稀有金属(英文版) 2017,36(11),865-871

Dye-sensitized solar cells based on Cr-doped TiO₂ nanotube photoanodes

M.M.Momeni

Department of Chemistry, Isfahan University of Technology

收稿日期：21 May 2015

基金：the financial support from Iranian Nanotechnology Society and Isfahan University of Technology (IUT) Research Council;

Dye-sensitized solar cells based on Cr-doped TiO₂ nanotube photoanodes

M.M.Momeni

Department of Chemistry, Isfahan University of Technology

Abstract：

The effect of chromium doping on the photovoltaic efficiency of dye-sensitized solar cells(DSSCs) with anodized TiO₂ nanotubes followed by an annealing process was investigated. Cr-doped TiO₂ nanotubes(CrTNs) with different amounts of chromium were obtained by anodizing of titanium foils in a single-step process using potassium chromate as the chromium source. Film features were investigated by scanning electron microscopy(SEM), X-ray diffraction(XRD), energy-dispersive X-ray spectroscopy(EDX), and ultraviolet-visible(UV-Vis) spectroscopy. It is clearly seen that highly ordered TiO₂ nanotubes are formed in an anodizing solution free of potassium chromate, and with a gradual increase in the potassium chromate concentration, these nanotube structures change to nanoporous and compact films without porosity. The photovoltaic efficiencies of fabricated DSSCs were characterized by a solar cell measurement system via the photocurrent-voltage(I-V) curves. It is found that the photovoltaic efficiency of DSSCs with CrTNsl sample is improved by more than three times compared to that of DSSCs with undoped TNs. The energy conversion efficiency increases from 1.05 % to 3.89 % by doping of chromium.

Keyword：

Dye-sensitized solar cell; Photoanodes; Nanotubes; Anodization;

Author： M.M.Momeni e-mail:mm.momeni@cc.iut.ac.ir;

Received： 21 May 2015

1 Introduction

Nowadays,the world is facing a major crisis with regards to the pollution of the earth and shortage of sustainable,safe,and environmental friendly energy resources.Photovoltaic cell can be regarded as one of the most credible and viable ways to face the problem,being the supply of energy from the sun about ten thousand times more than that mankind currently consumes [ 1] .Photovoltaic cell is a device that converts the energy from sunlight,which is an unlimited source of clean energy into electric energy.Additionally,photovoltaic cells do not require mechanical movement or movable parts to generate electricity.Photovoltaic cells can be pided into different categories depending on their working principles,production techniques,and used materials [ 2] .Compared to conventional silicon solar cells,which are costly,dye-sensitized solar cells (DSSCs) are expected to be promising alternative devices due to their low cost and simple fabrication process [ 3, 4] .

A typical DSSC device contains an anode made with a fluorine-doped tin oxide (FTO)/glass substrate covered with a nanocrystalline dye-impregnated TiO₂ film,a Ptbased counter electrode as the cathode,and a liquid electrolyte that closes the circuit [ 5, 6] .The total efficiency of the DSSC depends on optimization and compatibility of each of these constituents,in particular on the semiconductor film along with the dye spectral responses.Despite the advances in this field,conventional DSSC materials present some important drawbacks.While the nanoparticlebased electrode provides a large surface area which potentially provides drawbacks such as grain boundary losses (recombination) or hampered electron transfer (due to random walk effects).In order to overcome these effects,several approaches have been studied toward the use of ordered one-dimensional nanostructures [ 7] .These onedimensional structures are considered to significantly improve the electron transport time and reduce recombination effects.Another method to improve the electronic properties of TiO₂ is based on the introduction of some doping elements,to promote charge transfer and mediate recombination in TiO₂ structures [ 2] .

Recently,attempts were made to combine the advantages of ordered nanotube geometries with beneficial doping effects in DSSCs [ 2, 8] .During the last years,self-assembled TiO₂nanotube arrays grown by controlled electrochemical anodization have emerged as a vertically oriented architecture that is being actively investigated for various applications [ 9, 10] .Particular attention has been attracted for the utilization of anodized TiO₂ nanotubes as photoelectrodes in solar energy conversion,especially DSSCs [ 11] ,where the improved lightscattering properties (thus better light harvesting),potentially faster electron transport along the tube axis and particle interconnectivity of the TiO₂ nanotubes,could be effectively exploited.Proper control of the anodization parameters,including solution chemistry,applied potential,and anodization time,can enable tailoring of the tube structure,diameter,length,and wall thickness [ 12] .

In the present work,to increase the photovoltaic efficiency of anodized TiO₂-nanotube-based DSSC,selfordered Cr-doped TiO₂ nanotube (CrTN) photoanodes with different amounts of chromium were synthesized by a single-step anodization of titanium substrate in an organic bath of ethylene glycol (EG)-fluoride electrolyte containing various amounts of potassium chromate.The quantity effect of chromium in anodizing solution on DSSC performance of these samples was investigated.It is demonstrated a considerable enhancement in the DSSC performance for CrTN photoanode formed by anodic oxidation in anodizing electrolyte containing 5 mmol·L^-1 potassium chromate.

2 Experimental

All chemicals were of analytical grade without further purification before experiment,and solutions were prepared with distilled water.CrTNs with different amounts of chromium were synthesized by anodic oxidation of titanium.Square samples with dimensions of 20 mm×20 mm×1 mm and99.99%purity titanium were used as working electrodes in this experiment.The working electrodes were sealed with insulation resin,leaving only active surface with an area of 2.0 cm².

Before the anodizing,samples were first mechanically polished with different emery-type abrasive papers (with the following grades:60,80,600,1200,and 2500),rinsed in abath of distilled water and then chemically etched by immersing in a mixture of HF and HNO₃ acids for 30 s.The ratio of components HF/HNO₃/H₂O in the mixture was 1:4:5 in volume.The last step of pretreatment was rinsing with distilled water.

The anodizing process was carried out in an electrolytic cell using a titanium foil as anode and platinum foil with about 12 cm² geometric areas as cathode.Anodizing was performed in a solution of ethylene glycol (98 ml)containing 0.001 mol·L^-1 ammonium fluoride and 2 ml distilled water,followed by the dissolution with different concentrations of potassium chromate (K₂CrO₄).Anodizing was performed for 6 h at a constant potential of 60 V at room temperature.A controlled direct current (DC) power source(ADAK,PS405) supplied the required constant voltage.A schematic illustration of the pretreatment method of titanium sheets and process of producing CrTNs films on titanium is shown in Fig.1.

After anodization,the as-formed samples were annealed in oxygen atmosphere at 400℃for 2 h (2℃.min^-1).The concentration of potassium chromate in anodizing solution was 0,5,15,and 25 mmol·L^-1,leading to samples with varied chromium contents of 0 wt%,0.2 wt%,0.5 wt%,and0.8 wt%referred to as TNs,CrTNs l,CrTNs2,and CrTNs3,respectively.Table 1 summarizes the experimental conditions for four different samples.

The surface morphology of all samples were characterized by field emission scanning electron microscopy (FESEM,Hitachi S-4160,Japan),and the elemental composition was estimated by energy-dispersive X-ray spectroscopy (EDX).The crystalline phases were identified by X-ray diffractometer(XRD,Philips XPert).Diffraction patterns were recorded in the 2θrange from 20°to 80°at room temperature.The optical absorption of the samples was determined using a diffuse reflectance ultraviolet-visible (UV-Vis,DRUV-Vis) spectrophotometer (JASCO V-570).The values of the band gap energy (E_g) were calculated using following equation:

where h is Planck’s constant,v is the frequency of vibration,hv is the incident photon energy,A is a proportional constantαis the absorption coefficient per unit length,and n is 0.5 and 2.0 for a direct transition semiconductor and indirect transition semiconductor,respectively [ 13] .The band gap values were determined by extrapolating the linear region of the plot to hv=0.

For dye sensitization and solar cell construction,it was followed a procedure given in Ref. [ 2] .In briefly,after annealing,the samples were dye-sensitized with Ru-based N719 dye (cis-bis (isothiocyanato)-bis (2,2-bipyridyl 4,4-dicarboxylato) ruthenium (Ⅱ) bis-tetrabutylammonium)(Solaronix SA,Switzerland) by immersion in 0.3 mmol·L^-1ethanolic solution of dye N719 for 24 h,and the samples were used as the photoanode of the dye-sensitized solar cells.The preparation of platinum counter electrode (using 50 mmol·L^-1solution of H₂PtCl₆ in ethanol),cell sealing with 60μm thickness thermoplastic spacer,injection of electrolyte,and final sealing were successively performed.The currentvoltage characteristics of the cells were measured under air mass (AM) of 1.5 and 100 mW·cm^-2 simulated light radiations.Backside illumination was used to record the currentvoltage (Ⅰ-Ⅴ) characteristics of the solar cells.This configuration eliminates undesired influences that typically are inherent to (higher efficiency) front-side constructions based on lift-off process and glueing with TiO₂ nanoparticle [ 2] .

Fig.1 Schematic presentation of pretreatment method of titanium sheets and process of producing CrTNs on titanium foils.(TCO transparent conductive oxide)

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Table 1 Anodizing solutions for synthesized samples at anodizing condition of 60 V,6 h,and room temperature

3 Results and discussion

The morphology of anodized samples in electrolytes containing different concentrations of potassium chromate was observed by SEM.Figure 2 shows FESEM images of TNs,CrTNsl,CrTNs2,and CrTNs3,which clearly shows the formation of films on the surface of titanium.In Fig.1a,b,TNs and CrTNsl display vertically ordered nanotube arrays of which the surface is open.It can be seen that TNs and CrTNs 1 consist of a layer of tubes with a diameter in the range of 90-150 nm and wall thickness of 20-40 nm.In Fig.2c,no nanotubes form.Porous films form instead(CrTNs2).It can be said that when the potassium chromate concentration in anodizing solution increases to15 mmol·L^-1 (Fig.2c),the formed nanotube array becomes very non-uniform,implying that an appropriate concentration of potassium chromate is important for the nanotube array structures.FESEM images of CrTNs3 in Fig.2d show that a compact film,without holes and porosity,forms on the surface of titanium.From the crosssectional view of CrTNs1 in Fig.3,it can be seen that the formed nano tubes are parallel-aligned,opened on top,and have a length in the range of 16μm.

Fig.2 SEM top-view images of samples formed by anodic oxidation in an ethylene glycol electrolyte containing different concentrations of potassium chromate:a 0 mmol·L^-1 (TNs),b 5 mmol·L^-1 (CrTNs1),c 15 mmol·L^-1 (CrTNs2),and d 25 mmol·L^-1 (CrTNs3)

Fig.3 Cross-sectional images of CrTNsl sample with different magnifications

Fig.4 XRD patterns of as-prepared TNs,TNs annealed at 400℃,as-prepared CrTNs1,and CrTNsl annealed at 400℃

Figure 4 shows the XRD patterns of the undoped TNs and CrTNsl samples annealed at 400℃.It can be seen that undoped TNs and CrTNs films exhibit characteristic features of anatase TiO₂,but no characteristic peak attributed to chromium oxide or Cr component can be found in the XRD patterns,implying that Cr is incorporated into the crystal lattice of TiO₂,or chromium oxide is highly dispersed,and its size is very small [ 14, 15] .It has been considered that metal dopants can be conveniently incorporated into TiO₂lattice when their ionic radii are identical or nearly identical to that of Ti⁴⁺ [ 15] .Since the ion radius of chromium ion is much close to that of Ti⁴⁺,it is easy for Cr ion to substitute Ti⁴⁺into TiO₂ lattice [ 14, 15] .Because chromium oxide reflections are not observed in the diffraction patterns,EDX spectra of films were examined,and the results are shown in Fig.5.It is seen that the films mainly consist of Ti,O,and Cr,and their contents are shown in Table 2.The EDX data of CrTNsl (Fig.5b) show apeak around 0.4 and 0.5 keV,and another intense peak appears at 4.5 and 4.9 keV for Ti.The peaks due to chromium are clearly distinct at 0.5,0.6,and5.4 keV.These results confirm that Ti,O,and Cr exist in the catalyst structure.The occurrence of traces of contaminants such as carbon and fluorine from precursors are also observed.The presence of c arbon originates from absorption of carbon from the ethylene glycol.It is believed that carbon species adsorb onto the surface during anodizing,and subsequent heat treatment induces diffusion into the crystal structure of TiO₂ [ 13] .With the increase in the potassium chromate concentration in the electrolyte,the content of chromium increases.

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Table 2 Average elemental compositions of samples obtained by taking 10 spots in EDX analysis (at%)

Fig.5 EDX spectra of different samples:a TNs and b CrTNsl

The optical absorption of the different samples was studied.Regarding the recorded UV-Vis spectra data,the absorption edge of CrTNs3 is closer to the visible light region than that of undoped TNs.Figure 6 shows the plot of (αhv)^1/2 versus hv employed to calculate the band gap value of different TNs samples.The band gap energies decrease progressively from～3.2 eV for undoped TNs to～2.82,2.71,and 2.30 eV for CrTNs (CrTNsl,CrTNs2,and CrTNs3,respectively).Compared with undoped TiO₂,all of the CrTNs samples exhibit a redshift of absorption edge,and a decrease in band gap with the increase of Cr concentration was observed.These results suggest that chromium is incorporated into the crystalline network of TiO₂.This incorporation could generate a new Fermi level between bad gap due to strain of fields caused by lattice match between Cr and TiO₂ [ 14, 15, 16] .These results confirm that TiO₂ lattice is doped with chromium.

Fig.6 Plot of (αhv)^1/2 versus hv employed to calculate band gap values of different CrTNs samples

The different samples were assembled to solar cells as described in the experimental section.I-V characteristics of the DSSCs with various photoanodes (dye-adsorbed TiO₂ electrode/electrolyte/platinum counter electrode) are shown in Fig.7a,and the detailed photovoltaic parameters are listed in Table 3 [ 17, 18, 19, 20, 21, 22] .

The DSSC with undoped TNs exhibits a short-circuit current (J_sc)of 2.54 mA·cm^-2 and a fill factor (FF) of0.553,yielding an overall energy conversion efficiency (η)of 1.05%.When the TiO₂ nanotubes were doped with chromium,the values ofηincrease to 3.89%for CrTNsl and thereafter decrease.The higher conversion efficiency of CrTNs (CrTNsl) can be attributed to the combined effect of two factors:the Cr doping and the one-dimensional micro structure.It is well known that theηvalue of DSSCs is significantly affected by the amount of dye loading to photoanodes [ 2] .Also for achieving high efficiency,an optimal value of chromium is required.With the further increase in the amount of chromium in the solution,however,the photovoltaic efficiency decreases to 0.65%.This can be attributed to the decrease in the amount of dye loading due to the reduced surface area.The results show that CrTNsl has better performance than TNs and CrTNs fabricated using other Cr concentrations.The values of open-circuit voltage (V_OC),J_SC,FF,andηof the DSSCs vary with the concentration of chromium.By incorporation of chromium into the TiO₂,J_SC and V_OC values change.Clearly,for CrTNsl,higher solar cell efficiency is obtained compared to the other samples.A comparison of DSSC photovoltaic parameters of the present study with similar studies is presented in Table 4.

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Table 3 Photovoltaic parameters of DSSCs with different photoanodes

J_SC short-circuit current,V_OC open-circuit voltage,FF fill factor,ηefficiency

Fig.7Ⅰ-Ⅴcharacteristics under AM 1.5 solar light illuminations for different samples a andⅠ-Ⅴcharacteristics of DSSC with WT2 photoanode in dark and in presence of light b

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Table 4 Comparison of DSSC photovoltaic parameters of present study with similar studies

Theηvalue of the dye-sensitized solar cell is determined byJ_SC,V_OC,FF of the cell and the intensity of the incident light (I_s)

The V_OC value is determined by the energy difference between the Fermi level of the solid under illumination and the Nernst potential of the redox couple in the electrolyte.However,the experimentally observed V_OC for various sensitizers is smaller than the difference between the conduction band edge and the redox couple.This is generally due to the competition between electron transfer and charge recombination pathways [ 23] .The FF value is defined as the ratio of the maximum power (P_max) obtained with the device and the theoretical maximum power(P_th=J_SCV_OC).The FF value can then take values between 0 and 1.It reflects electrical and electrochemical losses occurring during operation of the DSSC [ 23] .

Typical current-voltage characteristics (Ⅰ-Ⅴcurve) of DSSC with CrTNsl photoanode in dark and in presence of light is shown in Fig.7b.Without illumination,no current flows through the external circuit.With incident light,I-V curve shifts up,indicating that there is external current flow from the solar cell.

4 Conclusion

Back-illuminated DSSCs were fabricated with different CrTNs.CrTN samples were successfully synthesized through a simple,facile,and novel anodization process in a single-step process using potassium chromate as the Cr source.The morphology and structure were characterized by FESEM,XRD,and EDX.By tuning the starting molar concentration of potassium chromate,different chromium doping levels are obtained.With Cr content increasing,the tube morphology disappears.Diffuse reflectance spectra show an improvement in the visible absorption relative to undoped TNs samples.The DSSCs based on CrTNs show a higher efficiency (3.89%) compared to that on undoped TNs(1.05%) due to the increase in the photocurrent density.

Acknowledgments The author would like to acknowledge the financial support from Iranian Nanotechnology Society and Isfahan University of Technology (IUT) Research Council.

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