Improving the photovoltaic performance of dye sensitized solar cells based on a hierarchical structure with up/down converters

Abstract

A hierarchical structure composed of porous TiO₂:Al₂O₃:Eu³⁺ nanoparticles (NPs) and vertically grown one-dimensional TiO₂:Er³⁺,Yb³⁺ nanorods (NRs) on fluorine doped tin oxide (FTO) substrates coated with a TiO₂:graphene (G) seed layer was investigated for use in photoanodes for dye sensitized solar cells (DSSCs). The DSSCs assembled with this hierarchical structure exhibit an outstanding power conversion efficiency of 4.58%, which is superior to that of the devices based on pure TiO₂. This high performance can be attributed to the spectrum modifications achieved by utilizing the upconversion (UC) material TiO₂:Er³⁺,Yb³⁺ and the downconversion (DC) material Al₂O₃:Eu³⁺, which facilitate the light harvesting of solar cells via converting near infrared (NIR) and ultraviolet (UV) radiation to visible emission, respectively. Moreover, the TiO₂:G layer provides faster electron transport from TiO₂ to FTO for the high carrier mobility of G. Moreover, the one-dimensional nanorod structure can offer direct electrical pathways for photogenerated electrons as well as enhance the light scattering capabilities of photoanodes. This study indicates that the TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ hierarchical structure has the potential to improve the performance of DSSCs.

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1. Introduction

With the development of society, fossil fuels are being excessively exploited, which is causing a world energy crisis and environmental pollution. Renewable energy utilization is an effective solution for the world energy crisis and environmental pollution. Dye sensitized solar cells (DSSCs) have recently received attention as a renewable energy source for their low cost, easy fabrication procedures, safety, and non-toxicity, showing great advantages and broad application prospects.^1,2 In general, DSSCs are a type of photoelectrochemical system consisting of a porous nanocrystalline TiO₂ film sensitized by dye for absorbing incident light, a redox electrolyte, and a platinum counter electrode that acts as a catalyst for the redox couple regeneration reaction.³ Considerable efforts have been devoted to DSSCs over the past decade.^4–7 However, the conversion efficiency of DSSCs is still limited by many factors, such as electron injection efficiency, light harvesting efficiency, and the rate of charge recombination.^8–10

Nanocrystalline TiO₂ deposited on a conductive substrate, fluorine doped tin oxide (FTO), as a photoanode of DSSCs plays a significant role in light harvesting and the transfer of photoinduced carriers.^11,12 To date, the highest efficiency of DSSCs has reached 13% with porous TiO₂ film.¹³ The usual porous nanocrystalline TiO₂ film offers a high surface area to absorb a sufficient amount of dye. However, the porous structure is not favorable to electron transport, due to the multiple transport paths between different grain boundaries.¹⁴ One-dimensional nanostructures such as nanorods or nanotube arrays have been adopted to address this issue by providing direct electrical pathways for electrons along the direction of the nanorod, as well as increased electron lifetimes.^15–17 It is noteworthy that one-dimensional TiO₂ nanostructures grown alone on a substrate have an insufficient exposed surface area for the adsorption of dye.¹⁸ Therefore, we used a composite structure containing TiO₂ nanorods (NRs) and nanoparticles (NPs) as a photoanode to resolve the issues existing in each case.

In DSSCs, the most conventionally used ruthenium dyes, such as N719, usually absorb light only in the visible range due to the relatively large optical bandgap of 1.8 eV, resulting in a great deal of energy loss in the ultraviolet and infrared regions.^19,20 An effective way to avoid the loss of photo energy is the utilization of upconversion (UC) or downconversion (DC) materials, which aim to shift the incident solar spectrum to better match the absorbed spectrum of the dye so as to enhance the conversion efficiency of the solar cells.^11,21,22 Lanthanide ions are often used as UC or DC luminescence centers due to their intra 4f transitions.²³ Photon UC is a process in which the sequential absorption of two or more low energy photons leads to the emission of a high energy visible photon.²⁴ Lanthanide ions of Er³⁺, Ho³⁺, Tm³⁺, Pr³⁺, and Tb³⁺ have attracted much attention in the investigation of upconverters, and Yb³⁺ ion is usually co-doped as a sensitizer to increase the near infrared (NIR) absorption strength of the upconverter.²⁵ Among these UC materials, the Er³⁺/Yb³⁺ couple has been the most researched pair to date.²⁶ In addition, downshifting has also been applied for the improvement of solar cell efficiencies, and Eu³⁺ is a well-studied lanthanide ion for its ultraviolet (UV) absorption and red emission.²⁷

In this paper, the main idea is to utilize upconversion and downconversion of the UV and IR parts of sunlight into the region of dye absorption, as well as to use graphene (G) to achieve faster electron transfer, which is efficient for enhancing the performance of DSSCs. Thus, a multifunctional TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ composite structure is applied to construct photoanodes of DSSCs. Here, we considered the effect of TiO₂:G compact film between FTO and TiO₂ nanorod arrays. Wang et al. reported that DSSCs based on TiO₂ nanorod arrays grown on a seeded-FTO substrate have longer electron lifetimes, which could reduce electron recombination and lead to enhanced performance of the cells.²⁸ The presence of G facilitates the electron transfer to FTO due to its excellent electron mobility at room temperature.²⁹ Furthermore, the introduction of TiO₂:Er³⁺,Yb³⁺ NRs can not only benefit electron transport, but achieve light confinement. Also, UV light can be used by cells through the DC luminescence process of Al₂O₃:Eu³⁺ in TiO₂ paste. Therefore, the performance of DSSCs will be effectively improved.

2. Experimental

2.1 Preparation of TiO₂:G seed layer on the FTO substrate

FTO glasses were used as the substrate and were pretreated by ultrasonic cleaning in acetone, ethanol and deionized (DI) water for 20 min in turn, followed by rinsing with distilled water and, lastly, drying at 80 °C. To synthesize the TiO₂:G sol, 5 ml tetrabutyl titanate (Ti(OBu)₄) was dissolved in 15 ml ethanol and 0.5 ml glacial acetic acid; then 0.25 ml nitric acid, 5 ml ethanol and 0.5 ml deionized water were added to the solution. In addition, 20 mg commercial G (XF001W) was also dispersed in the above solution with ultrasonic dispersion for 2 h. The TiO₂:G seed layer was prepared on the FTO substrate by spinning the sol. The samples were then dried and finally annealed at 500 °C for 1 h.

2.2 Synthesis of TiO₂:Er³⁺,Yb³⁺ nanorod arrays

The TiO₂ nanorod arrays were prepared using a hydrothermal method reported previously.³⁰ Typically, 0.5 ml of (Ti(OBu)₄) was added to a mixed solution of 15 ml DI and 15 ml HCl (38 wt%) under stirring at room temperature in a Teflon-lined stainless steel autoclave. Moreover, in order to synthesize the TiO₂:Er³⁺,Yb³⁺ nanorods, Er(NO₃)₃·6H₂O and Yb(NO₃)₃·6H₂O were dissolved in a transparent solution with mole ratios of Ti⁴⁺ to Er³⁺ and Yb³⁺ of 1 : 0.01 : 0.01 and 1 : 0.01 : 0.05 under vigorous stirring at room temperature. After stirring for 30 min, the FTO substrates coated with the TiO₂:G seed layer were placed in the Teflon-lined autoclave and heated at 150 °C for 6 h in an electric oven. Then the samples were cooled to room temperature, followed by rinsing with DI water.

2.3 Synthesis of Al₂O₃:Eu³⁺ NPs

Eu³⁺ doped Al₂O₃ NPs with different doping concentrations (1 mol%, 2 mol% and 3 mol%) were synthesized by a precipitation method. Al(NO₃)₃·9H₂O and Eu(NO₃)₃·6H₂O were dissolved in DI water under vigorous stirring at room temperature, then ammonium hydroxide was added to the transparent solution to adjust the pH to 3 (solution a). Also, NH₃HCO₃ was added to ammonia aqueous solutions to obtain a pH of 10; some PEG, acting as a dispersant, was also added to the solution (solution b). Then saline solution was dripped slowly into the above solution b under stirring for 1 h. The precipitate was collected by centrifugation at 5000 rpm and washed with distilled water several times to remove the excess solvent and impurities. After drying in a hot air oven at 100 °C for 24 h, the obtained brown powder was annealed at 1200 °C for 4 h in a muffle furnace and white powders of Al₂O₃:Eu³⁺ were finally obtained.

2.4 DSSCs assembly

To prepare the TiO₂:Al₂O₃:Eu³⁺ composite film electrodes, 10 mg of Al₂O₃:Eu³⁺ was added to TiO₂ paste according to our previous report.²⁷ The photoanode pastes were prepared by the doctor blade method after the synthesis of TiO₂:Er³⁺,Yb³⁺ NRs grown on FTO substrates covered by a TiO₂:G layer. Then the TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ hierarchical structure was annealed at 450 °C for 30 min. After cooling to 80 °C, in order to absorb the dye, the photoelectrodes were immersed into an ethanol solution of 0.36 mM N719 ruthenium dye for 24 h at room temperature; they were then finally washed with ethanol to remove the nonchemisorbed dye and were dried in air. Pt (OPV–Pt–S) electrode was prepared by a spin-coating method and annealed at 450 °C for 30 minutes. Subsequently, the photoanode and the Pt counter electrode were fixed together with a hot-melt film spacer, followed by injecting electrolyte (OPV-MPN-I) into the space between the electrodes to assemble the DSSCs. Fig. 1 shows the fabrication process of DSSCs based on the TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ hierarchical structure. In this paper, a comparative study of the photovoltaic performance of DSSCs with different photoelectrodes was carried out. The photoelectrode structures of the DSSCs are as follows:

Fig. 1 The fabrication process of DSSCs based on the TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ hierarchical structure.

TiO₂/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂:Al₂O₃:Eu³⁺ and TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺.

2.5 Characterization

The structures of Al₂O₃:Eu³⁺ and TiO₂:Er³⁺,Yb³⁺ were investigated by X-ray diffraction (XRD, D8-Advance, Bruker). The photoluminescence (PL) of the samples was measured using the 7-FRSpec fluorescence spectrometer made by Saifan, using a Xe lamp as the excitation source. The surface and cross-section morphologies of the photoanode structures were examined with a field emission scanning electron microscope (FE-SEM, Quanta FEG250). The photocurrent response to time and the transient open-circuit potential measurements were performed by cycles of light switching on and off under a Xe lamp as the light source. The absorption spectrum of the composite electrodes sensitized by N719 was tested to confirm the effectiveness of the converters. Also, incident-photon-to-current-conversion efficiency (IPCE) measurements were carried out in a standard three-electrode system with 1 M NaOH aqueous solution as the supporting electrolyte, in which composite structures coated on FTO, platinum wire and Ag/AgCl act as the working electrode, the counter electrode electrode and the reference electrode, respectively. The I–V characteristics of the DSSCs were measured with an Agilent B2901A source/meter under a Xe lamp. The irradiation areas of the working electrode were about 0.16 cm². All these measurements were conducted at room temperature.

3. Results and discussion

As shown in Fig. 2, the crystal structures of Al₂O₃:Eu³⁺ and TiO₂:Er³⁺,Yb³⁺ can be confirmed by XRD. From the XRD patterns of Al₂O₃:Eu³⁺ powder (Fig. 2(a)) and TiO₂:Er³⁺,Yb³⁺ NRs grown on FTO (Fig. 2(b)), the main diffraction peaks are indexed to Al₂O₃ (JCPDS no. 86-1410) and rutile phase TiO₂ (JCPDS no. 21-1276) respectively, and no diffraction peaks related to other oxide phases are observed, indicating that rare earth ions might have been incorporated into Al₂O₃ and TiO₂ lattice. In Fig. 2(a), the broad and low intense peaks indicate the smaller size of the particles. Also, it can be observed in Fig. 2(b) that the TiO₂:Er³⁺,Yb³⁺ crystal growth is (004)-oriented and shows good crystallinity.

Fig. 2 XRD patterns of (a) Al₂O₃:Eu³⁺ powder and (b) TiO₂:Er³⁺,Yb³⁺ NRs grown on FTO.

The room temperature PL spectra of Al₂O₃:Eu³⁺ and TiO₂:Er³⁺,Yb³⁺ have been acquired to illustrate the DC and UC process, respectively. The emission spectrum of Al₂O₃:Eu³⁺ under 320 nm excitation is presented in Fig. 3(b). It is a known fact that Eu³⁺ emission is observed under UV (320 nm) excitation, which is not absorbed by Eu³⁺. Thus, the dopant is excited indirectly by effective energy transfer from Al₂O₃ to Eu³⁺. In Fig. 3(b), the sharpest and most intense emission peak in the red spectral region was observed in Al₂O₃:Eu³⁺, and the emission peaks at 577 nm, 590 nm, 598 nm, 617 nm and 653 nm can be attributed to the ⁵D₀–⁷F_J transitions (J = 0–3) of Eu³⁺. Also, a weak, broad emission around 400 nm to 450 nm could be related to the intrinsic defect emission in the Al₂O₃ host. The inset in Fig. 3(b) shows the PL spectra of Al₂O₃:Eu³⁺ with different doping concentrations. It is obvious that the PL intensity of Eu³⁺ decreased as the Eu³⁺ concentration increased.

Fig. 3 (a) Absorption of N719; (b) PL spectrum of Al₂O₃:Eu³⁺ (inset: PL spectra of Al₂O₃:Eu³⁺ with different doping concentrations) and (c) PL spectrum of TiO₂:Er³⁺,Yb³⁺ (inset: PL spectrum of TiO₂:Er³⁺,Yb³⁺ with different doping concentrations).

The PL spectrum of Er³⁺,Yb³⁺ co-doped TiO₂ was obtained with 980 nm laser excitation. For the TiO₂:Er³⁺,Yb³⁺, Er³⁺ acts as an activator and Yb³⁺ has been demonstrated to act as a sensitizer with a absorption of 980 nm, corresponding to the ²F_5/2–²F_7/2 transition. The visible luminescence spectrum of TiO₂:Er³⁺,Yb³⁺ shown in Fig. 3(c), reveals two emission bands in the green and red spectral regions due to the intra-4f transitions of rare earth ions; the PL intensities are TiO₂:1 mol% Er³⁺, 1 mol% Yb³⁺ > TiO₂:1 mol% Er³⁺, 5 mol% Yb³⁺ (shown in the inset). The green emission bands observed between 520 nm and 550 nm correspond to the ²H_11/2–⁴I_15/2 and ⁴S_3/2–⁴I_15/2 transitions of Er³⁺ ions. The stronger red emission from 650 to 680 nm is attributed to the ⁴F_9/2–⁴I_15/2 transition of Er³⁺ ions. It is clear that the NIR light (980 nm) can be shifted to visible light by TiO₂:Er³⁺,Yb³⁺. Therefore, in consideration of PL intensity, TiO₂:1 mol% Er³⁺, 1 mol% Yb³⁺ and Al₂O₃:1 mol% Eu³⁺ were chosen as up/downconversion materials to convert NIR and UV radiation to visible emission, respectively; this will reduce the energy loss in the ultraviolet and infrared region and broaden the absorption of N719 (Fig. 3(a)).

Fig. 4 shows the surface and cross-section SEM images of the as prepared photoanodes, FTO/TiO₂/TiO₂ NRs/TiO₂ and FTO/TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺. From the top view of the photoanode, FTO/TiO₂/TiO₂ NRs/TiO₂ exhibits a uniform surface morphology with high porosity (Fig. 4(a)), as well as FTO/TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ film (Fig. 4(c)). Also, the cross-sectional SEM images of both photoanodes (Fig. 4(b) and (d)) show that the thickness of the composite films is about 12 μm, while the length of the NRs is 1 μm, as shown in the inserts.

Fig. 4 SEM images of (a) top view and (b) cross-section of FTO/TiO₂/TiO₂ NRs/TiO₂ (the inset in (b) shows a higher resolution image); SEM images of (c) top view and (d) cross-section of FTO/TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ (the inset in (d) shows a higher resolution image).

The photocurrent responses of the samples were examined by light on–off cycles, as shown in Fig. 5, which indicate the reproducibility and stability of photoresponses. From Fig. 5, a steady and fast response can be achieved during the light on and off cycles. The photocurrent of samples rapidly increased to a constant value when the light was switched on and decreased sharply as soon as the light was switched off, indicating good reproducibility. The trend was repeated with 30 s on–off cycles, which showed that all the samples possess a stable photoresponse for 200 s under illumination. Obviously, the photocurrent of TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ electrode presented a stronger photoresponse than that of the other samples under white light. The enhancement in the photocurrent suggests that this composite structure exhibits greater photo-induced electrons and higher transfer efficiency. Therefore, the photocurrent and overall conversion efficiency of the DSSCs will be increased efficiently.

Fig. 5 Photocurrent–time testing curves of TiO₂/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂:Al₂O₃:Eu³⁺ and TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺.

As for the semiconductor electrode, the photo-induced electronic hole pairs will separate and spread to the solution/electrode interface under illumination, establishing an electric double layer and generating an open circuit voltage, which reflects the number of carriers transferring to the surface of the film. Fig. 6 shows the transient open-circuit potential of the TiO₂ and TiO₂:G film under the intermittent illumination of Xe light. It is noted that the TiO₂:G film exhibited a photopotential of 0.065 V, which was larger than that of the TiO₂ film (0.044 V). The enhanced open-circuit potential implied an increase of carriers in the TiO₂:G layer, indicating faster separation and transport rates of electrons and holes. The slower decay responses of the TiO₂:G compact layer indicate that the recombination is reduced, which is in agreement with the results of the photocurrent response.

Fig. 6 Variation of the open circuit potential of the TiO₂ and TiO₂:G films.

To confirm the effectiveness of the converters, the absorption of photoanodes sensitized with N719 was carried out, and the result is presented in Fig. 7. It was found that the absorption intensity is enhanced after the incorporation of the up/down converter. The TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ hierarchical structure shows an optimized absorption. To further quantify the relationship between the photovoltaic performance and the incident light wavelength, IPCE measurements were conducted on TiO₂/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂:Al₂O₃:Eu³⁺ and TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺, as shown in Fig. 8. In comparison to pristine TiO₂ electrode, the other constructors exhibit higher photon responses in the visible light region. In addition, the IPCE values of the electrodes containing rare earth ion doped materials slightly increased in the UV light region due to the increased light absorption, which is in agreement with the I–T results.

Fig. 7 The absorption spectra of photoanodes sensitized by N719.

Fig. 8 IPCE spectra of TiO₂/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂, TiO₂:G/TiO₂ NRs/TiO₂:Al₂O₃:Eu³⁺ and TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺.

In order to further verify the influence of G and the UC/DC materials, solar cells based on the TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ hierarchical structure were fabricated and their photovoltaic performances were investigated. Fig. 9 shows the photocurrent–voltage performances of DSSCs with different photoelectrodes. The short-circuit current density (J_SC), open-circuit voltage (V_OC), fill factor (FF) and the power conversion efficiency (η) of these DSSCs were summarized in Table 1. From these parameters, the cells incorporating the TiO₂:G compact layer directly on FTO glass (TiO₂:G/TiO₂ NRs/TiO₂) show enhancements in their photocurrents and conversion efficiencies compared to the cell based on TiO₂/TiO₂ NRs/TiO₂ due to the presence of G, which acted as a charge pathway which can reduce the charge recombination rate and enhance the transport of electrons. Also, the DSSCs based on the TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ hierarchical structure showed optimal performances, with J_SC = 10.38 mA cm⁻² and η = 4.58% when utilizing DC and UC materials, respectively. It is known that N719 dye has a strong absorption around 550 nm. Al₂O₃:Eu³⁺, which can absorb UV light and convert it to visible light, has emissions in the green and red regions. Also, the NIR light of the solar spectrum can be shifted to the visible region by TiO₂:Er³⁺,Yb³⁺. These results are shown in Fig. 3. The effect of DC-luminescence and UC-luminescence from 600 and 700 nm reduces the major loss of light in the UV and NIR regions in the cell without being used. Moreover, the conversional luminescence, especially the green bands (from 536 to 565 nm) coincide with the best absorption wavelength of the N719 dye. Consequently, UV and NIR irradiation can be absorbed by the N719 dye in the DSSCs through the effect of spectral conversion with the addition of Al₂O₃:Eu³⁺ powder and TiO₂:Er³⁺,Yb³⁺ NRs in the photoanodes, widening the light absorption range of the cell; thus, the light to electricity efficiency is effectively enhanced.

Fig. 9 Photocurrent–voltage curves of DSSCs based on different photoanodes.

Table 1 Photovoltaic parameters (short-circuit current density, open-circuit voltage, fill factor, and efficiency) of the DSSCs using different photoelectrode structures

Photoanodes	JSC (mA cm^−2)	VOC (V)	FF	η (%)
TiO2/TiO2 NRs/TiO2	7.98	0.71	0.60	3.38
TiO2:G/TiO2 NRs/TiO2	8.32	0.73	0.60	3.62
TiO2:G/TiO2 NRs/TiO2:Al2O3:Eu^3+	8.94	0.72	0.60	3.87
TiO2:G/TiO2:Er^3+,Yb^3+ NRs/TiO2:Al2O3:Eu^3+	10.38	0.72	0.61	4.58

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4. Conclusions

In conclusion, a hierarchical structure in the order of TiO₂:G/TiO₂:Er³⁺,Yb³⁺ NRs/TiO₂:Al₂O₃:Eu³⁺ has been successfully prepared for use as photoanodes in DSSCs. This hierarchical structure provides a sufficient exposed surface area for the adsorption of dye and direct electrical pathways for photogenerated electrons. In addition, the DSSCs based on these composites possess an excellent capacity to expand the light absorption of solar cells via converting UV and NIR radiation to visible emission by Al₂O₃:Eu³⁺ and TiO₂:Er³⁺,Yb³⁺. Moreover, the addition of G to TiO₂ compact film contributes to the increase in the short circuit current density due to the faster electron transfer to FTO. Thus, an optimal efficiency of 4.58% was achieved for DSSCs with this hierarchical structure.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 61106059, 11304120, 61504048, and 21505050), the Encouragement Foundation for the Excellent Middle-aged and Young Scientist of Shandong Province (Grant No. BS2014CL012), the Science-Technology Program of Higher Education Institutions of Shandong Province (Grant No. J14LA01), the Natural Science Foundation of Shandong Province (Grant No. ZR2013AM008).

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