Incorporation of Mn2+ and Co2+ to TiO2 nanoparticles and the performance of dye-sensitized solar cells

Applied Surface Science 283 (2013) 975–981

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Incorporation of Mn2+ and Co2+ to TiO2 nanoparticles and the performance of dye-sensitized solar cells A.E. Shalan a,b,∗ , M.M. Rashad a a Electronic and Magnetic Materials Division, Advanced Materials Department, Central Metallurgical Research & Development Institute (CMRDI), P.O. Box 87 Helwan, 11421, Cairo, Egypt b Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-, Alexander-University of Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 12 February 2013 Received in revised form 10 July 2013 Accepted 12 July 2013 Available online 20 July 2013 Keywords: Dye sensitized solar cells Cobalt doped titania electrode Manganese doped titania electrode Photoelectrochemistry

a b s t r a c t Dye-sensitized solar cells were fabricated using (Mn and Co) M-doped TiO2 electrodes which were successfully synthesized via the hydrothermal method. Furthermore, the effect of Mn2+ and Co2+ ions content on the properties of TiO2 electrodes was studied. The materials were characterized by XRD, TEM/HRTEM, EDS, BET specific surface area (SBET ), pore-size distribution by BJH, UV–Vis Spectroscopy, and their photoconversion efficiencies were evaluated using I–V characterization, IPCE and EIS. X-ray diffraction results reveal both undoped and M-doped TiO2 structure without any impurity phase. The X-ray diffraction patterns of the (Mn and Co) ions doped TiO2 is almost the same as that of pure TiO2 , showing that (Mn and Co) have little influence on the formation of anatase titania. The influence of dopant (Mn, Co) ions on band energetics and photoelectrochemical properties of nanostructured TiO2 electrodes was investigated. The total trap densities were remarkably increased as TiO2 electrodes were doped with (Mn and Co). Experiment results showed that the content of M-doped TiO2 plays an important role in the photoelectrochemical properties. The conversion efficiency was decreased with (Mn and Co)-doped TiO2 electrodes under irradiation of 100 mW/cm2 white light due to the high change of flat band edge and the charge recombination which happened related to trap density of TiO2 electrodes with (Mn and Co) ions doping. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cells are currently attracting widespread academic and commercial interests for the conversion of sunlight into electricity with low cost and high efficiency [1–4]. Excellent photo-electron conversion efficiencies have been reported for regenerative photo electrochemical cells based on nanocrystalline titanium dioxide (TiO2 ) films, sensitized by ruthenium-dyes using iodide/triiodide (I− /I3 − ) redox couple in an organic solvent as the electrolyte [5,6]. A number of methods have recently been used on the synthesis of nanosized TiO2 , such as sol–gel, spray pyrolysis and hydrothermal techniques [7]. Besides these techniques, hydrothermal is an alternative, cheap and simple method for preparation of nanostructure material [8]. The hydrothermal method is a novel technique of producing metal-ions doped oxides in a sealed hightemperature and high-pressure environment [9]. It offers many advantages of energy conservation, simple preparation, perfect

∗ Corresponding author. Tel.: +20 2 2501 0640x43, fax: +20 2 2501 0639. E-mail addresses: [email protected], genral [email protected] (A.E. Shalan). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.055

nucleation, rapid reaction, and low operation temperature. The modification of TiO2 by means of metal doping can affect the crystallization process, influencing the photocurrent efficiency of DSSCs. Numerous metal ions have been investigated as potential dopants, including iron [10], chromium [11], manganese [12], cobalt [13], etc. It has been reported that there are a large number of surface states in nanocrystalline TiO2 electrodes, which are energetically located below the TiO2 conduction band with an exponential distribution [14–16]. The recombination between the injected electrons and the oxidized redox couples occurs principally via trap states rather than via the conduction band [17,18]. So, these surface states are deleterious to the performance of DSSCs since they trap carriers and promote charge recombination [19]. Metal doping showed several drawbacks: thermal instability of doped TiO2 , electron trapping by metal centers and poor overall conversion efficiency [20,21]. We think that it is necessary and significant to investigate the behaviors of (Mn and Co)-doped TiO2 electrodes in DSSCs. In this work, we report the synthesis of (Mn and Co)-doped TiO2 colloids by hydrothermal method and the fabrication of undoped TiO2 and M-doped TiO2 electrodes. The pure TiO2 and M-doped TiO2 electrodes were sensitized with ruthenium dye N719 (Ru[L2 (NCS)2 ],

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L = 2,2 -bipyridine-4,4 -dicarboxylic acid) and their photoelectrochemical properties were studied. The influence of (Mn and Co) content on the electrochemical and photoelectrochemical properties of the M-doped TiO2 electrodes was investigated. 2. Experimental 2.1. Materials and methods 2.1.1. Materials Titanium isopropoxide [Ti (OCH (CH3 )2 )4 ] (99.99%) purchased from Sigma–Aldrich; was used to synthesize anatase TiO2 nanopowders via hydrothermal method. Moreover, pure ammonium hydroxide (33%), (Fluka) was employed as a base to attain pH value 7. Different sources of metals such as cobalt chloride [CoCl2 ] as a source of Co+2 and manganese chloride [MnCl2 ] as a source of Mn+2 purchased from Sigma–Aldrich were used as the doping metals materials up to the hilt of the hydrothermal preparation method. Fluorinated tin oxide (FTO) (Asahi Glass Co. Ltd.) and indium tin oxide (ITO) glass substrates were bought from SOLEMS, 10 and 70 /cm2 , cleaned with soap water, mili-Q water, acetone and ethanol (99.5%) for 10 min before use. The substrates were dried under an N2 flux and finally cleaned for 20 min in a UVsurface decontamination system (Novascan, PSD-UV) connected to an O2 gas source. O2 (BIP quality) and N2 (BIP quality, <0.02% O2 ) were purchased from Carburos Metalicos (Air Products) and used at <0.5 bar pressure. The dye solution was prepared by dissolving   0.17 mM cis-di (thiocyanato) bis (2,2 -bipyridyl-4,4 -dicarboxylate) ruthenium- (II) (N719, Solaronix SA, Switzerland) in dry ethanol (from Sigma–Aldrich). An electrolyte was made with 0.1 M Lithium iodide (LiI) (Aldrich), 0.1 M Iodine (I2 ) (Aldrich), 0.6 M tetrabutylammonium iodide (Fluka), and 0.5 M tert-butylpyridine (Aldrich) in dry acetonitrile (Fluka). 2.1.2. Methods (preparation of TiO2 and M-TiO2 nanoparticles) 25 ml titanium isopropoxide [Ti (OCH (CH3 )2 )4 ] was added to 100 ml distilled water under vigorous stirring for 10 min, and homogeneity mixed with ammonium hydroxide till pH 7. After stirring for another 10 min to improve the homogeneity, the mixed solution was transferred to a 150 ml Teflon-lined stainless steel autoclave. The autoclaves were kept in an electric oven at 100 ◦ C for 24 h. After cooling down to room temperature, samples were washed with deionized water several times and then dried at 80 ◦ C for 10 min. Furthermore, samples of (Mn, Co)-doped TiO2 nanoparticles were prepared by the hydrothermal technique. First, titanium isopropoxide was dissolved in distilled water, and an appropriate amount of (MnCl2 or CoCl2 ) was dissolved in deionized water. The (MnCl2 or CoCl2 ) solution was then slowly poured into the titanium isopropoxide solution by stirring with ammonium hydroxide till pH 8 and pH 10 for Mn+2 and Co+2 , respectively. The well-mixed solution was transferred to an autoclave at 100 ◦ C for 24 h. 2.1.3. Preparation of photoanodes Nanoporous TiO2 film was prepared according to our previous work [7]. In a typical method, 1.0 g of TiO2 nanopowders were mixed with 1.0 ml distillated water, 5 ml absolute ethanol, and stirred using hot plate magnetic stirrer for 10 h. Fluorinated tin oxide (FTO) glass substrates were cleaned with soap water, mili-Q water, acetone and ethanol (99.5%) for 10 min before use. The substrates were dried under an N2 and finally cleaned for 20 min in a UV-surface decontamination system. Deposit a TiO2 blocking layer using titanium tetrachloride (TiCl4 ). Cover the FTO glass with tape as spacer and then coated with TiO2 paste by doctor blade method. The thickness of titania film can be controlled by changing the concentration of the paste and the layer numbers of the adhesive tape (Scotch, 50 ␮m). The films were dried under ambient conditions

and then annealed at 450 ◦ C for 30 min to remove the binders in the paste and to increase the crystallinity of the nanorods. Then, the films were cooled down to 80 ◦ C for dye sensitization. Dye sensitization was achieved by immersing the TiO2 nanoparticles electrodes in a 0.3 mM N719 dye (Solaronix) in ethanol solution overnight, followed by rinsing in ethanol and drying in air. 2.1.4. Fabrication of DSSCs The sensitized TiO2 film was rinsed with ethanol and assembled with a platinum covered FTO electrode (TEC 15, 15 /cm2 ) containing a hole into a sandwich-type configuration using sealing technique. The counter electrode was prepared by adding 50 nm layer of platinum (Pt) on the FTO surface using spin coating pyrolysis technique. The two electrodes sealed with a 25 ␮m thick polymer spacer (Surlyn, DuPont). The void between the electrodes then filled with an iodide/tri-iodide based electrolyte, containing 0.1 M lithium iodide, 0.1 M iodine, 0.6 M tetrabutylammonium iodide, and 0.5 M tert-butylpyridine in dry acetonitrile in 1:1 acetonitrile propylene carbonate by firm press, via air pump vacuum backfilling through a hole pierced through the Surlyn sheet. The hole then sealed with an adhesive sheet and a thin glass to avoid leakage of the electrolyte. The resulting cell had an active area of ∼0.25 cm2 . 2.2. Physical characterization The crystallite phases present in the different annealed samples were identified by X-ray diffraction (XRD) on a Brucker axis D8 diffractometer with crystallographic data software Topas 2 using Cu-K␣ ( = 1.5406 Å ) radiation operating at 40 kV and 30 mA at a rate of 2◦ /min. The diffraction data were recorded for 2 values between 20◦ and 80◦ . Transmission electron microscopy (TEM) and High resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDS) were performed with a JEOL-JEM-1230 microscope. Nitrogen adsorption–desorption isotherms were obtained on an ASAP 2020 (Micromeritics Instruments, USA) nitrogen adsorption apparatus. All the samples degassed at 180 ◦ C prior to Brunauer–Emmett–Teller (BET) measurements. The BET specific surface area (SBET ) was determined by a multipoint BET method using the adsorption data in the relative pressure P/P0 range of 0.05–0.25. Desorption isotherm was used to determine the pore-size distribution using the Barret–Joyner–Halender (BJH) method. The nitrogen adsorption volume at P/P0 = 0.995 was used to locate the pore volume and average pore size. The UV–Vis absorption spectrum was deliberated by a UV–VIS–NIR scanning spectrophotometer (Jasco-V-570 Spectrophotometer, Japan) using a 1 cm path length quartz cell. Photocurrent-voltage J-V characteristic curves measurements were investigated using the solar simulation which carried out with a Steuernagel Solarkonstant KHS1200. Light intensity was adjusted at 1000 W/m2 with a bolometric Zipp & Konen CM-4 pyranometer. Calibration of the sun simulator was made by several means: with a calibrated S1227- 1010BQ photo diode from Hamamatsu and a mini spectrophotometer from Ava-Spec 4200. The AM1.5 simulated sunlight reference spectrum was according to an ASTM G173 standard. Solar decay and I–V-curves were measured using a Keithley2601 multimeter connected to a computer and software. The photoelectric conversion efficiency () is calculated according to Eq. (1): % =

Jsc Voc FF × 100 Pin

(1)

where, the fill factor (FF) is the ratio between the maximum output power density available (Jm Vm ) and the maximum power

A.E. Shalan, M.M. Rashad / Applied Surface Science 283 (2013) 975–981

peaks that have arisen from the main phase of anatase TiO2 , there were some additional peaks in XRD patterns of the (Mn, Co)-doped samples, which were assigned to CoTiO3 and Mn2 O3 ; as depicted in Fig. 1.

(*) Anatase TiO 2, (+) CoTiO3, (o) Mn2O3

*

+

Intensity (cps)

+

*

+

o

+

*

*

*

(c) Co-TiO2

3.2. Microstructure Fig. 2 presents the TEM/HRTEM microphotographs of TiO2 powders. The microphotographs emphasize that sizes of anatase grains decrease with the (Mn, Co) doping. It is noted that the particles having certain homogeneity in size and nearly spherical shape. TEM images show that the anatase grain sizes decrease from ca. 13 nm for pure TiO2 powder to ca. 7 nm for Mn-doped TiO2 powders and increase again to 15.2 nm for Co-doped TiO2 powders which agreed with the XRD data.

(b) Mn-TiO2

o

977

(a) TiO2 (NPs)

3.3. EDS analysis 20

30

40

50

60

70

80

2θ degree Fig. 1. XRD patterns of (a) TiO2 (NPs), (b) Mn-TiO2 , (c) Co-TiO2 .

combining short-circuit and open-circuit situations (Eq. 2) and it describes the “squareness” of the J–V curve. FF(%) =

Jm Vm × 100 Jsc Voc

(2)

The incident monochromatic photoelectric conversion efficiency (IPCE) analyses was carried out using a QE/IPCE measurement system from Oriel at 10 nm intervals between 300 and 700 nm, where a monochromator was used to obtain the monochromatic light from a 300 W Xe lamp. The IPCE is defined as Eq. (3): IPCE(%) =

12400 × JSC (Acm−2 ) (nm) × Pin (Wcm−2 )

(3)

We use a calibrated photodiode (S1227-1010BQ from Hamamatsu) before each IPCE analyses. In the above two formulas,  is the global efficiency, VOC , JSC , and FF are open circuit voltage, short circuit current density, and fill factor, respectively. Pin and  are the light energy and wavelength of the incident monochromatic light, respectively. The measurements repeated three times for each sample, and the experimental error was found to be within ca. 5%. Impedance measurements (EIS) were performed with a computer-controlled potentiostat (EG&G, M273) equipped with a frequency response analyzer (EG&G, M1025). The frequency range is 0.005–100 kHz. The magnitude of the alternative signal is 10 mV. Unless otherwise stated, all impedance measurements were carried out under a bias illumination of 100 mW/cm2 (global AM 1.5, 1-sun) from a 450-W xenon light source. The obtained spectra were fitted with Z-View software (v2.1b, Scribner Associate, Inc.) in terms of appropriate equivalent circuits.

The EDS spectra and chemical compositions of pure TiO2 , Mn-TiO2 and Co-TiO2 are shown in Fig. 3 (a)–(c), respectively. Quantitative analysis of the EDS spectrum showed that the main elements present in the pure TiO2 were Ti and O and the atomic ratio of titanium to oxygen was closely equal to the stoichiometric of 1:2. For Co-TiO2 , the main elements were Ti, O and cobalt and For Mn-TiO2 , the main elements were Ti, O and manganese. The atomic ratio of titanium to oxygen was also closely equal to 1:2 for both doping metal ions. The cobalt and manganese content in the as-synthesized Mn-TiO2 and Co-TiO2 was determined to be 4.21 wt% (9.6 at.%) for manganese and was 6.5 wt% (12.6 at%) for cobalt, which indicates that the nanoparticles obtained were doped with both cobalt and manganese. 3.4. Optical properties Fig. 4 shows the UV–Vis spectra of pure TiO2 and the M-TiO2 . The optical absorption edges of the (Mn+2 , Co+2 ) doped TiO2 shifted to a lower energy in the visible-light region compared to that of undoped TiO2 . This result indicates the narrowing of band gap upon doping (Mn+2 , Co+2 ) into TiO2 . All the produced nanopowders were highly transparent with the doping metals; the high transparency of the produced powder is attributed to the generation of donor levels accompanying an addition of doping metals. Subsequently, the optical band gap energy (Eg ) was determined by Tauc’s equation [22–24], given as (˛h) = A(h − Eg )

n

(4)

3. Results and discussion

where ˛ was calculated from the transmittance data, A is an energy-independent constant, h is the photon energy (h is a blank constant and  is wavenumber), B is the edge width parameter dependent on structural disorder of powders, and the exponent n depends on the type of transition. The optical band-gap value determined 4.1 and 4.4 eV for TiO2 doped with Mn and Co, respectively was higher than the ones usually found in the literature (around 3.2 eV) [23]. This difference may be attributed to the small particle size of the powder characterized in this work.

3.1. Crystal structure

3.5. Brunauer–Emmett–Teller (BET) measurements

Structural characterization of the pure TiO2 , (Mn and Co)-doped TiO2 powders was performed by X-ray diffraction (XRD). Fig. 1 displays the XRD patterns for pure TiO2 and M-doped TiO2 powders. The XRD patterns are the same with that of standard anatase TiO2 (JCPDS card number 04-0477). The results indicated that the crystal phase of the prepared powders was single anatase. The crystallite size was ∼13 nm for pure nanoparticles, ∼7 nm for Mn doped and ∼15.2 nm for Co doped. It should be noticed that besides the XRD

Analyzing the hysteresis loop shapes of nitrogen isotherms gives information about pore structure in TiO2 and M-TiO2 samples. This method based on adsorption–desorption isotherms and more specifically on the hysteresis region, where multilayer adsorption occur. The as-prepared TiO2 and M-TiO2 show a type-II adsorption–desorption isotherm, confirming that the sample is mesoporous [25]. The surface area and the average pore radius were calculated as method reported elsewhere [26]. The surface

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A.E. Shalan, M.M. Rashad / Applied Surface Science 283 (2013) 975–981

Fig. 2. TEM micrographs of the produced powders (a) TiO2 (NPs), (b) Mn-TiO2 , (c) Co-TiO2 .

area of pure TiO2 was 99.30 m2 /g, larger obtained for Co-TiO2 and lower than Mn-TiO2 powders because of its smaller crystallite size [27]. Particularly Mn-TiO2 exhibited the largest surface area of 171.18 m2 /g. The porous photoelectrode surface permits a better wetting of the film by the electrolyte and finally results in a perfect penetration of the iodide/triiodide redox couple into the film pores, which in turn favor the interaction between the oxidized dye and regeneration of the sensitizer’s ground state. Thus, the obtained results show that the as-prepared TiO2 and M-TiO2 coating demonstrates a great potential for use as electrode material in dye-sensitized solar cell. Table 1 list the physical properties of TiO2 and M-TiO2 nanoparticles

which can evaluated from the SBET measurements, as it found the difference in specific surface areas, porosity, and relative anatase crystallinity in both materials. The porosity and surface area were influenced by the doping materials. 3.6. Photovoltaic characteristics Fig. 5 shows the comparison of J–V characteristics of the as prepared TiO2 , manganese and cobalt doped TiO2 nanoparticles electrodes. The average photovoltaic parameters of TiO2 and M-TiO2 measured under illumination AM 1.5 simulated sunlight (100 mW/cm2 ) are summarized in Table 2. The overall conversion

Table 1 Physical properties of TiO2 (NPs), Mn-TiO2 and Co-TiO2 . Sample

SBET (m2 /g)

Crystallite size (nm)

Pore volume (cm3 /g)

Average pore size (nm)

Porosity (%)

BJH Adsorption (nm)

BJH Desorption (nm)

TiO2 (NPs) Mn-TiO2 Co-TiO2

99.30 171.18 78.59

13.0 7.0 15.2

0.24 0.41 0.16

9.75 9.72 7.11

65 83 40

8.25 9.35 5.55

6.50 7.66 4.02

A.E. Shalan, M.M. Rashad / Applied Surface Science 283 (2013) 975–981

979

100

O

Weight%

Atomic%

90

O K Ti K Totals

39.00 61.00 100.00

65.68 34.32

80

O

a

Cu

Ti

Ti

Cu Si

(b) 0.3 Mn-doped TiO2 (c) 0.3 Co-doped TiO2 (c)

70

Intensity (T%)

Ti

(a) TiO2 nanoparticles

Element

(b) (a)

60 50 40

Cu 30

0

5

10

O

20

Kev

10 200

O

Ti Mn

Element

Weight%

Atomic%

Mn K O K Ti K Totals

4.21 40.13 55.66 100.00

9.60 61.78 28.62

Si Ti

Mn

0

b

Cu

Ti

5

10

Kev

O

Ti O

Element

Weight%

Atomic%

Co K O K Ti K Totals

6.50 40.00 53.50 100.00

12.60 58.40 29.00

Co

Ti

Ti

Co Si

0

c

Cu

5

10

300

400

500

600

Wavelength (nm) Fig. 4. UV–visible transmittance spectrum T% of (a) TiO2 (NPs), (b) 0.3 Mn-TiO2 , (c) 0.3 Co-TiO2 .

density, because it can be improved by the suppression of charge recombination. It was found that the recombination was the dynamic reason for decreased photovoltage [28]. In this study, the photoelectric conversion efficiency of the (Mn and Co) doped TiO2 electrodes mainly was less than as-prepared TiO2 due to the flat band potential changed high (positive shift) and the trap densities don’t have remarkable decrease [29]. Moreover, it should be noticed that the factors affecting the photoelectric conversion are the pore structure, the trap densities and dye loading. 3.7. Incident photon to current conversion efficiency (IPCE) The IPCE is defined as the number of electrons generated by light in the external circuit divided by the number of incident photons. The IPCE values of N719 sensitized pure TiO2 and M-doped TiO2 electrodes were measured and shown in Fig. 6. It can be seen that the (Mn and Co)-doped TiO2 electrodes show pretty lower photoelectrical response than the pure TiO2 electrode throughout the visible wavelengths. The major IPCE values were from the UV region and expended its spectral response to visible light region. The low 10

Kev

(a) η=5.27%

Fig. 3. EDS spectra and their corresponding quantitative elemental analysis (inset) of (a) pure TiO2 , (b) Mn-TiO2 and (c) Co-TiO2 , respectively.

2

Current mA/cm

efficiencies of the DSSCs were 5.27%, 1.85% and 1.06% for undoped TiO2 , Mn-TiO2 and Co-TiO2 , respectively. One important factor determining photovoltage efficiency is the difference between the flat band potential and the redox potential of electrolyte. Our experimental results showing that the flat band edge of TiO2 electrodes changed high (positive shift) with (Mn and Co) doping. On the other hand, photovoltage is also related to trap

8

6

(b) η=1.85%

4

(c) η=1.06%

2 Table 2 Comparison of the I–V characteristics of DSSCs made from TiO2 (NPs), Mn-TiO2 and Co-TiO2 . Sample

Voc

Jsc (mA/cm2 )

FF

 (%)

Active area (cm2 )

TiO2 (NPs) Mn-TiO2 Co-TiO2

0.750 0.656 0.600

9.417 4.241 3.12

74.68 66.74 57.11

5.27 1.85 1.06

0.25 0.20 0.23

0 0.0

0.2

0.4

0.6

0.8

Voltage (V) Fig. 5. Comparison of the I–V characteristics of DSSCs made from (a) pure TiO2 , (b) Mn-TiO2 and (c) Co-TiO2 .

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A.E. Shalan, M.M. Rashad / Applied Surface Science 283 (2013) 975–981

the electrode contact under illumination. The EIS data in this work are helpful for understanding of all the fabricated solar cells, and also can explain well the existing interfaces in DSSCs.

80

(a)

4. Conclusions 60

QE%

(b) 40

(c) 20

0 250

300

350

400

450

500

550

600

650

700

750

800

Wanelength (nm) Fig. 6. IPCE of DSSCs made from (a) pure TiO2 , (b) Mn-TiO2 and (c) Co-TiO2 using N719 ruthenium dye.

IPCE values for M-TiO2 could be attributed to the substantial effect of recombination and back electron transfer, which resulted in low harvesting efficiency observed in the solar cells.

M-doped TiO2 nanoparticles were synthesized with a hydrothermal method. The effects of (Mn and Co) doping into TiO2 nanoparticles were studied and their solar cell performances were compared with those of undoped TiO2 nanoparticles. It was found that the manganese and cobalt were introduced into the interstitial sites of TiO2 lattice and contributed to the shift of conduction band. The crystallite size was ∼13 nm for pure nanoparticles, ∼7 nm for Mn doped and ∼15.2 nm for Co doped. The surface area of pure TiO2 was 99.30 m2 /g, larger than that of Co-TiO2 but lower than MnTiO2 because of its smaller crystalline sizes. The band gap energy of the formed powder was from 3.7 to 4.4 eV. The (Mn and Co)doped TiO2 nanospheres showed a less performance compare to those of undoped TiO2 due to the increasing in trap state densities and decreased dye adsorption. Furthermore, the performance of the (Mn and Co)-TiO2 could be attributed to shorter electron life time in the TiO2 nanoparticles and the depletion electron-injection due to bad matching of the LUMO of dye molecules and the conduction band of TiO2 nanoparticles. The highest conversion efficiency of 5.27% was obtained with the undoped TiO2 electrode under irradiation of 100 mW/cm2 white light and about 1.85% and 1.06% for Mn-TiO2 and Co-TiO2 , respectively lower than that of pure TiO2 electrodes.

3.8. Electrochemical impedance spectroscopy (EIS) analysis Electrochemical impedance spectroscopy (EIS) is a powerful tool to clarify the electronic and ionic transport processes in DSSCs [30]. The internal resistances of the three kinds of photoelectrodes (pure TiO2 , Mn-TiO2 and Co-TiO2 ) were studied using Nyquist plot in order to investigate the electron transfer at the TiO2 /dye/electrolyte interface as shown in Fig. 7. The present study employs EIS as a diagnostic tool for analyzing in particular photovoltaic performance changes detected on dye-sensitized solar cells. An experimental model is presented interpreting the frequency response in terms of the fundamental electronic and ionic processes occurring in the photovoltaic device. A high electron accumulation must be occurred because photogenerated electrons are not extracted at

(a) TiO2, (b) Mn-TiO2

100

(c) Co-TiO2

- Z'' Ohm

80

(a) (b)

60

(c)

40

20

0 0

50

100

150

200

250

300

Z' Ohm Fig. 7. Nyquist diagrams of the impedance spectra of (a) DSSCs based on pure TiO2 , (b) DSSCs based on Mn-TiO2 and (c) DSSCs based on Co-TiO2 .

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