Photovoltaic Performance Improvement of Dye-Sensitized Solar Cells Based on Mg-Doped TiO2 Thin Films

Photovoltaic Performance Improvement of Dye-Sensitized Solar Cells Based on Mg-Doped TiO2 Thin Films

Electrochimica Acta 129 (2014) 459–462 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 129 (2014) 459–462

Contents lists available at ScienceDirect

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% [7]. 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 [8]. The doped TiO2 nanomaterials has been investigated for more than ten years [9]. 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 [10]. Doping a metal or nonmetal into TiO2 could change the band edge or surface states of TiO2 [11]. 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 [14], Cr-[15], Yb-[16], and Zn-doped [17] 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) [20]. Liu and Feng [21] reported Nb- and Ta-doped TiO2 nanomaterials for fabrication of DSSCs, respectively. The Nb-doped TiO2 photoanode [22] 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

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Ta-doped TiO2 nanowire arrays [23] 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.

(101)

Intensity(a.u)

460

4--0.5mol%Mg-doped 3--1.0mol%Mg-doped 2---2.0mol%Mg-doped 1---TiO2 (200)

(103)

(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.

20

30

40

50

60

70

80

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) [26], 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

(1)

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

O1s

TiO2

(529.8eV)

3.1. Characterization of TiO2 and Mg-doped TiO2 Ti2p

Intensity(a.u)

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

(458.4eV)

C12p

0

(284.8eV) C1s

200

400

600

800

1000

Binding Energy(eV) Fig. 2. X-Ray photoelectron spectra (XPS) of TiO2 .

1200

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1Hz

5000Hz

0.0

(a)

461

0.0% 0.5% 1.0% 2.0%

-0.5 -1.0 -1.5

Intensity(a.u)

O1s

I'(uA)

-2.0

Ti2p

-2.5 -3.0 -3.5

Mg2p C1s

-4.0

50.2eV

-4.5 -5.0

0

200

400

600

800

1000

1200

-5.5

0

Binding energy(eV)

2

4

6

8

10

12

14

16

18

I"(uA)

(b)

Mg2p,50.2eV

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

Intensity(a.u)

3.3. Effect of the charge transport

46

48

50

52

54

56

Binding energy(eV)

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 [31].

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

10

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

C-2(uF-2cm4)

8

6

2.0% 1.0% 0.5% 0.0%

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

4

Samples

Dye loading(× 10−7 molcm−2 )

Jsc (mAcm−2 )

Voc(mV)

ff



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

2

0 -1.0

-0.8

-0.6

-0.4

-0.2

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.

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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 .

20

Potocurrent(mAcm-3)

15

2.0% 1.0% 0.5% 0.0%

10

References

5

0 0

100

200

300

400

500

600

700

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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