Enhancing performance of ZnO dye-sensitized solar cells by incorporation of multiwalled carbon nanotubes
© Chang et al; licensee Springer. 2012
Received: 30 November 2011
Accepted: 5 March 2012
Published: 5 March 2012
A low-temperature, direct blending procedure was used to prepare composite films consisting of zinc oxide [ZnO] nanoparticles and multiwalled carbon nanotubes [MWNTs]. The mesoporous ZnO/MWNT films were fabricated into the working electrodes of dye-sensitized solar cells [DSSCs]. The pristine MWNTs were modified by an air oxidation or a mixed acid oxidation treatment before use. The mixed acid treatment resulted in the disentanglement of MWNTs and facilitated the dispersion of MWNTs in the ZnO matrix. The effects of surface property and loading of MWNTs on DSSC performance were investigated. The performance of DSSCs was found to depend greatly on the type and the amount of MWNTs incorporated. At a loading of 0.01 wt%, the acid-treated MWNTs were able to increase the power conversion efficiency of fabricated cells from 2.11% (without MWNTs) to 2.70%.
KeywordsZnO nanoparticle multiwalled carbon nanotube composite film dye-sensitized solar cells conversion efficiency
Dye-sensitized solar cells [DSSCs] are considered as promising third-generation solar energy devices because of their low fabrication cost, compatibility with flexible substrates, and practicable high conversion efficiency [1, 2]. The maximum conversion efficiencies attained by DSSCs so far (approximately 11%), although considerably lower than those of silicon solar cells, may meet the requirements of many practical applications [1, 3]. Further improvement in the conversion efficiency of DSSCs is possible, as theoretical prediction of the maximum conversion efficiency of DSSCs is approximately 20% .
A DSSC is a photoelectrochemical system in which a porous nanostructured oxide film with adsorbed dye molecules acts as the photoanode and plays a significant role in converting photons into electrical energy. It has been shown that the performance of DSSCs is closely related to the structure of the photoelectrode film . A rational design of the photoelectrode film structure may result in better light harvesting and electron transport. Zinc oxide [ZnO] has been used widely for the fabrication DSSC photoanodes. ZnO is regarded as an attractive alternative to titanium dioxide [TiO2] because it has a similar band gap level to that of TiO2 while possesses a higher electron mobility and more flexibility in synthesis and morphologies [6, 7]. Among the nanostructures investigated for the fabrication of DSSC photoelectrode films, nanoparticles are most widely used. This is likely due to a high specific surface area provided by nanoparticle-based films. Another reason is probably the ease of preparation of nanoparticles through simple chemical solution methods and the ease of film formation through the doctor-blade method.
Nanoparticle-based films can provide a large surface area, but the existence of numerous boundaries in the nanoparticle network may hinder the transport of electrons in photoelectrode and thus limit energy conversion efficiency of DSSCs. Incorporating one-dimensional [1-D] nanostructures into nanoparticulate films may overcome the problem by providing direct pathways for electron transport . Carbon nanotubes [CNTs], a type of 1-D nanostructure, possess several unique properties including hollow and layered structures, a high aspect ratio, excellent electrical and thermal conductivity, high mechanical strength, and a large specific surface area . Incorporating CNTs into ZnO electrodes should provide highly conductive paths to the ZnO nanostructures, thereby promoting faster transport of photo-induced electrons in DSSCs and higher current. The combination of CNTs with ZnO nanoparticles is thus a promising approach to boost conversion efficiency of DSSCs. In fact, this approach has been used to improve the performance of TiO2-based DSSCs [10–12]. However, the effect of CNTs on ZnO nanoparticle-based DSSCs has not been reported before. The major barrier for the application of CNTs in DSSCs is the insolubility of CNTs in most solvents. To obtain homogeneous dispersion of CNTs, CNTs need to be pre-treated before mixing them with nanoparticles. The air oxidation treatment has been found to completely remove amorphous carbon and metal oxide impurities from CNTs . The mixed acid treatment not only effectively purifies CNTs but also leads to the formation of oxygen-containing groups, mainly carboxylic, on the graphitic surface [14, 15]. The carboxylic groups thus formed facilitate the exfoliation of CNT bundles and therefore the dispersion of CNTs. The oxygen-containing groups should also improve the interfacial bonding between CNTs and ZnO nanoparticles.
In this study, DSSC photoanodes were fabricated using nanostructured films based on commercial ZnO nanoparticles. To study the effects of incorporating CNTs on device performance, two different types of multiwalled carbon nanotubes [MWNTs], i.e., oxidized MWNT [O-MWNT] (or oxidized in air) and acid-MWNT (mixed acid treated), were prepared and blended with ZnO nanoparticles at various levels. The effects of air oxidation and mixed acid treatments on the morphologies of MWNT were studied using transmission electron microscopy [TEM], and the morphologies of the prepared ZnO/MWNT composite films were investigated by using field emission scanning electron microscopy [FE-SEM]. The energy conversion efficiency [η] and the electrochemical impedance of the fabricated DSSCs were also determined.
Pristine MWNTs that were 10 to 30 nm in diameter and 5 to 15 mm in length were purchased from Nanotech Port (Genesis Nanotech Corporation, Tainan, Taiwan, Republic of China). These MWNTs were produced via a chemical vapor deposition method. To obtain the O-MWNT, the pristine unmodified-MWNT [U-MWNT] was purified by thermal treatment in air at 550°C for 45 min to remove amorphous carbon and residual metal catalysts. The carboxylic group-modified MWNT [acid-MWNT] was prepared by treating the O-MWNT with a sulfuric acid/nitric acid mixture (3:1, v/v) at 50°C for 2 h under ultrasonication. After the mixed acid treatment, the acid-MWNT was collected by filtration and then washed with distilled water and methanol .
Commercial ZnO nanoparticles (UniRegion Bio-Tech, Taiwan, Republic of China) that were about 20 nm in size were dispersed in equal proportion of distilled water and tert-butanol to form ZnO pastes. The MWNTs were added at this step when ZnO/MWNT composite films were to be made. The nanoporous films were prepared by applying the pastes onto commercial fluorine-doped tin oxide [FTO] substrates (Nippon Sheet Glass, 8 to 10 Ω/□, 3 mm thick) (Nippon Sheet Glass Company, Tokyo, Japan) by the doctor-blade method using adhesive tapes as a frame and spacer. The active electrode area was 0.25 cm2, and the films had a thickness of 8 μm. The resulting films were then gradually heated and annealed at 150°C for 1 h to remove organic materials in the paste and to increase crystallinity of ZnO. After being cooled to room temperature, the porous films were sensitized by immersing them into a dye solution that contained 0.5 mM of cis-bis (isothiocyanato) bis-(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium (Solaronix, N719) (Solaronix SA, Aubonne, Switzerland) in a mixed solvent consisting of equal parts of acetonitrile and tert-butanol. The dye-adsorption time lasted for 2 h. The dye-loaded electrodes were then rinsed with acetonitrile and dried in the air. The counter electrode was made of FTO glass, onto which a nanocrystalline platinum [Pt] catalyst was deposited by decomposition of H2PtCl6 at 400°C for 20 min. The ZnO photoanode and the counter electrode were sealed together with a 60 μm-thick hot-melting spacer (DuPont, Surlyn, Shanghai, People's Republic of China), and the inner space was filled through a hole with a volatile electrolyte composed of 0.1 M lithium iodide, 0.6 M 1, 2-dimethyl-3-propylimid- azolium iodide (PMII, Merk Ltd., Taiwan, Republic of China) 0.05 M I2 (Sigma-Aldrich China, Inc., Shanghai, People's Republic of China), and 0.5 M tert-butylpyridine (Sigma-Aldrich China, Inc., Shanghai, People's Republic of China) in acetonitrile.
The surface morphologies and dimensions of the ZnO/MWNT composite films were characterized using a FEI Nova230 FE-SEM (FEI Inc., Hillsboro, Oregon). TEM images of MWNTs were taken by employing a JEOL-JEM-1230 transmission electron microscope (Jeol (Beijing) Co., Ltd., Beijing, China). X-ray diffraction [XRD] patterns were obtained by using a diffractometer (PANalytical X'Pert PRO) (Spectris Instrumentation and Systems Shanghai Ltd., Shanghai, People's Republic of China) with Cu Kα radiation. The thickness of the ZnO nanocrystalline layer was measured by using a microfigure measuring instrument (Surfcorder ET3000, Kosaka Laboratory, Kosaka, Japan). The η was measured under a white light source (YSS-100A, Yamashita Denso Company, Tokyo, Japan), which gave an irradiance of 100 mW cm-2 at the equivalent of air mass [AM] 1.5 on the surface of the solar cell. The irradiance of simulated light was calibrated using a silicon photodiode (BS-520, Bunko Keiki Co., Ltd, Tokyo, Japan). The evolution of the electron transport process in the cell was investigated using electrochemical impedance spectroscopy [EIS]. Impedance spectroscopy was performed using an electrochemical analyzer (Autolab PGSTAT30) (EcoChemie, Utrecht, The Netherlands). The impedance measurements were carried out by applying a direct current bias at open circuit voltage [VOC] and an alternating current voltage with amplitude of 10 mV in a frequency range from 10-2 to 105 Hz under AM 1.5 illumination.
Results and discussion
Effects of incorporating MWNTs on J-V characteristics of DSSCs
Short-circuit photocurrent density (mA/cm2)
Open circuit voltage (volt)
Energy conversion efficiency (percent)
ZnO/0.01 wt% O-MWNT
ZnO/0.01 wt% acid-MWNT
Effects of incorporating MWNTs on the electron transport properties of fabricated cells
Charge-transfer resistance at the ZnO/electrolyte interface [Rk] (ohm)
Electron transport resistance in the ZnO network [Rw] (ohm)
Ratio of Rk to Rw
Mean electron lifetime (ms)
Electron diffusion coefficient in the photoanode (10-4 cm2 s-1×)
Effective electron diffusion length (micrometer)
ZnO/0.01 wt% O-MWNT
ZnO/0.01 wt% acid-MWNT
As shown in Table 1 the ZnO/O-MWNT device had the highest JSC value, but its overall η was lower than that of the ZnO/acid-MWNT device. This was due to the low VOC value of the ZnO/O-MWNT device, which was likely resulted from the severe aggregation of O-MWNTs in the ZnO matrix. Because O-MWNTs formed bundles (Figure 2c), only the outer surface of the bundle was in contact with ZnO nanoparticles. The interior of the O-MWNT bundle was most likely exposed directly to electrolyte, resulting in the highest recombination loss (lowest Rk as shown in Table 2) and consequently the lowest VOC. Therefore, despite its highest JSC, the ZnO/O-MWNT device had lower η than the ZnO/acid-MWNT cell.
DSSCs based on ZnO/MWNT composite films were fabricated using a low-temperature, direct blending procedure and compared with the pure ZnO nanoparticle-based cells. The cell performance was found to depend on the loading and surface modification of MWNTs. At a loading of 0.01 wt%, both O-MWNT and acid-MWNT could enhance the η of DSSCs, which was attributed to higher JSC values. However, the acid-MWNT modified cell had higher η. This is because the acid-MWNTs were disentangled and better dispersed in the ZnO matrix, leading to less recombination loss as compared to the O-MWNTs containing cells. The solar cell containing 0.01 wt% of acid-MWNT yielded the highest η, which was characterized by the following parameters: JSC = 5.68 mA/cm2, VOC = 0.66 V, FF = 0.72, and η = 2.70%.
dye-sensitized solar cells
multiwalled carbon nanotubes
electrochemical impedance spectroscopy
field emission scanning electron microscopy
fluorine-doped tin oxide
- J SC :
short-circuit photocurrent density
transmission electron microscopy
- V OC :
open circuit voltage
The authors acknowledge the financial support from Bureau of Energy, Ministry of Economic Affairs, Taiwan (project no. A455DR2110). The authors also thank Prof Chung-Wen Lan at the Department of Chemical Engineering, National Taiwan University for the instrument support.
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