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

doi:10.1016/j.electacta.2014.02.129

Electrochimica Acta

Volume 129, 20 May 2014, Pages 459-462

https://doi.org/10.1016/j.electacta.2014.02.129 Get rights and content

Abstract

Mg salts [Mg(NO₃)₂·6H₂O]-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.

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], [2], [3], [4], [5], [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 (V_oc) 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 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 Ta-doped TiO₂ nanowire arrays [23] showed an improved open-circuit 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.

Section snippets

Experimental

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%

Characterization of TiO₂ and Mg-doped TiO₂

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%

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 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

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The TiO₂ nanorods (NRs) have been synthesized from colloidal titanate by using hydrothermal route. Three different mol.% ratios of Mg metallic ions are doped to study its influences in the growth of TiO₂ NRs. For this, the synthesized TiO₂ NRs including doped samples were examined by as usual analytical techniques and the obtained outcomes abstraction has given. While introducing Mg dopant, the generated constrain stress in the TiO₂ NRs is substantiated from the marginal shift in the (1 1 0) preferentially oriented crystallographic plane. Apart from this, there is no changes witnessed in rutile crystallographic structures of TiO₂ NRs. With respect to the presence of constrain stress during the growth of TiO₂ NRs under Mg doping, the specific surface to volume ratio is changed with the diameter of the NRs and morphology. The reduced energy discontinuity region width in k-space region signifies that the capacity of absorption of TiO₂ NRs is shifted to visible region by Mg doping. Herein the above said physical property changes in TiO₂ NRs is mainly due to Mg ions and is identified from EDS analysis. Eventually, after seeing anticipated results in TiO₂ NRs by Mg doping, all the samples are used as a photoelectordes in dye-sensitized solar cells (DSSCs). Here, the 2 mol.% Mg doped TiO₂ NRs photoelectrode shows an improved short-circuit current density (13.6 mA cm⁻²), open-circuit photo voltage (0.72 V), fill factor (0.65) and power conversion efficiency (PCE) of 6.36%. This obtained result again reiterates that there will be a soul in modified TiO₂ NRs for DSSCs.
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The Raman spectra of undoped and 1, 2, 3 wt % Mg doped TiO2 are shown in Fig. 2(b). The peaks at 143, 195, 398, 512 and 632 cm−1 are indexed to the anatase phase [24]. No peak for any other phase of TiO2 or any oxide of magnesium was observedin accordance with XRD results.
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This has increased the recombination rate and reduced the efficiency [43]. Even, it is reported in literature that large amount of Mg decreased the efficiency of TiO2 [44]. According to Shalini, efficiency will be improved by increasing electron concentration, as well as Jsc value.
Dye sensitized solar cell (DSSC) is an emerging energy harvesting tool which converts direct sunlight into electrical energy. These cells have much better properties in contrast with silicon based solar cells because of their flexible nature, light weight, low cost, environment friendly nature, and involvement of a simple manufacturing process. Since, a photoanode is the backbone of DSSC, we synthesized a pure and 1% manganese (Mn) doped titanium dioxide (TiO₂) films by sol-gel method which are irradiated with silver (Ag) ions at two different concentrations (2 × 10¹⁴ and 4 × 10¹⁴) ions-cm⁻². X-ray diffraction revealed that Mn doping followed by Ag irradiation transformed TiO₂ from pure anatase to rutile phase. Ultraviolet–visible spectroscopy exposed the reduction in band gap of TiO₂ film during this doping and irradiation process. Therefore, absorption is enhanced with red shift in UV-range. When these films are used as a photoanode in DSSC, 1% Mn doped TiO₂ film exposed with Ag at the concentration of (2 × 10¹⁴) ions-cm⁻² exhibited maximum efficiency of 2.40%.
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Photovoltaic Performance Improvement of Dye-Sensitized Solar Cells Based on Mg-Doped TiO2 Thin Films

Abstract

Introduction

Section snippets

Experimental

Characterization of TiO2 and Mg-doped TiO2

Conclusions

Photobio. A: Chem.

J. Photochem. Photobio. A: Chem.

J. Colloid. Interf. Sci.

Chem.Rev.

J. Phys.Chem.B

J. Phys. Chem. C

Sol. Energy.

Nature.

[J]. Science

Nano Lett.

Langmuir

Nano. Lett.

J. Phys. Chem. C.

J. Mater. Chem.

J. Phys, Chem. B

Phys. Chem. Chem. Phys.

J. Phys. Chem. C.

Phys. Chem. Chem. Phys.

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

Characterization of TiO₂ and Mg-doped TiO₂