Abstract
The composite films which consist of amorphous carbon, carbon nanotube, and iron nanoparticles were prepared by ethanol catalytic combustion in atmospheric environment. The as-prepared composite films have good electrocatalytic activity and high conductivity which is due to their particular structure. The efficiency of the composite films based dye-sensitized soar cells (DSSCs) is closed to that of the Pt based one. Most importantly, the DSSC employing the composite films presents a higher FF than those of Pt based solar cell. In addition, it is a simple method for mass production of composite films counter electrode (CE) which is expected to reduce the cost of fabricating DSSCs.
1. Introduction
Since the report by O’Regan and Grätzel in 1991 [1], DSSCs have been regarded as promising devices for energy applications because of their advantages of low cost, ease of manufacture, and a relatively high photo-to-current conversion efficiency [2]. Moreover, the solar energy conversion efficiency of DSSCs with a ruthenium sensitizer and Pt counter electrode reaches 13.1% [3].
Counter electrode serves to transfer electrons from external circuit to triiodide and iodine in the redox electrolyte. Pt has been widely used as the counter electrode in DSSCs. Although Pt exhibits excellent catalytic activity for triiodide reduction and good electric conductivity, it is extremely expensive. Therefore, some nanosized carbon materials, such as hard carbon spherule [4], carbon nanotubes [5], mesoporous carbon [6], and ordered multimodal porous carbon [7], have been investigated extensively as counter electrode to replace the FTO/Pt electrode. However, the application of composite films which consist of amorphous carbon, carbon nanotube, and iron nanoparticles as counter electrode in DSSCs has not been reported yet. Herein, the composite films were prepared by ethanol catalytic combustion in atmospheric environment. The as-prepared composite films as the counter electrode exhibit a good photovoltaic performance of DSSCs.
2. Experimental
2.1. Fabrication of Composite Film CEs
The preparation of composite electrode was conducted in the ethanol catalytic combustion (ECC) process. A small amount of thiophene was added to 100 mL anhydrous alcohol and then we added right amount of ferrocene to the solution which poured into the alcohol lamp at last. In this experiment, FTOs were employed as substrate which were fixed on the small lift then placed in the flame. The distance between the FTOs and the alcohol lamp wick was 6 cm. After several seconds, the composite films were obtained on FTOs.
2.2. Assembly of DSSCs
The clean FTO was treated with 40 mM TiCl4 aqueous solution at 70°C for 30 min. Then the TiO2 was coated on the FTO substrates by screen printing technique, followed by sintering at 450°C for 30 min. The thickness of the TiO2 film was about 6.6 μm which was controlled by the times of the screen printing technique. The TiO2 film was annealed at 450°C for 30 min and then was soaked into the TiCl4 aqueous solution with the concentration of 0.2 M at 70°C for 30 min. Following annealing for 30 min at 450°C, the TiO2 film was cooled to 80°C and then immersed into a solution of the dye of cis-di (thiocyanato)-N-N′-bis (2,2′-bipyridyl-4-carboxylic acid-4′-tetrabutylammonium carboxylate) ruthenium (II) (N-719) overnight. To prepare FTO/Pt electrode, the Pt catalyst was deposited on the FTO glass by coating with a drop of H2PtCl6 solution with repetition of the heat treatment at 450°C for 30 min. The electrolyte was LiI (0.5 M), I2 (0.05 M), 1-propy 1-2, 3-dimethylimidazolium iodide (DMP, 0.3 M), and 4-ter butylpyridine (4-TPB, 0.3 M) in an acetonitrile solution which was introduced into the cell gap between the anode and counter electrode. The active area of the cell was 0.25 cm2.
2.3. Characterization and Measurements
The morphologies of the composite films were characterized using the scanning electron microscopy (SEM, Zeiss ULTRA 55) and transmission electron microscopy (TEM, JEOL 2100). The composition of the as-prepared films was analyzed by the energy dispersive X-ray spectra (EDX, Zeiss ULTRA 55), Raman spectroscopy (Renishaw 2000), and thermogravimetric analysis (TGA, SDIA8H).
Electrochemical impedance spectroscopy (EIS) measurements were recorded using electrochemical workstation with bias DC voltage of −0.6 V over the frequency range of 0.1 Hz to 100 kHz. The incident photo-to-current conversion efficiency (IPCE) was measured by using a lab-made IPCE setup in institute of plasma physics. The photocurrent-voltage (I-V) curves were measured by the electrochemical workstation under an illumination of xenon lamp. The irradiance is 100 mW/cm2 which is measured by a radiometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University).
3. Results and Discussion
Figures 1 and 2 show the typical SEM and TEM images of the composite films, respectively. The SEM shows that the composite film is made up of nanoparticles and nanotube. From the TEM, we can see clearly the structure of the carbon nanotube, metal nanoparticles, and amorphous carbon. Moreover, we can see that the metal nanoparticles are wrapped in the surface of nanotube and the amorphous carbon.
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For further analysis, we have characterized the composite films by Raman spectroscopy. Figure 3 shows that the G-band (1586.4 cm−1) is much higher than its D-band (1339 cm−1) which reveals that the composite films have a high graphitization degree. Moreover, there are some peaks in 0 to 400 cm−1 named radial breathing vibration mode (RBM) which shows that the composite films contain single-walled carbon nanotubes. Figure 3(b) is the enlarged of RBM in Figure 3(a). From Figure 3(b), we can see that the frequency of the peaks is 209 cm−1, 222 cm−1, 230 cm−1, and 268 cm−1 which correspond to the diameter of single-walled carbon nanotubes at 1.12 nm, 1.05 nm, 1.02 nm, and 0.87 nm, respectively.
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Figure 4 shows the EDX spectrum of the composite films. From the EDX spectrum, we can see that the sample consists of C, O, Fe, and a very small amount of Sn. The existence of Sn is probably from the FTO.
In order to determine the content of carbon nanotube, iron nanoparticles, and amorphous carbon, we have analyzed the composite films by TGA (Figure 5). The TGA carve declines slowly from 20°C to 191.7°C which corresponds to the reduction of water of the composite films. In this process, the weight of the sample is loss of 5%. When the temperature is more than 254.68°C, the slope of TGA carve increases which shows that the amorphous carbon reduces. When the temperature is from 330°C to 509.2°C, the slope of the TGA carve continues to increase and the weight loss of the sample is sped up; the mass loss is about 34.4%. When the temperature is more than 509.2°C, the slope of the TGA carve reduces till 758.3°C which corresponds to the reduction of carbon nanotube. In this process, the mass loss is about 5.5%. The rest of the sample is iron and iron oxide which is about 54.16%.
IPCE characteristics of the DSSCs with composites films counter electrode and Pt are shown in Figure 6. The maximum IPCE value at 540 nm of the DSSCs with composite films counter electrode is approximately 72%, and those of the DSSC with the Pt are 76.3%. Adsorbed spectra of the DSSCs with composite films counter electrode are lower than those of the DSSC with the Pt in the range 380–650 nm and both the adsorbed spectra change almost unanimously at longer wavelengths above 650 nm. In general, the IPCE of the DSSCs with composite films counter electrode is comparable to that of the DSSC with FTO/Pt counter electrode.
To investigate the electrochemical activity of the as-prepared composite films, electrochemical impedance spectra (EIS) were measured. Figure 7 shows the equivalent circuit for the DSSCs, where is the ohmic serial resistance mainly related to the FTO glass, CPE1 and CPE2 are the capacitance of electrical double layer, respectively, is the charge-transfer resistance of the counter electrode and electrolyte, is the recombination resistance of the anode electrode and the electrolyte, and Ws is the Warburg impedance of the electrolyte. As shown in Figure 7, can be determined by the impedance at high frequency of around 100 kHz, while and are derived from the semicircles in the high and middle frequency ranges, respectively, through fitting the plots with Z-VIEW software. The resistance elements , , CPE1, and Ws values are summarized in Table 1. Obviously, the charge-transfer resistance of the composite films is much lower than Pt. It means that the composite films have an excellent electrocatalytic activity and high conductivity which is likely attributed to the big effective contact area of the composite films and the electrolyte and the high conductivity of the carbon nanotube. The composite electrodes exhibit larger CPE1 values than that of the Pt, which means that the composite electrodes have longer electronic life [8]. However, the Ws values of the composite electrode is larger which is likely due to the thicker layer along the composite electrode, as compared with the Ws in a thin layer of the smooth FTO/Pt electrode [9, 10].
Figure 8 shows photocurrent-voltage characteristics of DSSCs using the composite electrodes and the Pt electrode as the counter electrode and the corresponding parameters are listed in Table 2.
As we can see in Table 2, as reference, the DSSCs with a typical FTO/Pt as counter electrode has an open-circuit voltage () of 0.8072 V, short-circuit current density () of 12.91 mA cm−2, fill factor (FF) of 0.5316, and conversion efficiency () of 5.54%. When composite electrode is used as the counter electrode, the , , FF, and of DSSC can reach as high as 0.7940 V, 10.70, 0.5603, and 4.76%, respectively. Compared with Pt electrode, the , , and are lower, but the FF is larger, which should be attributed to the effective contract area of the electrolyte and the composite films counter electrode.
4. Conclusions
In summary, the composite films were obtained by ethanol catalytic combustion in atmospheric environment and were used directly as the counter electrode for DSSCs. It is demonstrated that the photovoltaic performance of the DSSCs with the composite films as the counter electrode is excellent and compared to that of the DSSCs with FTO/Pt counter electrode, which is attributed to their good electrocatalytic activity and high conductivity. A detailed understanding of the relationship between the electron transfer and electrolyte diffusion needs further investigation.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was partially supported by Key Project of Beijing Natural Science Foundation (3131001), Key Project of Natural Science Foundation of China (91233201 and 61376057), Key Project of Beijing Education Committee Science and Technology Plan (KZ201211232040), State 863 Plan of MOST of PR China (2011AA050527), Beijing National Laboratory for Molecular Sciences (BNLMS2012-21), State Key Laboratory for New Ceramic and Fine Processing of Tsinghua University (KF1210), Key Laboratory for Renewable Energy and Gas Hydrate of Chinese Academy of Sciences (y207ka1001), Beijing Key Laboratory for Sensors of BISTU (KF20131077208), and Beijing Key Laboratory for photoelectrical measurement of BISTU (GDKF2013005).