Hierarchical TiO2 Submicrorods Improve the Photovoltaic

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Hierarchical TiO2 submicrorods improving photovoltaic performance of dye-sensitized solar cells Daipeng Guo, Shengqiang Xiao, Ke Fan, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01671 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Sustainable Chemistry & Engineering

Hierarchical TiO2 submicrorods improving photovoltaic performance of dye-sensitized solar cells

Daipeng Guo, † Shengqiang Xiao, † Ke Fan, *,† and Jiaguo Yu*,†

†

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan, 430070, P. R. China Fax: 0086-27-87879468, Tel: 0086-27-87871029 K. Fan. E-mail: [email protected]; J. Yu. E-mail: [email protected]

1

ACS Paragon Plus Environment


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Abstract Hierarchical TiO2 submicrorods (HTRs) assembled by tiny nanoparticles and nanorods were synthesized using titanate glycolate rod as a self-template through a facile hydrothermal method. The as-prepared hierarchical TiO2 submicrorods possessed a higher surface area (103 m2 g-1) than P25 nanoparticles (NPs) (55 m2 g-1). Composite photoanodes in dye-sensitized solar cell (DSSC) were prepared by integrating the prepared HTRs and P25 NPs, and the photovoltaic conversion efficiency of 8.09% was obtained, obviously higher than that (5.37%) of the pristine P25 NPs-based photoanode. The incorporated hierarchical TiO2 submicrorods in the hybrid photoanode film showed three functions in enhancing photovoltaic performance of DSSCs: (i) increasing the specific surface area for effectively adsorbing dye molecules, (ii) enhancing the light harvesting efficiency and (iii) accelerating electron transport rate of the films. This work highlighted the importance of the structure tuning of photoanode and exhibited an efficient strategy for enhanced energy conversion of DSSCs. Keywords: TiO2, Hierarchical submicrorods, Composite photoanode, Dye-sensitized solar cells, Photovoltaic performance.

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1. Introduction Dye-sensitized solar cell (DSSC), as one of the candidates to substitute silicon-based solar cells, is very promising in the third-generation photovoltaic devices due to its easy fabrication, low cost, stability and relatively high energy conversion efficiency.1 Normally, a DSSC contains three main components: a dye-adsorbed nanocrystalline titanium oxide photoanode, I-/I3- electrolyte solution, and platinized (Pt) counter electrode.2,3 The principle of DSSC can be expressed as follows: after light absorption, the dye molecules generate photo-induced electrons, and then these electrons are injected into the conduction band (CB) of TiO2 and are collected by fluorine-doped tin oxide (FTO) glass, afterwards the electrons move through external circuit to Pt counter electrode. The I- at electrolyte solution will be transformed to I3- to regenerate the excited dye sensitizers, while I3- ions can be reduced to I- by the electrons at Pt counter electrode.4,5 Typically, TiO2 nanoparticles (NPs), which possess a high specific surface area for adsorbing dye molecules, are widely applied in the photoanodes of DSSCs. However, the visible light scattering of such photoanodes based on TiO2 NPs is usually weak due to the small particle size (10-50 nm), a significant part of input light directly transmits through the TiO2 NPs film without being harvested and utilized.6 Additionally, the electron traps existing in grain boundaries between nanosized particles lead to the low electron transfer rate and serious charge recombination loss in the film of TiO2 NPs, which limit the efficiency of DSSCs.7,8 To enhance the efficiency of DSSCs, most of the state-of-the-art photoanodes are concentrated on 3



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developing various advanced electrode materials with two or multi-advantages such as outstanding light-scattering effect, large amount of adsorbed dye, porous structures and/or fast electrons transfer capability.9-19 Among these materials for photoanodes, one-dimensional

(1D)

nanomaterials,

e.g.,

nanowires,20,21

nanorods,22,23

nanofibers,24,25 and nanotubes26,27 become more and more popular in DSSCs due to their enhanced light scattering and fast electrons transfer capability. For instance, Wang et al.23 prepared a DSSC using the TiO2 nanorods as overlayer of photoanodes, which showed strong light scattering ability and supplied a fast and lengthened charge carrier transport channel, resulting in dramatically enhanced photovoltaic conversion efficiency. However, the 1D nanostructured materials (mainly nanowires, nanorods) usually have small specific surface areas which lead to insufficient dye loading compared to nanoparticles.28-30 For example, Bai et al.28 introduced ZnO nanowires into the TiO2 nanoparticles to improve the photovoltaic conversion efficiency, but resulting in decreased specific surface area and less dye loading because the size of ZnO nanowires is much larger. Up to now, the preservation of 1D structure without sacrificing the surface area for photoanode of DSSCs is still challenging. Fortunately, materials with hierarchical structure, in which building blocks, e.g., nanoparticles (0D), nanofibers, nanorods, nanowires and nanotubes (1D), nanosheets (2D) self-assemble into highly ordered structures with various morphologies, have attracted large attention in DSSCs because of the porosity and large specific surface areas.31-33 The high surface areas are beneficial for adsorbing dye molecules and the porous structure provides an accessible diffusion pathway for electrolyte. Li et al.31 prepared 4


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a three-dimensional hierarchical TiO2 microsphere structure assembled by numerous nanorods and found that DSSCs based on this hierarchical microstructure showed greatly enhanced photovoltaic conversion efficiency due to a higher specific surface area for effectively adsorbing dyes and enhanced light scattering. Based on the above considerations, we successfully prepared a novel 1D anatase hierarchical TiO2 submicrorod (HTR) with a high specific surface area (103 m2 g-1) through a facile hydrothermal method. The HTRs assembled by numerous tiny nanorods and nanoparticles were mixed with P25 NPs to fabricate composite photoanodes to enhance the photovoltaic conversion efficiency of DSSCs. Because of the increased adsorbed dye amount, enhanced light harvesting and facilitated charge transfer derived from the incorporated HTRs, the composite photoanode containing 20 wt% of HTRs showed the highest photovoltaic performance of 8.09% conversion efficiency, which was far superior to the pristine P25 NPs-based photoanode. Our work was proven to be an efficient and promising approach for improving the photovoltaic efficiency of DSSCs.

2. Experimental section 2.1. Materials Tetrabutyl titanate (TBT, 98%, analytical reagent), ethylene glycol, sodium hydroxide (NaOH, analytical reagent), ethanol and poly (ethylene glycol) (PEG 2000) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ruthenium dye (N719) was bought from Solaronix. Lithium iodide (LiI, 99.99%), iodine (I2, 99.99%), and 5



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tert-butylpyridine were got from Sigma-Aldrich. 1-propyl-3-methylimidazolium iodide was obtained from Suzhou Zhongsheng Chemical Co., Ltd., and anhydrous acetonitrile was came from Shanghai Chemical Reagent Factory of China. P25 is commercial-grade TiO2 powder (Degussa AG, 80% anatase and 20% rutile). 2.2. Synthesis of HTR The HTR was prepared based on our previous work.34 Typically, TBT (2 mL) and ethylene glycol (200 mL) were heated at 150 oC in an oil bath for 2 h. After washed with distilled water several times, the precipitates were dried at 80 oC for 6 h. Subsequently, 0.2 g of the dried titanium glycolate precursor was put into 40 mL of 0.1 M NaOH aqueous solution, and then the stirred solution was moved to a 50 mL Teflon-lined autoclave. The autoclave was sealed and kept at 180 oC for 24 h. The resulted white precipitates were centrifuged and washed with 0.1 M HCl aqueous solution and distilled water until pH = 7. Finally, the precipitates were heated at 80 °C for 6 h and calcined at 450 °C for 1 h for further characterization. 2.3. Assembly of NP/HTR composite film DSSCs The Nippon FTO glass (14-22 Ω/square, Japan) was washed ultrasonically with surfactant, ethanol, and acetone to obtain a clean surface. Five pastes containing different HTR contents were obtained by grinding the mixture of ethanol, PEG, and mixed commercial P25 NP and HTR for ca. 40 min. Through the doctor blade method, the pastes were then deposited on FTO glass to form the NP/HTR composite films. After that, the films were calcined at 450 oC for 30 min. The as-obtained NP/HTR composite films were denoted as P100R0, P90R10, P80R20, P50R50 and P0R100, 6


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based on the ration of HTRs (0, 10, 20, 50 and 100 wt%) in the mixture. Dye sensitization was done by dipping the composite films in 0.3 mM N719 ethanol solution at 50 oC for 12 h. The excess dye molecules of the dye-sensitized composite photoanode were then removed by anhydrous ethanol. Finally, the open solar cell was prepared by placing a Pt-coated FTO glass on the sensitized photoanode separated by a ca. 50 µm thick polymer spacer and the active area of the cell was 0.14 cm2. The electrolyte was then injected into the open cell, and the photovoltaic performance was immediately carried out. 2.4. Characterization The morphologies of the as-prepared HTRs and NP/HTR composite film were observed using a field emission scanning electron microscopy (FESEM, JEOL, JSM-7500) and a JSM-5610LV SEM. The transmission electron microscopy (TEM) images were observed through a JEM-2100F microscope. X-ray diffraction (XRD) results were gotten from a Rigaku D/Max-RB X-ray diffractometer with Cu Kα irradiation. Nitrogen isotherms were done through Brunauer-Emmett-Teller (BET) measurements from a Micromeritics ASAP 2020 nitrogen adsorption apparatus. UV-vis diffuse reflectance experiments were performed by a Shimadzu UV-2600 spectrophotometer. To determine the adsorbed dye loading on photoanodes, the dye-sensitized photoanodes were immersed into a 0.1 M NaOH water/ethanol solution (v/v=1:1) at room temperature for 1 h, then, the absorbance A was obtained by a Shimadzu UVmini-1240 spectrophotometer. The obtained solution can be analyzed by UV-Vis spectroscopy to determine the dye concentration c using beer’s Law (A = εcl); 7



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where A is the absorbance at the wavelength of 515 nm, l is the length of the cell and ε is the N719 absorptivity (1.41 × 104 dm3 mol−1 cm−1).35 The current-voltage (I-V) curves and the electrochemical impedance spectroscopy (EIS) with 10 mV AC amplitude were all measured on a CHI660C electrochemical workstation. All the cells were performed under a 100 mW cm-2 with a Newport 91160 solar simulator. The incident photoelectric conversion efficiency (IPCE) data were obtained using a 300 W Xe lamp (Newport, model no.6258) fitted with a monochromator (Irvine, CA, USA).

3. Results and discussion 3.1 Phase structure and morphology The crystalline features of the obtained samples were determined by XRD (shown in Figure 1). As noted, all the peaks in the XRD pattern of the pure HTR powders (Figure 1a) can be assigned to anatase TiO2 phase (JCPDS no. 21-1272) and no impurities can be detected. Figure 1b and c show the XRD patterns of P80R20 and P100R0 films on FTO glass respectively, no evident difference of XRD presents between these two films except that the intensity of the (110) peak at 2θ = 27.4 o (which belongs to the rutile TiO2 phase in P25, JCPDS No. 21-1276) of P80R20 is slightly weaker comparing to that of P100R0. This can be ascribed to the relatively lower content of P25 in P80R20 than that in P100R0, leading to a fewer content of rutile TiO2 phase in P80R20 film.

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Figure 1. XRD patterns of pure HTR powders (a), P80R20 film (b), P100R0 film (c) and FTO glass substrate (d).

The SEM image of the as-prepared HTR is shown in Figure 2a, demonstrating that the surface of the HTR is very rough, the diameter of the HTR is ca. 450 nm and the length is several micrometers. The SEM image with high magnification in Figure 2b distinctly depicts that the HTR is hierarchically composed of numerous tiny nanoparticles and nanorods with uniform diameter of ca. 30 nm. The hierarchical structure was further authenticated by the corresponding TEM image (Figure 2c). The HRTEM image of the TiO2 nanorods in the HTR with clear lattice fringes provides the evidence of high crystallization and the lattice spacing of 0.34 nm can be indexed as (101) of the anatase TiO2 phase (Figure 2d), which is consistent with the XRD.

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Figure 2. FESEM images (a, b), TEM image (c) and HRTEM image (d) of the as-prepared HTR.

The cross sections of the prepared samples were also investigated to obtain more information about the NP/HTR composite films. Taking P50R50 as the representation, the cross sectional SEM image in Figure 3a indicates that the NP/HTR composite film has about 25 µm thickness. The SEM with higher magnification in Figure 3b clearly shows that the HTRs are embedded in the film of P25 NPs, and a further observation reveals that there are plenty of large voids in the TiO2 composite films, which are supposed to facilitate the penetrability of electrolytes. Due to the hierarchical and 1D structure of the HTR observed from these SEM and TEM images, as illustrated in Figure 3c, the HTRs incorporated in P25 film can be expected to act as scattering 10


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centers with enhancing the light harvesting because of the relatively larger submicron size. Meanwhile, it is predicted that the 1D geometry of HTR could offer a direct pathway for long charge carrier transfer to reduce the recombination as well.

Figure 3.

Cross sectional SEM images (a) and (b) of the NP/HTR composite film

(P50R50) and the schematic illustration (c) of the NP/HTR composite film structure.

3.2 BET analysis and UV-vis reflectance spectra To further get an insight into the porous structure of NP/HTR composite film, the 11



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pore structures and specific surface areas of all the samples were obtained by BET measurements. Taking P100R0, P80R20 and P0R100 as examples, Figure 4a shows their nitrogen isotherms and the pore size distribution curves (inset in Figure 4a). All the

nitrogen

isotherms

for

the

three

samples

belong

to

type

IV

(Brunauer-Deming-Deming-Teller (BDDT) classification),36,37 showing the mesopores structure existence. The type H3 hysteresis loops imply the slit-like pores derived from nanoparticles gathering.38 Furthermore, comparing with P100R0 and P0R100, P80R20 shows bimodal pore structure (~7.6 nm and~ 49 nm mesopores), which can be ascribed to the small inner pores of the additive HTRs and the large pores constructed by P25 NPs, respectively. These porous structures could facilitate the permeation of liquid electrolyte into the film of the composite photoanode. Moreover, as listed in Table 1, the BET values of the composite samples gradually increase from 55 to 103 m2 g-1 with increasing the HTR content from 0 to 100 wt%. The larger SBET and increased pore volumes are favorable for the adsorption of the dye molecules. UV-vis reflectance spectra of all the photoanodes in the range of 400-800 nm were shown in Figure 4b, the reflectance intensity gradually increases with the HTR content getting more, indicating that the as-prepared HTR indeed can enhance the light scattering and light harvesting. Because the hierarchical structure can be conducted as a light scattering center of the TiO2 porous films, the incident light can be strongly scattered by the HTRs with submicron size. And the path length of the incident light can be greatly extended, leading to an enhanced light harvesting for the DSSC.39-41 12


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Figure 4. N2 isotherm and the pore size curves (inset) of P100R0, P80R20 and P0R100 powders (a), and UV-vis reflectance spectra of all the photoanodes (b).

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Table 1. The data from BET measurements of the as-obtained samples SBET

Pore volume

Average pore size

(m2 g-1)

(cm3 g-1)

(nm)

P100R0

55

0.16

11.9

P90R10

61

0.23

13.8

P80R20

67

0.24

14.4

P50R50

80

0.26

13.0

P0R100

103

0.28

10.9

Samples

3.3. Electrochemical impedance spectroscopy

Nyquist plots were obtained to study the internal charge transfer and recombination in DSSCs. Figure 5a and 5b present the electrochemical impedance spectroscopy (EIS) results of P100R0, P80R20 and P0R100-based DSSCs. In the equivalent circuit (inset in Figure 5a), Rs, R1/R2 and CPE represent the series resistance, the internal resistance and the constant phase element respectively. The semicircles in the high-frequency region, intermediate-frequency region and low-frequency region are assigned to the charge transfer resistance (R1) at the platinum counter electrode/electrolyte interface, the electron transfer resistance (R2) at the TiO2/dye/electrolyte interface, and the diffusion process of I-/I3- in the electrolyte, respectively.42-44 The parameters are summarized in Table 2. It can be found that Rs becomes larger with the HTR content increasing in the photoanodes, indicating that 14


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the interfacial contact between TiO2 and FTO becomes poorer through the incorporation of HTR. However, the change trend of R2 displays the advantage of HTR in the films. Usually, a larger R2 indicates a larger interfacial electron transport resistance and higher current loss.45 The values of R2 for different DSSCs are as follows: P80R20 (25.0 Ω) < P100R0 (30.4 Ω) < P0R100 (90.2 Ω). The DSSC based on P80R20 has a smaller interfacial resistance than the DSSC based on P100R0, suggesting that HTRs incorporated in P25 NPs contribute to the reduction of charge transport resistance, because the 1D structure of HTR facilitates the interfacial charge transport and lowers the charge recombination. Moreover, the pore size distribution in the TiO2 ﬁlm is also essential for the electron transport.46,47 The P80R20 with bimodal pore-size distribution (inset in Figure 4a) shows the largest average pore size (14.4 nm, Table 1), which could facilitate the diffusion of the electrolyte and the regeneration of the dye, resulting in accelerating electron transport. In contrast, the P0R100 DSSC has the biggest interfacial resistance R2 due to the small inner mesopores in the HTR and the poor contact between the HTRs, which can suppress the diffusion of the electrolyte and the transfer of electrons, resulting in larger R2.

Another interesting parameter, electron lifetime, was derived from the following equation (1):

τe = 1/ (2π fmax)

(1)

where τe is the electron lifetime and fmax gotten from the bode phase plot is the maximum frequency.48,49 As shown in Table 2, the electron lifetimes are as following: 15



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P0R100 (0.63 ms) < P100R0 (19.7 ms) < P80R20 (29.0 ms). Longer electron lifetime means a longer electrons transport distance and efficient charge transfer process. From the above investigations, one can see that the DSSC based on P80R20 has the lowest transport resistance and longest electron lifetime, suggesting that the P80R20-based DSSC indeed is an ideal cell.

Figure 5. Nyquist plots (a) and Bode phase plots (b) of P100R0, P80R20 and P0R100 photoanodes of DSSCs under illumination. 16


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Table 2. The parameters derived from the EIS measurements. Rs

R2

fmax

τe

(Ω)

(Ω)

(Hz)

(ms)

P100R0

13.0

30.4

8.1

19.7

P80R20

13.7

25.0

5.5

29.0

P0R100

14.2

90.2

254

0.63

Samples

3.4 Photocurrent-voltage characteristics and IPCE analysis The I-V characteristics of the as-prepared solar cells were investigated and represented in Figure 6 and Table 3. The film thicknesses of all photoanodes are similar and have been optimized in the design of photoanode for DSSCs. It can be seen that the I-V characteristics of DSSCs strongly rely on the content of HTRs. Isc of DSSCs firstly increases and then decreases with increasing the HTR content, and the DSSC based on P80R20 has the highest Isc of 16.5 mA cm-2. As discussed earlier, the P90R10 and P80R20 films have higher surface areas (seen in Table 1) and stronger light scattering (seen in Figure 4b) than the P100R0 film, therefore, the higher current densities of DSSCs based on P90R10 and P80R20 are due to the more dye loaded, stronger light scattering and the fast electron transfer provided by 1D structure of HTRs. However, when the content of HTRs in the photoanode is more than 20 wt%, e.g., the P50R50 and P0R100-based DSSCs show relatively lower photocurrent densities compared to P100R0 even though they have more dye loaded and stronger light scattering ability. This is due to their poor interface contacts with FTO, leading 17



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to less collected photo-generated electrons. Open-circuit voltages (Voc) of the DSSCs dependent on different HTR contents are compared in Table 3, the Voc of a DSSC belongs to the difference between Fermi level of TiO2 photoanodes and the redox potential of I-/I3- solution. 50,51 It can be found that the Voc values of all the DSSCs in this research are similar. Compared with the pristine one, Voc values of P90R10-based DSSC and P80R20-based DSSC are slightly larger, which could be explained by the negative shift of Fermi level, resulting from the less electron-hole recombination and the consequent increase of electron density in conducting band of TiO2. The conversion efficiency η of DSSCs firstly increases and then decreases with increasing the HTR content from 0 to 100 wt%. Significantly, the P80R20-based DSSC shows the maximum η of 8.09%. Moreover, the adsorbed dye amounts on all the photoanodes are shown in the Table 3, one can see that the adsorbed dye amount increases with increasing the HTR content, which is consistent with the SBET trend. From the above investigations, this improvement of DSSCs can be ascribed to the advantages from the multifunction of the incorporated HTRs, including the comprehensive effect of the good dye loading, the strong light scattering effect, the consequently lower charge transfer resistance and the longer electron lifetime.

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Figure 6. I-V curves of DSSCs made of P100R0, P90R10, P80R20, P50R50 and P0R100.

Table 3. Parameters of DSSCs based on P100R0, P90R10, P80R20, P50R50 and P0R100. Samples

Film thickness

Isc

Voc

(µm)

(mA cm-2)

(V)

P100R0

23

11.6

0.67

0.69

5.37

0.25

P90R10

24

14.1

0.68

0.71

6.87

0.48

P80R20

25

16.6

0.69

0.70

8.09

0.62

P50R50

24

10.4

0.67

0.71

4.96

0.90

P0R100

25

7.1

0.68

0.67

3.26

1.11

FF

η（%）

Dye loading (10-7 mol cm-2)

19



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Figure 7 displays the normalized IPCE characterization of DSSCs made of P100R0, P90R10, P80R20, P50R50 and P0R100. The IPCE firstly increases and then decreases with the HTR content increasing. Particularly, the P80R20 composite photoanode shows the best IPCE performance, confirming that the incorporation of moderate amount of HTRs (20 wt%) into TiO2 films indeed can improve the photovoltaic performance of DSSCs.

Figure 7. Normalized IPCE of the DSSCs based on P100R0, P90R10, P80R20, P50R50 and P0R100 film electrodes.

4. Conclusion In summary, 1D anatse hierarchical TiO2 submicorods were successfully prepared and used in the photoanode film of DSSCs. To optimize the performance of DSSCs, the effect of the relative NP/HTR ratio on the performance of solar cells was 20


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investigated. The optimized DSSC exhibited a photoelectric conversion efficiency of 8.09% at the 20 wt% HTR in the composite photoanode film, which was much higher than that of 5.37% for pristine P25 NPs based one. The improvement can be assigned to the multifunction of incorporated HTRs, including enhanced light harvesting, increased dye loading, and accelerated electron transport rate of the photoanode films. This work will offer a new perspective for performance enhancement of DSSCs.

Acknowledgement This study was supported by the NSFC (51272199, 21433007 and 51320105001), 973 program (2013CB632402), the Natural Science Foundation of Hubei Province (2015CFA001), the Fundamental Research Funds for the Central Universities (WUT: 2015-III-034) and Innovative Research Funds of SKLWUT (2015-ZD-1).

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For Table of Contents Use Only

Hierarchical TiO2 submicrorods improving photovoltaic performance of dye-sensitized solar cells Daipeng Guo, Shengqiang Xiao, Ke Fan*, Jiaguo Yu*

HTRs can improve photoelectric efficiency of DSSC owing to enhanced adsorbed dye amount, light harvesting and the electron transfer capability.

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Hierarchical TiO2 Submicrorods Improve the Photovoltaic

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