Chinese Chemical Letters  2014, Vol.25 Issue (06):953-956   PDF    
Analysis on dye-sensitized solar cells based on Fe-doped TiO2 by intensity-modulated photocurrent spectroscopy and Mott-Schottky
Qiu-Ping Liua,b     
* Corresponding authors at:a School of Pubic Policy and Management, Tsinghua University, Beijing 100084, China;
b Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Abstract: The pure TiO2 and Fe salts [Fe(C2O4)2•5H2O]-doped TiO2 electrodes were prepared by the hydrothermal method. The pure TiO2 or Fe-doped TiO2 slurry was coated onto the fluorine-doped tin oxide glass substrate by the Doctor Blade method and then sintered at 450℃. The Mott-Schottky plot indicates that the flat band potential of TiO2 was shifted positively after Fe-doped TiO2. The positive shift of the flat band potential improves the driving force of injected electrons from the LUMO of the dye to the conduction band of TiO2. This study shows that photovoltaic efficiency increased by 22.9% from 6.07% to 7.46% compared to pure TiO2, and the fill factors increased from 0.53 to 0.63.
Key words: Dye sensitized solar cells     Titanium dioxide     Fe-doped film     Photovoltaic performance     Flat band potential    

1. Introduction

Dye-sensitized solar cells (DSSCs) are considered to be a promising renewable source energy [1, 2, 3]. Recently,the photoelectric conversion efficiency of DSSCs has reached 12.3% [4]. However,the cells still suffer significant energy loss problems. For example,the recombination between the injected electrons and the oxidized dye or ions in the electrolyte has smaller open-circuit voltage (Voc) than the theoretical value,which results in a rapid decrease of the conversion efficiency [5]. The doped method for TiO2 film has been researched,and the relationship of different dyes for doped TiO2 has been investigated on the increase of conversion efficiency of cells [6, 7, 8].

Doped metal atoms into semiconducting material is a commonly adopted method,such as conduct band (CB) position and trap/defect level distribution in TiO2 [8, 9, 10]. Here,we report our investigation on introducing Fe into TiO2 nanocrystals and the fabrication of DSSCs with photoanodes of Fe-doped TiO2 nanocrystals. We found that the flat band potential of the TiO2 photoanode shifted positively by Fe-doped film. The Mott- Schottky plots confirmed this,and was further supported by the improved electron transport properties [11, 12, 13, 14].

2. Experimental

Fe-doped TiO2 at 0.5 mol%,1.0 mol%,2.0 mol% and pure TiO2 was synthesized by the hydrothermal method. Acetic acid (3 mL), tetrabutyl titanate (3 mL),butanol (20 mL) were mixed under constant stirring. A mixture of butanol (15 mL) and distilled water (1 mL) was then added to the above two solutions. After stirring continuously for 0.5 h,the mixture was transferred into an autoclave for the hydrothermal process at 240 ℃ for 5 h. After cooling to room temperature,the concentrated colloid contained 12% TiO2. The dopant precursor,corresponding to a level of 0.0 mol%,0.5 mol%,1.0 mol%,and 2.0 mol% of the doped (Fe(C2O4)3·5H2O),was added to the tetrabutyl titaniate (molar ratio of Fe and Ti: 0.5:100,1:100,2:100) to start the hydrolysis reaction. Except for the added dopant,the preparation of Fe-TiO2 colloid followed the procedure described above.

Dye-sensitized solar cells were prepared by the following procedure: Fluorine-doped tin oxide (FTO) conductive glass (20V/sq,Hake New Energy Co.,Ltd. Harbin) was cleaned by being scoured with surfactant,treated with ultrasonic washing,and swilled with deionized water. The clean conductive glass substrate was then coated with a thin layer of TiO2 and Fe-doped by the Doctor Blademethod andwas sintered at 450 ℃ for 30 min. To fabricate the DSSCs,weused a double-layer structure electrode.A3 mmthick film of TiO2 was first coated onto the FTO and then further coated by a 3 mm thick second layer of Fe-doped TiO2. This double-layer structure can retard the electron recombination occurring in the double layer region. When being cooled to 100 ℃,the film coated substrate was immersed in N3 ethanol solution. The electrolyte was composed of 0.05 mol/L iodine (I2),0.5 mol/L lithium iodide (LiI), and 0.05 mol/L tert-butylpyridine dissolved in 3-methoxypropionitrile. A platinized counter electrode constituted a sandwich-like, open cell to form the test cell. In order to test intensity-modulated photocurrent spectroscopy (IMPS) and Mott-Schottky analysis, working electrodes used a 3 mm TiO2 layer or 3 mm Fe-doped TiO2 layer.

3. Results and discussion 3.1. Characterization of TiO2 and Fe-doped TiO2

The XRD analysis of the TiO2 and Fe-TiO2 samples sintered at 450 ℃ is shown in Fig. 1. The XRD results also indicate that no second phase is detected in both undoped TiO2 and Fe-doped film. The fact that the structure of the Fe-doped TiO2 film did not change much confirmed the Fe3+ ions must have been doped into the TiO2 lattice successfully.

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Fig. 1.XRD patterns of TiO2 and Fe-doped TiO2.

Fig. 2 shows the HR-TEM images of the TiO2 and 1.0 mol% Fedoped TiO2. A particle size of 11-14 nm was observed in the HRTEM. The observed spacing between the lattice planes of the Fe3+- doped TiO2 and TiO2 was observed as 0.318 nm,0.321 nm for the (1 0 1) plane of the anatase crystal,respectively. SEM images of the TiO2 and 1.0 mol% Fe-doped TiO2 are shown in Fig. 3 and have similar morphologies and uniform particle size distributions. The average particle size is about 13 nm.

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Fig. 2.HRTEM images of pure TiO2 (a) and 1.0 mol% Fe-doped TiO2 (b) nanoparticles.

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Fig. 3.SEM images of TiO2 (a) and 1.0 mol% Fe-doped TiO2 (b).

3.2. Band structure analysis

The Mott-Schottky plot (MS plot) involves measuring the capacitance of the space charge region (Csc) as a function of electrode potential under depletion conditions and is based on the Mott-Schottky relationship of a semiconductor represented by Eq. (1),

where Csc is the charge space capacity; ND is the carrier density; ε is the relative electric permittivity; ε0 is the electric permittivity of vacuum; K is the Boltzman constant; ε is the elementary charge; T is the absolute temperature; Efb is flat band potential; A is the active surface and E is the potential.

The simple Mott-Schottky theory predicts that a straight line in the dCsc/dE plotwith constant intercept at Efb is independent of time and polarization. Fig. 4 shows the Mott-Schottky plots for the TiO2 and Fe-doped TiO2 thin filmelectrodes. The defect density ND can be derived from the gradient dCsc/dE and the intercept with the potential axis yields the flat band potential Efb. The Efb of the pure TiO2 and the 0.5 mol%,1.0 mol%,2.0 mol%Fe-doped TiO2 electrode is about -0.73 V (vs. SCE),-0.61 V (vs. SCE),-0.55 V (vs. SCE),and -0.71 V (vs. SCE),respectively. It is easy to expect fromthe positive shift of the CB (conduct band) that the electron injection efficiency based on dyes with high LUMO can be improved by the Fe-doping, because the energy difference between the LUMOof the dye and the CB of TiO2 is enlarged. The enhanced Efb increases the energy gap between the CB of TiO2 and the LUMO of dye,which results in an increased injection driving force of electrons and increased the electron injection efficiency fromthe LUMOof the dye to the TiO2CB.

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Fig. 4.Mott-Schottky plots for TiO2 and different doped amount of Fe-doped TiO2.

3.3. Effect of the charge transport

Fig. 5 shows the transport character of electrons in the DSSC by intensity modulated photocurrent spectroscopy (IMPS). The electron transport time τd,which represents the average time intervals from the generation to collection of electrons,can be calculated from τd = (2πfmin)-1 where fmin is the frequency of the minimum point in the IMPS semicircle. The electron transport time (τd) for the 1.0 mol% Fe-TiO2 and pure TiO2 electrodes are 3.9 ms and 5.8 ms,respectively. The shorter transport time of the 1.0 mol% Fe-doped TiO2 indicates a faster electron transport rate,which is favorable to the improvement of charge-collection efficiency.

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Fig. 5.Complex plane plots of the TiO2 and the Fe-doped TiO2 cells obtained from IMPS measurements.

3.4. Photovoltaic performance

Fig. 6 shows the optimum photocurrent-voltage curves of the DSSC based on the pure TiO2 and three different dopant content of Fe-doped TiO2 photoelectrodes under a light intensity of 100mWcm-2 at AM 1.5. The photovoltaic parameter is listed in Table 1. FF,VOC and JSC are fill factor,open circuit photovoltage,and short circuit photocurrent,respectively. For the pure TiO2 based on DSSCs,JSC,VOC,ff,and η are 18.4 mA cm-2,615 mV,0.51,and 6.07%, respectively. However,after 1.0 mol% Fe-doped TiO2,the performance of DSSC based on TiO2 was enhanced. As illustrated in Fig. 6 and Table 1,the conversion efficiency of cells increases drastically after 1.0 mol% Fe-doped,and resulted in the maximum of η (7.46%). The dye-loading amount shown in Table 1 is similar for both of the films,and confirmed that the enhancement of photocurrent for 1.0 mol% Fe-doped TiO2 is not due to the increase of the dye adsorption. The FF (fill factor) is improved by 18.9% from 0.53 to 0.63. The increase of FF in TiO2 based on DSSCs after Fe-doping could be generally ascribed to the trivalent Fe3+ ions doped into the TiO2 lattice which were occupied by quadivalent Ti4+ causing a increased net in the electron concentration,and thus increased the electrical conductivity of Fe-doped TiO2. The FF is also another important parameter to determine the performance of DSSCs, which depends on the series internal resistance. The improved open circuit voltage was ascribed to the Fe3+ ions incorporated in the anatase lattice owing to the Fe3+-doped TiO2 electrode with a higher conduction band.

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Fig. 6.Current-voltage curves of DSSC under illumination AM 1.5 and in the dark. TiO2 photoanode and Fe-doped TiO2.

Table 1
Photovoltaic characteristics of the DSSC based on TiO2 and Fe-doped photoanodes.

4. Conclusion

In conclusion,four different dopant amounts of Fe-doped TiO2 were synthesized by the hydrothermal method. It was determined that the photoelectric conversion efficiency of dye-sensitized solar cell reaches the maximum (7.46%) when the dopant concentration was 1.0 mol%,which was improved by 22.9% compared with undoped TiO2,and the fill factor increased from 0.53 to 0.63. This was due to two main effects: increased injection efficiency of electrons from the LUMO of the dye to the conduction band of TiO2 and the fast electron transport rate as measured by IMPS. A positive shift of the flat band is also responsible for the improvement of conversion efficiency.

Acknowledgment

This work was supported by National Research Fund for High-Tech Research and Development of China Program (No.2007AA05Z439).

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