Electrochimica Acta 129 (2014) 459–462
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Photovoltaic Performance Improvement of Dye-Sensitized Solar Cells Based on Mg-Doped TiO2 Thin Films QiuPing Liu a,b,∗ a
School of Pubic Policy and Management, Tsinghua University, 100084, P. R. China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China b
a r t i c l e
i n f o
Article history: Received 15 December 2013 Received in revised form 21 February 2014 Accepted 21 February 2014 Available online 10 March 2014 Keywords: Titanium dioxide Mg-doped ﬁlm hydrothermal method ;X-ray photoelectron spectroscopy Photovoltaic performance ﬂat band potential
a b s t r a c t Mg salts [Mg(NO3 )2 ·6H2 O]-doped TiO2 electrodes prepared well-optimized by the hydrothermal method. To prepare the working electrode, the TiO2 or Mg-doped TiO2 slurry was coated onto the ﬂuorine-doped tin oxide glass substrate by the doctor blade method and was then sintered at 450 ◦ C. X-ray photoelectron spectroscopy (XPS) data indicated that the doped Mg ions exist in form of Mg2+ , which can play a role as e− or h+ traps and reduce e− /h+ pair recombination rate, The Mott-Schottky plot indicates that the Mg-doped TiO2 photoanode shifts the ﬂat band potential positively. 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 show a photovoltaic efﬁciency of 7.12%, which is higher than that of the undoped TiO2 thin ﬁlm (5.62%) and increase short-current by 26.7% from 14.9 mA to 19.1 mA. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction TiO2 is applied into DSSCs owing to its good properties of high chemical stability, low toxicity, and ideal position of the conduction band edge. DSSCs are considered to be a promising renewable source energy device because of advantages such as mechanical robustness, light weight of the glass-less collector, and favorable “differential kinetics” [1–6].The photoelectric conversion efﬁciency of DSSCs has reached 12.3% . However, the cells still suffer a series of energy losses. For example, the recombination between the injected electrons and the oxidized dye or ions in the electrolyte may cause a reduction of approximately 300 mV of open-circuit voltage (Voc ) compared to the theoretical value, leading to a rapid decrease in the conversion efﬁciency. Thus, different approaches such as the light scattering effect on the photoanode, different dye, Doped TiO2 has been investigated concerning the increase of the conversion efﬁciency . The doped TiO2 nanomaterials has been investigated for more than ten years . Some papers reported that the n-doped nanostructured Titania electrode-based DSSCs showed a superior
∗ Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.electacta.2014.02.129 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
efﬁciency with stability, relative to that of the commercial TiO2 . Doping a metal or nonmetal into TiO2 could change the band edge or surface states of TiO2 . Until now, most of the doped TiO2 nanomaterials has been explored for photocatalysis. To the best of our knowledge, there are only a few papers reported in which doped TiO2 nanomaterials were used as photoanodes in DSSCs (including nitrogen-doped TiO2 ) [12,13]. For the metal-doped TiO2 nanomatrials, Al, W-codoped , Cr-, Yb-, and Zn-doped  TiO2 have been attempted to be applied as the photoanodes of DSSCs. The doped TiO2 effects, however, do not seem so pronounced by comparison to the corresponding undoped TiO2 photoanode. The energy conversion efﬁciency remained either unchanged or a little improvement. Doped metal atoms into semiconducting material [18,19] is a commonly adopted method such as conduct band (CB) position and trap/defect level distribution in TiO2 . Recently, Lindqiust and co-workers also demonstrated that n-doped TiO2 ﬁlms prepared by reactive DC magnetron sputtering displayed an improved conversion efﬁciency, particularly in the visible wavelength(450-500 nm) . Liu and Feng  reported Nb- and Ta-doped TiO2 nanomaterials for fabrication of DSSCs, respectively. The Nb-doped TiO2 photoanode  exhibited a positive shift of the ﬂat band potential of TiO2 . By contrast, although Ta and Nb are in the same element group and have one more electron than Ti, DSSCs based on
Q. Liu / Electrochimica Acta 129 (2014) 459–462
Ta-doped TiO2 nanowire arrays  showed an improved opencircuit voltage of 0.87 V due to a negative shift of the TiO2 Fermi level. Recently, Mg2+ doped nanostructure TiO2 electrodes with higher conduction band position have been investigated [24,25]. However, there is no detailed study about the mechanism of electron lifetime and electron transport in the Mg2+ -doped TiO2 samples. In this study, TiO2 was doped with Mg in the range of 0.5 mol%, 1.0 mol%, 2.0 mol% to control the junction characteristics, enhance the charge transport and reduce the rate of charge recombination. shift of ﬂat band potential of the TiO2 photoanode with Mg-doped was systematically investigated by Mott-Schottky plots.
4--0.5mol%Mg-doped 3--1.0mol%Mg-doped 2---2.0mol%Mg-doped 1---TiO2 (200)
(105) (204) (211) (116)(220) (215)
4 3 2 1
2. Experimental 10
0.5 mol%,1.0mol%,2.0mol% Mg-doped TiO2 and undoped TiO2 was synthesized by using the hydrothermal method. Acetic acid (3 mL), butanol (20 mL), tetrabutyl titanate (3 mL) was mixed under constant stirring. A mixture of butanol (15 mL) and distilled water (1 mL) was then added to the above solution. After stirring continuously for 0.5 h, the mixture was transferred into an autoclave for the hydrothermal process at 240 ◦ C for 6 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%, 2.0 mol% of the doped (Mg(NO3 )2 ·6H2 O), was added to the tetrabutyl titanate (molar ratio of Mg and Ti: 0.5:100, 1:100, 2:100) to start the hydrolysis reaction. The obtained sample was denoted as Mg-doped TiO2 . Dye-sensitized solar cells were prepared as the following procedure: Fluorine-doped tin oxide (FTO) conductive glass (20 /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 TiO2 and Mg-doped by the doctor blade method and was sintered at 450 ◦ C for 30 min. To fabricate the DSSCs, we used a double-layer structure electrode. A ﬁlm of TiO2 was ﬁrst coated onto the FTO and then further coated by a second layer of Mg-doped TiO2 . This double-layer structure can retard the electron recombination occurring in the double layer region. When being cooled to 100 ◦ C, the ﬁlm was immersed in N3 ethanol solution. The electrolyte was composed of 0.05 M iodine (I2 ), 0.5 M LiI, and 0.05 M tert-butylpyridine dissolved in 3 methoxypropionitrile. A platinized counter electrode constituted a sandwichlike 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 m TiO2 layer or 3 m Mg-doped TiO2 layer.
2 degree) Fig. 1. XRD Patterns of TiO2 and three different doped amount of Mg-doped TiO2 .
sample. It is clear that Mg ions were doped successfully into the TiO2 . 3.2. Band structure analysis It is broadly accepted that band bending is negligible for nanoparticles with very small sizes (e.g.<6 nm) , but band bending may exist in nanoparticles with diameters (>10 nm) nanometers. Mott-Schottky analysis of impedance for the TiO2 and Mg-doped TiO2 nanocrystalline ﬁlms was carried out for the measurement of Vfb [27–30], which could approximately denote the position of the CB edge in TiO2 nanoparticles. The Mott-Schottky plot (MS plot) involves measuring the capacitance of the space charge region (Csc ) as a function of electrode potential under depletion condition and is based on the Mott-Schottky relationship of a semiconductor Eq. (1), (C sc ) − 2 = 2(E − Efb − kT/e)/ND εε0 eA2
where Csc is the charge space capacity, ND is the carrier density, ε the relative electric permittivity, ε0 the electric permittivity of vacuum, e the elementary charge, K the Boltzman constant, T the absolute temperature, Efb ﬂat band potential, E the potential and A is the active surface. The ﬂat band potential and electron density of the electrode can be calculated from the intercept and the slope, respectively.
3. Results and discussion
3.1. Characterization of TiO2 and Mg-doped TiO2 Ti2p
Fig. 1 shows the crystalline properties of the TiO2 and 0.5 mol%, 1.0 mol%, 2.0 mol% Mg-doped TiO2 . The result indicated that Mg-doped TiO2 sintering at 450 ◦ C had polycrystalline structures consisting of anatase TiO2 (JCPDS, No. 21-1272) phase characterized with primary (101), (200), and (211) peaks. The particle sizes calculated from the Scherrer equation were 10-15 nm. X-ray photoelectron spectroscopy was used to investigate the chemical composition and electronic structure of the 1.0 mol% Mgdoped TiO2 . Fig. 2 shows the typical full XPS spectrum of TiO2 . The TiO2 ﬁlm is composed mainly of Ti, O. Fig. 3(a) shows the typical full XPS spectrum of Mg-doped TiO2 . The ﬁlm is composed mainly of Ti, O, and Mg. The photoelectron peak of Mg2p (50.2 eV) can be observed in Fig. 2 (b) and indicates the presence of Mg2+ in the
Binding Energy(eV) Fig. 2. X-Ray photoelectron spectra (XPS) of TiO2 .
Q. Liu / Electrochimica Acta 129 (2014) 459–462
0.0% 0.5% 1.0% 2.0%
-0.5 -1.0 -1.5
-2.5 -3.0 -3.5
Fig. 5. Complex plane plots of the TiO2 and the Mg-doped TiO2 cells obtained from IMPS measurements.
3.3. Effect of the charge transport
Fig. 3. XPS survey spectra of Mg-doped TiO2 (a) and Mg2p (b).
The simple Mott-Schottky theory predicts that straight line in the dCsc /dE plot with constant intercept at Efb is indepent of time and polarization. Fig. 4 shows the Mott-Schottky plots for the TiO2 and Mg-doped TiO2 thin ﬁlm electrodes. 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% Mg-doped TiO2 electrode is about -0.71 V (vs. SCE), -0.69 V (vs. SCE), -0.58 V (vs. SCE), -0.62 V (vs. SCE) respectively. It is easy to expect from the positive shift of the CB that the electron injection efﬁciency and hence the short-current for DSSCs based on those dyes with high LUMO can be improved by the Mg-doped,because the energy difference between the LUMO of the dye and the CB of TiO2 is enlarged .
IMPS is a useful method to study charge transport. Fig. 5 shows a complex plane plot of the IMPS spectrum for the TiO2 and three different dopant amount Mg-doped TiO2 thin ﬁlms. The response appears in the fourth quadrant of the complex plane and displays one semicircle,i.e., it is a single time constant process in IMPS measurements where the frequency at the apex of the semicircle can be related to the time constant of the process. The time constant ( D ) can be calculated from D = (2fmin )−1 , where fmin is the frequency at the bottom of the semicirle observed in the IMPS plots [32–34]. The minimum frequency of the IMPS at the imaginary plot, which gives an estimate of the average time that photoinjected electrons need to reach the back contact. For ﬁlms with comparable ﬁlm thickness and dye loading, such as those investigated here, the electron transit time should enable a valid comparison of the electron transport in the ﬁlms. The D for 0.5 mol%,1.0 mol%,2.0 mol% Mg-doped TiO2 and undoped TiO2 the 1.0mol% Mg-doped TiO2 and TiO2 electrodes are 3.67 ms,3.22 ms,4.12 ms and 4.67 ms, respectively. From the measured D value, one can estimate that the electron transport properties are different between the TiO2 and 1.0 mol% Mg-doped TiO2 thin ﬁlms. In the 1.0 mol% Mg-doped TiO2 ﬁlm, electron transport is much faster than un-doped TiO2 ,which resulted in increased charge-collection efﬁciency and thus increase photocurrent density. 3.4. Photovoltaic performance
The current-voltage curves of the DSSCs based on the TiO2 and four different dopant amount Mg-doped TiO2 ﬁlms are shown in Fig. 6. The photovoltaic parameters are shown in Table 1. Under light intensity of 100 mW·cm−2 at AM1.5. The dye-loading amount shown in Table 1 is similar for both of the ﬁlms, suggesting that the
2.0% 1.0% 0.5% 0.0%
Table 1 Photovoltaic characteristics of the DSSC based on TiO2 and Mg-doped TiO2 photoanodes.
Dye loading(× 10−7 molcm−2 )
Jsc (mAcm−2 )
TiO2 Mg/TiO2 = 0.5 Mg/TiO2 = 1.0 Mg/TiO2 = 2.0
1.58 1.61 1.63 1.54
14.92 14.77 19.10 18.56
640 595 615 605
0.665 0.549 0.605 0.569
6.35 5.62 7.12 6.40
Polarization(V.vs.SCE) Fig. 4. Mott-Schottky plots for TiO2 and different doped amount of Mg-doped TiO2 .
Jsc : short circuit current density; Voc : open circuit voltage; : overall energy conversion efﬁciency;FF: ﬁll factor.
Q. Liu / Electrochimica Acta 129 (2014) 459–462
efﬁciency(7.12%) which was improved by 26.7%.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 measured by IMPS. A positive shift of the ﬂat band is also responsible for the improvement of conversion efﬁciency.The result show the possibility of improvement conversation efﬁciency by Mg-doped TiO2 .
2.0% 1.0% 0.5% 0.0%
Potential(mV) Fig. 6. Current-voltage curves of DSSC under illumination AM1.5 and in the dark. TiO2 photoanode (solid line) and Mg-doped TiO2 (dashed line).
enhancement of photocurrent for Mg-doped TiO2 is not due to the increase of the dye absorption. Therefore, the enhanced efﬁciency is mainly ascribed to the increased JSC , which is improved by 28.0% from 14.92 to 19.10. The increased of JSC in TiO2 based on DSSCs after Mg-doped could be generally ascribed to two main factors:(1)The divalent Mg2+ ions was doped into the TiO2 lattice and occupied the quadivalent Ti4+ causing a increased net in the electron concentration, and thus increased the electrical conductivity of Mg-doped TiO2 .(2)The injection efﬁciency must be faster than the relaxation of the excited state of the sensitizer dye. The more positive ﬂat band in the former case is the result of the effective driving force for the photoelectron, which can lead to a larger electron injection efﬁciency from the LUMO of dye to the conduction band of the semiconductor. The ﬁll factor is another important parameter to determine the performance of DSSCs, which depends on the series internal resistance. From Table 1, an improved ff is not observed for DSSCs based on the 1.0 mol% Mg-doped TiO2 thin ﬁlm. The D of 1.0 mol% Mgdoped TiO2 and TiO2 is 3.22 ms and 4.67 ms respectively, which is conﬁrmed by IMPS. Therefore, it suggest that charge transport in 1.0 mol% Mg-doped TiO2 ﬁlm faster than that in TiO2 . Doped TiO2 can create a donor level, which increases the concentration of the carriers and reduces the ﬁlm resistance [35–38]. 4. Conclusions DSSCs based on four different dopant amount Mg-doped TiO2 exhibit high short circuit photocurrent. Compared with the undoped TiO2 ﬁlms, DSSCs with 1.0mol% Mg-doped photoanode successfully achieved the maximum energy conversion
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