Abstract
Al doped SnO2 microspheres were prepared through hydrothermal method. As-prepared SnO2 microspheres were applied as photoanode materials in dye-sensitized solar cells (DSCs). The properties of the assembled DSCs were significantly improved, especially the open-circuit voltage. The reason for the enhancement was explored through the investigation of dark current curves and electrochemistry impedance spectra. These results showed that the Al doping significantly increased the reaction resistance of recombination reactions and restrained the dark current. The efficient lifetime of photoexcited electrons was also obviously lengthened.
1. Introduction
Dye-sensitized solar cells (DSCs) have been actually promoted by the development of nanocrystal materials, especially the preparation of TiO2 nanocrystals with different morphology [1, 2]. High photoelectric conversion efficiency up to 14.3% has been obtained [3]. However, the band gap of TiO2 is about 3.2 eV which causes obvious catalyzing properties under UV light. The organic solvent in DSCs might be slightly decomposed by TiO2 nanoparticles on the photoanode under sunlight. As an alternative to TiO2, SnO2 has been extensively investigated as a photoanode material in DSCs. SnO2 has a wider band gap (about 3.6 eV) than that of TiO2 which was inactive to the organic solvent in DSCs. And SnO2 has high electron mobility (about 150 cm2V−1s−1) which is a benefit for the collection of the photoexcited electrons in the photoanode [4, 5]. However, the band edge of conduction band edge of SnO2 is −4.5 eV (vacuum level) which causes serious back reaction between the electrons in the conduction band and the oxide ions in the electrolyte. It is an efficient way to inhibit these back reactions by doping SnO2 with other metal elements. Duan et al. doped SnO2 nanoparticle with Al and found the tuning of the conduction band and suppression of charge recombination [6]. Li et al. prepared Zn-doped SnO2 nanocrystals to obtain longer electron lifetimes and higher dye loading [7].
In this work, we prepared Al doped SnO2 hollow microspheres. The electron recombination was efficiently restrained and the photoelectrical conversion efficiency was significantly enhanced compared with that of pure SnO2.
2. Experimental
2.1. Preparation of Pure SnO2 Power and Al Doped SnO2 Power
SnO2 microspheres were synthesized using the hydrothermal method as follows. For the preparation of SnO2 power, 0.8 g of stannous chloride dihydrate (SnCl2·2H2O) was dissolved in 80 mL deionized water. D-Glucose was used as the soft template. The content of D-glucose was 7.024 g. The former mixture was stirred for 30 min. at room temperature. The resulting well-distributed mixture was transferred into a 100 mL autoclave for hydrothermal reaction at 180°C. The hydrothermal reaction time was 16 h. After the autoclave was cooled to room temperature, the product was collected by centrifugation and washed with distilled water and ethanol several times. The obtained sample was dried at 60°C overnight. The resulting brown power was calcined at 550°C for 1 h in air to obtain the final product. For preparing the Al doped SnO2 sample, aluminum(III) chloride (AlCl3) was added to the precursor solution. The addition amount of Al was controlled to be 0.5%, 1.0%, 1.5%, and 2.0% (at%) of Sn content in the solution. The preparation processes of Al doped SnO2 power were controlled to be the same as that described above except for the addition of AlCl3. The obtained samples of pure SnO2 and Al doped SnO2 powder were denoted as pure SnO2, 0.5% Al doped SnO2, 1.0% Al doped SnO2, 1.5% Al doped SnO2, and 2.0% Al doped SnO2, respectively.
2.2. Fabrication of DSSC Based on Pure SnO2 and Al Doped SnO2
To prepare the working electrode, SnO2 or Al doped SnO2 slurry was covered on fluorine-doped tin oxide (FTO) glass (1 × 2 cm2, 15 Ωsq−1, Opvtech) using a doctor blade technique and then sintered at 450°C for 30 min. After cooling to 80°C, the samples were immersed in 5 × 10−4 mol L−1 ethanol of N719 dye for 24 h. Pt counter electrode was prepared by spreading 5 mM H2PtCl6 aqueous solution on an FTO glass substrate, followed by pyrolyzation at 390°C for 15 min. The mixture of 0.6 M dimethylpropylimidazolium iodide, 0.1 M iodine, 0.5 M 4-tert-butylpyridine, and 0.1 M lithium iodide in methoxyacetonitrile was prepared as the electrolyte of DSCs.
2.3. Characterization and Optical Measurements
The crystalline phase of the samples was characterized by DX-2700 X-ray diffractometer (XRD) with monochromatized Cu K irradiation. The morphology was studied using a JSM-7001F field emission scanning electron microscope (FE-SEM) and JEM 2100 transmission electron microscope (TEM). XPS measurements were performed in Thermo Scientific ESCALAB 250 station (Thermo Fisher Scientific, Massachusetts, USA). Photocurrent density-voltage () characteristics were measured using a Keithley 2440 Source Meter under AM 1.5 G illumination from a Newport Oriel Solar Simulator with an intensity of one sun. The incident light intensity was calibrated with a standard Si solar cell provided by Newport Oriel. The active cell area of the assembled DSCs was 0.25 cm2. An electrochemistry workstation (IM6) was used to investigate the electrochemical impedance spectra (EIS) of DSCs. This measurement was also carried out with the same structured DSCs as that used in the former experiments. The impedance measurement of DSCs was recorded under dark condition at the bias potential of −0.6 V over a frequency range of 0.1–1 MHz with an AC amplitude of 10 mV.
3. Results and Discussion
Figure 1(a) shows the morphology of the prepared Al doping SnO2 microspheres. The diameter is 300–500 nm. These SnO2 microspheres were piled up with homogeneous nanoparticles. The size of the particles is 20–40 nm. There are some broken microspheres which indicate that the prepared SnO2 is hollow microspheres. This structure is a benefit for the absorbing of dye and the diffusion of the electrolyte in DSCs. TEM was also carried out to confirm the hollow spheres structure of SnO2. The TEM of SnO2 microspheres is shown in Figure 1(b). The whole SnO2 spheres show almost the same darkness which indicates that the thickness at the center of SnO2 spheres is almost the same as that of edge. Therefore, the as-prepared SnO2 should be hollow spheres.
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Figure 2 shows the XRD spectra of SnO2 microspheres doped with different Al content. The prepared SnO2 microspheres correspond to the cassiterite structured SnO2 (JCPDS database card number 41-1445). There is almost no change in the XRD spectra with the addition of Al element which might be because the content of added Al element is too little to change the structure of SnO2. The crystalline particle size () could be estimated from the 110, 101, and 211 diffraction peak using the Scherrer equation [8]:where is the wavelength of the X-ray, is the full-width at half-maximum (FWHM), and is the Bragg angle in the diffraction pattern. The particle size was estimated to be about 17.0 nm for Al doped SnO2.
XPS were carried out to confirm the introduction of Al into the SnO2 hollow spheres. The survey scan spectra and the element narrow scan of 1.5% (at%) Al doped SnO2 are shown in Figure 3. The XPS of pure hollow microspheres were shown in the insert of the corresponding spectra. The binding energies of O 1s and Sn 3d show no obvious shift after Al doping. However, a new weak peak appears at about 75 eV which corresponds to Al 2p. The content of Al was also estimated to be about 0.61% (at%). The content of Al is so little that the O 1s and Sn 3d have no change which is in accordance with the result of XRD.
(a)
(b)
(c)
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Figure 4(a) shows the curves of the DSCs assembled with Al doped SnO2 microspheres photoanode. The specific parameters of these curves were summarized in Table 1. The photoanode prepared with pure SnO2 microspheres shows low open-circuit voltage (, 179 mV) due to the low conduction band edge. increased from 179 mV to 474 mV when the addition of Al increased from 0 to 1.5% (at%). At the same time, the short circuit current density () was also enhanced significantly from 8.05 mA cm−2 to 11.48 mA cm−2. The photoelectric conversion efficiency () increased from 0.46% to 3.04%. However, and both decreased when the addition of Al further increased to 2.0% (at%). The increase of should be due to the inhibition of back reaction after the doping of Al. Figure 4(b) shows the dark current-voltage curves. The dark current density becomes weaker and weaker with the increase of Al doping content. The depression of dark current should be one key reason for the increase of .
(a)
(b)
Electrochemical impedance spectroscopy (EIS) is an efficient method to investigate the recombination process of the photoexcited electrons. EIS was carried out on the SnO2 and Al doped SnO2 under dark condition. The bias potential is −0.6 V. The Nyquist plots are shown in Figure 5(a). The Nyquist plots were fitted using the equivalent circuit shown in the insert of Figure 5(a). In the equivalent circuit, represent the series resistors during the transport of electrons. correspond to the resistance during the charge-transfer processes occurring at the counter electrode/electrolyte interface. should correspond to the recombination of electron at the SnO2/electrolyte interface. From the fitting result of Nyquist plots, it can be seen that and show few changes before and after Al doping. However, increases from 35 Ω cm−2 to 127 Ω cm−2 after Al doping. This indicates that the photoelectron recombination is efficiently inhibited. Figure 5(b) shows the Bode phase plots of the pure SnO2 sphere film and the Al (1.5%, at%). There are two electrochemical processes, and , which occur at high frequency (103 Hz–105 Hz) and low frequency (1 Hz to 103 Hz), respectively. corresponds to the charge-transfer processes occurring at the counter electrode/electrolyte interface [9, 10]. should correspond to the charge-transfer processes occurring at the SnO2/electrolyte interface. The characteristic frequency of may reflect the electron lifetimes () of the injected electrons [11]. The lifetimes () of the photoexcited electron in the photoanodes were determined using the following equation:
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(b)
The characteristic frequencies of these photoanodes, SnO2 and Al doped SnO2, are 3.1 and 1.2 Hz, respectively. According to (2), the electron lifetimes () were calculated to be about 51 ms and 132 ms for the SnO2 and Al doped SnO2 electrodes, respectively. It can be seen that Al doping can enhance the efficient electron lifetime of SnO2 electrodes. This result is in accordance with that of dark current experiments (shown in Figure 4).
4. Conclusions
Al doped SnO2 microspheres were prepared through hydrothermal method using glucose as template. The SnO2 microspheres were piled up with SnO2 nanoparticles. As-prepared SnO2 microspheres were applied as photoanode materials in dye-sensitized solar cells. The results showed that the Al doping significantly restrained the dark current and improved the open-circuit voltage of the cells. Electrochemistry impedance spectra showed that the reaction resistance of recombination reactions increased sharply after doping of Al and the efficient lifetime of photoexcited electrons was lengthened. The total photoelectrical conversion efficiency was improved from 0.46% to 3.04% after Al doping.
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
This work was supported by the Natural Science Foundation of China (nos. 51304062, 21403056, and U1404202).