polypropylene composite plate decorated with poly(3,4-ethylenedioxythiophene) as efficient counter electrodes for dye-sensitized solar cells

Accepted Manuscript Flexible carbon nanotube/polypropylene composite plate decorated with poly(3,4ethylenedioxythiophene) as efficient counter electrodes for dye-sensitized solar cells Jeng-Yu Lin, Wei-Yen Wang, Shu-Wei Chou PII:

S0378-7753(15)00158-5

DOI:

10.1016/j.jpowsour.2015.01.142

Reference:

POWER 20578

To appear in:

Journal of Power Sources

Received Date: 3 December 2014 Revised Date:

15 January 2015

Accepted Date: 23 January 2015

Please cite this article as: J.-Y. Lin, W.-Y. Wang, S.-W. Chou, Flexible carbon nanotube/polypropylene composite plate decorated with poly(3,4-ethylenedioxythiophene) as efficient counter electrodes for dyesensitized solar cells, Journal of Power Sources (2015), doi: 10.1016/j.jpowsour.2015.01.142. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Flexible

carbon

nanotube/polypropylene

composite

plate

decorated with poly(3,4-ethylenedioxythiophene) as efficient

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counter electrodes for dye-sensitized solar cells

Jeng-Yu Lin,* Wei-Yen Wang and Shu-Wei Chou

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Department of Chemical Engineering, Tatung University, No. 40, Sec. 3, ChungShan North Rd., Taipei City 104, Taiwan

[*]

Corresponding author

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Fax: +886-225861939 E-mail: [email protected]

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Keywords: Dye-sensitized solar cell; Counter electrode; Composite plate; Carbon nanotube; Polypropylene; Poly(3,4-ethylenedioxythiophene)

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Abstract

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In this study, we fabricate an efficient, flexible and low-cost counter electrode (CE) composed of a plasma-etched carbon nanotubes/polypropylene

(designated as ECP) composite plate decorated with poly(3,4-ethylene (PEDOT)

for

dye-sensitized

solar

cells

(DSCs).

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dioxythiophene)

The

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PEDOT-decorated monolithic ECP CEs are fabricated via series of processes including high-temperature refluxing, thermal compression, oxygen plasma etching, and electropolymerization. The bottom ECP plate is used to replace conventional transparent conducting oxide (TCO) as a conductive substrate, and the

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top PEDOT layer is employed as catalyst for I 3 ─ reduction. According to the extensive electrochemical measurements, the as-fabricated flexible PEDOT coated

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ECP CE demonstrates a Pt-like electrocatalytic for I 3 ─ reduction. The DSC based on

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the flexible PEDOT-decorated ECP CE yields impressive energy conversion efficiency of 6.82% (or 6.77% even after the bending test), which is comparable to that of the DSC using the Pt CE (7.20%) under similar device architecture conditions. Therefore, the PEDOT-decorated ECP based CEs show the possibility of serving as low-cost and flexible CEs for efficient DSCs.

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Introduction

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Dye-sensitized solar cells (DSCs) have been intensively studied as promising energy conversion devices for clean renewable energy due to their low cost,

simple manufacturing procedure, and efficient photovoltaic

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performance [1]. The main advantage of DSCs over silicon-based photovoltaic

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cells is the possibility for the roll-to-roll scalable production when flexible substrates are employed. Because of the characteristics of light weight and variable geometry, flexible solar panels can offer the following applications, including serving as a mobile power source for portable electronic devices and

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easy installation on buildings [2].

Generally, a typical DSC is composed of three components: a

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dye-sensitized nanocrystalline TiO 2 photoanode, an I3 ─ /I─ -containing redox

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electrolyte, and a counter electrode (CE) with a catalytic layer coated on fluorine-doped tin oxide (FTO) glass substrate. Among them, CE is a crucial component in DSCs, which essential functions are to promote the electrons transport from the external circuit back to the I3 ─ /I─ -containing electrolyte and accelerate the reduction reaction of I3 ─ /I─ to provide sufficient I─ species for the regeneration of the oxidized dye molecules. Up to date, a platinized layer on a 3

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transparent fluorine-doped tin oxide (FTO) has still been considered as the most effective CE due to its excellent electrocatalytic activity toward I3 ─ reduction

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and high electrical conductivity. Nevertheless, noble metal Pt is rare, expensive, and easy to be corroded by the I3 ─ /I─ -containing electrolyte. Moreover, efficient Pt catalytic layer on FTO substrates is usually fabricated via high-temperature

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pyrolysis, which is beyond the sustaining capability of conventional conductive

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plastic substrates such as indium doped tin oxide polyethylene terephthalate (ITO/PET) and indium doped tin oxide polyethylene naphthalate (ITO/PEN), and thus restricts the realization of flexible DSCs. Furthermore, the current production of transparent conducting oxides such as FTO and ITO on glass or

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plastic substrates involves high-cost vacuum processes [3]. Therefore, it is imperative to develop Pt-free and TCO-free flexible CEs for low-cost flexible

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DSCs.

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To date, lots of materials have been reported to be the potential substitutes for the conventional Pt CE like carbonaceous materials (activated carbon [4,5], graphite [6], carbon nanotubes (CNT) [7,8], graphene [9], etc.), conductive organic

polymers

(polyaniline

(PANI)

[10,11],

polypyrrole

[12],

poly(3,4-ethylene dioxythiophene) (PEDOT) [13,14], etc.), and inorganic oxides [15], nitrides [16], sulfides [17,18], selenides [19,20], and carbides [21]. 4

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Nevertheless, most of the aforementioned alternatives are still deposited on the TCO current collectors to show the comparable electrocatalytic activities to Pt

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CE. Hence, it still remains a huge challenge to seek for a totally TCO-free and Pt-free CEs with excellent electrical conductivity and electrocatalytic activity for the further development of low-cost flexible DSCs. For instance, Chen et al.

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[5] employed an industrial flexible graphite sheet and activated carbon as

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conductive substrate and electrocatalytic material, respectively. The DSC based on this TCO- and Pt-free CE achieved impressive power conversion efficiency (PCE) of 6.46%. Sun et al. [22] further in situ chemically polymerized PANI thin film onto a flexible graphite (FG) sheet. When the thickness of PANI on

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the FG substrate was up to 330 nm, an optimized PCE of 7.36% was obtained for the PANI/FG based DSC. Nagarajan et al. [23] electroploymerized highly

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electrocatalytic PEDOT onto the conductive reinforced exfoliated graphite

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(REG) substrate, and the DSC assembled with the PEDOT/REG CE showed a superior PCE of 5.7% to that of the DSC using the Pt CE (4.4%). Hashmi et al. [24] deposited single-walled carbon nanotube onto PET foil (designated as SWCNT/PET) as a conductive substrate, and subsequently to be coated with PEDOT via electropolymerization. The PEDOT/SWCNT/PET sheet was tested as CEs in DSCs, yielding impressive PCE up to 7% and excellent 5

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electrocatalytic activity in terms of low charge-transfer resistance of 0.4 Ω cm 2. Consequently, it is imperative to combine conductive TCO-free flexible

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substrate and highly electrocatalytic Pt-free catalysts together for developing cost-effective flexible CEs.

In this current work, we first synthesized low-cost, conductive, flexible

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carbon nanotubes/polypropylene (CNT/PP, designated as CP) composite plates

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as free-standing conductive substrates to replace the high-cost TCO substrates via a series of processes including refluxing, thermal compression, and oxygen plasma

treatment.

Nevertheless,

the

electrocatalytic

activity

of

the

plasma-etched CNT/PP (designated as ECP) plate was not satisfactory and was

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still inferior to that of the conventional Pt CE. To further improve the electrocatalytic activity of the ECP plates, highly electrocatalytic PEDOT thin was

deposited

onto

the

conductive

ECP

plates

via

facile

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film

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electropolymerization approach. The resultant PEDOT-ECP CE displayed excellent electrical conductivity, electrocatalytic activity as well as flexibility. Therefore, the DSC based on the highly flexible PEDOT-ECP CE displayed impressive PCE of 6.82% (or 6.77% even after bending), which was comparable to that of the DSC assembled with the conventional Pt/FTO CE (7.20%).

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Experimental

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Preparation of CNT/PP composite plate Prior to the synthesis of CNT/PP (CP) composite plate, functionalized

multiwall CNTs (with diameter of ca. 90 nm diameter and length of ca. 10 µm)

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purchased from Golden Innovation Business Company were dispersed in

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toluene (50 ml) and ultrasonically sonicated for 90 min to make a dispersion solution. Moreover, 200 mg PP with average molecular weight of 190000 Da was also dissolved and refluxed in toluene. Then, these two dispersion solutions were mixed together for 150 min and finally dried in a vacuum oven at 60oC for

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24 h to finally remove the solvent. The resultant powders were mashed, and then modeled into a thin plate with a thickness of ca. 0.3cm by a compression

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molding operated at temperature of 215oC and pressure of 1000 psi for 12 min.

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It should be noted that different CP composite plates were prepared by adjusting the weight percentage of CNTs within the composites. The CP composites based on the CNT content of 33, 43, 50, and 56wt% is designated as CP-33, CP-43, CP-50, and CP-56, respectively. To allow CNTs to partially merge from the PP matrix, the PP capping layer on the top surface of the as-prepared CP composite plates was further etched by 7

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oxygen plasma reactor (OEM-6 SOLID STATE POWER GENERATOR) [25]. Prior to the plasma treatments, the as-prepared CP plates were firstly placed

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into the chamber, followed by evacuation to 25 mTorr. Then, oxygen gas was introduced and the pressure in the chamber was controlled at 100 mTorr. After

plates are designated as ECP plates in this study.

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Preparation of PEDOT-coated CEs

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that, the glow discharge was ignited at 75 W for 15 min. The plasma-etched CP

PEDOT thin film was electropolymerized on the FTO glass substrates or plasma-etched CP plates in an aqueous solution consisting of 20 mM 3,4-ethylenedioxythiophene (EDOT) monomer, 15 mM sodium dodecyl sulfate

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and 20 mM lithium perchlorate via a conventional cyclic voltammetry (CV) method. The electropolymerization of PEDOT was carried out in a

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three-compartment cell, in which an ECP plate or a FTO substrate, a Pt wire,

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and a saturated Ag/AgCl electrode served as a working electrode, a counter electrode, and a reference electrode, respectively. The electropolymerization was performed within the potential interval between 0.3 V and 1.1 V vs. Ag/AgCl at a sweep rate of 50 mV s -1 by using a computer- controlled

electrochemical

analyzer,

CHI6081D

(CH

Instrument).

After

the

electropolymerization, the PEDOT-coated FTO and ECP substrates is 8

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designated as PEDOT and PEDOT-ECP CE, respectively; which were rinsed by deionized water and dried at 60℃ for 12 h. As a reference, Pt coated on FTO

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substrates was prepared via thermal pyrolysis. The loading of Pt was ca. 10 µg cm -2 . Assembly of DSCs

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The 2 cm × 1.5 cm FTO glass substrate (8 ohm square -1, 3.1 mm thick,

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Nippon Sheet Glass) was employed as the anode substrate. The commercial nano-TiO2 paste (20 nm, ETERDSC Ti-2105A, Eternal Chemical Co.) was coated onto the cleaned FTO glass substrates by using a semi-automatic screen printer (ATMA, AT45PA) to form a dense-nanocrystalline TiO 2 film with 12µm

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thickness, and then 4 µm scattering TiO2 layer was subsequently printed on its top surface using another TiO2 paste (ETERDSC Ti-2325, Eternal Chemical Co.). The

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TiO 2 anodes with an active area of 0.16 cm 2 were obtained after the TiO 2

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bilayer film gradually sintered under an air flow at 150 °C for 10 min, 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min. After being cooled down to 80 °C, the TiO 2 anodes were soaked in an ethanol solution composed of 0.3 mM N719 dye for 12 h, followed by rinsing with ethanol and drying with cool air flow. The DSCs were assembled by sandwiching the dye-sensitized TiO2 anodes and various CEs with an electrolyte (1 M 1.3-dimethylimidazolium iodine, 9

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0.5 M 4-tert-butylpyridine, 0.15 M iodine, and 0.1 M guanidine thiocyanate in 3-methoxypropionitrile solvent) using a 60µ m thick hot-melt Surlyn spacer

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(Dupont, USA) Characterizations

Fourier transform infrared spectra (FTIR) of samples were recorded based

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on a Perkin Elmer Spectrum Gx FTIR Spectrometer with attenuated total

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reflection (ATR) device. The surface morphologies of the samples were characterized using a field-emission scanning electron microscopy (FESEM; JSM-7600F). The simple bending tests for the samples were performed by home-made clamping apparatus. Surface electrical resistivity of samples was

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obtained via four-point probe method by using a Keithley 2400 source meter. To study the electrocatalytic activity and electrochemical kinetic of the CEs,

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CV and Tafel polarization curve, and electrochemical impedance spectroscopy

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(EIS) were performed with the aforementioned CHI 6081D electrochemical analyzer and ZAHNER electrochemical workstation, respectively. CV tests were conducted in a three-electrode electrochemical cell, in which an as-prepared CE as the working electrode, a Pt foil as an auxiliary electrode, and a Pt wire as a reference electrode. The CVs were recorded in a 3-methoxypropionitrile solution containing 50 mM LiI, 5 mM I2 , and 0.5 M 10

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LiClO 4 within the potential range between -0.8 V to 0.8 V vs. Pt at different scan rates. The Tafel polarization curves and EIS spectra were recorded by

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scanning the dummy cell (Pt CE/redox electrolyte/various CE) from 106 Hz to 0.1 Hz with 10 mV ac amplitude at open-circuit condition under dark situation. The resultant impedance spectra were further analyzed by using Z-view

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software. The photocurrent-voltage curves of DSCs were recorded with a

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computer-controlled Keithly 2400 digital source meter under exposure of a Newport solar simulator, which was calibrated to 1 sun light density (AM 1.5G, 100 mW cm -2 ) with a radiant power/energy meter (Oriel, 70260).

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Results and Discussion

Figure S1 illustrates the CP-33 composite plates before and after plasma

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etching treatment. After the plasma treatments, the formation of deeper

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interconnected channels based on the CNT frameworks became visible, indicating that the PP capping layer on the as-prepared CP plates was successfully removed and therefore the embedded CNTs were exposed. Fig. 1 shows the surface morphologies of the ECP composite plates with differenct CNT contents. It can be found that all PP capping layers were successfully removed after the plasma treatments for 15 min. The surface morphologies of 11

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all ECP plates were almost the same, except that the exposed CNT filaments increased with increasing the CNT content within the ECP composite plates. To

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further verify that CNTs were susscessfully emedded into the PP matrix, the CNTs, PP, and as-prepared ECP-33 composite were characterized by FTIR. Fig.

S2 presnets the FTIR spectra of the CNTs, PP, and ECP-33 composite. As can

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be seen in the FTIR spectrum of CNTs in Fig. S2a, two main absorption bands

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located at 1543 and 1228 cm -1 of CNTs are apparently observed, which basically originate from the carbon nanotube backbone of stretching [26]. Moreover, the bands located at 1720 cm -1 can be ascribed to the C=O stretching vibrations, and the vibrations at 1398 cm -1 is mainly assigned to the hydroxyl

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groups bending deformation of the carboxylic groups [27,28]. As for the charasticistic peaks of PP (seen in Fig. S2b), the absorption peaks at 2955, 2922,

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2873, and 2843 cm -1 are found, in which the absorption peaks at 2955 and 2873

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cm -1 can be attributed to CH 3 asymmetric and symmetric vibrations, and the peaks at 2922 and 2843 cm -1 can be associated with the CH 2 asymmetric and

symmetric stretching vibration [29,30]. As depicted in the FTIR spectra of ECP-33 composite (Fig. S2), all of the absorption peaks originating from CNTs and PP are found. This indicates that CNTs were susscessfully embbed within the PP polymer matrix as a CP composite, which is consistent to the observation 12

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from SEM characterizations. To investigate the effect of CNT contents on the physical properties of the

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ECP plates, sheet resistance values (σ) of various ECP plates were measured, and the obtained values were summaried in Table 1. As seen in Table 1, it can be clearly observed that the resultant sheet resistance value decreases with

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increasing the CNT content within the ECP composite plates, revealing that the

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incorporation of high-content CNTs in the PP polymer matrix can be responsible for the remarkable enhancement in the electrical conductivity of the ECP plates. Additionally, series of bending tests for the ECP plates with various CNT contents were carried out to ensure if the ECP plates possess sufficient

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capability to stand bending without fracturing. As illustrated in Fig. 2, various ECP plates were bent via adjusting the distance between two clamps in the

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testing apparatus. Among all ECP plates, the ECP-33 plate demonstrates the best

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bending capability, which can stand bending without fracturing when the distance between two clamps decreases from 2 cm to 1 cm, and then fractures as the distance further decreased to 0.5 cm. Nevertheless, it can be found that the bending capability of ECP plates decreases with increasing the CNT content in the ECP plates. For instance, both of ECP-50 and ECP-56 plates cannot stand the bending while decreasing the distance between two clamps from 2 cm to 13

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only 1.5 cm and therefore fracture. This signifies that the increase in the CNT content in the ECP plates would be inversely associated with their bending

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capability. Therefore, controlling the CNT content in the ECP plates would be a crucial factor for fabricating highly flexible and conductive ECP plates.

Fig. 3a shows the photocurrent-voltage curves of the DSCs based on

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various ECP plates and Pt/FTO as CEs. The corresponding photovoltaic

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parameters including short-current density (J sc ), open-circuit voltage (Voc ), fill factor (FF), and PCE are also summarized in Table 1. It can be observed that all of the DSCs assembed with ECP-based CEs have comparatively similar V oc values of around 650 mV, which is relatively lower than that of the DSC based

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on the Pt/FTO CE. This may be possibly explained due to the absorption of the I3 ─ /I─ -containing redox electrolyte on CNTs [31]. Moreover, the DSCs with the

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ECP based CEs have improved J sc and FF values as increasing CNT content in

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ECP plates, therefore resulting in the enhancement of η values. Among of all DSCs assembled with the ECP based CEs, the device using the ECP-56 CE demonstrates the best photovoltaic performance. It shows a J sc of 14.69 mA

cm -2 , a Voc of 0.64, a FF of 0.36, and therefore results in PCE of 3.29%; however, which is still significantly lower compared to that of the DSC based on the Pt/FTO CE (7.20%), especially due to the significantly lower FF values. 14

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To clarify the reason for the poor photovoltaic performance of the DSCs assembled with the ECP based CEs, CV tests were conducted within the

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potential interval of -0.8 to 0.8 V vs. Pt at a scan rate of 10 mV s-1 . Fig. 3b presents the resultant CV curves of the Pt/FTO and ECP based CEs. As expected, two pairs of redox reactions are observed according to the previous

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study [32]. The relatively positive pair of peaks is basically responsible for the

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cathodic (R I) and anodic (O I) reactions of I 2 /I3─ , as illustrated in eqn. 1. The other redox pair is associated with the cathodic (R II) and anodic (O II ) reactions of I3 ─/I─ , as presented in eqn. 2.

3I2 + 2e- ↔ 2I3 −

−

(2)

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I3 + 2e- ↔ 3I−

(1)

Since the main function of a CE is to quickly speed up the reduction

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reaction of I3 ─ to I─ , the relatively negative redox pair involving in the

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electrocatalytic reaction of I3 ─ /I─ is therefore considered here. In general, the peak to peak separation (E pp ) of a redox pair is inversely related with the

electrochemical rate constant of a redox reaction [33]. Noticeably, all ECP based CEs have similar E pp value of ca. 580 mV, which is remarkably larger than that of the Pt/FTO CE. This signifies that all ECP based CEs have the poorer electrocatalytic activity for I3 − reduction compared to the Pt/FTO CE. 15

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Consequently, a larger overpotential is required to drive the reduction reaction of I3─ /I─ for the ECP based CEs than the Pt/FTO CE, thus resulting in the

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decrease in FF values shown in Fig. 3a [34,35]. Fig. 4 shows the FE-SEM images of the PEDOT-decorated ECP plates. It

is obviously found that the surfaces of the exposed CNTs of all ECP plates were coated

with

PEDOT.

Interestingly,

the

diameter

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evenly

of

the

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PEDOT-decorated CNTs significantly decreases from ca. 200 nm to ca. 95 nm with increasing the CNT content within the ECP plates. It reveals that the increase in the CNT content indeed increases the exposed surface area of CNTs in the ECP plates and therefore leads to the decrease in thickness of PEDOT

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coatings on the surface of each CNT. Additionally, it is worthy noted that the decreased thickness of PEDOT coating results in improving the porosity of the

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PEDOT-ECP plates. The porous nanostructure would be beneficial for the

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electrolyte penetration, and therefore be expected to reduce the diffusion resistance [26,32]. To further verify if the PEDOT were successfully electropolymerized onto the ECP plates, FTIR analyses were utilized to examine the PEDOT-ECP-33 CE. As can be seen the spectrum of PEDOT in Fig. S3, the peaks of 1346 and 1519 cm -1 can be assigned to the C-C and C=C stretching of the quinoidal structure of the thiophene ring, respectively [36]. 16

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Moreover, the vibrations observed at 1178, 1105, and 1089 cm -1 are associated with the C-O-C bond stretching in the ethylene dioxy-group [37]. Furthermore,

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the peaks located at 958, 845, and 704 cm-1 can be attributed to the C-S in the thiophene ring of bond stretching [38]. As for the spectrum of the PEDOT-ECP,

all of the characteristic absorption bands originating from PEDOT are displayed.

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Based on the FESEM and FTIR results, it is confirmed that PEDOT is

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successfully coated onto ECP plate, and the surface of the CNTs is evenly wrapped with the homogeneous PEDOT thin film.

Fig. 5a presents the CV curves of PEDOT-ECP CEs. It can be found that all PEDOT-ECP CEs present typical two-pair redox peaks and the shape of their

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CV curves is similar to that of the Pt/FTO CE, suggesting their Pt-like electrocatalytic activities for I3 − reduction. As further compared with the ECP

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CEs, the PEDOT-ECP CEs indeed demonstrate superior electrocatalytic

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activities. For instance, the Epp value of the PEDOT-ECP-43 CE (334 mV) is significantly lower than that of the ECP-43 CE (ca. 580 mV). This indicates that the coating of PEDOT on the ECP plates can improve their electrocatalytic activity for I3 ─ reduction. It is noteworthy that the estimated Epp value for the

PEDOT-ECP CE is found to decrease from 352 mV to 306 mV with increasing the CNT content from 33 wt% to 50 wt% in the PEDOT-ECP plates. Nevertheless, the Epp 17

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value for the PEDOT-ECP-56 reaches to 393 mV. In addition to the Epp value, the cathodic current density of R II reaction is another crucial factor to evaluate the

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electrocatalytic activity of a CE. As depicted in Fig. 5a, the cathodic current density gradually increases with increasing the CNT content from 33 wt% to 50 wt%,

and then significantly increases when the CNT content further increases to 56 wt%.

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The increase in the cathodic current density with increasing the CNT content can be

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explained by the fact that the porous nanostructured network of PEDOT-ECP can be well constructed when increasing the CNT content. The porous nanostructured network would not only provide increased active surface area for I3 ─ reduction, but also facilitate the electrolyte penetration within the CEs. To further elucidate

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the diffusion behavior of electrolyte within the PEDOT-ECP CEs, CV measurements at different scan rates were performed. Fig. S4 illustrates the CV curves of the

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PEDOT-ECP CEs recorded at various scan rates individually. It is worth to note that

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the cathodic peaks regularly and gradually shift to the relatively negative potential, and the resultant anodic peaks shift to the relatively positive potential, and the both anodic and cathodic current densities are found to be increased with increasing the scan rate for all PEDOT-ECP CEs. Fig. 5b summarizes the correlation of the cathodic and anodic peak current densities with the square root of scan rate for the PEDOT-ECP CEs. The good linear correlation is observed for all PEDOT-ECP CEs, 18

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signifying that the redox reactions on the surface of all PEDOT-ECP CEs are mainly controlled by the diffusion transport of redox species in the redox electrolyte. Such

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findings further reveal that the adsorption of iodide species is hardly affected by the redox reaction on the PEDOT-ECP CEs, suggesting no specific interaction between I3─/I─ redox couple and PEDOT-ECP CEs [12,39]. To investigate the diffusion

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behaviors of redox species within the PEDOT-ECP CEs, Randles-Sevcik theory as

the PEDOT-ECP CEs.

ipc = Kn1.5 ACD0.5v 0.5

(3)

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depicted in eqn (3) is utilized to estimate the diffusion coefficients of I3─ (D) within

Where ipc represents the cathodic peak current density of the reaction based on eqn (2),

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K is the constant 2.69×105, n is the number of electrons involving in charge transfer, A is the electrode area, C is the bulk concentration of I3─ species. As depicted in Table 2,

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the estimated diffusion coefficients for the PEDOT-ECP CEs increase with increasing

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the CNTs within the ECP plates. This signifies that the high content of CNTs within the PEDOT-ECP can facilitate the I3─/I─ species diffusion within the CEs, presumably arising from the formation of the three-dimensional and nanostructured network of PEDOT-ECP. To further evaluate the electrochemical reversible stability of PEDOT-ECP based CEs, long-term CV test for the PEDOT-ECP-43 CE was carried out. Fig. 6a presents the 100 consecutive CV scans of the PEDOT-ECP-43 19

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CE at the scan rate of 10 mV s -1 . With 100 consecutive scans, the shape of the CVs does not change significantly, and both redox peak current densities retain stable

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with increasing the cycle number, as illustrated in Fig. 6b. This signifies that the PEDOT-ECP-43 CE possesses excellent chemical and electrochemical stability, and the PEDOT thin film is coated tightly and homogeneously on the ECP-43 plate.

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To get the deep insights into the electrocatalytic kinetics of the PEDOT-ECP

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CEs and Pt/FTO CEs, EIS and Tafel polarization measurements were conducted with the dummy cell configuration composed of CE/redox electrolyte/Pt CE (Fig. S5a). Fig. 7a presents the Nyquist plots of the dummy cells based on the PEDOT-ECP and Pt/FTO CEs. For the symmetric device with two the same Pt CEs, the corresponding

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Nyquist plot displays two semicircles, which are assigned to the impedance associated with the charge transfer processes at the Pt CE/redox electrolyte interface and the

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Nernst diffusion impedance of the I3─/I─ redox couple within the electrolyte,

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respectively. When one Pt CE is substituted by a PEDOT-ECP based CE, a new semicircle is correspondingly observed within the middle frequency region in the Nyquist plot, which can be ascribed to the charge-transfer processes on the PEDOT-ECP/redox electrolyte interface [40,41]. Table 2 also summarizes the EIS parameters obtained from the simulated data of the Nyquist plots by the equivalent circuit shown in Fig. S5b. Since the same redox electrolyte is employed during the 20

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EIS tests, the R s values can be mainly influenced by the sheet resistance of the electrode. It can be observed that the Rs values of the PEDOT-ECP CEs

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decrease with the increase in highly conductive CNT content in the ECP plates. Due to the incorporation of highly conductive CNTs within the PP matrix, the

decrease in R s values can be expected, which confirms the variance in sheet

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resistance of ECP plates, as presented in Table 1. Moreover, the Rs values of the

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PEDOT-ECP based CEs are lower than that of the Pt/FTO CE, suggesting that the introduction of ECP plates instead of FTO glass substrates can effectively reduce the Rs value of the DSCs and therefore alleviate the energy loss on the CEs. Furthermore, the , R ct and ZN values of the PEDOT-ECP CEs decrease

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from 7.17 Ω to 2.15 Ω and 5.43 Ω to 2.13 Ω when the conductive CNT network (from 33 wt% to 56 wt%) are incorporated in the ECP plates, respectively. This

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signifies that the incorporation of conductive CNT network would enhance the

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electrical conductivity, facilitate the electrolyte diffusion, and speed up the electron transfer with in the PEDOT-ECP CEs. As depicted in Fig. 4, the PEDOT-ECP CE with the three-dimensional and nanostructured network is apparently presented. This kind of mesoporous nanostructure possesses the following strengths [17,39,42]. First, it can provide increased active surface area for I3─ reduction reaction, and the interconnected CNT framework can promote the electron transfer. Second, the 21

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ordered mesopores originating from the CNT framework can facilitate the redox electrolyte penetration within the CE and thus the wetted surface area of the

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PEDOT-ECP CE rises up; correspondingly, the Rct and ZN values go down. To confirm the observations from EIS tests, Tafel-polarization measurements were performed. Figure 7b shows the Tafel-polarization curves of dummy cells based

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on the PEDOT-ECP and Pt/FTO CEs. The curves present logarithmic current density

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as a function of potential, and the exchange current density (J o ) can further be determined based on the intersection of the cathodic branch and the equilibrium potential line. The estimated J0 values for the individual CEs are summarized in Table

[43,44]. J0 =

RT nFRct

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2. In theory, J0 value is inversely dependent on the Rct value, as presented in Eqn. (4)

(4)

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where R is the gas constant, T is the absolute temperature, F is the Faraday constant,

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and n represents the number of electrons involving in the redox reaction at the electrolyte/CE interface. It is noteworthy that the J0 values of the PEDOT-ECP CEs

significantly increase with the increase in CNTs content in the ECP plates. The variance in J0 values is in the order of PEDOT-ECP-33 CE (0.68 mA cm-2) <

PEDOT-ECP-43 CE (1.55 mA cm-2) < PEDOT-ECP-50 CE (1.96 mA cm-2)< PEDOT-ECP-56 CE (2.83 mA cm-2). The variance tendency of the Rct value estimated 22

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from the J0 value for the PEDOT-ECP CEs is in accordance with that obtained by the EIS measurements. Moreover, the limiting diffusion zone located at the very high

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potential zone is highly correlated with the transport of electrolyte, in which the limiting current density (Jlim) can be determined from the intersection of the cathodic

branch with the y axis. The variance in the corresponding Jlim values is found to be in

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the order of PEDOT-ECP-33 CE (0.98 mA cm-2) < PEDOT-ECP-43 CE (1.45 mA

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cm-2) < PEDOT-ECP-50 CE (1.55 mA cm-2) < PEDOT-ECP-56 CE (1.60 mA cm-2). Basically, Jlim is positively related with the diffusion coefficient (D) of I3─ in terms of Eqn. (5), in which δ represents the spacer thickness, C is the I3─ concentration, n represents the number of electrons involved in the I3─ reduction, and F is the Faraday

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constant. As a result, the variance tendency of the expected diffusion coefficient (D) of I3─ in the PEDOT-ECP based CEs is on the order of PEDOT-ECP-33 CE <

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PEDOT-ECP-43 CE < PEDOT-ECP-50 CE < PEDOT-ECP-56 CE. This can be

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ascribed to the increased ordered mesopores originating from the CNT framework, for providing lots of open channels for the transport of redox species within the CEs. This observation is in well accordance with the aforementioned CV results. J lim =

2nFC

δ

D (5)

The excellent electrocatalytic activity and electrochemical stability of the PEDOT-ECP based CEs make them potentially suitable to serve high-performance 23

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CEs in DSCs. To directly assess this, the DSCs with the Pt/FTO and different PEDOT-ECP CEs were fabricated. Fig. 8 displays the comparison of the

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photocurrent-voltage characteristics of the devices, and the corresponding photovoltaic parameters are also tabulated in Table 2. As can be seen in Table 2, both of the J sc and FF values for the DSCs with PEDOT-ECP CEs appartently

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increase when increasing the CNT content within the ECP paltes. The more

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CNT networks incorporated wthin the PP matrix not only enhances the electrical conductivity, but also introduces more surface area for decorating with highly electrocatalytic PEDOT thin film. Consequently, the increased electrocatalytic active sites for charge transfer across the CE/electrolyte

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interface could be provided for the efficient reduction of I3─ to I─ species [26]. This could promote the regeneration of oxidized dye at the photoanode, and therefore

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result in an improvement of J sc value. The striking enhancement of FF for the

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PEDOT-ECP based DSCs can be ascribed to the reduced internal resistance of the devices since the decreased electrical conductivity, charge-transfer resistance, and diffusion resistances are verified with increasing the CNT content within the ECP plates by the aforementioned series of electrochemical anlyses [34,45]. As a result, the DSC using the PEDOT-ECP-56 CE has a J sc of 14.99 mA cm -2, a V oc of 0.76, a FF of 0.63, and results in the best PCE of 7.17% 24

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among all PEDOT-ECP based devices, which is comparable to that of the DSC with the Pt/FTO CE (7.19%) and relatively superior to that of the device with

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the PEDOT/FTO CE (6.43%) as shown in Fig. S6. It should be noteworthy that the DSC based on the flexible PEDOT-ECP-43 CE (the inset in Fig. 8) can still display a J sc of 14.49 mA cm -2 , a V oc of 0.75, a FF of 0.62, and therefore

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achieved an acceptable PCE of 6.82%. Nevertheless, the aforementioned

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photovoltaic tests were performed with the CEs without bending. The DSCs assembled with the PEDOT-ECP-43, PEDOT-ECP-50 and PEDOT-ECP-56 CEs bened with the homemade bending device by decreasing the distance between two clamps from 2 cm to 1.5 cm for five times were tested. The photovoltaic

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performance of the DSC with the PEDOT-ECP-56 CE can not be measured due to the severe fracture. Fig. S7 presents the corresponding photovoltaic curves of

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the DSCs based on the PEDOT-ECP-43 and PEDOT-ECP-50 CE. It can be

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observed that the cell efficiency of the device with the bended PEDOT-ECP-50 CE only achieves 0.46% due to the cracking on the PEDOT-ECP-50 CE. Compared with the bended PEDOT-ECP-50 CE, the DSC assembled with the bended PEDOT-ECP-43 CE still exhibits the impressive PCE of 6.77%. On the basis of flexibility and photovoltaic performance, the flexible PEDOT-ECP-43 CE could be considered a promising Pt- and TCO-free flexible CE material for 25

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low-cost DSCs.

Conclusions

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In summary, we successfully fabricated PEDOT-decorated ECP plates as Ptand TCO-free, conductive, flexible CEs for low-cost DSCs via a series of processes including

refluxing,

thermal

compression,

oxygen

plasma

etching,

and

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electropolymerization for the first time. Due to the insufficient electrocataytic activity

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of ECP based CEs, PEDOT thin films deposited onto the surface of exposed CNTs within the ECP plates indeed improved their electrocatalytic activity for I 3─ reduction. Moreover, it was found the content of CNTs within the ECP played another crucial role to affect the electrical conductivity and electrocatalytic

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activity of PEDOT-decorated ECP CEs as well as bending capability. The DSC assembled with the PEDOT decorated ECP CE with 43 wt% CNT content not only

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achieved impressive photovoltaic conversion efficiency of 6.82% (or 6.77% even

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after the bending test) comparable to that based on the Pt/FTO CE (7.20%), but also demonstrated unique feature of excellent bending capability. This work demonstrates the possibility of fabricating Pt- and TCO-free flexible CEs in DSCs, which is highly desirable to reduce the manufacturing cost of DSCs while providing the opportunity for application in fully flexible DSCs.

26

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Acknowledgements

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This research was supported by National Science Council in Taiwan (NSC 102-2221-E-036-034) and the Ministry of Science and Technology in Taiwan (MOST

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103-3113-M-110-001 and MOST 103-2221-E-036-014-MY3).

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Supporting Information

FESEM images of CP composite plate before and after plasma treatment, FTIR spectra of PP, CNT, ECP and PEDOT-decorated ECP, CVs of I3─/I─ for the PEDOT-ECP CEs at various scan rates, the configuration of dummy cell, the

DSCs

based

on

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equivalent circuits used for fitting the EIS results, and the photovoltaic curves of the the

PEDOT/FTO,

bended

PEDOT-ECP-43

and

fended

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PEDOT-ECP-50 CEs.

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Captions of Tables and Figures

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Table 1 Sheet resistance values of ECP-33, ECP-43, ECP-50, ECP-56 and Pt/FTO CEs, and the corresponding photovoltaic parameters of the DSCs based on them.

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Table 2 Photovoltaic parameters of the DSSCs based on various CEs and electrochemical parameters from CV and EIS measurements.

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Figure 1 FESEM images of (a) ECP-33, (b) ECP-43, (c) ECP-50, and (d) ECP-56 plates. Figure 2 Bending tests for (a) ECP-33, (b) ECP-43, (c) ECP-50, and (d) ECP-56 plates.

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Figure 3 (a) Photovoltaic performances of the DSSCs based on various ECP-based and Pt/FTO CEs. (b) CVs of I2/I3─ system for the various ECP-based and Pt/FTO CEs at a scan rate of 10 mV s-1. Figure 4 FESEM images of (a) PEDOT-ECP-33, (b) PEDOT-ECP-43, (c) PEDOT-ECP-50, and (d) PEDOT-ECP-56 CEs. The insets present the corresponding magnified images.

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Figure 5 (a) CVs of I2/I3─ system for the various PEDOT-ECP-based and Pt/FTO CEs at a scan rate of 10 mV s-1. (b) Plot showing the correlation between peak current densities and scan rate for the four different PEDOT-ECP-based CEs. Figure 6 (a) A total of 100 consecutive CVs for I2/I3─ redox system using the PEDOT-ECP-43 CE at a scan rate of 10 mV s−1, and (b) the relationship between the cycle number and the redox current densities for the PEDOT-ECP-43 CE.

Figure 7 (a) Nyquist plots and (b) Tafel polarization curves of the dummy cells based on the various PEDOT-ECP-based and Pt/FTO CEs. Figure 8 Photovoltaic performances of the DSCs based on the various PEDOT-ECP-based and Pt/FTO CEs. The inset in Fig. 8 shows the photograph for the 35

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as-prepared PEDOT-ECP-43 CE.

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Table 1 Sheet resistance values of ECP-33, ECP-43, ECP-50, ECP-56 and Pt/FTO CEs, and the corresponding photovoltaic parameters of the DSCs based on them. Jsc / mA cm-2

Voc / V

FF

η (%)

ECP-33 ECP-43 ECP-50

5.70 ± 0.66 2.02 ± 0.19 1.29 ± 0.19

10.76 13.54 14.07

0.64 0.65 0.66

0.17 0.19 0.20

1.16 1.71 1.89

ECP-56 Pt/FTO

1.08 ± 0.11 7.87 ± 0.08

14.69 15.16

0.64 0.77

0.36 0.62

3.29 7.20

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σ / Ω cm-2

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Table 2 Photovoltaic parameters of the DSCs based on various CEs and electrochemical parameters from CV and EIS measurements. D / cm2 s-1

Rs /Ω

Rct /Ω

ZN /Ω

PEDOT-ECP-33 PEDOT-ECP-43 PEDOT-ECP-50

352 334 306

23.16 17.04

7.17 4.44

5.43 4.40

PEDOT-ECP-56 Pt/FTO

393 250

2.17 × 10-6 3.84 × 10-6 4.65 × 10-6 8.67 × 10-6

16.54 16.72 25.37

2.85 2.15 4.90

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3.84 2.13 0.06

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—

Jsc /mA cm-2

Voc / V

FF

η (%)

14.01 14.49

0.74 0.75

0.57 0.62

6.29 6.82

14.71 14.99 15.16

0.76 0.76 0.77

0.62 0.63 0.62

6.88 7.17 7.20

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Highlights 1. ECP plate was fabricated via series of chemical/mechanical processes. 2. ECP decorated with PEDOT showed improved catalytic activity for I3- reduction. 3. The DSC with the bended PEDOT-ECP CE reached an efficiency of 6.77%.

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4. PEDOT-ECP CE could serve as a promising Pt- and TCO-free flexible CE.

ACCEPTED MANUSCRIPT Supporting Information for “Flexible carbon nanotube/polypropylene composite plate decorated with poly(3,4-ethylenedioxythiophene) as efficient counter

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electrodes for dye-sensitized solar cells”

Jeng-Yu Lin,* Wei-Yen Wang and Shu-Wei Chou

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Department of Chemical Engineering, Tatung University, No. 40, Sec. 3, ChungShan North Rd., Taipei City 104, Taiwan

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Corresponding author

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[*]

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Fax: +886-225861939 E-mail: [email protected]

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Fig. S6 Photovoltaic performances of the DSCs based on the PEDOT/FTO CE. The corresponding photovoltaic parameters are also presented in the inset of Fig. S6.

16

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PEDOT-ECP-43 after bending PEDOT-ECP-50 after bending

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Current density / mA cm

-2

14

8 6 4

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0 0.0

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0.4

0.6

0.8

Voltage / V

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Fig. S7 Photovoltaic performances of the DSCs based on the bended PEDOT-ECP-43 and PEDOT-ECP-50 CEs.

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Fig. S1 FESEM images of (a) CP-33 and (b) ECP-33 plates.

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Fig.S2 FTIR spectra of CNT, PP, and ECP-33.

Fig. S3 FTIR spectrum of PEDOT-coated ECP-33 CE. s-2

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(b)

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Fig. S4 Plots showing the correlation between current density and scan rate for (a) PEDOT-ECP-33, (b) PEDOT-ECP-43, (c) PEDOT-ECP-50, and (d) PEDOT-ECP-56 CEs.

Fig. S5 (a) The configuration of dummy cell used for EIS and Tafel measurements. (b) The equivalent circuit model used for fitting the EIS results of the dummy cells. Rs: serial resistance; Rct(Pt): charge-transfer resistance of Pt/FTO; Rct(CE): charge-transfer resistance of the CEs; CPE(Pt): constant phase element of Pt/FTO; CPE(CE): constant phase element of the CEs; ZN: Nernst diffusion resistance.

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