Carbon nanotube counter electrode for high-efficient fibrous dye-sensitized solar cells
© Huang et al.; licensee Springer. 2012
Received: 2 December 2011
Accepted: 17 April 2012
Published: 17 April 2012
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© Huang et al.; licensee Springer. 2012
Received: 2 December 2011
Accepted: 17 April 2012
Published: 17 April 2012
High-efficient fibrous dye-sensitized solar cell with carbon nanotube (CNT) thin films as counter electrodes has been reported. The CNT films were fabricated by coating CNT paste or spraying CNT suspension solution on Ti wires. A fluorine tin oxide-coated CNT underlayer was used to improve the adherence of the CNT layer on Ti substrate for sprayed samples. The charge transfer catalytic behavior of fibrous CNT/Ti counter electrodes to the iodide/triiodide redox pair was carefully studied by electrochemical impedance and current-voltage measurement. The catalytic activity can be enhanced by increasing the amount of CNT loading on substrate. Both the efficiencies of fibrous dye-sensitized solar cells using paste coated and sprayed CNT films as counter electrodes are comparative to that using Pt wires, indicating the feasibility of CNT/Ti wires as fibrous counter electrode for superseding Pt wires.
Flexible dye-sensitized solar cells are the subject of active research as a good power supply for portable and integrated equipment . Particularly, fibrous dye-sensitized solar cells (F-DSCs) based on various fibrous substrates (i.e., metal wires, optical fiber, carbon fiber, etc.) have attracted increasing attentions due to their unique structures for omnidirectional light absorption and weavable characteristic [2–4]. The metal-based F-DSCs have the advantages of low bulk resistance, low cost and easy fabrication. Recently, considerable efforts have been focused on the cost effective fibrous photoanode. It can be constructed by covering the metal wire substrate with titanium dioxide particle films, nanotube arrays, or nanowires films via simple dip-coating, spray techniques , or electrochemical-eroding Ti wires [6, 7]. However, less attention has been paid to the fibrous counter electrode (CE). Commonly, the F-DSCs were assembled by twisting Pt wire CE and photoande together, and the conversion efficiency (Eff) of 5.8% and 1.86% could be achieved using liquid electrolyte  and solid electrolyte , respectively . However, the Pt wire was an extremely expensive CE for the practical applications of F-DSCs, which gave rise to the strong request for new cheaper materials to substitute Pt.
Carbon materials are considered to be excellent substitutes of Pt for their good reduction ability to tri-iodine ions in electrolyte. Various kinds of carbon, such as activated carbon , carbon nanotubes (CNT) [11–14], graphite , hard carbon spherules  and carbon black  have been studied. Due to their advantages of high electrical conductivity, chemical stability and high surface area, these carbon materials have been proven to be competitive with Pt as CE used in flat DSCs .
In this study, multiwalled CNT is firstly chosen as the catalytical material of CE for F-DSCs. Fibrous CEs are fabricated by two different methods, brushing CNT paste or spraying CNT suspensions on Ti wire substrate. Because the CE should be convolved on photoanode, the adherence of the active layer on wire substrate is of great importance. Fluorine tin oxide (FTO) was sprayed on CNT layer to improve the contact between CNTs and enhance the adherence of CNT layer which cycles to the surface of Ti wire. The impact of CNT loading amount and fabrication methods on the electrochemical catalytic activity of the CEs have been studied by electrochemical impedance spectra (EIS) and current-voltage measurement. The F-DSCs were assembled using dye-sensitized TiO2 nanotube (TNT)-coated Ti wire as photoanode and CNT-coated Ti wire as CE. The best energy conversion efficiency of 4.18% has been achieved under AM 1.5-G illumination.
The TiO2-nanotube photoanode was fabricated by anodic oxidation . Ti wires (Φ = 0.3 mm, purity 99.7%) with length of 4 cm were first ultrasonically cleaned in ethanol and then electrochemically polished. The polished Ti wires were electrochemically eroded in ethylene glycol electrolyte containing 0.2 wt% NH4F and a small amount of deionized water, with an applied voltage of 50 V. The length of TNT can be adjusted by controlling the reaction time. The TNT-coated Ti wires were ultrasonically treated after anodic oxidation for several minutes and annealed at 450°C for 3 h for crystallization, then treated in 0.1 M TiCl4 aqueous solution at 70°C for 1 h, followed by re-annealing at 450°C for 30 min. After cooling down to 80°C, the TNT photoanodes were immediately immersed into 0.3 mM cis-bis (isothiocyanato) bis (2,2 = −bipyridyl-4,4 = −dicarboxylato) ruthenium(II) bistetrabutyl ammonium (N719, Dyesol, New South Wales, Australia) in ethanol for 24 h at room temperature.
To prepare a viscous CNT paste, 0.5 g CNT powder (donated by Prof. Fei Wei from Tsinghua University, P.R. China ) was ultrasonically dispersed in 300 mL ethanol, then 10 ml of terpineol, 0.3 ml of ethyl cellulose alcoholic solution and 0.2 ml of titanium isopropoxide used as binder were added into the solution. To improve the dispersion of CNTs, the mixed paste was ball-milled for 24 h. The as-prepared CNT paste was brush coated on Ti wires (Φ = 0.1 mm). After drying at 80°C in the air for 30 min, CNT CEs fabricated by brush coating method (BCNT) were obtained by annealing at 385°C for 20 min with a heating rate of 2°C/min.
Home-made reel equipment, which is able to automatically convolve the fibrous CE onto the photoanode, was used to assemble the F-DSCs. All the thread pitch distances of screwed CE were kept at 1 mm, according to our previous work . To fix the fibrous solar cell and to avoid electrolyte drying out, two pieces of PET were used to clamp the samples. A small amount of electrolyte was dropped into the gap between the two pieces of PET, and the electrolyte would flow along with Ti wire due to the capillary force. The electrolyte was composed of 0.6 M methylhexylimidazolium iodide, 0.05 M of iodine, 0.5 M tert-butylpyridine, and 0.1 M of lithium iodide in 3-methoxypropionitrile.
The morphology of CE was investigated by scanning electron microscopy (SEM, FEI XL30 SFEG). Dummy cells were composed of two identical fibrous CEs laid parallel to each other with the distance of 0.5 mm and clamped by two pieces of glass. Electrolyte was injected into the space between the two CEs. EIS and current-voltage characteristic of dummy cells were applied to test the catalytic properties of CEs. For photovoltaic measurement, the F-DSCs were exposed to the illumination of standard simulated sunlight of 100 mW cm−2 (AM 1.5 G, Oriel 91160A, Newport Corporation, Beijing, China). A 20 mm × 1 mm mask was used to screen stray light and ensure the light to vertically illuminate onto solar cells. The photocurrent-voltage curves of F-DSCs were recorded by IM6e electrochemical work station (Zahner Co., Germany). The frequency of the superimposed signal is from 100 kHz to 0.1 Hz with an AC amplitude of 10 mV. The effective illuminated area of one fibrous solar cell was calculated as the product of the Ti wire diameter and the cell length (2 cm), which was 0.06 cm2 for Ti wire with diameter of 0.3 mm.
The catalytic ability of carbon electrode is mainly influenced by the active surface area of the carbon layer. Increasing the loading amount of carbon materials on the substrate will directly raise the surface area, providing more sites for I3− reduction and, subsequently, improve the performance of the CEs. Normally, the thicknesses of the carbon layer on CEs are varied from several microns to more than 100 μm . However, too thick CNT layers would be easily crashed due to the distortion and extension during the convolving step in the assembly process of F-DSCs. Thus, the carbon layer thickness should be carefully controlled.
The fitted parameters of EIS results shown in Figure 3
Rct (Ω ·cm2)
SCNT_1,000s and BCNT show relatively low Rct of 22 Ω ·cm2 and 16 Ω ·cm2, respectively, giving rise to high J0 values, two orders larger than J0 of SCNT_120s. Thus, they have much better catalytic activity. Accordingly, the Pt CE shows the lowest Rct of 3.75 Ω ·cm2 and best catalytic performance.
Where Iph is the short circuit current without parasitic resistance, I0 is the reverse saturated current, Rs is the series resistance and Rsh is the shunt resistance. Rct at counter electrode introduces large Rs equivalently. Normally, I0 is much smaller than Iph in several orders and Rsh is much larger than Rs. So, the change of Rct could not lead to predominant change of the exponential part and Rs/Rsh. However, if the Rct is large enough, it will cause obvious decrease of photocurrent at short-circuit condition, which is the same as the cases of SCNT samples with only FTO coated on the CNT layer or very thin CNT layer.
The poor catalytic activity of FTO-coated CNT underlayer also leads to lower Voc, spraying CNTs on this underlayer make Voc increased quickly and it changed very slowly after spraying time of 360 s. Finally, the best average value of Eff of 3.14% is obtained while the spraying time is 1,000 s.
Photovoltaic parameters of photocurrent-voltage curves shown in Figure 6
Length of TNT (μm)
According to our previous work , the efficiency of F-DSC would increase along with the length of TNT when the length is shorter than 40 μm. Further increasing the length, the TNTs would easily collapse. Photoanode with 38 μm TNT was used to obtain a relatively high Eff. As seen from Table 2, the Eff s are 4.14% and 4.18% for F-DSCs using BCNT and SCNT as CEs, respectively, both reach 80% of the Eff of the cell with Pt CE. This result indicates the potential application of the CNT-coated Ti wires as the CE for F-DSCs. Furthermore, the SCNT CEs with FTO-coated CNT underlayer have better connection between CNT film and substrate than that of BCNT CEs. They can be reused because the CNTs would not easily crush from the substrate by distortion, while the SCNT samples with thicker activated layers show contrary result. Therefore, the CNT CEs fabricated by spray method are more practical.
The catalytic characters of CNT fibrous CEs prepared by brush coating and spraying methods have been carefully investigated by electrochemical impedance technique and current-voltage measurement. The catalytic activity of CNT films is enhanced with CNT loading amount as the results of the increasing inner surface area of CNT films. However, too thick CNT film which is prepared by brush coating method would lead to CNT particles pilling from substrate during cell assembly process, indicating that the CNT CEs fabricated by spray method are more practical. Both the efficiencies of F-DSCs with CNT CEs prepared by brush coating and spraying methods (spraying time is 1,000 s) exceeded 4% and reached 80% of that of F-DSCs with Pt CE, indicating the application potential of CNT fibrous CE as the substitute of Pt wire.
The authors appreciate the financial supports of National Natural Science Foundation of China (grant nos. 20725311, 51072221 and 21173260), National Key Basic Research Program (973 project, grant no. 2012CB932903) and the Knowledge Innovation Program of the Chinese Academy of Sciences (grant no. KJCX2-YW-W27). Thanks for the donation of carbon nanotube sample from Prof. Wei Fei of Tsinghua University.
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