Morphological Influence of Polypyrrole Nanoparticles on the Performance of Dye–Sensitized Solar Cells

Electrochimica Acta 155 (2015) 263–271

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Morphological Influence of Polypyrrole Nanoparticles on the Performance of Dye–Sensitized Solar Cells Ling-Yu Chang a , Chun-Ting Li b , Yu-Yan Li b , Chuan-Pei Lee b , Min-Hsin Yeh b , Kuo-Chuan Ho a,b, * , Jiang-Jen Lin a, ** a b

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

A R T I C L E I N F O

A B S T R A C T

Article history: Received 9 August 2014 Received in revised form 6 December 2014 Accepted 19 December 2014 Available online 24 December 2014

Polypyrrole (PPy) with various morphologies were synthesized by chemical oxidative polymerization and further used as the counter electrode (CE) in dye–sensitized solar cells (DSSCs). The conventional anionic surfactant, docusate sodium salt (AOT), cationic surfactant, cetylmethyl ammonium bromide (CTAB), and the newly developed polymeric dispersant, poly(oxyethylene)–imide (POEM), were employed in the PPy synthesis. Scanning electron microscopy images (SEM) revealed diversified morphologies of the synthesized PPy nanoparticles with irregular sheet (IS), hierarchical nanosphere (HNS), and nanosphere (NS), corresponded to the choice of the commercial surfactants AOT and CTAB, as well as the home–made POEM, respectively. Fourier transform spectroscopy (FT–IR) and X–ray diffraction (XRD) analysis were used to confirm the PPy structures and crystalline properties. When fabricated into films and used as CE in DSSCs, the PPy–HNS demonstrated the superior cell efficiency of 6.71
0.16% to those of PPy–IS (5.46
0.31) and PPy–NS (6.31
0.24), respectively. The excellent electrocatalytic ability of PPy–HNS was essentially attributed to its high electrochemical surface area (Ae), which was quantitatively calculated through a rotating disk electrode system by using the Koutecky– Levich equation. Brunauer–Emmett–Teller (BET) surface area measurement, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were also used to substantiate the explanation for the DSSC performances. ã 2014 Elsevier Ltd. All rights reserved.

Keyword: Conducting polymer Dye–sensitized solar cells Polymeric dispersant Poly(oxyethylene)–imide (POEM) Polypyrrole

1 Introduction Dye–sensitized solar cells (DSSCs) are important power sources, as they are expected to provide a partial answer to many environmental and energy problems. In addition, these types of cells have many advantages, such as low material cost, easy fabrication, and reasonably high efficiency for energy conversion [1–3]. Various efforts have been made to enhance the overall performance of DSSCs, such as modifying the morphology of nanocrystalline semiconductor [4–6] and the molecular structures of organic sensitizers [7,8] as well as using low–volatility electrolytes [9–12] and new counter electrode (CE) materials [13–24]. In DSSCs, the CE is one of the most important and indispensable components. The reactions at the CE rely on the type of redox

* Corresponding author. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040. ** Corresponding author. Tel.: +886 2 3366 5312; fax: +886 2 3366 5237. E-mail addresses: [email protected] (K.-C. Ho), [email protected] (J.-J. Lin). http://dx.doi.org/10.1016/j.electacta.2014.12.127 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.

couple in the electrolyte. Traditionally, the I/I3 couple has been adopted as the redox mediator. CE of DSSC is usually made of platinum (Pt), which has high conductivity, stability and catalytic activity for the reduction of I3 ions. However, Pt is an extremely rare metal and is thus a factor to be considered for cost–effective fabrication of a DSSC, although it shows a high catalytic activity for the reduction of I3 ions and an excellent electronic conductivity. Replacement of Pt with other cheaper catalytic materials is required for the reducing the production cost of the DSSCs, especially when the production is in mass scale. Several carbonaceous materials, e.g., activated carbon [13], carbon fiber [14], graphitic/grapheme [15], multi– walled carbon nanotube (MWCNT) [16], and single–walled carbon nanotube (SWCNT) [17] have become potential materials to substitute Pt. The inorganic transition metallic compounds, e.g., NiS [18], MoS2/C [19], and CoS/CNT [20], have also been proposed to replace the Pt for the CEs of DSSCs, because of their low cost, high conductivity and good catalytic ability for the reduction of I3 ions. Meanwhile, conducting polymers are also considered as one of the alternatives for Pt for the CEs of DSSCs; The conducting

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polymers, such as poly(3,4–ethylenedioxythiphene) (PEDOT) [21], polyaniline (PANI) [22], and polypyrrole (PPy) [23–28], are promising CE materials used in DSSC, because of their unique properties, such as inexpensive material, good conductivity, stability and catalytic activity for the reduction of I3 ions [21–28]. Among all the conducting polymers, PPy is of special interest due to its remarkable conductivity and electrochemical catalytic properties [29] for the application in field emission display device [30], sensors [31], and DSSCs [23–28]. Gong et al. [23], incorporated reduced graphene oxide into PPy matrix and used it as the CE of DSSCs to obtain a cell efficiency of 8.14%. Jeon et al. [24], synthesized PPy nanospheres by chemical polymerization and applied them to the CE of DSSCs, which achieved a cell efficiency of 7.73%. Wu et al. [25], also constructed PPy nanospheres for the use of the CE in DSSCs, and they obtained a cell efficiency of 7.66%. Makris et al. [26], fabricated a PPy–based CE for the quasi–solid– state DSSC by electrochemical deposition, and obtained a cell efficiency of 4.60%. Some reports indicated that the morphologies and the structures of PPy can be tuned by incorporating the surfactant or polymer–based stabilizer in the synthesized processes and being applied as the CE in DSSCs [27]. Peng et al. [28], have reported a composite film of PPy nanorod network and carbon black nanoparticles as the catalytic layer on the CE in DSSC, and achieved a cell efficiency (h) of 7.2%, this was due to the good catalytic property of PPy for reducing I3 and the higher surface active area from its nanorod network structure. Recently, we reported the synthesis of a novel amphiphilic polymer, i.e., poly (oxyethylene)–imide (POEM), which can function as the surfactant (or stabilizer) to well control the morphologies and sizes of metal nanoparticles [32]. Accordingly, the POEM seems a promising surfactant (or stabilizer) for synthesizing the PPy nanoparticles containing different nanostructures. In this study, we successfully synthesize three kinds of PPy nanoparticles containing different morphologies via a simple surfactant-assisted solution process. By incorporation of the anionic-type surfactant (docusate sodium salt, AOT), the cationic-type surfactant (acetylmethyl ammonium bromide, CTAB), and our home-made amphiphilic-type polymer (POEM), the synthesized PPy nanoparticles resulted in different architectures of irregular sheet (IS), hierarchical nanosphere (HNS), and nanosphere (NS), respectively. The DSSCs using the CEs with PPy–IS, PPy–HNS and PPy–NS exhibited cell efficiencies of 5.46
0.31, 6.71
0.16, and 6.31
0.24%, respectively. The main novelty of our study is to report the morphological variance of the PPy nanoparticles via using different types of surfactants (anionic-type, cationic-type and amphiphilic-type), and the DSSC performance and electrocatalytic activities of the corresponding PPy CEs are also investigated. Another novelty is to introduce, for the first time, our home-made amphiphilic-type polymer (POEM) as the surfactant for PPy synthesis; it is illustrated that PEOM is suitable for the synthesis of conducting polymer-based nanoparticles. Among all PPy CEs, the PPy–HNS CE renders its DSSC the highest efficiency of 6.71
0.16%, which is 90% compared to that of the cell with Pt CE. The PPy–HNS CE also suggests a low-cost and high efficient substitution of Pt CE. Moreover, the simple surfactant-assisted solution process of PPy nanoparticles is beneficial to large-scale production and marketing of DSSCs in the future.

0.95 meq g1 with a formula of 39 oxyethylene and 6 oxypropylene units). Monomer, 4,40 –oxydiphthalic anhydride (ODPA, 97% purified by sublimation), titanium (IV) tetra–iso–propoxide (TTIP > 98%), acetonitrile (ACN, 99.99%), isopropyl alcohol (IPA, 99.5%), cetylmethylammonium bromide (CTAB, 99%), and docusate sodium salt (AOT, 99%) were obtained from Aldrich Chemical Co. Tetrahydrofuran (THF, 95%) and ethyl alcohol (99.5%) were purchased from Teida Chemicals. Pyrrole (Py, 98%), lithium iodide (LiI, chemical grade), iodine (I2, chemical grade), and poly(ethylene glycol) (PEG, 20,000 MW) were obtained from Merck Chemical Co. 4–Tert–butylpyridine (TBP, 96%) was obtained from Acros Chemicals. Ferric chloride (FeCl3, 99.5%), dimethyl sulfoxide (DMSO, 99.7%) and 3–methoxypropionitrile (MPN, 99%) were obtained from Riedel–deHaën (Fluka). Hydrochloric acid (HCl, 37%) and 2–methoxyethanol (99.95%) were obtained from Sigma–Aldrich. The TiO2 colloidal solution was prepared by a sol–gel method. TTIP (69.10 g, 0.24 mole) was added to 430 ml of 0.1 M nitric acid aqueous solution with continuous stirring and heating at 85  C for 8 h. When the resultant colloid was cooled down to room temperature, it was transferred to an autoclave (Parr 4540, USA); the temperature of the autoclave was maintained at 240  C for 12 h. The prepared TiO2 colloid was concentrated to 10 wt%; the obtained TiO2 had particles of about 20 nm in diameter. Large-sized TiO2 (PT–501 A, 99.74%, 15 m2 g1 surface area, 100 nm diameter) was purchased from Ya Chung Industrial Co. Ltd. Taiwan, for using as scattering particles in the TiO2 photoanode. 2.2. Synthesis of PPy nanoparticles and preparation of the PPy films A home-made sufactant, poly(oxyethylene)–imide (POEM), was prepared by a standard synthesizing prodedure according to our previous report [12]. The number-average molecular weight (Mn) was measured by gel permeation chromatography technique (GPC; Waters apparatus 515HPLC pump; 717 auto sampler; 2410 refractive index detector; the waters style gel columns set: HR2 and HR4E were used with a 1.0 ml/min flow rate of THF, calibrated by polystyrene standards (Showa Denko, Shodex Standard SM-105)) to obtain Mn = 13,000 and PDI = 2.13. Scheme 1 represents the synthesis of poly(oxyethylene)–imide (POEM) by two–step process of forming POE–amidoacids and POEM at elevated temperature [12]. Py was purified by distillation under reduced pressure and stored in a refrigerator at about 4–8  C before use. Three kinds of monomeric solutions were separately prepared by mixing the 0.06 mole (4.03 g) purified–Py monomer with 0.02 mole of different surfactants (AOT, CTAB or POEM) in 200 ml aqueous solution under stirring for 1 h. The 0.12 mole of FeCl3 oxidant

2. Experimental 2.1. Materials Poly(oxyethylene)–diamine or poly(oxypropylene– oxyethylene–oxypropylene) segmented polyether of bis(2–aminopropyl ether) (POE2000, 2,000 MW) was a solid and water–soluble compound (waxy solid, melting point 37–40  C, amine content

Scheme 1. Synthesis of poly(oxyethylene)–imide (POEM) by two–step process of forming POE–amidoacids and POEM at elevated temperature.

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solution is prepared with 10 ml distilled water and stirred for 15 min. Then the oxidant solution was dropwisely added into the monomeric solutions individually. The polymerization was carried out for 4 h at room temperature. The product was collected by centrifugation (10,000 rpm for 30 min as a period). The collected solid was then washed with ethanol several times under the same centrifugation conditions. The black PPy powders were dried in a vacuum drier at 60  C for 12 h [24]. FT–IR and XRD analyses were used to verify the formation and clarify the crystalline of synthesized–PPy nanoparticles, respectively. The PPy nanoparticles synthesized by incorporating surfactants of AOT, CTAB, and POEM show the productivity of 3.66 g, 3.39 g, and 2.98 g, respectively. From which, the quantitative yields of PPy nanoparticles were 91, 84 and 74%, respectively, which can be determined by the weight percentage of each product to its pyrrole monomer. The as–prepared PPy nanoparticles were dispersed in methanol solution and then coated onto the fluorine–doped SnO2 conducting glass (FTO, 7 V sq.1, visible transmittance 380%, NSG America Inc. USA) as the CE of DSSC by using the doctor blade technique with a thickness of 5 mm. The obtained FTO/PPy CEs were heated to 60  C for 2 hrs in vacuum to remove the solvent. To improve the electrical conductivity of the PPy films, the FTO/PPy CEs were treated with HCl vapor for 30 min [24,33–35]. The procedure of PPy synthesis is shown in Fig. 1. 2.3. Fabrication of DSSCs The FTO was first cleaned with a neutral cleaner, and then washed with DI–water, acetone, and IPA, sequentially. The conducting surface of the FTO was treated with a solution of TTIP (1.0 g, 0.0035 mol) in 2–methoxyethanol (3.0 g, 0.039 mol) for obtaining a good mechanical contact between the conducting glass and TiO2 film, as well as to isolate the conducting glass surface from the electrolyte. TiO2 paste was coated onto the treated conducting glass by using the doctor blade technique. The dried TiO2 film was gradually heated to 450  C in an oxygen atmosphere, and subsequently sintered at that temperature for 30 min. The TiO2 photoanode of the DSSC was composed of a 14 mm thick TiO2 layer and a scattering layer of 4.5 mm thickness. After sintering at 450  C and cooling to 80  C, the TiO2 photoanode was immersed in a 3  104 M solution of N719 dye (Solaronix S.A., Aubonne, Switzerland) at room temperature for 24 h. N719 dye was dissolved in a mixing solvent of ACN and n–butanol (volume ratio of 1:1). The thus prepared FTO/TiO2/dye electrode was placed on a FTO/PPy CE, keeping the two electrodes separated by a 25 mm–thick surlyn1

Fig. 1. Schematic illustration of the procedure used to synthesize PPy nanoparticles.

265

film (SX1170–25, Solaronix S.A., Aubonne, Switzerland). The two electrodes were sealed by heating. A mixture of 0.1 M LiI, 0.6 M DMPII (Solaronix S.A., Aubonne, Switzerland), 0.05 M I2, and 0.5 M TBP in MPN/ACN (volume ratio of 1:1) was used as the electrolyte. The electrolyte was injected into the gap between the electrodes by capillarity. 2.4. Measurements and instruments The surface morphology of PPy nanopaticles and of PPy films were observed by a scanning electron microscope (SEM, NanoSEM 230, NovaTM), atomic force microscopy (AFM, Innova SPM, Vecco, USA), and microfigure measuring instrument (Surfcorder ET3000) separately. For AFM analysis, the non-contact mode probe is purchased from Nanoworld AG, Swizerland. According to the manufacturer's specification, the probe features a rectangular cantilever with a triangular free end and a tetrahedral tip (height = 1015 mm, diameter of a curvature < 20 nm). Brunauer–emmett–teller (BET) surface areas and total pore volumes of the PPy nanopaticles were asquired using a Micromeritic analyzer (ASAP 2000, Micromeritic Co., USA). Crystal phase and crystallinity of PPy nanopaticles were analyzed by X–ray diffraction (XRD, MO3XHF, Mac). The Fourier transform infrared (FT–IR) spectra of PPy nanoparticle was recorded on a Perkin Elmer Spectrum 100 FT–IR. The surface of the DSSC was covered by a mask allowing an area of 0.16 cm2 for light illumination. The DSSC was illuminated by a class–A quality solar simulator (XES–301 S, AM1.5G, San–Ei Electric Co., Ltd.). Incident light intensity (100 mW cm2) was calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc.). Photocurrent density–voltage curves of the DSSCs were obtained with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco–Chemie, the Netherlands). Electrochemical impedance spectra (EIS) were obtained by the above–mentioned potentiostat/galvanostat, equipped with an FRA2 module, under a constant light illumination of 100 mW cm2. The frequency range explored was 10 mHz to 65 kHz. The applied bias voltage was set at the open–circuit voltage of the DSSC, between the CE and the FTO/TiO2/dye working electrode, starting from the short–circuit condition; the corresponding AC amplitude was 10 mV. The impedance spectra were analyzed using an equivalent circuit model [36,37]. The catalytic ability of the CEs was measured using cyclic voltammetry (CV). CV was carried out in a three–electrode electrochemical system in an ACN-based electrolyte containing 10.0 mM LiI, 1.0 mM I2 and 0.1 M LiClO4. Various FTO/PPy CEs with 1 cm2 performed as the working electrode separately; a Pt foil and a Ag/Ag+ electrode served as the counter electrode and reference electrode, respectively. The scan rate of CV measurement was set at 100 mV s1. Here, the Ag/Ag+ reference electrode worked in an ACN-based buffer solution containing 0.01 M AgNO3 and the calibrated potential of the Ag/Ag+ reference electrode is 0.492 V versus normal hydrogen electrode (NHE). A rotating disk electrode (RDE) based on a glassy carbon electrode (GCE, working area: 0.2472 cm2, Part #AFE7R9GCGC, PINE Instrument Company) was coated with a thin film of PPy–IS, PPy–HNS and PPy–NS by drop coating (10 mL, 100 mg catalyst) and dried at ambient temperature. The RDE coated with the film of PPy–IS, PPy–HNS and PPy–NS CE had served as the working electrode, and a Pt wire and an Ag/Ag+ electrode were employed as the auxiliary electrode and the reference electrode, respectively. The RDE system was equipped with a modulated speed rotator (PINE Instrument Company) and was connected to a potentiostat (model 900B, CH Instruments). The electrolyte used was an ACN based solution, containing 0.1 M LiClO4 and 1.0 mM TBAI3. All potentials reported were referred to the reference electrode of Ag/ Ag+. Five linear sweep voltammetric (LSV) curves (not shown) were acquired for each of the above films of RDE, by controlling rotating

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speeds (50, 100, 200, 400, and 600 rpm), at a scan rate of 1 mV s1 (scan range: -0.1 to 0.5 V vs. Ag/Ag+). A corresponding current, i, could be obtained from each of the LSV curves at formal potential (E0). Thus, plots of i1 vs. v1/2 were made for the RDE with PPy–IS, PPy–HNS and PPy–NS film. Incident photon–to–current conversion efficiency (IPCE) curves were obtained at the short–circuit condition. The light source was a class A quality solar simulator (PEC–L11, AM1.5G, Peccell Technologies, Inc.); light was focused through a monochromator (Oriel Instrument, model 74100) onto the photovoltaic cell. The monochromator was incremented through the visible spectrum to generate the IPCE (l) as defined below, IPCE (l) = 1240 (JSC/lf)

PPy–HNS and PPy–NS exhibit similar broad characteristic peaks in the 2u region of 15 to 30 , suggesting that the resulting polypyrrole nanoparticles are poorly crystalline in nature. This

(1)

where l is the wavelength (nm), JSC is the short–circuit photocurrent density (mA cm2) recorded with a potentiostat/ galvanostat, and f is the incident radiative flux (W m2) measured with an optical detector (Oriel Instrument, model 71580) and power meter (Oriel Instrument, model 70310). 3. Results and discussion 3.1. Scanning electron microscopy (SEM), Atomic force microscopy (AFM) and Brunauer–Emmett–Teller (BET) analyses Fig. 2 shows the SEM images of various PPy nanoparticles in the form of powder. In Fig. 2a and its inset, the PPy–IS nanoparticles (obtained in the presence of AOT surfactant) show the irregular nanosheet structure with an average sheet thickness of about 15 nm. In Fig. 2b, the PPy–HNS nanoparticles (obtained in the presence of CTAB surfactant) show the hierarchical nanospherical structure with an average diameter of 100200 nm. In Fig. 2c, the PPy–NS (obtained in the presence of POEM surfactant) shows the nanospherical structure with an average diameter of 150200 nm. By careful examination on the insets of Fig. 2b and Fig. 2c, a PPy– HNS nanoparticle possesses an obviously rough surface, while a PPy–NS nanoparticle owns a relatively smooth surface. On the other hand, a tapping–mode AFM analysis was used to quantify the surface roughness of various PPy in the form of thin film at lower resolution, as shown in Fig. 3. The values of the root mean square (RMS) surface roughness of the PPy films with PPy–IS, PPy–HNS and PPy–NS were estimated to be 32.8, 63.4 and 54.4 nm, respectively. To clarify the effect of surface area on the morphology of the PPy nanoparticles, surface areas of powdered PPy nanoparticles were calculated using the BET method and the values of surface area are listed in Table 1. The PPy–IS film has least BET surface area (34.80 m2 g1), compared to those of films with PPy–HNS (61.18 m2 g1) and PPy–NS (58.42 m2 g1). These differences in surface areas are obviously due to the differences in the morphologies of the PPy nanoparticles. Some reports had already identified that the CE with the high roughness would enhance its electrocatalytic activity for I/I3 redox reaction [38]. Therefore, the attractive performance of the DSSC with PPy–HNS CE could be expected due to its superior surface roughness and surface area. 3.2. Fourier transform infrared spectroscopy (FT–IR) and X–ray diffraction (XRD) analyses FT–IR spectra of PPy nanoparticles and Py monomer are presented in Fig. 4a. The FT–IR spectrum of PPy shows the characteristic bands attributable to the C–H in–plane deformation vibration at 1032 cm1, C–C asymmetric stretching vibration at 1408 cm1, ring–stretching mode of Py ring at 1572 cm1 [39]. XRD patterns of PPy nanoparticles are shown in Fig. 4b. The PPy–IS,

Fig. 2. SEM images of PPy nanoparticles with different morphologies,. (a) PPy–IS, (b) PPy–HNS and (c) PPy–NS. The high magnifications images are also shown in the insets.

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267

Fig. 3. AFM images of PPy films (a) PPy–IS, (b) PPy–HNS and (c) PPy–NS.

agrees well with the results reported in literature [40]. The maximum peak for all the PPy nanoparticles was observed at 23.9 , which indicates that the peak may arise from the p-p interaction of PPy chains [41]. 3.3. Photovoltaic performance of the DSSCs with CE films of PPy nanoparticles having various morphologies Fig. 5a shows the photocurrent density–voltage curves of the DSSCs with CEs having the films of PPy nanoparticles of various morphologies, i.e., IS, HNS and NS. The photocurrent density– voltage (I–V) characteristics of the DSSCs were measured at the illumination of 100 mW cm2. The corresponding parameters of open–circuit voltage (VOC), short–circuit current (JSC), fill factor (FF), and cell efficiency (h) are listed in Table 1. The photovoltaic parameters of the DSSC with the PPy–IS in its CE are, VOC = 0.70
0.01 V, JSC = 11.52
0.81 mA cm2,FF = 0.67
0.01, and h = 5.46
0.31%. When the CE of the DSSC has the PPy–HNS, the

photovoltaic parameters are, VOC = 0.70
0.01 V, JSC = 16.49
0.10 mA cm2, FF = 0.58
0.01, and h = 6.71
0.16%. The photovoltaic parameters of the DSSC with the PPy–NS in its CE are, VOC = 0.70
0.01 V, JSC = 14.10
0.06 mA cm2, FF = 0.63
0.02, and h = 6.31
0.24%. Thus, the DSSC using the CE with PPy–HNS shows the highest cell efficiency with reference to those of the cells with CEs having PPy–IS and PPy–NS, and gives a slightly lower cell efficiency with the bare DSSC having the sputtered Pt CE (75 nm thickness), which achieves a cell efficiency of 7.47% under the same illumination conditions (see Fig. S1 in the electronic supplementary information). Based on the BET and AFM results, PPy-HNS exhibits the largest surface area (61.18 m2 g1) and highest roughness (63.40 nm), respectively, which benefit the electrocatalytic activity for I/I3 redox reaction; this has apparently led to a decrease in the charge transfer resistance at the CE of the pertinent DSSC, thereby to a high JSC, and ultimately to a high cell performance. From the BET surface areas, the lower surface area of the film of PPy–IS reflected the lower electrocatalytic activity for

Table 1 Photovoltaic parameters of the DSSCs with CEs having the films of PPy–IS, PPy–HNS and PPy–NS, measured at 100 mW cm2 light intensity. The table also shows the corresponding values of the surface area (SBET), active surface area (Ae) and standard heterogeneous rate constant (k0) were obtained through BET and RDE analysis. Counter Electrode PPy–IS PPy–HNS PPy–NS

SBET (m2 g1)

h

Voc (V)

Jsc (mA cm2)

FF

(%)

k0 (x104 cm s1)

Ae (cm2)

34.80 61.18 58.42

5.46
0.31 6.71
0.16 6.31
0.24

0.70
0.01 0.70
0.01 0.70
0.01

11.52
0.81 16.49
0.10 14.10
0.06

0.67
0.01 0.58
0.01 0.63
0.02

8.46 8.64 8.15

0.22 0.35 0.32

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HNS CEs with favorable surface morphologies offered larger surface area, resulting in higher electrocatalytic activity for the reduction of I3 and I ions at the CE [15]. Moreover, electrochemical impedance spectroscopy (EIS) technique was employed to analyze the interfacial resistances in the DSSCs. Fig. 7 shows the EIS data of the DSSCs with the common configuration of FTO/TiO2/dye/electrolyte/PPy nanparticles/FTO, in which the PPy nanoparticle morphologies are IS, HNS and NS. The equivalent circuit is shown as an inset in the figure. In general, EIS spectrum of a DSSC shows three semicircles in the frequency range of 10 mHz to 65 kHz. The ohmic series resistance (Rs) is determined at the high frequency region where the phase is zero. The first and second semicircles in the middle frequency range represent the heterogeneous electron transfer resistance at the CE/electrolyte interface (Rct1) and TiO2/dye/electrolyte interface (Rct2), respectively [36,37]. The Warburg diffusion process of I/I3 in the electrolyte (Rdiff) is represented by the third semicircle. Rdiff is virtually overlapped by Rct2 due to the short length for I– diffusion available within the thin spacer used (25 mm thick), and owing to the low viscosity of the solvents used in our electrolyte (viscosities of ACN and MPN are 0.37 cp and 1.60 cp, respectively) [12]. In Fig. 7, the similar small Rs values of 16.5 ohm can be clearly obtained from all DSSCs devices, which indicated the successful electron transport through the FTO/CE interface. This implies the good adhesion between the FTO and various PPy catalytic films separately [42]. For further investigating the interfacial electron transfer phenomenon between the CE and I/I3 electrolyte, the Rct1 values of the DSSCs coupled with PPy–IS, PPy–HNS and PPy–NS CEs showed 10.25, 3.82 and 4.49 ohm cm2, respectively. From

Fig. 4. (a) FT–IR spectra of PPy nanoparticles and Py monomer; (b) XRD patterns of different PPy nanoparticles.

I3 reduction and consequently the lower JSC of the cell (Table 1). Thus, it obviously shows that the cell efficiency strongly depend on the morphologies and porous surface of the PPy nanoparticles in their CEs. Fig. 5b shows the IPCE curves of the DSSCs with CEs having the films of PPy–IS, PPy–HNS, and PPy–NS, there were measured by monitoring the photocurrent at different wavelengths. The results of IPCE measurement are consistent with the JSC values of the DSSCs with CEs having the films of PPy–IS (11.52
0.81 mA cm2), PPy–HNS (16.49
0.10 mA cm2) and PPy– NS (14.10
0.06 mA cm2). These observations are in consistency with those obtained from the cyclic voltammetry (CV) analysis for the three types of CEs, as can be understood in the following. 3.4. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses The catalytic properties of the CEs were studied by CV. Fig. 6 shows the CV curves of the CEs with PPy–IS, PPy–HNS, and PPy–NS. Each CV curve shows an anodic peak current density (Ipa) and a cathodic peak current density (Ipc), corresponding to the oxidation of I ions and the reduction of I3 ions, respectively. The magnitude of the Ipc represents the catalytic ability of a CE for I3 reduction. It is clear that the value of the Ipc for the CE with PPy–HNS is higher than those of the CEs with PPy–IS and PPy–NS; this confirms the highest electrocatalytic ability and highest current value of the CE with PPy–HNS, among all the CEs. Consequently, the rough PPy–

Fig. 5. (a) Photocurrent density–voltage curves and (b) IPCE spectra of the DSSCs with different CEs having the films of PPy–IS, PPy–HNS and PPy–NS, separately, measured at 100 mW cm-2 light intensity.

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269

by using Koutecky Levich equation, which relates the current (i) to the rotating speed (v); the equation can be written as follows [44]: 1 1 1 ¼ þ i nFAe k0 C 0:62nFAe D2=3 n1=6 v1=2 C

Fig. 6. CV curves of the CEs with PPy–IS, PPy–HNS and PPy–NS.

which, the least Rct1 of the DSSC with PPy–HNS CE shows the best electrocatalytic ability of PPy–HNS CE than the others. The best electrocatalytic ability of PPy–HNS CE triggers the most amount of I3 reduction, and the highest JSC of the pertinent DSSC can be well explained. However, the electrocatalytic ability of each PPy film caused the difference of Rct2, which shows an order of PPy– IS > PPy–NS > PPy–HNS. It can be understood that the higher electrocatalytic ability causes the more retardation of the electron recombination reaction at the TiO2/dye/electrolyte interface [43]. 3.5. Rotating disk electrode for the determination of electrocatalytic abilities at PPy–IS, PPy–HNS and PPy–NS CEs

(2)

where i is the current, n = 2 is the electrons involved in the reaction, F is Faraday's constant, Ae is the active surface area, CI3- = 1.0 mM is the concentration of I3–, k0 is the standard heterogeneous rate constant, D is the diffusion coefficient of I3–(3.62  106 cm2 s1), n = 5.0  103 cm2 s1 is the kinematic viscosity of ACN, and v is the angular velocity converted from the rotating speed. The parameters of k0 and Ae of these three films are listed in Table 1. The results indicate that the PPy–HNS film with hierarchical particles rendered a larger active surface area for reducing the I3 (Ae = 0.352 cm2), compared to those of PPy–IS (Ae = 0.216 cm2) and PPy–NS film (Ae = 0.315 cm2). The highest value of Ae for PPy–HNS film was attributed to its inherent porous structure and high specific surface area (61.18 m2 g1), compared to those of PPy–IS (34.80 m2 g1) and PPy–NS (58.42 m2 g1). The standard heterogeneous rate constant, k0, can be considered as the heterogeneous rate constant at the CE/electrolyte interface at the formal potential. On the other hand, support the result that all these films are made of the same materials, ppy, thus having the same value of k0. The nearly constant values of k0 for films of PPy–IS (8.46  104 cm s1), PPy–HNS (8.64  104 cm s1), and PPy–NS (8.15  104 cm s1) are due to the intrinsic property of the same material, PPy, in them. 3.6. Conductivities of the CE films of PPy nanoparticles having various morphologies

In a DSSC, the good catalytic layer on the CE should possess both a high electrocatalytic activity and a high electroactive surface area. However, CV cannot distinguish these two parameters at the same time but just reveal the overall electrocatalytic ability of the CE. These two parameters, the standard heterogeneous rate constant (k0) and the electroactive surface area (Ae) can be determined simultaneously by using a RDE, through linear sweep voltammetry. Fig. 8 shows plots of i1 vs. v1/2 for the RDE with thin films of PPy–IS, PPy–HNS and PPy–NS. The values of k0Ae and Ae were calculated for these films from the intercept and the slope of the plot, respectively, by using Koutecky Levich equation, after fitting the data in the plots. The values of k0 and Ae were extracted

The conductivities of the CE films of PPy nanoparticles having various morphologies can be used to further explain the results mentioned above, and the corresponding conductivities can be recorded by four–point probe instrument. The synthesized–PPy CEs leading to the attractive electrical conductivities of PPy–IS (11.8 S cm1), PPy–HNS (9.5 S cm1), and PPy–NS (10.3 S cm1), which are attributed to their promising synthesizing procedures. The higher electrical conductivity of the CEs directly relates to the lesser electrons trapped in the PPy films, which results in the higher FF value of the DSSCs. The corresponding FF values of the DSSCs coupled with PPy–IS (0.67
0.01), PPy–HNS (0.58
0.01), and PPy–NS (0.63
0.02) show the good consistency with their electrical conductivities. On the other hand, the higher electrical

Fig. 7. EIS spectra of the DSSCs with various CEs of PPy–IS, PPy–HNS and PPy–NS, measured at 100 mW cm-2 (the characteristic frequencies are also indicated).

Fig. 8. Plots of i-1 vs. v-1/2 for the films of PPy–IS, PPy–HNS and PPy–NS.

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conductivity benefits the elelctrochemical properties of the CEs obtained from CV and EIS analyses mentioned above, i.e., the better electrocatalytic activity (Ipc) and lower charge transfer resistance (Rct1). However, the PPy–HNS film with least electrical conductivity can provide the largest Ipc and lowest Rct1 values, which can be attributed to its highest effective surface area (Ae) and surface roughness so as to compensate the energy loss caused by the unfavorable electrical conductivity. Therefore, the highest DSSC efficiency based on the cell with PPy–HNS CE is expected.

[7]

[8]

[9]

[10]

4. Conclusions We synthesized the PPy nanoparticles with various morphologies by using different surfactants of AOT, CTAB, and the homemade POEM; accordingly, three different morphologies, irregular sheet (IS), hierarchical nanosphere (HNS) and nanosphere (NS), were observed by SEM, respectively. Through drop coating on FTO substrate, the PPy–HNS film possesses larger BET surface area of 61.18 m2 g1 and RMS surface roughness of 63.40 nm, than the films of PPy–IS (34.80 m2 g1; 32.80 nm) and PPy–NS (58.42 m2 g1; 54.4 nm). The PPy–HNS based CE shows the relatively high electrocatalytic ability for the reduction of I3–and the lowest charge transfer resistance (Rct1), as compared to PPy–IS and PPy–NS. The RDE analysis confirms that the CE with the PPy– HNS film has the largest electroactive surface area among the three films. Ultimately, the DSSC with a CE of PPy–HNS film exhibits a cell efficiency (h) of 6.71
0.16%, superior to the other two films fabricated in the devices. Compared to the relevant literatures about PPy CEs in DSSCs, we synthesized two novel structures (irregular sheet and hierarchical nanosphere) of PPy nanoparticles and applied them as the electrocatalytic materials for the CEs in DSSCs. In this study, we provided various and detailed electrochemical analyses, including CV, EIS, and RDE, for investigating the electrocatalytic ability of synthesized PPy CEs. Especially for the RDE analysis, the values of the standard heterogeneous rate constant and electroactive surface area of various synthesized PPy CEs for I3 reduction were determined for the first time. Acknowledgements We acknowledge the financial supports received from the Ministry of Economic Affairs (101-EC-17-A-08-S1-205) and the Ministry of Science and Technology (MOST) of Taiwan. Some of the instruments used in this study were made available through the financial support of the Academia Sinica, Nankang, Taipei, Taiwan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2014.12.127.

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