Layer-by-layer deposition of CNT⁻ and CNT⁺ hybrid films for platinum free counters electrodes of dye-sensitized-solar-cells

Abstract

A nanocomposite film of polyaniline blend poly(sodium 4-styrenesulfonate)/multiwall CNT (PANI-PSS/MWCNT) and poly(diallydimethyl ammonium chloride)/multiwall CNT (PDADMAC/MWCNT) has been prepared as a highly-efficient catalytic material for the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The as-prepared CE is constructed by a nanoscale method of layer-by-layer (LbL) assembly based on the electrostatic interactions between negatively (PANI-PSS/MWCNT) and positively (PDADMAC/MWCNT) charged nanoparticles. The (CNT⁻/CNT⁺)₄ multilayer CE displayed remarkably enhanced electrocatalytic activities toward the reduction reaction and low charge-transfer resistance (R_ct) of 0.3 Ω at the CE/electrolyte interface. The DSSC with (CNT⁻/CNT⁺)₄ multilayer-CE showed the highest fill factor (0.71) and cell efficiency (9.13%) compared to our other electrodes.

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1. Introduction

Nowadays, energy conversion devices have attracted attention due to their great potential as power sources like dye-sensitized solar cells (DSSCs). As an alternative to silicon solar cells, DSSCs are considered to be low cost and environmentally friendly. A DSSC mainly consists of a dye sensitized photoanode, a redox mediator of I⁻/I₃⁻ and counter electrode (CE).^1–3 To improve the performance of DSSCs, much effort has been given to each of the constituent components, including the photoanode,⁴ dye molecules,⁵ electrolytes⁶ and CE.⁷ In typical DSSCs, the CE plays a crucial role for achieving a high performance i.e. excellent electrocatalytic ability and high conductivity. Due to this dual role, CEs quickly transport electrons from the electrode substrate to the electrolyte and effectively catalyze the iodide–triiodide (I⁻/I₃⁻) redox reaction in the electrolyte. Platinum (Pt) is a well-known noble metal and utilized as the CE material for DSSCs. However, there are also shortcomings of using Pt, i.e. highly expensive and scarcity as a precious metal. Extensive studies have been made on Pt alternatives as the catalytic materials for the CEs of DSSCs, including conductive polymers,^8–12 inorganic metal compounds,¹³ carbon-based materials,^14–18 and composite materials.¹⁹ Notably, carbon based materials have shown efficient performance and achieved significant improvement in the last few years including graphene,¹⁶ nanocarbon,^17,18 carbon black,²⁰ activated carbon,²¹ single-walled carbon nanotube (SWCNT),²² and multi-walled carbon nanotube (MWCNT)²³ etc.

Since 1991,²⁴ CNTs attracted a tremendous attentions due to their unique nanostructure and fascinating properties like high conductivity, high chemical stability, good catalytic ability and low-cost. Choi et al. prepared CNTs CE by screen printing technique and showed efficiency of 5.94%.²⁵ Ramasamy et al. describe the use of spray coated multi-wall CNTs on FTO glass substrate as CEs for I₃ reduction with an efficiency of 7.59%.²³ To enhance the electrocatalytic activity of CNTs, Lee et al. reported CNTs based CE by doctor blade method²⁶ which showed an enhanced photovoltaic performance (7.7%). There are other methods used for manufacture CEs such as chemical vapor deposition.²⁷ Recently, studies focused on the chemical deposition, electrode position, and spin-coating of graphene/polyaniline composite,²⁸ carbon fiber/PEDOT:PSS,²⁹ graphene/polyaniline hybrid³⁰ etc. Typically, in CEs of DSSCs, the interfacial resistance between conducting polymer and carbon material is relatively high because of their weak bonding which gives low charge-transfer kinetics. To control over the CNTs assembly in thin films, the layer-by-layer (LBL) technique was used in multilayers,³¹ where positive and negative layers banded with each other.³²

In this paper, we prepared a new CE for DSSCs, prepared by LBL concept of negatively charged PANI-PSS/MWCNT (anionic) and positively charged PDADMAC/MWCNT (cationic) by polyelectrolyte multilayers technique. The obtained power conversion efficiency (PCE) of our prepared (CNT⁻/CNT⁺)₄ multilayer-CE based DSSC is 9.13% under AM 1.5G simulated solar irradiation of 100 mW cm⁻². Notably, the variation in the thickness towards the fabrication of CNT⁻/CNT⁺ nanocomposite film significantly accelerates the overall PCE of the DSSC. Moreover, the superior performance (CNT⁻/CNT⁺)₄ multilayer-CE is the cost-effective way for highly efficient CEs of DSSCs.

2. Experimental details

Materials and device fabrication

To prepare the CEs of DSSCs, we utilize LBL technique of polyelectrolyte self-assembly as shown in Fig. 1. MWCNTs were dispersed with cationic or anionic polyelectrolytes. First, the anionic PANI-PSS was synthesized by previously reported literature.³³ In detail, MWCNTs (5 mg) were mixed with a solution of 100 mL solution of PANI-PSS content with weight concentration 0.15% (w/w). To attain the homogeneous suspension, the mixed solution was ultrasonicated in an ice bath for 60 min. Second, the preparation of cationic MWCNTs modified with PDADMAC was done by mixing 5 mg of MWCNTs in 100 mL of PDADMAC content with concentration of weight 0.16% (w/w). The solutions were stirred and sonicated for 60 min. Finally, these solutions were used for LBL self-assembly to make the CNTs into thin films deposited on the surface of a cleaned fluorinated tin oxide (FTO, 15 Ω cm⁻², 90% transmittance in the visible light, Nippon Sheet Glass Co.) glass by dip coating method. To achieve multilayered films the FTO glass was dipped sequentially in solutions of cationic and anionic. For the generation of positively charged and strong adhesion of CNTs on FTO glass, we coated 6 layers of PDADMAC-PSS on FTO substrate. Then the first layer of anionic PANI-PSS/MWCNT was deposited by simply dipping the as-prepared sample into a PANI-PSS/MWCNT solution. The next layer of cationic PDADMAC/MWCNT also transferred onto the substrate by dipping the sample into a PDADMAC/MWCNT solution. The alternate LBL films of each positive and negative charge polyelectrolytes were achieved by repeating this process until the desired number of layers was achieved. Afterwards, the films were dried in a stream of nitrogen and stored as depicted in Fig. 1.

Fig. 1 Schematic illustration of the fabrication procedure of (CNT⁻/CNT⁺)_n multilayer films on FTO glass-CE.

The working electrodes (WEs) were made of TiO₂ LBL hierarchical nanosheets (TiO₂ LHNs) paste by doctor blading. The TiO₂ LHNs was fabricated by the previously reported method.³⁴ Then the TiO₂ LHNs paste was prepared according to the previously paper,³⁵ the TiO₂ LHNs powder (0.9 g) was added to the solution containing ethanol/water (4/1 mL), and acetylacetone (0.16 mL) under vigorously stirring for 3 h. Then, the prepared TiO₂ paste was applied on pre-treated FTO glass by doctor-blade method and sintered at 500 °C for 30 min to achieve crystallization using a muffle furnace. After that, TiO₂ films with a thickness of 10 μm were sensitized in a N719 dye solution (0.5 mM in 1 : 1 ethanol and acetonitrile solution) for at least 12 h at 60 °C in a sealed beaker. Afterwards, the WE was assembled. After an electrolyte (0.05 M I2, 0.1 M LiI (Adamas-beta), 0.6 M 1 methyl-3-butylimidazolium iodide (TCl), 0.1 M guanidiniumthiocyanate (TCl), and 0.5 M 4-tert-butylpyridine (TCl) mixed in 3-methoxypropionitrile solution (Alfa Aesar)) was filled, the J–V performance of the DSSCs was investigated using a solar-simulator illumination (Newport, USA).

3. Results and discussion

Material characteristics

The high resolution transmission and scanning electron microscopy image of the PANI-PSS/MWCNT, PDADMAC/MWCNT and (CNT⁻/CNT⁺)_n multilayer view of DSSCs prepared in this work is shown in Fig. 2. Pristine MWCNTs are hollow multiwall objects including an outer diameter of 10–20 nm. The LBL deposition of nanoparticles into thin films, adequate electrostatic charges must be present at their surface for stable adsorption. Therefore, pristine MWCNTs need to be modified with either cationic or anionic polyelectrolytes before deposition for example PDADMAC and PANI-PSS in this paper. TEM images in Fig. 2a and b shows a uniform coverage of PDADMAC (3.3 nm) layer over the randomly distributed MWCNT network. While images in Fig. 2c and d clearly shown the PANI-PSS (9.5 nm) formation around the surface of the MWCNTs. PANI-PSS was used to improve the adsorption of the MWCNTs into multilayer thin films.³⁶ The SEM image Fig. 2e depicts the morphology of deposited films which discloses a uniform coverage.

Fig. 2 (a–e) SEM and TEM images of PDADMAC/MWCNT, PANI-PSS/MWCNT and CNT⁻/CNT⁺ multilayer composite. (f) UV-vis absorption spectra of CNT⁻/CNT⁺ multilayer deposited on FTO glass with various bilayers. Inset shows a plot of absorbance at 750 nm vs. bilayer numbers.

Multilayer preparation can enhance the absorbance, as shown in Fig. 2f, the UV/Vis absorbance spectra. The inset Fig. 2f shows that the (CNT⁻/CNT⁺)_n layers is morphologically homogeneous in all aspects. The (CNT⁻/CNT⁺)_n spectrum contains a strong absorption band at 750 nm, due to the polyaniline in the films PANI-PSS as polyelectrolytes versus PANI/PSS used to modified the MWCNTs and also all (CNT⁻/CNT⁺)_n multilayer display an increase in absorbance with the wavelength.

Fig. 2f shows the maximum absorption occur ∼750 nm for all four samples and the intensity of this peak has been increased with the number of (CNT⁻/CNT⁺)_n multilayer.

Electrocatalytic and electrochemical characteristics

The electro-catalytic activity of the CEs was evaluated via cyclic voltammetry (CV) for the reduction of triiodide ions in detail. Fig. 4a shows cyclic voltammograms of the four-electrode electrochemical cell based of (CNT⁻/CNT⁺)_n multilayer CEs toward redox reactions of iodide/triiodide species. The oxidation of iodide happens at the photo electrode and the reduction of triiodide occurs at the counter electrode, which are ascribed to the redox reaction of I₃⁻ + 2e → 3I⁻.³⁷ In DSSC based of (CNT⁻/CNT⁺)₄ multilayer CE, a pair of oxidation (3.083 eV) and reduction (−4.079 eV) peaks are detected. The peak positions and shapes of the (CNT⁻/CNT⁺)_n CEs, affecting the DSSC performances as shown in Fig. 4b and c. Therefore, the electro-catalytic activity for the (CNT⁻/CNT⁺)₄ CE appears to be the best, in which its catalytic current density is the highest. Additionally, Fig. 4d clearly shows that the (CNT⁻/CNT⁺)₄ CE displays a smaller peak-to-peak separation (E_pp) value than others, indicating a faster charge transfer kinetics than reference CEs under the same experimental condition.³⁸ The high current density of (CNT⁻/CNT⁺)₄ may be attributed to its layered structure. Notably, the multilayer films display more layered structure for I⁻/I₃⁻ transportation and redox reactions, giving a faster reaction kinetics for iodide.³⁸ For comparison study we also prepared Pt electrodes by deposition of ca. 20 μL cm⁻² of H₂PtCl₆ solution (2 mg of Pt in 1 mL of ethanol) and sintered at 400 °C for 15 min as shown in Fig. 6a. The peaks current density and E_pp of both electrodes are shown in Fig. 6b. The (CNT⁻/CNT⁺)₄-CE achieves highest current density with smaller E_pp to that of Pt-CE. The smaller E_pp is more effective to improve the catalytic activity of CEs. The enhanced current density and decreased E_pp values suggests that (CNT⁻/CNT⁺)₄-CE has good catalytic activity, which might be the main reason for improved electrocatalytic performance of DSSCs.

Fig. 3 (a) Schematic illustration of a DSSC device using the (CNT⁻/CNT⁺)_n multilayer-CE. (b) Photocurrent density vs. voltage of DSSCs from the (CNT⁻/CNT⁺)_n multilayer-CEs. (c–f) PCE, FF, V_OC and J_SC of (CNT⁻/CNT⁺)_n multilayer-CE, respectively.

Fig. 4 Cyclic voltammograms for the (CNT⁻/CNT⁺)_n multilayer-CE, the electrolyte was acetonitrile solution containing 10 mM LiI, 5 mM I₂, and 0.1 M LiClO₄. Pt electrode is used as auxiliary electrode and Ag/AgCl works as reference electrode, the scan rate is 50 mV s⁻¹.

In general, DSSCs have a diode-like element; the simple equivalent circuit consists of the current source, diode, capacitances, and the resistances. The relationship between current I and the voltage V at its terminals is given by

I_o = I_PH − I_D − I_SH (1)

in other words, the output current I_o is calculated bywhere
where, I₀, V, q, n, T, k and R_SH are dark saturation current of the diode, voltage, elementary charge, ideality factor, temperature, Boltzmann constant and shunt resistance through the back electron transfer, respectively.

In order to obtain additional information about the resistance of the DSSC based (CNT⁻/CNT⁺)_n-CE, electrochemical impedance spectroscopy (EIS) was employed.

Typically, Nyquist plot consists of two or three semicircles. The intercept of first semicircle on real axis in high frequency range (10⁶ to 10⁵ Hz) represents series resistance (R_s) which is related to FTO sheet resistance, contact resistance and external circuit resistance. The charge transfer resistance (R_ct) at the electrode/electrolyte interface relates to the intrinsic catalytic properties of the counter electrode materials for the reduction of I₃⁻ to I⁻.³⁹ The DSSC fabricated with the (CNT⁻/CNT⁺)_n-CE (n = 1, 2, 3 or 4) and (CNT⁻/CNT⁺)₄-CE was measured to compare the performance of (CNT⁻/CNT⁺)_n-CE (n = 1, 2 or 3) (Fig. 5a). The R_ct decreased clearly from (CNT⁻/CNT⁺)₁ to (CNT⁻/CNT⁺)₄-CE as shown in Fig. 5b and c. The redox reaction resistance (R_ct) of the CE affects the FF and PCE of cells in a negative way.^38,40,41 Therefore, DSSC device using the (CNT⁻/CNT⁺)₄-CE exhibits better performances than those of (CNT⁻/CNT⁺)_n (n = 1, 2 or 3)-CE, showing an elevated electrocatalytic activity at higher bilayers.^42,43 The comparative EIS analysis curves of (CNT⁻/CNT⁺)₄ and Pt-CE are shown in Fig. 6c. It can be seen that the R_ct of the as-prepared (CNT⁻/CNT⁺)₄-CE (0.3 Ω) is lower than that of the Pt-CE (0.7 Ω), which indicates the superior catalytic activity of (CNT⁻/CNT⁺)₄ that it has. The EIS results are in good agreement CV data.

Fig. 5 EIS analysis of (CNT⁻/CNT⁺)_n multilayer-CE and equivalent circuit models. R_s, R_ct, C_dl and Z_w are serial resistance, charge-transfer resistance of electrode, double layer capacitance and diffusion impedance, respectively.

Fig. 6 (a) Schematic illustration of a DSSC device using the (CNT⁻/CNT⁺)₄ multilayer and Pt-CE. (b) Photovoltaic performance of (CNT⁻/CNT⁺)₄ multilayer and Pt-CE based DSSCs. (c) CV for the (CNT⁻/CNT⁺)₄ multilayer and Pt-CE. (d) EIS analysis of (CNT⁻/CNT⁺)₄ multilayer and Pt-CE.

The photocurrent density–voltage (J–V) characteristics of DSSCs based on (CNT⁻/CNT⁺)_n multilayer CE were tested under 100 mW cm² AM 1.5G illumination, using a black metal mask with the active cell area of 0.15 cm² as shown in Fig. 3a. All experimental curves are shown in Fig. 3b and their relative photovoltaic parameters are summarized in Table 1. As the number of (CNT⁻/CNT⁺) CE increased to four layers, the PCE is increased, because of high photocurrent density (J_SC). The enhanced efficiency is attributed to the elevated interfacial area in (CNT⁻/CNT⁺)₄ multilayer CE, which can accelerate the triiodide recovery and therefore promote electron-transfer kinetics. In a related context, the detailed PV parameters of the devices are shown in Table 1. The light-to-electric PCE and FF were calculated according to the equations:³⁸

where V_max and J_max are the voltage and the current density under the maximum power output in the J–V curves, respectively, J_SC is the short-circuit current density (mA cm⁻²), V_OC is the open-circuit voltage (V), and P_in is the incident light power. The solar cells employing (CNT⁻/CNT⁺)₄ multilayer CEs showed highest efficiency of 9.13%, as compare to (CNT⁻/CNT⁺)₃, (CNT⁻/CNT⁺)₂ and (CNT⁻/CNT⁺)₁ bilayer showed PCE of 8.43%, 7.79% and 7.51%, respectively. The obtained data clearly demonstrate that employing (CNT⁻/CNT⁺)₄ multilayer CEs achieved high performance for solar cells. The comparative current density–voltage curves of (CNT⁻/CNT⁺)₄ and standard Pt CEs based DSSCs are shown in Fig. 6d. Table 1 shows the DSSC device performances with (CNT⁻/CNT⁺)₄ and Pt-CE, the solar cell fabricated with (CNT⁻/CNT⁺)₄-CE has achieved the PCE 9.13%, J_SC of 18.03 mA cm⁻² and FF of 0.71%, which were better to the cell with Pt CE 8.40%, 17.16 mA cm⁻² and 0.68%, respectively. The enhancement in J_SC and FF leading the DSSCs PCE performance is high.

Table 1 Photovoltaic performance of (CNT⁻/CNT⁺)_n multilayer and Pt-CE based dye-sensitized solar cells (DSSCs)

Counter electrode	JSC (mA cm^−2)	VOC (V)	FF	PCE (%)	Rs (Ω)	Rct (Ω)
(CNT^−/CNT^+)1	16.79	0.68	0.65	7.51	4.2	2
(CNT^−/CNT^+)2	17.05	0.69	0.67	7.97	3.7	1.2
(CNT^−/CNT^+)3	17.67	0.69	0.69	8.43	3.5	0.8
(CNT^−/CNT^+)4	18.03	0.71	0.71	9.13	3.3	0.3
Pt	17.16	0.72	0.68	8.40	4.4	0.96

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4. Conclusions

In this work, we fabricated (CNT⁻/CNT⁺)_n multilayer CEs in DSSCs by a LBL technique. The (CNT⁻/CNT⁺)₄ CEs exhibited good electrocatalytic performance for the redox couple and specific lower charge transfer resistance (R_ct) values at the CE/electrolyte interface. The (CNT⁻/CNT⁺)₄ device-based DSSC displayed the highest values of PCE (9.13%) and FF (0.71), which could be attributed to the low R_ct at the (CNT⁻/CNT⁺)₄-CE/electrolyte interface. The LBL method offers a novel easy synthesis, versatile electrical and photovoltaic performances for excellent CE material of DSSCs.

References

1. B. O. Regan and M. Grätzel, Low-cost, high efficiency solar cell based on dye-sensitized colloidal TiO₂ films, Nature, 1991, 353, 737–740.
2. M. Grätzel, Photoelectrochemical cells, Nat. Biotechnol., 2001, 414, 338–344.
3. M. Wu and T. Ma, Platinum-Free Catalysts as Counter Electrodes in Dye-Sensitized Solar Cells, ChemSusChem, 2012, 5, 1343–1357.
4. R. Jose, V. Thavasi and S. Ramakrishna, Metal Oxides for Dye-Sensitized Solar Cells, J. Am. Ceram. Soc., 2009, 92, 289–301.
5. M. Ohta, N. Koumura, K. Hara and S. Mori, Concerted effect of large molecular dyes and bulky cobalt complex redox couple to retard recombination in dye-sensitized solar cells, Electrochem. Commun., 2011, 13, 778–780.
6. Y. Fang, J. Zhang, X. Zhou, Y. Lin and S. Fang, “Soggy sand” electrolyte based on COOH-functionalized silica nanoparticles for dye-sensitized solar cells, Electrochem. Commun., 2012, 16, 10–13.
7. G. R. Li, F. Wang, J. Song, F. Y. Xiong and X. P. Gao, TiN-conductive carbon black composite as counter electrode for dye-sensitized solar cells, Electrochim. Acta, 2012, 65, 216–220.
8. K.-M. Lee, W.-H. Chiu, H.-Y. Wei, C.-W. Hu, V. Suryanarayanan, W.-F. Hsieh and K.-C. Ho, Effects of mesoscopic poly(3,4-ethylenedioxythiophene) films as counter electrodes for dye-sensitized solar cells, Thin Solid Films, 2010, 518, 1716–1721.
9. R. Trevisan, M. Döbbelin, P. P. Boix, E. M. Barea, R. Tena-Zaera, I. Mora-Seró and J. Bisquert, PEDOT Nanotube Arrays as High Performing Counter Electrodes for Dye Sensitized Solar Cells. Study of the Interactions Among Electrolytes and Counter Electrodes, Adv. Energy Mater., 2011, 1, 781–784.
10. P. Balraju, M. Kumar, M. S. Roy and G. D. Sharma, Dye sensitized solar cells (DSSCs) based on modified iron phthalocyanine nanostructured TiO₂ electrode and PEDOT:PSS counter electrode, Synth. Met., 2009, 159, 1325–1331.
11. G. Yue, J. Wu, Y. Xiao, J. Lin, M. Huang and Z. Lan, Application of Poly(3,4-ethylenedioxythiophene):Polystyrenesulfonate/Polypyrrole Counter Electrode for Dye-Sensitized Solar Cells, J. Phys. Chem. C, 2012, 116, 18057–18063.
12. J. Wu, Q. Li, L. Fan, Z. Lan, P. Li, J. Lin and S. Hao, High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells, J. Power Sources, 2008, 181, 172–176.
13. S. Cho, S. H. Hwang, C. Kim and J. Jang, Polyaniline porous counter-electrodes for high performance dye-sensitized solar cells, J. Mater. Chem., 2012, 22, 12164.
14. G. R. Li, J. Song, G. L. Pan and X. P. Gao, Highly Pt-like electrocatalytic activity of transition metal nitrides for dye-sensitized solar cells, Energy Environ. Sci., 2011, 4, 1680.
15. K. M. Lee, C. Y. Hsu, P. Y. Chen, M. Ikegami, T. Miyasaka and K. C. Ho, Highly porous PProDOT-Et₂ film as counter electrode for plastic dye-sensitized solar cells, Phys. Chem. Chem. Phys., 2009, 11, 3375–3379.
16. A. Kaniyoor and S. Ramaprabhu, Thermally exfoliated graphene based counter electrode for low cost dye sensitized solar cells, J. Appl. Phys., 2011, 109, 124308.
17. P. Poudel, A. Thapa, H. Elbohy and Q. Qiao, Improved performance of dye solar cells using nanocarbon as support for platinum nanoparticles in counter electrode, Nano Energy, 2014, 5, 116–121.
18. S. Gagliardi, L. Giorgi, R. Giorgi, N. Lisi, T. Dikonimos Makris, E. Salernitano and A. Rufoloni, Impedance analysis of nanocarbon DSSC electrodes, Superlattices Microstruct., 2009, 46, 205–208.
19. J. Y. Lin, C. Y. Chan and S. W. Chou, Electrophoretic deposition of transparent MoS₂-graphene nanosheet composite films as counter electrodes in dye-sensitized solar cells, Chem. Commun., 2013, 49, 1440–1442.
20. T. N. Murakami, S. Ito, Q. Wang, M. K. Nazeeruddin, T. Bessho, I. Cesar, P. Liska, R. Humphry-Baker, P. Comte, P. t. Péchy and M. Grätzel, Highly Efficient Dye-Sensitized Solar Cells Based on Carbon Black Counter Electrodes, J. Electrochem. Soc., 2006, 153, A2255.
21. K. Imoto, K. Takahashi, T. Yamaguchi, T. Komura, J.-I. Nakamura and K. Murata, High-performance carbon counter electrode for dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells, 2003, 79, 459–469.
22. Y. Xiao, J. Wu, G. Yue, J. Lin, M. Huang and Z. Lan, Low temperature preparation of a high performance Pt/SWCNT counter electrode for flexible dye-sensitized solar cells, Electrochim. Acta, 2011, 56, 8545–8550.
23. E. Ramasamy, W. J. Lee, D. Y. Lee and J. S. Song, Spray coated multi-wall carbon nanotube counter electrode for tri-iodide (I³⁻) reduction in dye-sensitized solar cells, Electrochem. Commun., 2008, 10, 1087–1089.
24. S. Iijima, Helical microtubules of graphitic carbon, Nat. Biotechnol., 1991, 354–356.
25. H. J. Choi, J. E. Shin, G.-W. Lee, N.-G. Park, K. Kim and S. C. Hong, Effect of surface modification of multi-walled carbon nanotubes on the fabrication and performance of carbon nanotube based counter electrodes for dye-sensitized solar cells, Curr. Appl. Phys., 2010, 10, S165–S167.
26. W. J. Lee, E. Ramasamy, D. Y. Lee and J. S. Song, Efficient dye-sensitized solar cells with catalytic multiwall carbon nanotube counter electrodes, ACS Appl. Mater. Interfaces, 2009, 1, 1145–1149.
27. J. G. Nam, Y. J. Park, B. S. Kim and J. S. Lee, Enhancement of the efficiency of dye-sensitized solar cell by utilizing carbon nanotube counter electrode, Scr. Mater., 2010, 62, 148–150.
28. C.-Y. Liu, K.-C. Huang, P.-H. Chung, C.-C. Wang, C.-Y. Chen, R. Vittal, C.-G. Wu, W.-Y. Chiu and K.-C. Ho, Graphene-modified polyaniline as the catalyst material for the counter electrode of a dye-sensitized solar cell, J. Power Sources, 2012, 217, 152–157.
29. S. Hou, X. Cai, H. Wu, Z. Lv, D. Wang, Y. Fu and D. Zou, Flexible, metal-free composite counter electrodes for efficient fiber-shaped dye-sensitized solar cells, J. Power Sources, 2012, 215, 164–169.
30. G. Wang, W. Xing and S. Zhuo, The production of polyaniline/graphene hybrids for use as a counter electrode in dye-sensitized solar cells, Electrochim. Acta, 2012, 66, 151–157.
31. G. Decher, Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites, Science, 1997, 277, 1232–1237.
32. H. Chen and S. Dong, A method to construct polyelectrolyte multilayers film containing gold nanoparticles, Talanta, 2007, 71, 1752–1756.
33. C.-W. Kuo and T.-C. Wen, Dispersible polyaniline nanoparticles in aqueous poly(styrenesulfonic acid) via the interfacial polymerization route, Eur. Polym. J., 2008, 44, 3393–3401.
34. J. Zhu, J. Wang, F. Lv, S. Xiao, C. Nuckolls and H. Li, Synthesis and self-assembly of photonic materials from nanocrystalline titania sheets, J. Am. Chem. Soc., 2013, 135, 4719–4721.
35. W. Sun, T. Peng, Y. Liu, W. Yu, K. Zhang, H. F. Mehnane, C. Bu, S. Guo and X. Z. Zhao, Layer-by-layer self-assembly of TiO₂ hierarchical nanosheets with exposed {001} facets as an effective bifunctional layer for dye-sensitized solar cells, ACS Appl. Mater. Interfaces, 2014, 6, 9144–9149.
36. E. Detsri and S. T. Dubas, Interfacial polymerization of polyaniline and its layer-by-layer assembly into polyelectrolytes multilayer thin-films, J. Appl. Polym. Sci., 2013, 128, 558–565.
37. J. D. Roy-Mayhew, D. J. Bozym, C. Punckt and I. A. Aksay, Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells, ACS Nano, 2010, 4, 6203–6211.
38. L. Han, N. Koide, Y. Chiba, A. Islam, R. Komiya, N. Fuke, A. Fukui and R. Yamanaka, Appl. Phys. Lett., 2005, 86, 213501.
39. C. Xu, J. Li, X. Wang, J. Wang, L. Wan, Y. Li, M. Zhang, X. Shang and Y. Yang, Synthesis of hemin functionalized graphene and its application as a counter electrode in dye-sensitized solar cells, Mater. Chem. Phys., 2012, 132, 858–864.
40. D. W. Zhang, X. D. Li, H. B. Li, S. Chen, Z. Sun, X. J. Yin and S. M. Huang, Graphene-based counter electrode for dye-sensitized solar cells, Carbon, 2011, 49, 5382–5388.
41. S. Das, P. Sudhagar, V. Verma, D. Song, E. Ito, S. Y. Lee, Y. Kang and W. Choi, Amplifying Charge-Transfer Characteristics of Graphene for Triiodide Reduction in Dye-Sensitized Solar Cells, Adv. Funct. Mater., 2011, 21, 3729–3736.
42. G. Wang, W. Xing and S. Zhuo, Nitrogen-doped graphene as low-cost counter electrode for high-efficiency dye-sensitized solar cells, Electrochim. Acta, 2013, 92, 269–275.
43. B. Lee, D. B. Buchholz and R. P. H. Chang, An all carbon counter electrode for dye sensitized solar cells, Energy Environ. Sci., 2012, 5, 6941.

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