Enhanced photovoltaic properties of dye-sensitized solar cell based on ultrathin 2D TiO2 nanostructures

Enhanced photovoltaic properties of dye-sensitized solar cell based on ultrathin 2D TiO2 nanostructures

Accepted Manuscript Title: Enhanced photovoltaic properties of dye-sensitized solar cell based on ultrathin 2D TiO2 nanostructures Author: Putao Zhang...

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Accepted Manuscript Title: Enhanced photovoltaic properties of dye-sensitized solar cell based on ultrathin 2D TiO2 nanostructures Author: Putao Zhang Zhiqiang Hu Yan Wang Yiying Qin Xiao Wei Sun Wenqin Li Jinmin Wang PII: DOI: Reference:

S0169-4332(16)30157-X http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.010 APSUSC 32528

To appear in:

APSUSC

Received date: Revised date: Accepted date:

10-11-2015 21-1-2016 1-2-2016

Please cite this article as: P. Zhang, Z. Hu, Y. Wang, Y. Qin, X.W. Sun, W. Li, J. Wang, Enhanced photovoltaic properties of dye-sensitized solar cell based on ultrathin 2D TiO2 nanostructures, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced photovoltaic properties of dye-sensitized solar cell based on ultrathin 2D TiO2 nanostructures Putao Zhang1,3, Zhiqiang Hu3, Yan Wang3, Yiying Qin3, Xiao Wei Sun*2, Wenqin Li1,

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Jinmin Wang*1

1. School of Environmental and Materials Engineering, College of Engineering, Shanghai

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Second Polytechnic University, Shanghai 201209, China.

E-mail: [email protected], Tel: +86-21 50217725, Fax: +86-21 50217725.

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2. School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798.

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E-mail: [email protected], Tel: +65-67905369, Fax: +65-67933318. 3. Institute of New Energy Material, Dalian Polytechnic University, Dalian 116034,

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China.

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Abstract:

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Ultrathin two-dimensional (2D) TiO2 nanostructures with a thickness of ~5 nm and a specific surface area of 257.3 m2·g-1 were synthesized by a hydrothermal process.

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The 2D TiO2 nanostructures and P25 nanoparticles were introduced as scattering

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layer and underlayer to construct a bi-layer photoanode in a dye-sensitized solar

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cell (DSSC). The as-prepared DSSC exhibits an enhanced power conversion efficiency (5.14%), which is 23.9% higher than that of pure P25 DSSC (4.15%).

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Electrochemical impedance spectroscopy (EIS) indicates that DSSCs based on P25-2D TiO2 nanostructures show a longer life time and a larger recombination

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resistance. The enhanced photovoltaic properties are attributed to the excellent

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light scattering capability and high capacity for dye adsorption of 2D TiO2

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nanostructures, which makes them a promising candidate as an efficient scattering layer in high-performance DSSCs.

Keywords: 2D, TiO2, dye-sensitized solar cell

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1. Introduction

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Solar energy is a kind of clean and sustainable energy. Along with the decreasing availability of fossil fuels, development and utilization of solar energy is more

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important. Since the groundbreaking report by O’Regan and Grätzel in 1991 [1],

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dye-sensitized solar cells (DSSCs) have received great attention due to their low cost, environmental friendliness and easy fabrication [2-8], which is regarded as

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one of the promising alternatives to traditional silicon-based solar cells. To date,

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high power conversion efficiency of DSSC of over 13% has been achieved [9,10]. Generally, a standard DSSC has three main components: a dye-sensitized

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photoanode, iodide/triiodide (I-/I3-) redox electrolyte, and a counter electrode

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[11,12]. Amongst, photoanode is a key component of DSSC, which determines the

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light absorption ability, charge collection and diffusion efficiency [13]. As known, the heart of photoanode is a wide band-gap semiconductor nanocrystalline film. TiO2 is one of the most promising semiconductor materials, playing a central role in DSSC due to its appropriate electronic band structure, non-toxicity, high chemical inertness and low cost. Various methods have been used to synthesize nanostructured TiO2, such as solvent thermal [14], sol-gel [15] and hydrothermal [16] methods. To improve the light harvesting ability of DSSCs, large-size (100-400 nm) TiO2 nanostructures have been employed as scattering layers due to their effective

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light scattering ability [17], so TiO2 nanoparticles with a particle size of ~200 nm is often used as a scattering layer [18]. Besides, Wu et al.[19] prepared hierarchical

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TiO2 flowers as the scattering layer, which shows an efficient scattering effect. Sun et al.[20] introduced hierarchical fastener-like TiO2 spheres as the scattering layer,

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and Shi et al.[21] prepared hollow TiO2 boxes as the scattering layer. Although the

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above-mentioned TiO2 nanostructures improve the performances of solar cells due

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to the enhanced light scattering ability, these nanostructures usually have low surface area, resulting in a low adsorption of dye molecules [22]. To further

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enhance the photovoltaic properties of DSSCs, both light scattering and dye adsorption should be improved. Hence, new structures shoud be introduced into

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DSSCs. Ultrathin two-dimensional (2D) nanostructures are expected to play thus a

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role because their 2D structure can effectively scatter light and their ultrathin

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structure is favorable for dye adsorption. Recently, 2D nanostructures including graphene [23-25], MoS2 [26-28], WS2

[29] and NaxCoO2·yH2O layers [30], have attracted considerable attention due to their unique structures and properties. Ultrathin 2D nanostructures are different from thick nanosheets, because the size on the thickness dimension is far less than the other two dimensions, which makes them unique physical and chemical properties [31,32]. Herein, we report a photoanode based on ultrathin 2D TiO2 nanostructures with a thickness of ~5 nm, which exhibits an enhanced power conversion efficiency.

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2. Experimental section 2.1. Material synthesis

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All the reagents were analytical grade and without any further purification. 2D TiO2 nanostructures were synthesized by a previous reported hydrothermal method

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[33]. In a typical synthesis, 2.2 mL of titanium isopropoxide (TTIP) was added into

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1.3 mL of concentrated HCl solution under vigorous magnetic stirring at room

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temperature to obtain solution A; 0.4 g of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO20-PP70-PEO20, Pluronic P123) was added into

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8.0 mL of ethanol under ultrasonic stirring for 30 min to obtain solution B. Solution B was added into solution A and stirred for another 30 min. Then, 3.0 mL

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of above solution (A+B) was mixed with 20.0 mL of ethylene glycol (EG) and

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subsequently transferred into a 45-mL Teflon-lined stainless steel autoclave.

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Hydrothermal reaction was conducted at 160 oC for 30 h. Then, the white precipitate was collected by centrifugation, and then washed with ethanol and distilled water for three times, respectively. After drying in an oven at 80 oC for 24 h, white product was obtained for further characterization. 2.2. Preparation of DSSCs

Fluorine-doped tin oxide (FTO) substrates were cleaned by sonication in acetone, deionized water and ethanol for 15 min, respectively. After blowing dry by N2, FTO substrates were immersed into 0.04 mol·L-1 TiCl4 at 70 oC for 30 min and dried at 80 oC for 15 min. Afterwards, the TiCl4-treated FTO substrates were

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calcined at 400 oC for 30 min. P25 paste was prepared by mixing 1.0 g of P25, 5.7 mL of 10wt% ethanol solution of ethyl cellulose, 0.3 mL of acetylacetone, 2 drops

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of octylphenol polyoxyethylene ether (OP-10) emulsifiers, 5.2 mL of ethanol and 4.4 mL of α-terpineol in an agate mortar and then grinding for 1 h. 2D TiO2 paste

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was prepared by the same process. The paste was coated onto the FTO substrate

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using a screen printing method, P25 paste was coated first followed by the 2D

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TiO2 paste coating, P25 paste screen printing two times and 2D TiO2 paste screen printing one time. Then, the substrate was heated at 275, 325 and 375 °C each for 5

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min, 450 °C for 15 min and 500 °C for 30 min in air. After cooling down to 80 °C, a photoanode was obtained. The photoanode was immersed in 0.50 mmol·L-1 N719

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dye for 24 h. The counter electrode was prepared by spin-coating of 0.02 mol·L-1

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H2PtCl6 isopropyl alcohol solution onto FTO glass, and then heating at 400 °C for

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15 min. The photoanode, electrolyte and counter electrode constituted a sandwich-like cell. The electrolyte solution composed of 0.05 mol·L-1 I2, 0.10 mol·L-1 LiI, 0.60 mol·L-1 N-methyl-N-butyl imidazolium iodide (BMII) and 0.50 mol·L-1 4-tert-butylpyridine (TBP) in acetonitrile. The schematic illustration for the preparation of the DSSC is shown in Fig. 1. We have prepared DSSCs using P25, 2D TiO2 and P25-2D TiO2 photoanodes for comparison. It is worth mentioning that the photoanode thicknesses of these three DSSC cells are the same for a fair comparison. This is achieved through repeated screen printings (three

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times). The P25-2D sample was obtained by screen printing P25 two times and 2D TiO2 one time.

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2.3. Characterizations The crystal structure information of the sample was characterized by X-ray

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diffraction (XRD), using Cu Kα (λ=0.15418 nm) radiation with a 2θ range from 20

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to 80° and scanning speed of 8o min-1. The morphology and structure of the sample

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were examined using field-emission scanning electron microscope (FESEM, S-4800, Shimadzu Corporation) and transmission electron microscopy (TEM,

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Tecnai F20, FEI). The specific surface area and pore size distribution of the samples were measured by a Brunauer-Emmett-Teller analyzer (BET, TriStar II

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Plus 2.02, MicroActive). The UV-Vis absorption spectrum was obtained by a

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UV-Vis spectrophotometer (UV-2600, Shimadzu Corporation).

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Photocurrent-voltage (J-V) and electrochemical impedance spectroscopy (EIS)

curves

were

measured

with

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electrochemical

workstation

(Autolab,

PGSTAT302N, Metrohm) under a light source for AM 1.5 radiation (Sun 2000 Solar Simulator, Abet Technologies). 3. Results and discussion

3.1. X-Ray diffraction analysis The XRD pattern of the as-prepared 2D TiO2 nanostructures is shown in Fig. 2. The characteristic diffraction peaks at 2θ=25.3, 37.8, 48.1, 53.5, 55.6 and 62.7° correspond well to (101), (004), (200), (105), (211) and (204) planes of anatase

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TiO2 (JCPDS No. 21-1272). Anatase TiO2 has proven photoelectric properties [20,34]. Hence, we used anatase 2D TiO2 in photoanodes of DSSCs.

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3.2. SEM and TEM images In order to get the fine structure of the sample, FESEM and TEM were used to

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characterize the sample, and the results are shown in Fig. 3. Thin 2D

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nanostructures can be found in the low-magnification FESEM image (Fig. 3a). Many pores exist among the 2D nanostructures, which is conducive to creating a

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large specific surface area. Thus a structure is hopeful to scatter light and adsorb

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more dye molecules, resulting in an ehhanced photoelectrical performance of DSSC. Under high magnification, ultrathin 2D nanostructures can be observed

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more clear (Fig. 3b), the thickness of the 2D TiO2 nanostructures is ~5 nm. Fig. 3c

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shows the TEM image of the 2D TiO2 nanostrutures. Transparent ultrathin 2D

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nanostructures can be observed due to their small thickness. The length of the 2D nanostructures is ~100 nm. We can clearly see that the 2D nanostructures are not flat, however, they have many folds, which shows that the 2D TiO2 nanostrutures are easily to curve. In the HRTEM image (Fig. 3d), some lattice fringes with d-spacing of 0.35 nm can be seen, which corresponds to the (101) planes of anatase TiO2. This is consistent with the XRD result. The selected area electron diffraction (SAED) pattern in the inset of Fig. 3d shows clear diffraction rings, which indicates that the 2D TiO2 nanostrutures are polycrystalline. 3.3. Specific surface area and pore size distribution

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Generally, the large specific surface area and big pore volume are essential factors to enhance the photoelectric conversion performance of DSSCs due to the resulting

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excellent dye adsorption capacity. Meanwhile, big pore volume is advantageous to the electrolyte injection and effective contact between electrolyte and TiO2 film. To

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verify the effect of 2D TiO2 nanostructures on specific surface area and pore size,

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BET surface area and Barrett-Joyner-Halenda (BJH) measurements were

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performed, the nitrogen adsorption-desorption isotherms and pore size distribution curve are shown in Fig. 4. Fig. 4a shows nitrogen adsorption-desorption isotherms,

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which is consistent with the type IV isotherms. With the value of P/P0 varies from 0.7 to 1.0, indicating the mesoporous structure of the sample. The isotherms show

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high adsorption (nearly approaching 1.0) at high relative pressure range, it can be

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inferred that the sample is highly mesoporous and macroporous. We can also see

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from Fig. 4b, the pore diameter centralize distribution is ~36 nm that is very large compared to the thickness of the 2D nanostructures. Furthermore, the specific surface area of the 2D TiO2 nanostructures reaches to 257.3 m2·g-1, which is much higher than that of commercial P25 (~50.0 m2·g-1) [35]. 3.4. Photovoltaic properties of DSSCs Photovoltaic properties of solar cells based on bilayer-type photoanode using 2D TiO2 nanostructures as auxiliary scattering particles and P25 as underlayer were measured. For comparison, solar cells based on pure P25 and pure 2D TiO2 were prepared and measured under the same conditions, the results are shown in Fig. 5.

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The specific values are listed in Tab. 1. The DSSC based on P25-2D TiO2 reveals an open-circuit voltage (Voc) of 0.72 V, shout-circuit current density (Jsc) of 11.34

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mA·cm-2 and overall conversion efficiency (η) of 5.14%. The open-circuit voltages for the three kinds of solar cells are basically identical. P25-2D TiO2 based solar

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cell presents a higher shout-circuit current density of 11.34 mA·cm-2, 23.7% and

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30.8% larger than pure P25 and pure 2D TiO2 based solar cells, respectively. Such

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an increase should be attributed to the strong light scattering ability and superior dye adsorption ability of 2D TiO2 nanostructures [36]. As shown in Fig. 6, for

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P25-2D TiO2 based solar cell (Fig. 6a), light utilization and light propagation path among 2D TiO2 nanostructures can be enhanced. For P25 photoanode based solar

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cell (Fig. 6b), more light may lose due to a higher transmittance, resulting in a

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lower photocurrent. For pure 2D TiO2 photoanode (Fig. 6c) based solar cell,

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affected by strong reflecting and scattering of 2D nanostructures, more light loss occurs at the interface of 2D TiO2 nanostructures and FTO glass. Thus, light cannot get into the inside of TiO2 layer effectively, which is a reason that η of pure 2D TiO2 solar cell is lower than that of pure P25 solar cell even though 2D TiO2 nanostructures have a much larger specific surface area than P25. The EIS Nyquist and Bode plots of the DSSCs based on three different photoanodes are shown in Fig. 7. In Fig. 7a, small semicircles located in high frequency region correspond to charge transfer resistance at Pt/electrolyte (R1) interface, while those located in middle region correspond to charge transfer

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resistance at electrolyte/dye/TiO2 (R2) interfaces [37,38]. In addition, Rs represents the total series resistance of the cell, which includes the sheet resistance of FTO

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substrate and the contact resistance between the FTO substrate and TiO2 film. As shown in Tab. 2, the values of Rs and R1 for the three solar cells are basically

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identical, while there are apparent differences for R2 which are 41.5, 32.6 and 27.7

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Ω for P25-2D TiO2, P25 and 2D TiO2 based solar cells, respectively. The similar

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values of Rs for the three kinds of cells were ascribed to the same FTO substrate and similar direct contact between the FTO substrate and TiO2 film layer, as Rs is

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determined by the sheet resistance of FTO substrate and the contact resistance between FTO substrate and TiO2 film. Likewise, R1 is related to the charge transfer

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in interface between the electrolyte and counter electrode, showing little difference

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due to the same Pt counter electrode and electrolyte. However, P25-2D TiO2 based

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DSSC shows a higher value of R2 than those of P25 and 2D TiO2 based DSSCs, which implies that the electron recombination resistance is increased at TiO2/electrolyte interface. This result may because a densely aggregated dye molecules layer is formed on the surface of 2D TiO2 nanostructures, which could inhibit tri-iodide ions from reaching the surface of 2D TiO2 nanostructures and thereby increase the recombination resistance [39]. From the Bode plots (Fig. 7b), we can get the information of electrons lifetime (τe), the specific value of τe can be estimated from the equation τe = 1/2πfmax, where the fmax is characteristic frequency of the maximum phase shift. As shown in Tab. 2, the τe values for P25-2D TiO2,

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P25 and 2D TiO2 based solar cells are 18.7, 7.2 and 8.2 ms, respectively. The results indicate that it is an effective way to enhance the photovoltaic properties of

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DSSC by introducing 2D TiO2 nanostructures in photoanode as the scattering layer. 3.5. UV-Vis absorption spectra

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The large surface of 2D TiO2 nanostructures may provide abundant sites for dye

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adsorption. In order to verify this, the dye-desorption experiments were conducted.

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The dye molecules were desorbed from the dye-sensitized photoanodes of P25, P25-2D TiO2 and pure 2D TiO2, corresponding UV-Vis absorbance spectra are

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shown in Fig. 8. We can see that the blue absorption curve is low, which shows that P25 photoanode loads less dye molecules than P25-2D TiO2 or pure 2D TiO2

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photoanode. The calculated concentrations of desorbed dye are 9.8×10-8, 1.59×10-7

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and 2.35×10-7 mol·cm-2 for P25, P25-2D TiO2 and pure 2D TiO2 photoanodes,

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respectively. This explains that the introduction of large specific surface area of 2D TiO2 nanostructures can increase the adsorption of dye molecules, which contributes to the improvement of photovoltaic properties of the DSSC based on P25-2D TiO2. However, the photovoltaic properties of the DSSC based on pure 2D TiO2 photoanode is not the best, which can be explained that its light harvesting ability is the worst (see Fig. 6c). The results also indicate that the light harvesting ability is dominant compared with the dye adsorption ability for enhancing the photovoltaic properties of DSSCs containing 2D TiO2 nanostructures. 4. Conclusions

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In summary, we have synthesized ultrathin 2D TiO2 nanostructures via a simple hydrothermal process. Compared with P25 nanoparticles, the as-prepared 2D TiO2

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nanostructures have a thickness of only 5 nm and a larger surface area of 257.3 m2·g-1, exhibiting a superior light scattering effect and better dye-adsorption ability.

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When introduced 2D TiO2 nanostructures as the auxiliary scattering layer, the

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power conversion efficiency is significantly improved to 5.14%, 23.9% higher than

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that of the DSSC based on pure P25 photoanode. The results indicate that the power conversion is significantly enhanced by simply introducing 2D TiO2

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nanostructures into the photoanode of DSSC as the auxiliary scattering layer. Conflict of interest

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Acknowledgments

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The authors declare no competing financial interest.

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We would like to thank support from National Natural Science Foundation of China (NSFC) (No. 61376009, No. 51590902), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. 2013-70), “Shu Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (No. 13SG55), Innovation Program of Shanghai Municipal Education Commission (No. 13ZZ138), Science and Technology Commission of Shanghai Municipality (No. 14YF1410500),

Shanghai

Young

Teacher

Supporting

Foundation

(No.

ZZEGD14011), National High Technology Research and Development Program

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863 (No. 2006AA05Z417), Science and Technology Platform Construction Project

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of Dalian (2010-354).

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Figure captions

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Fig. 1. Schematic illustration for the preparation of P25-2D TiO2 photoanode solar cell. Fig. 2. XRD pattern of the as-prepared 2D TiO2 nanostructures.

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Fig. 3. (a) FESEM, (b) higher-magnification FESEM, (c) TEM and (d) HRTEM images

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of 2D TiO2 nanostructures. Inset in Fig. 3d: SAED pattern of the 2D TiO2 nanostructures.

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Fig. 4. (a) Nitrogen adsorption-desorption isotherms and (b) pore-size distribution curve of 2D TiO2 nanostructures.

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Fig. 5. J-V curves of DSSCs based on P25-2D TiO2, pure P25 and pure 2D TiO2. Fig. 6. Schematic structures of photoanodes based on (a) P25-2D TiO2, (b) pure P25 and

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(c) pure 2D TiO2 showing different scattering effects.

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Fig. 7. (a) Nyquist plot and (b) Bolt plot of DSSCs based on P25-2D TiO2, P25, and 2D

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TiO2 photoanodes. Inset in Fig. 7a shows the equivalent circuit model. Fig. 8. Absorption spectra of dye molecules desorbed from the dye-sensitized photoanodes of P25, P25-2D TiO2 and 2D TiO2.

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Tables

Jsc (mA·cm-2)

FF

P25-2D TiO2

0.72

11.34

0.63

P25

0.73

9.17

2D TiO2

0.72

8.67

η (%)

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Voc (V)

5.14

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DSSC type

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Tab. 1. Effect of different photoanodes on the photovoltaic properties of the DSSCs.

4.15

0.63

3.93

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0.62

Rs (Ω)

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DSSC type

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Tab. 2. EIS parameters of the DSSCs determined by fitting the experimental data to the equivalent circuit model (see inset of Fig. 7a).

R1 (Ω)

R2 (Ω)

fmax (Hz)

τe (ms)

P25-2D TiO2

36.1

4.8

41.5

8.5

18.7

P25

36.6

3.8

32.6

22.0

7.2

2D TiO2

38.2

4.7

27.7

19.5

8.2

Highlights

1. Two-dimensional (2D) TiO2 nanostructures were synthesized. 2. 2D TiO2 nanostructures were used as scattering layer in DSSC. 3. Superior light scattering effect and dye-adsorption ability are achieved. 4. The as-prepared DSSC shows enhanced photovoltaic properties.

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Graphical Abstract Ultrathin 2D TiO2 nanostructures with a thickness of ~5 nm and a specific surface

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area of 257.3 m2·g-1 were synthesized. The 2D TiO2 nanostructures and P25 nanoparticles were introduced as scattering layer and underlayer to construct a

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bi-layer photoanode in a dye-sensitized solar cell, which exhibits an enhanced

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power conversion efficiency.

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