Bifacial dye-sensitized solar cells from covalent-bonded polyaniline–multiwalled carbon nanotube complex counter electrodes

Journal of Power Sources 275 (2015) 489e497

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Bifacial dye-sensitized solar cells from covalent-bonded polyanilineemultiwalled carbon nanotube complex counter electrodes Huihui Zhang a, b, Benlin He a, b, *, Qunwei Tang a, b, *, Liangmin Yu a, c, * a b c

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, PR China Qingdao Collaborative Innovation Center of Marine Science and Technology, Ocean University of China, Qingdao 266100, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


PANieMWCNT complexes are synthesized by a reflux method.
PANieMWCNT complexes are employed as cost-effective CE materials.
The incident light from rear side can compensate for the light from anode.
The DSSC employing PANie8 wt‰ MWCNT complex CE shows a bifacial efficiency of 9.24%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 July 2014 Received in revised form 9 October 2014 Accepted 10 November 2014 Available online 11 November 2014

Exploration of cost-effective counter electrodes (CEs) and enhancement of power conversion efficiency have been two persistent objectives for dye-sensitized solar cells (DSSCs). In the current work, polyanilineemultiwalled carbon nanotube (PANieMWCNT) complexes are synthesized by a reflux method and employed as CE materials for bifacial DSSCs. Owing to the high optical transparency of PANi eMWCNT complex CE, the incident light from rear side can compensate for the incident light from TiO2 anode. The charge-transfer ability and electrochemical behaviors demonstrate the potential utilization of PANieMWCNT complex CEs in robust bifacial DSSCs. The electrochemical properties as well as photovoltaic performances are optimized by adjusting MWCNT dosages. A maximum power conversion efficiency of 9.24% is recorded from the bifacial DSSC employing PANie8 wt‰ MWCNT complex CE for both irradiation, which is better than 8.08% from pure PANi CE. © 2014 Elsevier B.V. All rights reserved.

Keywords: Bifacial dye-sensitized solar cell Counter electrode Polyaniline Multiwalled carbon nanotube Electrocatalyst

1. Introduction

* Corresponding authors. Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, PR China. E-mail addresses: [email protected] (B. He), [email protected] (Q. Tang), [email protected] (L. Yu). http://dx.doi.org/10.1016/j.jpowsour.2014.11.046 0378-7753/© 2014 Elsevier B.V. All rights reserved.

Dye-sensitized solar cells (DSSCs), electrochemical devices directly converting solar energy into electricity, have attracted growing interests because of their advantages on easy fabrication process, relatively high power conversion efficiency, and environmental-friendliness [1e6]. As one of the most significant components in a DSSC device, Pt counter electrode (CE) is relatively expensive for future commercial application. By addressing this

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issue, it is a prerequisite to develop cost-effective CE candidates with superiorities of good electrocatalytic activity toward triiodides (I 3) reduction and rapid charge transfer ability. Among various alternatives, conducting polymers such as polyaniline (PANi) and carbon materials such as carbon nanotubes (CNTs) are preferred electrocatalyst materials [7e9]. The single utilization of PANi or CNT always generates associated problems. For example, PANi has excellent redox behaviors but the charge-transfer ability is relatively low because of its organic semiconductor nature [10], whereas CNT has an excellent electrical conduction, but the redox performances are unsatisfactory. A compromising route is to integrate PANi with CNT, for which blending or physical combination techniques are commonly used, leading to a larger interfacial resistance between PANi and CNT. Therefore, the electron transfer from CNT to PANi is significantly limited and the electrons can not fully participate in I 3 reduction reaction. After careful literature survey, the power conversion efficiencies of the DSSCs fabricated using traditional PANieCNT CEs are in the level of ~7% [11e15]. In our recent research [10,16e19], we have proposed a new concept of employing reflux technique to synthesize covalent bonded conducting polymerecarbon complexes with an aim of significantly enhancing the electron transfer ability and therefore power conversion efficiency of cell devices. Due to the covalent bonding between eNHe in conducting polymer and eC] in carbon, the electrons from external circuit can be facilely collected by carbon and transferred to conjugated structure of conducting polymer, therefore the efficiency has been elevated to ~8% [16e19]. Recently, bifacial technique of irradiating DSSC devices from either front (anode) or rear (CE) side has €tzel to help bring down the cost of solar energy been aroused by Gra production. To realize this technique of collecting sunlight from either side of a solar cell, transparent CEs are crucial in designing such bifacial DSSCs. PANi, a colored and transparent electrocatalyst, has been deposited on a conductive substrate for CE application by Zhao in bifacial DSSC assembly [20], yielding front and rear efficiencies of 6.54% and 4.26%, respectively. No matter which irradiation, the stepwise decrease in incident light intensity will result in incomplete irradiation of organic dyes and therefore unsatisfactory efficiency. We have pioneerly irradiated the cell device from both TiO2 anode and transparent PANi CE, giving a promising efficiency of 8.35%, which is enhanced by 24.6% compared to the front efficiency [21]. The incident light from transparent PANi CE has a compensation effect to the light from anode. In search for more robust and cost-effective CE for bifacial DSSC applications, here the aniline monomers are employed to solubilize multiwalled CNTs (MWCNTs) by a reflux process. The resultant PANieMWCNT after in-situ polymerization is a typical electron donoreacceptor complex, in which MWCNT can rapidly collect electrons from external circuit and transfer to PANi for I 3 reduction. The molecular structure, morphology, and electrochemical performances of the PANieMWCNTs complex CEs are thoroughly characterized to confirm the formation of covalent bonds between PANi and MWCNT. More importantly, the resultant PANieMWCNTs complex CEs are semitransparent, therefore they can employed as transparent CEs for bifacial DSSC applications. The light-to-electric conversion efficiencies of bifacial DSSC with PANie8 wt‰ MWCNT complex CE are as high as 9.24% for both irradiations. To the best of our knowledge, the measured efficiency is so far the highest record for the DSSCs with Pt-free CEs. 2. Experimental 2.1. Reflux synthesis of anilineeMWCNT complexes The anilineeMWCNT complexes were synthesized by a reflux process. In details, fresh aniline/MWCNT mixture at an MWCNT

dosage of 2, 4, 6, or 8 wt‰ was sealed in a three-neck flask filled with high-purity N2 gas. The mixture was refluxed in dark for 6 h at 184  C to obtain the target anilineeMWCNT complex, which was subsequently stored in a dark and cold atmosphere. 2.2. Fabrication of PANieMWCNT complex CEs The fabrication of PANieMWCNT complex CEs was carried out on an electrochemical workstation (CHI660E): In details, a cleaned fluorine doped tin oxide (FTO, 12 U square1) glass was used a working electrode, a Pt plate was a CE, and an Ag/AgCl was a reference electrode. The supporting electrolyte was 0.5 M H2SO4 aqueous solution dissolved anilineeMWCNT complex. A cyclic voltammetric method was employed at a sweep rate of 200 mV s1 for 50 repeating cycles. After deposition, the electrodes were rinsed by 0.5 M H2SO4 aqueous solution and deionized water, and then dried in vacuum at 60  C for 24 h. As a comparison, PANi-only CE was also prepared under the same conditions. 2.3. Assembly of DSSCs A layer of TiO2 nanocrystal anode film with a thickness of 10 mm was prepared by a sol-hydrothermal method and casted onto a cleaned FTO glass substrate. After being calcined at 450  C for 30 min, the resultant TiO2 films were further sensitized by immersing into a 0.5 mM ethanol solution of N719 dye (purchased from DYESOL LTD). The DSSC device was fabricated by sandwiching liquid electrolyte between a dye-sensitized TiO2 anode and a PANieMWCNT complex CE. A redox electrolyte consisted of 100 mM of tetraethylammonium iodide, 100 mM of tetramethylammonium iodide, 100 mM of tetrabutylammonium iodide, 100 mM of NaI, 100 mM of KI, 100 mM of LiI, 50 mM of I2, and 500 mM of 4-tert-butyl-pyridine in 50 ml acetonitrile. 2.4. Electrochemical characterizations The electrochemical performances were recorded on a conventional three-electrode cell comprising an Ag/AgCl reference electrode, a CE of platinum sheet, and a working electrode of FTO glass supported PANieMWCNTs complex. The cyclic voltammetry (CV) curves were recorded from 0.5 to þ1.3 V and back to 0.5 V. Before the measurement, the supporting electrolyte consisting of 50 mM LiI, 10 mM I2, and 500 mM LiClO4 in acetonitrile was degassed using nitrogen for 10 min. Electrochemical impedance spectroscopy (EIS) measurements were also carried out on the CHI660E electrochemical workstation (CHI660E, Shanghai Chenhua Device Company, China) in a frequency range of 0.01 Hz ~ 105 Hz and an ac amplitude of 5 mV at room temperature. The resultant impedance spectra were analyzed using the Z-view software. Tafel polarization curves were recorded on the same workstation by assembling symmetric dummy cell consisting of PANieMWCNT complex CEjelectrolytejPANieMWCNT complex CE. The curves were recorded by a scanning potential window of 1 ~ 1 V at a scan rate of 10 mV s1. 2.5. Photovoltaic tests The photovoltaic tests of the DSSC were carried out by measuring the currentevoltage (JeV) characteristic curves using an electrochemical workstation under irradiation of a simulated solar light from a 100 W Xenon arc lamp (XQ-500 W) in ambient atmosphere. The incident light intensity was controlled at 100 mW cm2 (calibrated by a standard silicon solar cell). A black mask with an aperture area of around 0.25 cm2 was applied on the surface of DSSCs to avoid stray light. Each DSSC device was

H. Zhang et al. / Journal of Power Sources 275 (2015) 489e497

measured five times to eliminate experimental error and a compromise JeV curve was employed.

491

0.30

Aniline MWCNT Aniline-MWCNT mixture Aniline-2 wt‰ MWCNT complex Aniline-4 wt‰ MWCNT complex Aniline-6 wt‰ MWCNT complex Aniline-8 wt‰ MWCNT complex

0.25 Absorbance (a.u.)

2.6. Other characterizations The morphologies of the resultant PANi, PANieMWCNT complex CEs, and pristine MWCNT were observed with a scanning electron microscope (SEM, S4800). Fourier transform infrared spectrometry (FTIR) spectra were recorded on a PerkinElmer spectrum 1760 FTIR spectrometer. The UVevis spectra were measured on a UV-3200 spectrophotometer by dissolving the samples in isopropanol. The fluorescence emission spectra were recorded at room temperature using a Fluorolog 3-P spectrophotometer. The emission spectrum was collected using a conventional setup at excitation wavelengths of 515 nm.

0.20 0.15 0.10 0.05 0.00 400

3. Results and discussion

550

600

650

700

Wavelength (nm)

750

800

700

complex in its ground state because MWCNT is a good electron acceptor, whereas aniline is a fairly good electron donor, as evidenced by the appearance of the new absorptions. Originating from aniline monomers, they suffer from dimmers, trimers, oligomers and conjugated PANi in the presence of APS and protonic acid [25]. During an electrochemical polymerization of PANieMWCNTs, each imine group (eNHe) has a lone-electron pair which can share with a carbon atom in conjugation structure of MWCNT (eC]) to form a covalent bond. There is a consensus that the chemical bonding between PANi and carbon materials can significantly accelerate the charge transfer [26,27]. Furthermore, previous research also validates that the resultant PANi chains are intertwisted to form agglomerated nanoparticles in traditional chemical or electrochemical polymerization because of the strong intramolecular and intermolecular hydrogen bonding [28,29], resulting in low electron delocalization, short-range charge-transfer and therefore poor electrical and electrochemical performances. The parallel bonding can enhance the well-aligned arrangement of PANi chains along the surface of MWCNT, which is expected to give extraordinary synergistic effect in accelerating charge transfer along PANi chains. Fig. 3 shows the FTIR spectra of PANi and PANieMWCNT complexes at various MWCNT dosages. The main absorption bands situated at ~795, 1296, 1480, and 1560 cm1 are attributed to the

b

Acetonitrile Isopropanol Carbon tetrachloride

Intensity (cps)

Intensity (cps)

Aniline MWCNT Aniline-MWCNT mixture Aniline-2 wt‰ MWCNT complex Aniline-4 wt‰ MWCNT complex Aniline-6 wt‰ MWCNT complex Aniline-8 wt‰ MWCNT complex

600

Fig. 2. UVevis absorption spectra of aniline, MWCNT, anilinee8 wt‰ MWCNT mixture and anilineeMWCNT complexes diluted in isopropanol at MWCNT dosages of 2, 4, 6 and 8 wt‰.

A prerequisite of fabricating robust PANieMWCNT complexes with rapid charge-transfer ability is the successful combination of aniline with MWCNT. Fig. 1(a) displays fluorescence emission spectra of pure aniline, MWCNT, anilineeMWCNT mixture, and the resultant anilineeMWCNT complexes diluted with isopropanol. The maximum emissions in isopropanol are at around 570 and 615 nm, whereas the fluorescence spectra of pure aniline, MWCNT, and anilineeMWCNT mixture are generally quenched. The fluorescence excitation spectra of the anilineeMWCNT complexes are quite different from the absorption spectra of the individual components and mixture, indicating the formation of a new lightabsorbing species. It is noteworthy to mention that the red shift of maximum emission, as is shown in Fig. 1(b), is in an order of acetonitrile, isopropanol, and carbon tetrachloride. This is attributed to their polarities to pep stacking resulted from the electrostatic interactions and hydrophobic effects [22]. To reveal the complexing mechanism of aniline monomers onto MWCNTs, UVevis adsorption spectra of aniline, MWCNT, anilineeMWCNTs mixture and anilineeMWCNT complexes with various MWCNTs dosages are diluted in isopropanol. As is shown in Fig. 2, no peak is observed in the UVevis spectrum ranging from 330 to 700 nm for aniline, MWCNT, and anilinee8 wt‰ MWCNTs mixture, however, new absorption peaks at about 350, 452, 518, and 559 nm are detected in anilineeMWCNT complexes, suggesting that anilineeMWCNT complexes have been successfully formed during the reflux process [23,24]. At elevated temperature, such as 184  C, MWCNT and aniline are believed to form a charge-transfer

a

500 Wavelength (nm)

550

600

650

700

750

800

Wavelength (nm)

Fig. 1. (a) Emission spectra of aniline, MWCNT, anilinee8 wt‰ MWCNT mixture, and anilineeMWCNT complexes in isopropanol at MWCNT dosages of 2, 4, 6, and 8 wt‰. (b) Emission spectra of anilinee8 wt‰ MWCNTs complex diluted in organic solvents: acetonitrile, isopropanol and carbon tetrachloride. The excitation wavelength was 515 nm.

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following vibration modes: bending of CeH (out-of-plane) on benzene ring (B), bending of CeH (in-plane), mode of N]quinoid ring (Q)]N, stretching of aromatic-N, stretching of NeBeN, and stretching of N]Q]N [30,31]. In comparison with PANi, the FTIR spectra of PANieMWCNT complexes determine the appearance of the 1034 cm1 band. The appearance of this vibration absorption band indicates a charge-transfer and a selective interaction of the MWCNT fragments with quinoid rings of PANi backbones [32,33]. A similar interaction between carbon materials and PANi quinoid rings has also been detected in PANi/single-walled CNTs and PANi/ graphene [16,19]. The electrochemically polymerized PANi is composed of nanofibers, as shown in Fig. 4(a) and (b). In comparison with the average diameter of around 30 nm for pristine MWCNT (Fig. 4(e) and (f)), the diameter of PANie8 wt‰ MWCNT complex (Fig. 4(c) and (d)) is approximately 50 nm. The increase in diameter means the aniline monomers have been attached on the surface of MWCNT for homogeneous in-situ polymerization. The close covalent bonding is expected to enhance charge-transfer ability between PANi and MWCNT. The transmittance of CE is an important factor affecting sunlight collection from rear side and therefore the overall conversion efficiency of the bifacial DSSC. Fig. 5 compares the optical transparencies of PANi and PANieMWCNT complexes with various MWCNT dosages. Apparently, all the PANieMWCNT complex CEs have higher optical transparencies in comparison with pure PANi electrode. Moreover, the transparency of resultant PANieMWCNT complex CE increases with MWCNT dosage. This is due to a fact that the alignment of PANi chains along MWCNT becomes better at higher MWCNT dosage because conjugated structure of PANi chain is facilely attached on MWCNT by covalent bonding. Therefore, the employment of PANie8 wt‰ MWCNT complex CE for bifacial DSSC is expected to generate promising power conversion efficiency. The CV curves of the resultant complex CEs in liquid electrolyte with PANi-only CE as a reference were recorded, as is shown in Fig. 6. The peak positions and shapes of the CV curves from PANieMWCNT complex CEs are very similar to those of Pt and PANionly CE [19], revealing that PANieMWCNT complex CEs have a similar electrocatalytic activity to Pt CE. Considering that the main task of a CE is to reduce I 3 species, therefore the reduction peak for  I 3 þ 2e / 3I can be employed to assess the electrocatalytic activity of PANieMWCNT complex CE. Notably, the combination of MWCNT with PANi increases the peak current density and decreases the peak-to-peak separation (Epp). The higher peak current density and lower Epp demonstrate an enhanced electrocatalytic

(e) Transmittance (%)

(d) (c) (b)

2500

2000

1500

795

1034

1296

1560 1480

(a)

1000 -1

Wavenumber (cm ) Fig. 3. FTIR spectra of (a) PANi and PANieMWCNT complexes at MWCNT dosages of (b) 2, (c) 4, (d) 6, and (e) 8 wt‰.

activity for I 3 reduction reaction. Apparently, the PANie8 wt‰ MWCNT complex CE has the highest electrocatalytic activity toward I 3 reduction. The increase in the number of bonding sites between PANi and MWCNT at MWCNT dosage of 1e8 wt‰ has a promotion effect on electrocatalytic performance of PANieMWCNT complex CE. Moreover, the ratio of Jox1/jJred1j is a parameter to elevate the reversibility of the redox reaction toward I/I 3 [34]. The obtained values from PANi, PANie2 wt‰ MWCNT, PANie4 wt‰ MWCNT, PANie6 wt‰ MWCNT, and PANie8 wt‰ MWCNT complex CEs are 1.095, 1.254, 1.116, 1.090 and 1.060, respectively. The closer Jox1/ jJred1j from PANie8 wt‰ MWCNTs complex CE to 1.0 indicates a  more reversible redox reaction for I 3 4 I . The rapid exchange  between I 3 and I facilitates the participation in subsequent circles and improves the photovoltaic performances. To elucidate the relationship between increased bonding sites and diffusion of iodide in a complex CE, RandleseSevcik theory is employed and presented [35]: 1=2

Jred ¼ Kn3=2 ADn v1=2 C0

(1)

where Jred is the peak current density of Red1 (mA cm2), K is 2.69  105, n is the number of electrons of reduction reaction, A is the electrode area (cm2), C0 represents the bulk concentration of I 3 (mol L1), Dn is the diffusion coefficient (cm2 s1). The diffusivity of PANi-only CE is 1.445  106 cm2 s1 which is comparable to 4.92  106 cm2 s1 of poly(3,4-ethylenedioxythiophene) (PEDOT) and 2.55  106 cm2 s1 of Pt CEs [36]. Interestingly, Dn is 6.042  106, 7.468  106, 1.174  105 and 1.298  105 cm2 s1 for PANie2 wt‰ MWCNT complex, PANie4 wt‰ MWCNT complex, PANie6 wt‰ MWCNT complex, and PANie8 wt‰ MWCNT complex, respectively. The results indicate that the elevated bonding sites between PANi and MWCNTs can accelerate both charge transfer and I/I 3 redox couples within PANieMWCNT complex CEs. From the stacking CV curves of PANie8 wt‰ MWCNT complex CE at different scan rates, one can find an outward extension of all the redox peaks (Fig. 7(a)). Linear relationships are observed by  plotting peak current density corresponding to I 3 4 I versus square root of scan rate, as shown in Fig. 7(b). This result indicates the oxidation/reduction reactions of I/I 3 redox couples within PANie8 wt‰ MWCNT complex CE are controlled by ionic diffusion in the electrolyte, and the transfer rate of both electrons and ions are fast enough for the reduction rate of I 3 on the surface of PANie8 wt‰ MWCNT complex CE. This result also suggests that the adsorption of I 3 species is hardly affected by the redox reaction on the PANie8 wt‰ MWCNT complex CE surface and no specific interaction occurs between I/I 3 and the CE [37]. Tafel polarization measurements are performed to explore the electrocatalytic activity and interfacial charge-transfer properties of the I/I 3 redox couples on a CE. Fig. 8 shows the logarithmic exchange current density (logJ) as a function of the potential (V) for   the reduction of I 3 ions. The Rct (charge-transfer resistance for I /I3 redox) may be correlated with the exchange current density (J0)  during the reduction of I 3 to I , which can be calculated from the intersection of the tangent line of the polarization curve and the extension of the linear segment to zero bias using the equation [38]:

J0 ¼ RT=nFRct

(2)

where R is the gas constant, T is absolute temperature, F is Faraday's constant, and n is the number of electron involved. From the Tafel polarization curves, it is apparent that the J0 follows an order of PANie8 wt‰ MWCNT > PANie6 wt‰ MWCNT > PANie4 wt‰ MWCNT > PANie2 wt‰ MWCNT > PANi, generating an Rct order of

H. Zhang et al. / Journal of Power Sources 275 (2015) 489e497

493

Fig. 4. SEM photographs of (a) & (b) PANi and (c) & (d) PANie8 wt‰ MWCNT complex CEs. (e) & (f) TEM images of pristine MWCNT. Images (b), (d), and (f) are magnified morphologies of images (a), (c), and (e), respectively.

PANi PANi-2wt‰ MWCNT complex PANi-4wt‰ MWCNT complex PANi-6wt‰ MWCNT complex PANi-8wt‰ MWCNT complex

Transparency (%)

80 60 40

Epp

8

-

4

0 -

-4 -

-8 400

500

600

700

800

900

1000

-

Ox1, 3I -2e=I3

20 0

-

Ox2, 2I3 -2e=3I2

-2

100

diffusion coefficient of I/I 3 couples in electrolyte. The diffusion coefficient (Dn) of redox couples is in linear with Jlim according to equation (3) [39]:

Current density (mA cm )

PANie8 wt‰ MWCNT < PANie6 wt‰ MWCNT < PANie4 wt‰ MWCNT < PANie2 wt‰ MWCNT < PANi. The intersection of the cathodic branch with the Y-axis can be considered as the limiting diffusion current density (Jlim), which is of dependence on the

Red1, I3 +2e=3I -0.5

-

0.0

PANi Red2, 3I2+2e=2I3 PANi-2 wt‰ MWCNT complex PANi-4 wt‰ MWCNT complex PANi-6 wt‰ MWCNT complex PANi-8 wt‰ MWCNT complex 0.5 Potential (V vs Ag/AgCl)

1.0

Wavelength (nm) Fig. 5. Optical transparency of FTO supported PANi and PANieMWCNT complexes.

Fig. 6. CV curves of PANi and PANieMWCNT complex CEs for I/I 3 redox species recorded at a scan rate of 50 mV s1.

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H. Zhang et al. / Journal of Power Sources 275 (2015) 489e497

a

b

12

Ox2 Ox1

9

-2

Current density (mA cm )

-2

Current density (mA cm )

9 6 3 0 -3

10, 20, 30, 40, 50, 60, and 70 mV s

-1

6 3 0 Red2

-3

-6

-6

-9

-9

-0.5

0.0 0.5 Potential (V vs Ag/AgCl)

Red1

3

1.0

4

5 6 7 -1 1/2 Squre root of scan rate (mV S )

8

9

Fig. 7. (a) CV curves of PANie8 wt‰ MWCNTs complex CE for I/I 3 redox species at varied scan rates, and (b) relationship between peak current density and square root of scan rate.

ions, NA is the Avogadro constant. From the stacking Tafel curves, it is apparently that Jlim and therefore Dn both follow an order of PANie8 wt‰ MWCNT > PANie6 wt‰ MWCNT > PANie4 wt‰ MWCNT > PANie2 wt‰ MWCNT > PANi, suggesting an enhanced diffusion kinetics for I/I 3 redox species in liquid electrolyte. The result also indicates an enhanced electrocatalytic activity for PANieMWCNT complex CE at increased MWCNT dosage, which is consistent with the CV analysis. The EIS spectra of DSSCs with the PANi-only and PANieMWCNTs complex CEs are shown in Fig. 9. A typical Nyquist characteristic is observed in the measured frequency range 102e105 Hz for all devices. According to the equivalent circuit (inset of Fig. 9(a)), the intercept on the real axis represents the series resistance (Rs), the semicircles in the frequency regions 103e105, 1e103 and 102e1 Hz are attributed to impedance related to charge-transfer processes occurring at the counter electrode/electrolyte (Rct1), at the TiO2/ dye/electrolyte interface (Rct2), and in Nernstian diffusion within the electrolyte (W), respectively. Constant phase element (CPE) is frequently used as a substitute for a capacitor in an equivalent circuit to fit the impedance behavior of the electrical double layer. These electrochemical parameters were obtained by fitting EIS spectra using a Z-view software and summarized in Table 1. There is a good agreement between measured and fitted curves. The PANie8 wt‰ MWCNT complex CE has the smallest Rct1, indicating the highest electrocatalytic activity toward I 3 reduction. Moreover, the order of W, diffusion resistance of I/I 3 , against MWCNTs

-2

log(Current density, A cm )

-1 -2

Jlim

J0

-3 -4 -5

PANi PANi-2 wt‰ MWCNT complex PANi-4 wt‰ MWCNT complex PANi-6 wt‰ MWCNT complex PANi-8 wt‰ MWCNT complex

-6 -7 -1.0

-0.5

0.0 Potential (V)

0.5

1.0

Fig. 8. Tafel polarization curves of symmetrical dummy cells fabricated with double PANi-only or PANieMWCNT complex CEs.

Dn ¼ lJlim =2ne0 CNA

(3)

where l is the spacer thickness, n is the number of electron involved, e0 is the electronic charge, C is the concentration of I 3

35 30

2

-Z'' (ohm cm )

25 20

CPE1

15

PANi PANi-2 wt‰ MWCNT complex PANi-4 wt‰ MWCNT complex PANi-6 wt‰ MWCNT complex PANi-8 wt‰ MWCNT complex Simulated line CPE2

Rs Rct1

10

Rct2

W

b

-70 -60 -50

Phase (deg)

a

-40 -30 -20

PANi PANi-2 wt‰ MWCNT complex PANi-4 wt‰ MWCNT complex PANi-6 wt‰ MWCNT complex PANi-8 wt‰ MWCNT complex

-10

5

0

0 0

5

10

15

20 2

Z' (ohm cm )

25

30

35

0.01

0.1

1

10 100 1000 10000 100000 Frequency (Hz)

Fig. 9. EIS spectra of the DSSCs from PANi-only and PANieMWCNT complex CEs: (a) Nyquist plots, (b) Bode phase plots. The inset gives the equivalent circuit by fitting the impedance data.

H. Zhang et al. / Journal of Power Sources 275 (2015) 489e497

dark current and therefore enhanced photovoltaic performance for their cells. To determine the dependence of electrons lifetime (t) on DSSCs from PANi-only and PANieMWCNT complex CEs, as shown in Fig. 9(b), we have calculated the lifetime using equation (4) [40] and the data are summarized in Table 1.

Table 1 The maximum frequency of the mid-frequency peak (fmax), electron lifetime (t) and related EIS parameters of the DSSCs employing various CEs. Rs Rct1 Rct2 W Frequency Τ (ms) (U cm2) (U cm2) (U cm2) (U cm2) (Hz)

CEs PANi PANie2 wt‰ complex PANie4 wt‰ complex PANie6 wt‰ complex PANie8 wt‰ complex

0.44 MWCNT 0.39

0.78 1.27

28.25 22.84

1.22 3.95

38.31 31.62

4.16 5.04

MWCNT 0.34

0.81

16.98

3.21

26.10

6.10

MWCNT 0.33

0.91

14.62

1.91

21.54

7.39

MWCNT 0.30

0.61

9.29

0.75

17.78

8.96

t ¼ 1=2pfmax

b

20

5 4

Current density (mA cm )

16 -2

-2

Current density (mA cm )

(4)

where fmax is the maximum frequency of the mid-frequency peak in the bode phase plots. The t in TiO2 anode for the cell device with pure PANi CE is 4.16 ms, which is lower than 5.04 ms for PANie2 wt ‰ MWCNT, 6.10 ms for PANie4 wt‰ MWCNT, 7.39 ms for PANie6 wt‰ MWCNT, and 8.96 ms for PANie8 wt‰ MWCNT. The optimal t in the DSSC from PANie8 wt‰ MWCNT CE can be attributed to its high kinetics for I 3 reduction and effective suppression reaction of the electrons in TiO2 conducting channels with I 3 . Till now, we can make a conclusion that the design of PANieMWCNT complex CEs is an efficient approach in accelerating electron transfer, elevating electron density, and enhancing photovoltaic performances. Fig. 10(a) shows characteristic JeV curves of the DSSCs irradiated from front side and the detailed photovoltaic parameters are summarized in Table 2. The cell device with PANie8 wt‰ MWCNT complex CE yield a maximum front h of 7.91% (Jsc ¼ 17.95 mA cm2, Voc of 0.675 V, and FF of 0.653) under simulated air mass 1.5 (AM1.5) global sunlight in comparable to 6.49% for the cell with pure PANi electrode. To determine the efficient limit, we have also synthesized PANie10 wt‰ MWCNT and PANie12 wt‰ MWCNT complex CEs. However, the MWCNT can not be completely dissolved in aniline at a higher MWCNT dosage such as higher than 8 wt‰ after reflux process. The phase separation between MWCNT

dosage is the same to that of Rct1. For the complex CEs at high MWCNT dosages, the increased number in covalent bonds can accelerate the electron transfer from MWCNT to PANi and partici pate in the reduction reaction of I 3 species, once I3 are reduced to I, they will diffuse to anode/electrolyte interface for dye recovery. Therefore, the enhancement in electrocatalytic activity at increased MWCNT dosage has an acceleration effect on the diffusion of redox species. The conclusions for the electrocatalytic activity and diffusion derived from EIS and CV data are consistent. Furthermore, the electrocatalytic performance of a CE greatly influences the impedance behavior at TiO2/dye/electrolyte interface. From the gradually decrease in Rct2 value by elevating MWCNT dosage, we can make a conclusion that the charge-transfer kinetics at TiO2/dye/ electrolyte interface is elevated. The result indicates that PANieMWCNT complex improves the reduction kinetics of I 3 ion. The recombination issue of I 3 species with electrons on conduction band of TiO2 nanocrystallite is reduced, resulting in a decreased

a

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4 0 0.0

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Fig. 10. JeV characteristics of the DSSCs employing pure PANi and PANieMWCNTs complex CEs for (a) front, (b) rear and (c) both irradiation.

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H. Zhang et al. / Journal of Power Sources 275 (2015) 489e497

Interestingly, a maximum h of 9.24% (Jsc ¼ 22.25 mA cm2, Voc of 0.691 V, and FF of 0.601) in Fig. 10(c) is measured under both irradiation for the cell with PANie8 wt‰ MWCNT complex electrode. To the best of knowledge, this is the highest h for I/I 3 redox couple based DSSCs with Pt-free CEs. Fast start-up and multiple start capabilities are two crucial requirements for the engines and vehicles driven by solar panels, in which robust CEs are core components in starting motors. As shown in Fig. 11(a), the photocurrent of the cell device employing PANie8 wt‰ MWCNT complex electrode has an abrupt increase at irradiation from front, rear, or both. No delay in time is recorded to start the solar cell, demonstrating that the PANie8 wt‰ MWCNT complex electrode is vigorous in collecting electrons and reducing I 3 species. Under durable irradiation in each “on” state, an attenuation in photocurrent density, suggesting a diffusion mechanism for I/I 3 redox couples. Moreover, start/stop cycling can be employed to evaluate the multiple start-up capability of a DSSC. After multiple start-up, the cell still remain properties in its initial level (Fig. 11(b)), which is an essential prerequisite for the application of DSSCs in engines or vehicles. Another important performance in selecting robust CEs is their long-term stability, the photocurrent density versus time plots over 3000 s demonstrate photocurrent stability on prolonged exposure to light irradiation. Although 3000 s-test is far from demonstrating the long-term stability for a real DSSC device, the preliminary result suggests a relatively good stability for the cell employing PANie8 wt‰ MWCNT complex electrode. Notably, the photocurrent density follows an order of bifacial > front > rear. The result further supports the compensation effect of incident light from PANie8 wt‰ MWCNT complex CE to the light from photoanode.

Table 2 Output photovoltaic characteristics of the DSSCs employing pure PANi and PANieMWCNTs complex CEs for front-, rear- and both-irradiation. h: power conversion efficiency; Jsc: short-circuit current density; Voc: open-voltage; FF: fill factor. CEs

Irradiation

h (%)

Voc (V)

Jsc (mA cm2)

FF

PANi

Both Front Rear Both Front Rear Both Front Rear Both Front Rear Both Front Rear

8.08 6.49 1.26 8.36 7.44 1.44 8.65 7.53 1.50 8.91 7.61 1.63 9.24 7.91 1.72

0.682 0.676 0.609 0.691 0.691 0.595 0.731 0.730 0.670 0.714 0.702 0.616 0.691 0.675 0.622

19.16 17.23 3.21 18.33 16.26 3.94 17.52 15.24 3.30 19.09 16.91 4.26 22.25 17.95 4.30

0.618 0.558 0.645 0.659 0.662 0.614 0.675 0.677 0.678 0.654 0.641 0.621 0.601 0.653 0.643

PANie2 wt‰ MWCNT complex

PANie4 wt‰ MWCNT complex

PANie6 wt‰ MWCNT complex

PANie8 wt‰ MWCNT complex

and PANieMWCNT complex can result in a high charge transfer resistance. The measured h values are 6.01% and 4.36% for the solar cells from PANie10 wt‰ MWCNT and PANie12 wt‰ MWCNT complex electrodes (not shown here), respectively. In respect of high optical transparency in visible region, simulated light can penetrate the rear side of DSSC devices and generate solar energy conversion. As shown in Fig. 10(b), an optimal rear h of 1.72% is recorded in the cell with PANie8 wt‰ MWCNT complex electrode, which is higher than 1.26% for the DSSC from PANi electrode. PANi has a lower transparency for visible light and therefore less incident light can penetrate the rear side, leading to a lower dye excitation, which is supported by a lower rear Jsc in comparison with front ones. Moreover, all the Voc values generated from front irradiation are higher than that from rear side. Due to a fact that intensity of incident light stepwisely decreases within TiO2 film, the electron distribution generated from excited dyes is therefore diminishing. From this point of view, the electron recombination with I 3 species   (I 3 þ 2eCB(TiO2) ¼ 3I ) from front irradiation is retarded in comparison with rear irradiation. The maximum Voc is determined by the difference between quasi Fermi energy of electrons in TiO2 and redox potential energy of electrolyte, whereas the real Voc of the DSSCs is smaller than this theoretical limit because of a backward reaction between photogenerated electron and I 3 species. Therefore, there is a low electron loss for front irradiation and a high Voc.

10 light on

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rear front both

In summary, we have demonstrated that reflux synthesis of PANieMWCNT complex is an effective strategy for designing costeffective and transparent CE materials and enhancing the photovoltaic performances of DSSCs. PANie8 wt‰ MWCNT complex electrode exhibits superior electrocatalytic activity toward I 3 species and charge-transfer ability than pure PANi electrode. The DSSC from PANie8 wt‰ MWCNT complex CE provides an impressive power conversion efficiency of 7.91%, 1.72%, and 9.24% for irradiations from front, rear, and both, respectively, superior to the cell performances from PANi CE. Moreover, the virtues on fast start-up, high multiple start capability, and good stability motivate the potential applications of such bifacial DSSCs in engines, vehicles and

Current density (mA cm )

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4. Conclusions

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6 4 2

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off

0

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1500 Time (s)

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Fig. 11. (a) The oneoff plots with irradiation from front, rear and both were achieved by alternately illuminating (100 mW cm2) and darkening (0 mW cm2) the DSSC device at an interval of 25 s and 0 V. (b) Photocurrent stabilities of the DSSC devices employing PANie8 wt‰ MWCNT complex CEs.

H. Zhang et al. / Journal of Power Sources 275 (2015) 489e497

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