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Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran. We present a new TiO2 morphology, featuring ...

J. Am. Ceram. Soc., 97 [9] 2873–2879 (2014) DOI: 10.1111/jace.13043 © 2014 The American Ceramic Society

Journal

Improved Efficiency of Dye-Sensitized Solar Cells Based on a Single Layer Deposition of Skein-Like TiO2 Nanotubes Majdoddin Mojaddami, Mohammad Reza Mohammadi,† and Hamid Reza Madaah Hosseini Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran high surface area and open structure would be sufficient to obtain a highly efficient photoactive layer. A photoanode based on nanosized TiO2 crystals with various geometries such as nanoparticles,6,12 ordered mesostructured materials,13,14 and one-dimensional structured materials, for example, nanorods, nanowires, and nanotubes (NTs),15,16 have been extensively explored. Among different morphologies of TiO2, one-dimensional nanostructures such as nanorods,17 nanofibers,18 nanowires,19 corn-like nanowires,20 and NTs21 are attractive due to their facile electron transport22 and enhanced visible light scattering and absorption.23 It is of interest to note that femtosecond–nanosecond studies demonstrated that the electron injection from the dye to TiO2 is faster for NTs than that of nanorods and nanoparticles.24,25 Generally, anodic oxidation26 and hydrothermal methods27 are employed to prepare TiO2 NTs for DSSC applications. The main advantage of hydrothermal method rests in its low-cost process. A challenge in applying randomly oriented TiO2 NTs in DSSCs is their agglomeration which results in poor dye adsorption. Many efforts have been aimed to enhance the active surface area of NTs, using different paste compositions and deposition methods. Xu et al. 27 prepared hydrothermally grown TiO2 NTs for DSSC application using commercial P25 powder. Although the NTs had high surface area of 316 m2/g, the cell showed poor efficiency of 0.37% due to low adsorption of dye molecules, as a result of agglomeration of NTs during deposition process. Tacchini et al.24 synthesized TiO2 NTs using commercial anatase-TiO2 powder by hydrothermal process. The cell had efficiency of 3.12%. An efficiency of 4.9% was reported for a DSSC made of hydrothermally grown TiO2 NTs using threefold layer deposition procedure.28 Furthermore, the highest efficiency of 7.6% was reported by Akilavasan et al.29 using hydrothermally grown TiO2 NTs. The NTs were firstly deposited on FTO substrate by electrophoretic method and then the electrode was treated with an ethanol solution of TiCl4. However, the cell without TiCl4 treatment had efficiency of 1.7%. We propose a concept of new arrangement of NTs as an alternative approach to enhance their light scattering, dye sensitization, and electron transport. The new arrangement of NTs involves sets of intertwined NTs with open structure, so called skein-like NTs. Such morphology is produced by arrangement modification mechanism via a straightforward hydrothermal process. This construct is used as the transparent and scattering layers simultaneously in DSSCs to improve dye sensitization, electron transport rate, and consequently the power conversion efficiency of DSSC. Photovoltaic characteristics of the solar cell were studied.

We present a new TiO2 morphology, featuring high surface area and open structure, synthesized by a two-step chemical route for the manufacture of dye-sensitized solar cells (DSSCs). This construct is sets of intertwined one-dimensional (1D) nanostructures (i.e., nanotubes), so-called skein-like nanotubes (NTs). Such morphology is produced by a combination of TiC oxidation and hydrothermal processes. The mesoporous TiO2 nanoparticles, as the product of TiC oxidation operation, is used as the precursor of hydrothermal process to grow the skein-like NTs. The effect of processing parameters of TiC oxidation and hydrothermal processes is studied. The skein-like morphology enables to eliminate the conventional three or fourfold layer deposition process by a single layer deposition of TiO2 NTs. The novel TiO2 morphology enhances photon capture of fabricated DSSC by exerting a triple function mechanism including improvement of light scattering, dye sensitization, and electron transport. The presented strategy demonstrates the feasibility of the new concept for improvement of cell efficiency by effective light management.

I.

Introduction

O

VER the past several years, TiO2 has become a focus of considerable researches as it possesses unique properties in different applications such as gas sensors,1 photo catalysts for water purification2 electrochromic displays3 self-cleaning coatings of windows and tiles4 and dye-sensitized solar cells (DSSCs).5 The main stream of the research on DSSCs has been focusing on development of materials which would enhance the conversion efficiency, simplify the production of the cell and assure their long-lifetime. To realize high-efficiency DSSCs, a high surface area of the nanostructured TiO2 layer is essential. In addition to the high surface area of the TiO2 layer, good connections between TiO2 grains as well as a good adhesion to the transparent conductive oxide coated glass are required to diminish the reactions of photogenerated electrons with tri-iodide species in electrolyte and to assure good electrical conductivity.6–8 Therefore, the optimization of the morphology of TiO2 layer according to the demands mentioned above is a prerequisite for the realization of high-efficiency DSSCs. At present, efficiencies of more than 11% can be obtained for DSSCs using electrolytes based on volatile organic solvents9 and a three or fourfold layer deposition of TiO2.10,11 The preparation procedure of fourfold TiO2 layer is complex, as different TiO2 precursors as well as different deposition techniques are required. Therefore, our aim is to simplify the preparation of TiO2 layer in such a way that solely a deposition of a single TiO2 film with

X.-D. Zhou—contributing editor

II.

Experimental Procedure

(1) Preparation of Skein-Like TiO2 NTs TiO2 NTs were grown by a two-step chemical route according to an analogous procedure reported in the literature.30 TiO2 nanoparticles were firstly synthesized by a reaction

Manuscript No. 34677. Received March 13, 2014; approved May 13, 2014. † Author to whom correspondence should be addressed. e-mail: [email protected]

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Journal of the American Ceramic Society—Mojaddami et al. Table I.

Preparation Condition of Synthesized TiO2 Nanotubes (NTs) by a Two-Step Method TiO2 nanoparticles by TiC oxidation

Sample

P-70-300 P-70-400 P-70-500 P-110-400 P*-110-400 P-130-200 P-70-400 P-130-200

TiO2 NTs by hydrothermal process

Oxidation Temperature (°C)

Heat-treatment temperature (°C)

Sample

Temperature (°C)

Time (h)

Reactor

70 70 70 110 110 130 70 130

300 400 500 400 400 200 400 200

A-70-300 A-70-400 A-70-500 A-110-400 A*-110-400 A-130-200 F-70-400 F-130-200

130 130 130 130 130 130 130 130

12 12 12 12 6 12 12 12

Autoclave Autoclave Autoclave Autoclave Autoclave Autoclave Flask Flask

(a)

between 0.7 g TiC (97%; Alfa Aesar, Lancashire, UK) and 8 mL HNO3 (65%; Merck, Darmstadt, Germany) at different temperatures (i.e., 70°C, 110°C, or 130°C) for 1 h under mild stirring. The collected precipitates were filtered, washed with distilled water and ethanol several times and finally heat treated at various temperatures for 30 min. In the second step, skein-like TiO2 NTs were grown by hydrothermal process under two different conditions (i.e., using Teflon-lined autoclave and balloon-shaped flask as the reactors). 0.4 g mesoporous TiO2 nanoparticles and 34 mL KOH solution (10M) (90%; Merck) were firstly mixed for 5 min under vigorous stirring and transferred into the reactor. The autoclave was heated at 130°C, whereas the balloon-shaped flask was sealed and heated in a hot oil bath at 130°C. The obtained powder was acid treated with H2SO4 (95%; Merck) under

(b)

Fig. 2. XRD patterns of as-synthesized mesoporous TiO2 nanoparticles by TiC oxidation with HNO3 at 70°C, 110°C, and 130°C.

(c)

Fig. 1. Field-emission scanning electron microscopy images of assynthesized mesoporous TiO2 nanoparticles by TiC oxidation with HNO3 at: (a) 70°C, (b) 110°C, and (c) 130°C.

Fig. 3. XRD patterns of heat-treated mesoporous TiO2 nanoparticles by TiC oxidation with HNO3: (a) P-70-300 (b) P-70-400 (C) P-70-500, (d) P-110-400, and (e) P-130-200.

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mild stirring for 30 min, washed with distilled water several times and finally dried at room-temperature naturally. Table I summarizes the preparation condition of synthesized TiO2 NTs.

(2) DSSCs Assemblage TiO2 pastes were prepared based on our previous work.31 A monolayer of TiO2 film was coated on an FTO substrate (7 Ω/sq) by spin coating technique. The deposited films, as

photoanode electrode, were sintered at 400°C for 2 h in air atmosphere. The TiO2 photoanodes were further dipped in a solution of 0.3 mM ruthenium (II) dye (Ruthenium 535bisTBA dye, or N719; Sigma Aldrich, Gillingham, UK) in dry ethanol for 15 h. The counter Pt-electrode was deposited on the FTO glass by coating with a drop of H2PtCl6 solution with repetition of the heat treatment at 400°C for 30 min. The dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket of 60 lm thickness made of

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4. Field-emission scanning electron microscopy images of skein-like TiO2 nanotubes: (a) A-70-300, (b) A-70-400, (c) A-70-500, (d) A-110400, (e) A*-110-400, and (f) A-130-200.

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Journal of the American Ceramic Society—Mojaddami et al.

Vol. 97, No. 9

N2 Adsorption-Desorption Characteristics of Mesoporous TiO2 Nanoparticles and A-70-400 Skein-Like TiO2 Nanotubes (NTs)

Sample

DP (nm)

rp,peak (nm)

VTot (cm3/g)

SBET (m2/g)

Mesoporous TiO2 nanoparticles Skein-like TiO2 NTs (A-70-400)

5.2 7.4

1.7 1.9

0.10 0.41

79.7 223.8

DP , mean pore diameter; rp,peak, mean pore radius calculated from BJH method; SBET, BET specific surface area; VTot., total pore volume calculated by BET method (P/P0 = 0.98).

the Surlyn Meltonix 1170 (Solaronix, Aubonne, Switzerland). The redox electrolyte, composed of 0.6M dimethylpropylimidazolium iodide, 0.1M LiI, 0.05M I2, and 0.5M 4-tertbutylpyridine in acetonitrile, was injected into the cell.

(3) Characterization and Measurements TiO2 nanoparticles and NTs were characterized by X-ray diffraction technique (XRD) using a STOE STADI P,  CuKa (k = 1.54060 A), field-emission scanning electron microscopy (FESEM) using a MIRA II TESCAN, Kohoutovice, Czech Republic, transmission electron microscopy (TEM) using a 100 kV Philips EM 208 (Eindhoven, the Netherlands) and Brunauer–Emmett–Teller equation (BET) and Barrette–Joynere–Helenda (BJH) measurements at 77 K using a Belsorp mini II analyzer (Cera Laboratory Technology, Tuggerah, Australia). Prior to BET measurement, powders were degassed for 10 h at 120°C with pressure of 0.1 Pa. The amount of adsorbed dye was determined by a spectroscopic method through measuring the concentration of desorbed dye on the TiO2 surface into solution of 0.1M NaOH using a JENWAY (Staffordshire, UK) 6705 UV–Vis Spectrophotometer. Photovoltaic measurements were performed under 1 Sun AM 1.5G using Zahner Cimps pcs solar simulator (Zahner, Kronach, Germany). III.

Results and Discussion

(1) Mesoporous TiO2 Nanoparticles Field-emission scanning electron microscopy images of mesoporous TiO2 nanoparticles prepared by TiC oxidation at 70°C, 110°C and 130°C are shown in Fig. 1. All samples had particle size in the range 20–40 nm. As mentioned in Section II(1), TiO2 nanoparticles were synthesized according to an analogous procedure reported by Shieh et al.30 However, they obtained TiO2 nanoparticles with particle size around 100 nm. Therefore, TiO2 nanoparticles with smaller particle size were achieved in this work. This can be related to stirring of the reaction solution, resulting in prevention of particle growth and agglomeration. Figure 2 shows XRD patterns of prepared TiO2 nanoparticles. The sample synthesized at 110°C and 130°C showed crystalline structure, whereas that of prepared at 70°C had almost amorphous structure. Moreover, no unreacted TiC compound and undesired phases were detected for all products. As can be seen in Fig. 3, the crystallinity of the samples was increased by a subsequent heat treatment. After heat treatment, the samples firstly syn-

Table III. Sample

A-70-300 A-70-400 A-70-500 A*-110-400 A-130-200

Fig. 5. Nitrogen adsorption–desorption isotherms and pore size distribution curves of mesoporous TiO2 nanoparticles and A-70-400 skein-like TiO2 nanotubes.

thesized by TiC oxidation at 70°C and 110°C had pure anatase structure, whereas that of firstly prepared at 130°C was partially transformed into rutile phase. It has been reported that anatase and rutile are generally regarded to be photocatalytic, whereas brookite is not.32

(2) TiO2 NTs Grown by Hydrothermal Process in Autoclave Field-emission scanning electron microscopy images of TiO2 NTs grown by hydrothermal process in autoclave are shown in Fig. 4. A-70-300 was a dense mixture of zero and onedimensional (1D) TiO2 nanostructures. This can be related to low crystallinity of synthesized nanoparticles. It is known that the more crystalline precursor, the more 1D growth in hydrothermal process.33,34 As P-70-400 and P-70-500 nanoparticles had crystalline structure, the resulted products after heat treatment (A-70-400 and A-70-500, respectively) showed 1D structure. Low magnification images of these products [Figs. 4(b) and (c)] demonstrate that the structure is a skeinlike morphology of NTs. The formation mechanism of this structure is slightly different from that of conventional hydrothermal process. It is known that chemical reaction between a solid and a liquid is solely accomplished on the surface of the solid. Therefore, surface area and surface features of the solid have crucial contribution in the reaction process and obtained product. In conventional hydrothermal process in which nano- or micrometer-size TiO2 powders are used as precursor, NTs formation takes place near the sur-

Photovoltaic Characteristics of Fabricated Dye-Sensitized Solar Cells Using Hydrothermally Grown Skein-Like TiO2 Nanotubes in Autoclave PCE (%)

2.10 5.53 3.14 4.31 4.36

    

0.062 0.075 0.244 0.109 0.230

Jsc (mA/cm2)

Voc (V)

FF

4.75 12.87 7.18 9.94 9.56

0.72 0.70 0.71 0.71 0.74

0.62 0.61 0.61 0.61 0.62

Adsorbed dye (10

6.70 6.26 7.51 4.30 4.50

8

mol/cm2)

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Fig. 6. Photocurrent density–voltage curves of fabricated dyesensitized solar cells using hydrothermally grown skein-like TiO2 nanotubes in autoclave.

face of particles resulted in agglomerated NTs. When mesoporous TiO2 nanoparticles are employed as the precursor of hydrothermal process, formation of NTs can initiates inside the pores. Therefore, the mesoporous structure can act as a scaffold for prevention of agglomeration of NTs. Such scaffold not only allows NTs formation, but also prevents their agglomeration. Consequently, the skein-like TiO2 NTs with high active surface area and open structure is obtained. As expected, A-70-400 sample had higher open structure than A-70-500 due to higher temperature of the latter sample. Therefore, heat treatment at high temperature on the one hand resulted in more crystalline material, however, on the other hand destroyed the mesoporous structure and decreased the surface area. Such destruction of mesoporous structure obstacles the so called scaffold role during hydrothermal process. The hydrothermally grown TiO2 NTs using mesoporous nanoparticle synthesized by TiC oxidation with HNO3 at 110°C followed by heat treatment at 400°C (i.e., A-110-400) showed the highest open structure of skein-like TiO2 NTs with small diameter in the range 15–25 nm [Fig. 4(d)]. This can be related to crystalline structure of the precursor of hydrothermal process (i.e., P-110-400), resulted in formation of TiO2 NTs with small diameter and open structure. The Nitrogen adsorption–desorption isotherms of mesoporous TiO2 nanoparticles and A-70-400 skein-like TiO2 NTs are shown in Fig. 5. Moreover, N2 adsorption–desorption characteristics of the powders is summarized in Table II. The surface area of the samples was 79.7 and 223.8 m2/g, respectively. The isotherm corresponding to both samples represents a combination of types II and IV corresponds to mesoporous materials. The pore size distribution curves (inset of Fig. 5) verify mesorporous structure of samples. The mesoporous TiO2 nanoparticles have an upper size restriction as the adsorption and desorption branches are closed in the pressure region near saturation.35 Such structure is predicted to easily adsorb dye molecules for DSSC applications regardless of deposition method. In addition, this structure can improve the scattering effect and, therefore, photon management of the cell due to micrometer size of the skeins-like NTs. As XRD pattern of as-synthesized TiO2 nanoparticles by TiC oxidation at 130°C showed a crystalline structure (see Fig. 2), it was heat treated at lower temperature of 200°C to preserve the mesoporous structure. The resultant hydrothermally grown TiO2 NTs (i.e., A-130-200) showed the skeinlike morphology with broad diameter size distribution in the range 20–50 nm [Fig. 4(f)]. The broad diameter size distribution can be explained by presence of rutile phase in the precursor of hydrothermal process (see Fig. 3). Because formation of TiO2 NTs from rutile phase is slower than that

(a)

(b) Fig. 7. (a) Field-emission scanning electron microscopy (FESEM) images of F-70-400 skein-like nanotubes (NTs) and (b) FESEM and TEM images of F-130-200 skein-like NTs.

of anatase structure.33 Consequently, the optimum oxidation and heat-treatment temperatures for production of skein-like TiO2 NTs with high open structure and narrow diameter size distribution are 110°C and 400°C, respectively (i.e., A-110400). To optimize the reaction time of hydrothermal process, one sample was also grown similar to the processing parameters of A-110-400 sample except that the reaction time was shortened down to 6 h (i.e., A*-110-400), as shown in Fig. 4(e). No significant change was observed in morphology of A-110-400 and A*-110-400 samples. Therefore, the reaction time of 6 h was determined as the optimum hydrother-

Fig. 8. Photocurrent density–voltage curves of fabricated dyesensitized solar cells using hydrothermally grown skein-like TiO2 nanotubes in glass flask.

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F-70-400 F-130-200

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Journal of the American Ceramic Society—Mojaddami et al.

Photovoltaic Characteristics of Fabricated Dye-Sensitized Solar Cells Using Hydrothermally Grown Skein-Like TiO2 Nanotubes in Glass Flask PCE (%)

Jsc (mA/cm2)

Voc (V)

FF

4.06  0.113 4.61  0.127

9.15 10.59

0.72 0.71

0.62 0.61

mal time for formation of skein-like TiO2 NTs with the highest open structure and narrow diameter size distribution. The photocurrent density–voltage (J–V) characteristics of fabricated DSSCs using the skein-like TiO2 NTs were illustrated in Fig. 6. In addition, the corresponding photovoltaic parameters such as short circuit current (Jsc), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (g) were summarized in Table III. As expected, all fabricated cells had almost the same Voc as this parameter depends on the Fermi level of the semiconductor.36 The Fermi level of all synthesized NTs is almost the same. A-70400 cell showed the highest PCE of 5.53% amongst all fabricated solar cells. This is one of the highest PCE reported in the literature for a monolayer DSSC made of 1D nanostructures without treatment with TiCl4 solution. However, this cell did not have the maximum amount of adsorbed dye. Such phenomenon can be related to the position of adsorbed dye molecules. In an open structure of skein-like NTs dye molecules adsorbed inside the pores, whereas for a dense structure of skein-like NTs dye molecules adsorbed on the surface of TiO2 NTs. In the latter case a layer of dye molecules, in the form of aggregated dyes, covers TiO2 NTs.37 Therefore, the excited electrons have to diffuse a long distance in the layer of dye molecules before injection into the NTs, resulting in increasing the probability of recombination phenomenon. Although A-70-300 and A-70-500 cells adsorbed high amount of dye molecules, due to dense structure of skein-like NTs, they had lower PCE than A-70-400 cell as a result of recombination process. As mentioned earlier, the porosity and open structure of A-130-200 cell was lower than those of A-70-400 and, therefore, the amount of adsorbed dye for the former cell was also lower than the latter cell. Therefore, A-130-200 cell had lower PCE than A-70-400 solar cell. It should be noted that A*-110-400 cell which had the highest open structure did not show the highest cell efficiency. This can be explained due to the presence of large porosities. The large porosities have three effects on electron transfer and recombination. First, they can increase diffusion length of electrons are passing through the TiO2 layer. Second, they act as electron trapping. Third, they can decrease the number of electron diffusion paths (i.e., NTs) toward FTO glass. Therefore, the efficiency of A*-110-400 cell decreased down to 4.30%.

Adsorbed dye (10

8

mol/cm2)

13.5 6.24

The photocurrent density–voltage (J–V) characteristics of F-70-400 and F-130-200 DSSCs were illustrated in Fig. 8. In addition, the corresponding photovoltaic parameters were summarized in Table IV. Both cells had almost the same Voc, as the Fermi level of synthesized NTs was almost the same. Dye adsorption of F-70-400 was much higher than all other samples. This is due to dense structure of skein-like NTs in which a layer of dye molecules covers the surface of TiO2 NTs. As explained earlier, such layer increased recombination and, therefore, decreased the cell efficiency. Although F-130-200 cell had the same dye adsorption as A-70-400 cell its PCE was lower than the latter solar cell. This can be related to difference of their morphology; controlling the electron transport. Comparing Figs. 7(b) and 4(b), it is evident that the diameter of F-130-200 NTs was around 35 nm, whereas it was around 20 nm for A-70-400 NTs. Therefore, the electron transport along NTs with thinner diameter is faster than that of NTs with thicker diameter. The more defects can be found in NTs with thicker diameter from the statistically point of view.

IV.

Conclusions

We present a monolayer DSSC containing new morphology of TiO2 NTs (i.e., skein-like NTs) as a transparent and scattering layer simultaneously. The skein-like NTs were grown by a two-step chemical route, including TiC oxidation and hydrothermal process. The mesoporous TiO2 nanoparticles, synthesized by TiC oxidation, were used as the precursor of hydrothermal method and the processing parameters were controlled to obtain the skein-like NTs. This construct enables effective photon capture by a triple function mechanism including significant increase in light scattering, dye sensitization, and electron transport. Moreover, such morphology can simplify the preparation of TiO2 solar cells in such a way that solely a deposition of a single film composed of skein-like NTs improved their photovoltaic parameters. The fabricated cell made of A-70-400 skein-like NTs showed the highest PCE of 5.53% and JSC of 12.87 mA/cm2 among all cells. The approach for introduction of skein-like NTs can open up new insight for effective light management in DSSC applications.

Acknowledgment (3) TiO2 NTs Grown by Hydrothermal Process in LowPressure Glass Flask To assess the feasibility for production of skein-like TiO2 NTs by a straightforward method using a low-pressure glass flask, two samples were synthesized under same condition as A-70-400 and A-130-200 samples, namely F-70-400 and F-130-200, respectively. Field-emission scanning electron microscopy images of F70-400 and F-130-200 samples are illustrated in Fig. 7. 1D nanostructures in the form of skein-like TiO2 NTs were fully grown. Furthermore, TEM image of F-130-200 [inset of Fig. 7(b)] verified that TiO2 NTs rather than nanowires were formed. The porosity and open structure of skein-like NTs grown in the flask is lower than those synthesized in the autoclave. This can be related to the pressure of the reactor. It can be concluded that skein-like TiO2 NTs with high open structure can be grown by hydrothermal process under high pressure.

M. Mojaddami and M.R. Mohammadi wish to thank the financial support from Iran National Science Foundation (INSF).

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