1)

3I2 + 2𝑒 − ↔ 2I3−

(2)

Eq. 1, which corresponds to the left redox peaks in Fig. 4, directly relates to the DSSC performance, whereas Eq. 2 has little effect on the performance. Comparing to Pt electrode, neat PANI exhibits lower reduction current density (JR), probably owing to its lower conductivity. However, JR increased as the PANI was incorporated with GNP

ACCEPTED MANUSCRIPT or MWCNT, and increased even more while both of them were incorporated. It is presumable that the MWCNTs may connect the separated GNPs, providing an "electric highway" for charge transfer, contributing to their lower sheet resistance as shown in Table 1. Furthermore, higher surface roughness providing more surface area (see Fig. 3) also facilitated the ionic charge transfer between the electrolyte and CE

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[49]. However, with higher content of MWCNT, the possible aggregation would raise

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the sheet resistance and drop JR. On the other hand, we also found the reduction

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voltage (VR) shown in Fig. 4 shifted positively with the incorporation of carbon additives. All the measured JR and VR are summarized in Table 3. Our results shows

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that PANI-G1C2 has higher reduction potential and current density than Pt but with wider peak to peak separation (∆Ep), which may be due to the nature of PANI that

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takes longer time to response the oxidation and reduction reactions. According to the literature, the lower the ∆Ep value, the better the catalytic activity is [50]. Interestingly,

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was incorporated.

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it can be seen that the ∆Ep of PANI decreases as a proper amount of GNP or MWCNT

CE

3.2 Performance of DSSCs with PANI composite CEs The J-V characteristics of DSSCs with PANI composite CEs are shown in Fig. 5 All the photovoltaic properties such

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that with Pt CE is also included for comparison.

as JSC, VOC, fill factor (FF), and PCE (η) are listed in Table 3. Notably, an obvious enhancement of JSC and VOC with the incorporation of GNP or MWCNT to the PANI electrodes is observed, where the incorporation of GNP is more effective (see Fig. 5a). As both GNP and MWCNT are incorporated, the synergistic enhancement in photovoltaic performance is found probably due to that MWCNTs have connected the separated GNPs for charge transfer. With increasing the content of MWCNT, the

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Fig. 4. CV plots of Pt and various indicated PANI composite films prepared by ED

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process.

Samples

JR (mA cm-2)

VR (V)

∆Ep (V)

Pt

-1.820

0.008

0.262

PANI

-1.337

-0.016

PANI-G1

-1.724

PANI-G2

-1.740

PANI-G3

-1.732

PANI-C1

-1.675

PANI-C2

-1.869

PANI-C3

-1.782

PANI-G1C1 PANI-G1C2

JSC (mA cm-2)

FF

η (%)

17.09±0.06

0.752±0.008

0.60±0.01

7.62±0.07

0.540

15.48±0.08

0.675±0.011

0.50±0.02

5.55±0.05

0.018

0.635

18.19±0.05

0.719±0.012

0.51±0.03

7.15±0.03

0.039

0.732±0.011

0.53±0.04

7.29±0.08

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VOC (V)

18.76±0.07

0.024

0.468

18.22±0.07

0.726±0.008

0.53±0.02

6.95±0.03

-0.008

0.547

17.33±0.08

0.705±0.006

0.53±0.03

6.23±0.04

0.031

0.460

18.78±0.09

0.726±0.010

0.53±0.04

7.21±0.08

0.026

0.397

18.07±0.09

0.724±0.008

0.53±0.03

6.94±0.05

-1.754

0.039

0.373

17.43±0.05

0.736±0.010

0.57±0.02

7.39±0.06

-1.874

0.073

0.413

18.21±0.06

0.780±0.007

0.54±0.02

7.67±0.05

-1.774

0.071

0.373

17.83±0.09

0.766±0.005

0.56±0.03

7.59±0.03

0.389

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CE

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PANI-G1C3

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Table 3. Properties of Pt and PANI composite CEs and their fabricated DSSCs.

photovoltaic performance was enhanced in the beginning and then reduced owing to the possible aggregation of MWCNTs. The initial increase of the photovoltaic performance and then decrease with the addition of carbon additives are also found for the PANI CEs incorporating neat MWCNTs (see Fig. 5b) and GNPs (see Fig. 5c)

ACCEPTED MANUSCRIPT respectively. The enhanced photovoltaic performance by incorporating carbon additives is almost parallel to their increase of JR and VR measured by CV plots (see Table 3). Notably, the DSSC fabricated with PANI-G1C2 CE has the best performance with JSC = 18.21±0.06 mA cm-2, VOC = 0.780±0.007 V, and η = 7.67±0.05%, which are all slightly higher than that of Pt cell (see Fig. 5a and Table 3). It may be attributed to

PT

its favourable morphology for reduction reaction although the sheet resistance is a little

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higher than Pt.

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By plotting JR versus JSC for all the PANI composite CEs prepared with the electric charge densities of 20, 30 and 40 mC cm-2, a roughly linear relationship is

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obtained as shown in Fig. 6a. It supports the hypothesis that the photocurrent of the DSSC is dependent on the reduction rate of I3− ions on CE, although it may also be

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affected by the transportation rate of I− /I3− redox couples in the liquid electrolyte and charge transfer in the dye/electrolyte interface, both of which have potential to deviate

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its linear relationship (R2 ≅ 0.72). Moreover, by plotting the VR versus VOC for all the PANI composite CEs, we also found a linear relationship as shown in Fig. 6b. VOC is mainly contributed by the electric potential difference between the conduction band edge (EC)

CE

of mesoporous TiO2 and the oxidation potential of I− /I3− redox couples in the electrolyte. By considering the charge recombination at the TiO2/electrolyte interface,

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Dürr et al [51] suggested the following equation to estimate VOC, VOC =

jinj kT EC − Eredox ln { }+ e qeket cox NC e

[3]

where jinj is the current density injected from the excited dyes to TiO2, q is the number of electrons transferred from TiO2 to the oxidized species of concentration cox with the transfer rate constant ket, e is elementary charge, and NC is the effective density of states in the conduction band. Linear relationship with the slop close to 1 shown in Fig. 6b indicates that

Fig. 5.

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CE

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PT

ACCEPTED MANUSCRIPT

J-V curves of the DSSCs with Pt and various indicated PANI composite CEs.

Fig. 6.

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CE

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PT

ACCEPTED MANUSCRIPT

Linear relationships of (a) JR versus JSC, (b) VR versus VOC, and (c) charge

transfer resistance at CE (R1) versus sheet resistance. The data include all the devices with the PANI composite CEs prepared at 20, 30 and 40 mC cm-2. Pt electrode is also included (blue circle) for comparison.

ACCEPTED MANUSCRIPT VR is equivalent to |𝐸𝑟𝑒𝑑𝑜𝑥 /𝑒| and strongly suggests the enhanced performance of CE barely affect the charge recombination at the TiO2/electrolyte interface. Notably, the performance of Pt electrode is obviously out of the linear relationship of the plots that belong to the PANI electrodes, which might be related to its lower resistance, higher reduction rate and smaller

PT

surface area. EIS technique is often used to characterize the kinetics of charges in DSSCs by

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analyzing the variation in impedance associated with the different interfaces of the device

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[52-54]. The Nyquist plots of the DSSCs with the PANI composite CEs under illumination are fitted well with Z-view software as shown in the Fig. 7a. Each resistance evaluated by the

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fitting result is listed in Table 4, where Rs, R1, R2 and R3 are the series resistance, charge

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transfer resistance at the electrolyte/CE interface, resistance at the TiO2/dye/electrolyte interface of DSSC, and the Nernstian diffusion resistance within the electrolytes [4]. As seen in Table 4, R1 decreases with the addition of GNP and MWCNT. By plotting the R1 versus

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the sheet resistance for all the PANI composite CEs as shown in Fig. 6c, an approximate linear relationship is obtained, indicating that the charge transfer resistance at electrolyte/CE interface is dependent on the sheet resistance of CE. By transforming the EIS results to the

CE

Bode phase plots shown in Fig. 7b, the charge transfer rate in the electrolyte/CE interface that can be estimated from the characteristic peak position at the right side of the plots shifts to

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higher frequency as GNP and MWCNT are incorporated.

The charge transfer lifetime (τr)

calculated by the equation of τr = 1/2πf shown in Table 4 has a similar trend as R1. After all, compared to the Pt cell, the DSSC with PANI-G1C2 CE has smaller R1 and shorter τr, which presumably contribute to its higher JSC and VOC (see Tables 2 and 3).

4. Conclusions In summary, we have successfully fabricated the PANI/GNP/MWCNT composite CE for DSSCs through electrochemical polymerization method. Incorporating GNP and/or MWCNT

(a) Nyquist plots and (b) Bode plots measured under illumination for the DSSCs

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Fig. 7.

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ACCEPTED MANUSCRIPT

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with various indicated PANI composite CEs. The EIS results were fitted with Z-view

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software by the equivalent circuit shown in (a).

R2 (Ω)

R3 (Ω)

τr (ms)

12.56

7.96

0.020

14.37

10.11

5.43

0.134

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Table 4. The EIS data of DSSCs with Pt and various PANI composite CEs.

12.71

8.67

0.058

8.56

14.10

9.49

0.025

26.67

10.69

15.58

9.85

0.033

PANI-C1

27.22

12.57

12.19

9.02

0.077

PANI-C2

26.66

9.43

14.46

8.09

0.031

26.77

9.89

14.76

9.94

0.040

27.62

10.18

14.22

9.15

0.021

PANI-G1C2

27.20

6.06

12.23

11.10

0.019

PANI-G1C3

27.21

8.35

15.30

10.41

0.022

RS (Ω)

R1 (Ω)

Pt

24.59

7.41

PANI

25.43

PANI-G1

28.35

PANI-G2

27.43

PANI-G3

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Samples

PANI-C3

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PANI-G1C1

CE

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10.32

to PANI results in lower charge transfer resistance and higher reduction potential of I3− / I− redox couples, which were confirmed by the EIS analysis and CV measurement. Moreover, by varying the content of GNP and MWCNT and the electric charge densities for electrochemical polymerization, a linear relationship of VR versus VOC is observed in addition

ACCEPTED MANUSCRIPT to roughly linear relations between JR and JSC, and between R1 and sheet resistance. Notably, higher JSC and VOC of the DSSC with PANI-G1C2 CE compared to the Pt cell is attributed to the faster charge transfer from PANI-G1C2 CE to electrolyte. Although the fill factor of the DSSC with PANI-G1C2 CE is lower, its PCE under 1 sun illumination is still comparable to

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the Pt cell.

Acknowledgments. The authors acknowledge the financial support of the National Council

in

Taiwan,

Republic

of

Grant

NSC

NU

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Research highlights

>Performance of DSSC with PANI composite counter electrode is akin to PT cells.

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> VOC of DSSC is dependent on the reduction potential of redox couples in electrolytes.

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CE

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> Jsc of DSSC is dependent on the reduction current density of redox couples.