Guangjie Zhanga,
Qingliang Liaoa,
Zi Qina,
Zheng Zhanga,
Xiaohui Zhanga,
Peifeng Lia,
Qinyu Wanga,
Shuo Liua and
Yue Zhang*ab
aDepartment of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
bKey Lab for New Energy and Nanotechnology, University of Science and Technology Beijing, Beijing 100083, People's Republic of China. E-mail: yuezhang@ustb.edu.cn
First published on 8th August 2014
A novel electrophoresis based dye adsorption technique is developed for the process of ZnO-nanorod-array dye-sensitized solar cells (DSSC). By applying a constant current electrophoresis procedure, ZnO electrodes were successfully sensitized by N719 dye, and the effects of varying the electrophoretic current and duration time on the performance of DSSC were investigated. It is found that abundant dye adsorption can be obtained within significantly reduced sensitization time by electrophoresis compared to standard immersion method. Thicker ZnO-nanorod-array film and lower electrophoretic current requires longer electrophoresis time to reach the optimal sensitization effect, while very long sensitization time will result in dye agglomeration, which is harmful to the performance of DSSC. The fast dye adsorption process is more advantageous as the ZnO-nanorod-array film becomes thicker because dye molecules can readily penetrate into the depth of ZnO film in a very short time, during which the formation of Zn2+/dye complex can be suppressed.
In the fabrication of DSSC, sensitization is of significant importance because it involves the formation of interface between dye molecules and the ZnO semiconductor, which is the key agency for electrons and holes to separate. Traditional dye adsorption is implemented by immersing ZnO electrodes into dye solutions for a period of time which can last for several hours or even days. For ZnO, an amphoteric oxide which can be corroded by most of the commercial dyes, it is especially harmful to have the electrodes soaking in acidic dye solutions for a long time. Enormous surface destruction and dramatic performance decline will occur when the ZnO electrodes are immersed in N719 dye for as short as 20 min.12 Few studies aiming to overcome this drawback have been reported, including developing new types of sensitizers13,14 and modifying the solvent used for dye adsorption.15 Recently, we have used ZnO nanoparticles16 and nanorod-array17 electrodes modified by Al2O3 or SiO2 layers to improve their stability in dye loading process. Herein, we developed a new sensitization technique with the use of electrophoresis to enhance dye adsorption. With the help of electric field, sufficient dye loading can be achieved within a couple of minutes. The effects of varying electrophoretic current and duration on the performance of DSSC are investigated. The electrophoresis method is advantageous compared to the traditional immersing method because the reduced sensitization time is helpful for reducing the corrosion of ZnO.
To determine the charge condition of dye molecules in ethanol, ZnO electrodes were connected to the positive or negative terminal of the constant current source. After electrophoresis with a current of 0.1 mA for 60 seconds, an interesting observation occurred when comparing the color changes of the electrodes. The right inset of Fig. 1b illustrates the photograph of ZnO electrodes sensitized under different conditions. As can be seen, the positively connected ZnO electrode (PC-ZnO) shows a dark red color, while the negatively connected ZnO electrode (NC-ZnO) shows practically no change compared with the bare ZnO (B-ZnO) electrode. When the ZnO electrode is immersed into the identical dye solution directly for 60 seconds (referred as I-ZnO), only a faint pink color is observed. Fig. 1b illustrates the absorption spectroscopy of the ZnO electrode sensitized by different methods. The PC-ZnO electrode shows the strongest absorption among the four samples, followed by the I-ZnO (60) and I-ZnO electrodes, which represent the ZnO electrode directly immersed into dye solutions at 60 °C for 60 min and 25 °C for 60 s, respectively. The inset table illustrates the dye adsorption amounts, which were calculated by the intensity of the absorption peak. The dye uptake amount of the PC-ZnO electrode was 2.04 × 10−7 mol cm−2, which was more than two folds than that of the I-ZnO (60) electrode. This unambiguously confirmed that an enhanced dye adsorption can be obtained on the positively charged electrode. In addition, with the electrophoresis current of 0.1 mA, the color of the electrodes gradually become darker as the electrophoresis time is prolonged, as presented in Fig. 1c. All these results indicate that N719 ethanol solution behaves like a colloid and the molecules are probably negatively charged because they tend to adsorb on the positive electrode. Consequently, all ZnO electrodes were set as positive electrodes during the electrophoresis process in our following study.
To further investigate the effect of electrophoresis on ZnO electrodes, scanning electron microscopy (SEM) was used to observe the surface morphology of the electrodes. As shown in Fig. 2a blank ZnO electrode without any sensitization typically consists of pristine ZnO nanorods, whose composition is detected by the energy dispersive X-ray spectroscopy (EDS) in the inset. After immersing in dye solution for 60 seconds, the surface morphology shows no changes (Fig. 2b) but trace Ru is evidenced by EDS, which reveals the presence of the N719 sensitizer. When the electrode is sensitized by electrophoresis with a current of 0.1 mA for 60 seconds, as shown in Fig. 2c, a thin layer of foreign matter is found in the spaces of nanorod arrays. Local EDS analysis demonstrates that these foreign matters contain high levels of Ru with an atom percentage of about 2.74%. The penetrated dense layer can also be observed in the cross-sectional view of the electrode. The formation of this layer may be attributed to the accumulation of dye molecules induced by the strong electric field, which forces numerous charged molecules to migrate towards ZnO electrode. In this circumstance, the adsorption sites on ZnO nanorods can saturate in a very short time. As electrophoresis time is prolonged, excess dye molecules will agglomerate in the pores of the electrode in response to the constant current. As the electrophoresis current is decreased to 0.01 mA, although Ru can still be detected on the surface of the ZnO electrode, no obvious foreign layers can be found both on the surface and the cross section of the electrode, which may indicate that the agglomeration of dye molecules is effectively diminished. Notably, as presented in Fig. 2b and d, no evident surface destruction of ZnO nanorods can be seen, as we previously observed,10 which implies that the time scale used for sensitization here is sufficiently short to avoid the corrosion of ZnO.
To compare the effect of different sensitization conditions on the performance of solar cells, J–V curves of the DSSC are plotted and photoelectric conversion efficiencies (η) are calculated. As can be seen in Fig. 3a and d, for the ZnO electrode with a thickness of ∼6 μm and an electrophoretic current of 0.1 mA, a decreasing η from 0.20% to 0.10% is presented with the sensitization time increasing from 15 to 90 seconds. This can be attributed to the agglomeration of dye molecules on the surface of ZnO-nanorod-array film, as identified by the SEM results, which is induced by the very large electrophoresis current. Although the electrode is equipped with abundant sensitizers, most of them are not directly anchored to ZnO, which will instead retard the electron injection efficiency. When the current is set at 0.01 mA, as shown in panel b and e of Fig. 3, the photovoltaic performance of the DSSC first increases and then reduces as the electrophoresis time is prolonged. The peak value achieved is 0.37% with the sensitization time of 90 seconds. Shorter and longer duration time will result in inadequate adsorption and a surplus pile of sensitizers, respectively, both of which can cause declined photocurrent. Moreover, better performance is achieved when the electrophoretic current is reduced from 0.1 mA to 0.01 mA. This again confirms that 0.1 mA is very large for the electrophoresis process, and a smaller current is desirable to avoid dye agglomeration.
For optimized DSSC performance, electrophoretic sensitization with an electrophoretic current of 0.001 mA was conducted, and the results are presented in the ESI.† As shown in Fig. S1,† photoelectric conversion efficiency as high as 0.42% has been obtained with a significantly prolonged electrophoresis time of 480 seconds. We further decrease the concentration of the dye solution to 10 mM and performed electrophoretic sensitization. Fig. S2† shows the J–V curves and photoelectric conversion efficiencies of DSSC. No discrepancy in results is observed between the situation where the dye concentrations of 30 mM and 10 mM were utilized. This is reasonable because we applied a constant current in the electrophoresis process, and the dye adsorption should be proportional to the current density and duration time. The influence of dye concentration can be neglected.
Traditional sensitization method in which ZnO electrodes are immersed in identical dye solution at 60 °C for a period of time has also been performed to further assess the effect of electrophoresis. As shown in Fig. 3c and f, photoelectric conversion efficiency as high as 0.48% can be achieved when the sensitization time is 30 minutes. The results of electrophoresis method are inferior to that of the traditional method. This is probably because the electrophoretic current applied here is still unsuitable for the ZnO-nanorod-array film with a thickness of ∼6 μm.
We then adopted ZnO-nanorod-array electrodes with a thickness of ∼10 μm for the sensitization process. With the electrophoretic current of 0.1 mA and 0.01 mA, the highest η obtained are 0.57% and 0.88%, respectively, as shown in Fig. 4d and e. A thicker ZnO-nanorod-array possesses larger surface area for dye molecules to attach, thus larger electrophoresis current and longer sensitization time is required to allow dye molecules to penetrate into the depth of the film. Fig. 4c presents the J–V behaviour of DSSC sensitized by the immersing method, where a short-circuit current density (Jsc) of 2.44 mA cm−2 and η of 0.86% are demonstrated (see Table 1) with an immersing time of 40 minutes.
Thickness of ZnO film (μm) | Sensitization time | Jsc (mA cm−2) | Voc (V) | Fill factor | η (%) |
---|---|---|---|---|---|
a Electrophoresis method with a current of 0.1 mA.b Electrophoresis method with a current of 0.01 mA.c Immersing method with 60 °C. | |||||
6 | 15a s | 0.76 | 0.53 | 0.41 | 0.20 |
6 | 90b s | 1.32 | 0.58 | 0.47 | 0.37 |
6 | 30c min | 1.67 | 0.60 | 0.48 | 0.48 |
10 | 60a s | 1.84 | 0.59 | 0.45 | 0.57 |
10 | 120b s | 2.78 | 0.62 | 0.51 | 0.88 |
10 | 40c min | 2.44 | 0.60 | 0.49 | 0.86 |
Unlike the case of 6 μm thick ZnO electrodes, 10 μm thick ZnO electrodes sensitized by electrophoresis method show comparable photovoltaic performance with the standard immersing method. This improvement can be attributed to two reasons. As the thickness of ZnO-nanorod-array film increases to ∼10 μm, the expanded surface area allows more dye molecules to adsorb on ZnO and thereby eliminates the agglomeration of dye molecules with the same electrophoresis current. In addition, it has been reported9 that acidic dye solution can dissolve ZnO into Zn2+ followed by the formation of Zn2+/dye complex on the surface of ZnO electrode. After a prolonged immersing time, although dye molecules can approach the depth of the ZnO-nanorod-array film and ensure sufficient dye adsorption, the outer layer of the ZnO film is inevitably corroded by dye solution. It is reasonable to infer that after immersing for 40 minutes, Zn2+/dye complex formed on the surface of the ZnO nanorods. Fortunately, for the electrophoresis process, the entire sensitization process is completed within hundreds of seconds, during which ZnO corrosion can be effectively reduced. From the SEM image presented in Fig. 2, no surface destruction can be observed for ZnO nanorods sensitized in N719 dye for as short as 60 seconds, which agrees with this speculation. As a result, the formation of Zn2+/dye complex is suppressed, and the performance of DSSC is improved.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05644b |
This journal is © The Royal Society of Chemistry 2014 |