Effective regulation of the micro-structure of thick P3HT:PC₇₁BM film by the incorporation of ethyl benzenecarboxylate in toluene solution

Abstract

In this work, ethyl benzenecarboxylate (EB) was creatively selected as the additive in a blend of poly(3-hexylthiophene)/phenyl-C71-butyric acid methyl ester (P3HT/PC₇₁BM) in non-halogenated solvent toluene (TL). With the optimized incorporating concentration of EB (i.e. 2 vol%) in toluene, a great power conversion efficiency (PCE) enhancement of 4.11% was achieved without thermal annealing, whereas a maximum PCE of 4.82% with thermal annealing was achieved under the same conditions. According to our systematic characterization results, we could conclude that we successfully regulated the micro-structures, including phase separation, the domain sizes of P3HT or PC₇₁BM and the crystallinity of P3HT by incorporating 2 vol% EB in TL solution. The effectiveness of EB as a TL additive in P3HT/PC₇₁BM can be interpreted based on its Hansen solubility parameters (HSPs) and its high boiling point.

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1 Introduction

Bulk-heterojunction (BHJ) polymer solar cells (PSCs) have gained tremendous attention in the last ten years due to their advantages of low cost, light weight, and flexibility compared to inorganic semiconductor solar cells.^1,2 Over 10% power conversion efficiencies (PCE) of PSCs have been achieved by using novel low-band-gap semiconductor polymers³ or by applying advanced architectures,^4,5 which is considerably closer to the PCE value that the commercialization of PSCs need.⁶ However, many disadvantages in the present fabrication process of PSCs restrict their commercialization, particularly the utilization of halogenated organic solvents such as chloroform (CF), chlorobenzene (CB), and ortho-dichlorobenzene (o-DCB), which are very toxic organic solvents and that can undermine human health and ruin the environment in the fabricating process of future commercialized PSCs.^7,8 Hence, it is of crucial importance to search for environmentally friendly solvents to replace the toxic halogenated solvents.

A spontaneous interpenetrating network structure is formed in the drying process of the solvents, and therefore the choice of processing solvent can have a notable impact on the resulting microstructure and thus on the photovoltaic performance. At the very beginning of studying PSCs, researchers tried to utilize toluene and xylene, which were widely used as processing solvents for polymer light-emitting diodes (PLEDs)⁹ to fabricate PSCs;¹⁰ however, the results demonstrated that the PCE of devices fabricated with CB was 2.6 times higher than that of devices fabricated with toluene,¹¹ and the photovoltaic performance of devices fabricated with o-DCB were considerably better than that of devices fabricated with xylene.¹⁰ Moreover, some other studies^12–15 also demonstrated that utilizing only one single type of non-halogenated solvent could not achieve as good a photovoltaic performance as that of devices fabricated with halogenated solvents.

It has been well known that blend solvents offer the possibility to tailor the parameters of solvents such as solubility, boiling point, and viscosity;^16–18 therefore, utilizing non-halogenated blend solvents or additives in non-halogenated solvents as the processing solvents of PSCs may be an effective method to achieve good photovoltaic performances. Alex K.-Y. Jen et al. reported high-efficiency PSCs based on PBDT-DNT/PC₇₁BM with a PCE > 6% using xylene as the host processing solvent and 2.5% 1,2-dimethylnaphthalene (1,2-DMN) as the additive;⁸ in addition, they reported a PCE > 7% for PSCs based on PIDTT-DFBT/PC₇₁BM with 1,2,4-trimethylbenzene (1,2,4-TMB) as the host processing solvent and 2.5% 1,2-DMN as the additive.⁸ Maojie Zhang et al.¹³ used a non-halogenated solvent as a processing solvent and N-methyl pyrrolidone (NMP) as a solvent additive to achieve a 6.6% PCE of PSCs based on P3HT/ICBA.

In this work, we utilized EB as an additive in a blend of P3HT/PC₇₁BM in non-halogenated solvent toluene. We demonstrated the reasons why EB was selected by the Hansen solubility parameters (HSPs)¹⁹ of related materials. The optimized doping concentration of the EB additive in toluene was 2%, and with this doping concentration we demonstrated a PCE of 4.11% without thermal annealing and a maximum PCE of 4.82% with thermal annealing. UV-Vis spectra, XRD, AFM and TEM characterization results indicated that a better phase separation was achieved as well as larger P3HT or PC₇₁BM domains and a larger crystallinity of P3HT were formed in the films cast from toluene solutions with the EB additive.

2 The choice of solvent additive

The addition of a small amount of high boiling point additive to the host processing solvent can significantly improve the phase separation of an interpenetrating network, which can be ascribed to the differential solubility of the additive and host solvent for polymer donor or fullerene derivative acceptors, and can lead to a longer drying time to give bigger crystallization.¹⁶ As has been published,^13,15,20 solvent additives had a great effect on fullerene solubility when they were incorporated into non-halogenated solvents. For instance, the solubility of PC₇₁BM in o-xylene can be greatly enhanced by the addition of 2.5 vol% 1-methylnaphthalene (1-MN).⁸ This phenomenon can be expounded by the Hansen solubility parameters (HSPs), which were first introduced by Hildebrand and Scott to describe and predict the solubility behavior of organic semiconductors. The Hildebrand parameter, δ, which indicates the miscibility of the components in a blended solution, was defined as the square root of the cohesion energy density, as shown in eqn (1), where E is the energy change of the evaporation and V is the molar volume.

δ² = δ_D² + δ_P² + δ_H² (2)

The Hildebrand parameter is mainly determined by 3 factors, as shown in eqn (2), namely, the atomic dispersive interactions (δ_D), the permanent dipole interactions (δ_P), and the molecular hydrogen bonding interactions (δ_H). The HSPs of the solvent and solute can be plotted in a 3-dimentional coordinate system with δ_D, δ_P and δ_H as the X, Y and Z axis, respectively. Therefore, the solubility of a solute in various solvents can be demonstrated by the Hansen sphere, where the HSP plot of the solute is located at the center of the sphere and the interaction radius R_o spans the regime within which the solute is dissolved. The distance between the HSPs of the solvent and solute is defined as R_a, as represented in eqn (3), in which the 1 and 2 represent the solute and solvent, respectively.

R_a² = 4(δ_D1 − δ_D2)² + (δ_P1 − δ_P2)² + (δ_H1 − δ_H2)² (3)

The ratio of R_a and R_o defines the relative energy difference (RED), which can be used to estimate the solubility of a solute in a particular solvent. For instance, the solubility of a solvent for a solute is high when the RED < 1, while a solute in a solvent is partially dissolved when RED = 1, and a solute in a solvent is insoluble when the RED > 1.^8,21

It can be found in ref. 19 that the δ_D, δ_P and δ_H of toluene are 18.0, 1.4 and 2.0, respectively, whereas the HSPs of PC₇₁BM are 20.2, 5.4 and 4.5.²² All the HSPs of toluene are located far away from that of PC₇₁BM, which indicates a bad solubility of PC₇₁BM in toluene. To improve the morphology of the active layers casted from the non-halogenated solvents, Alex K.-Y. Jen et al.⁸ incorporated 2.5 vol% 1,2-DMN additive to the o-xylene. They found a considerably finer D/A separation in the BHJ films cast from o-xylene solution with 2.5 vol% 1,2-DMN, which is very similar to that of BHJ films cast from o-DCB solutions. They ascribed the better morphology to the incorporation of 1,2-DMN, which increased the δ_D and δ_H of the solvents; in addition, the increased δ_D and δ_H led to a decreased RED and a better miscibility with PC₇₁BM. Inspired by this concept, we found that the δ_P and δ_H of EB were 6.2 and 6.0, respectively, which are relatively high values, and so it can be used as the additive to increase the δ_P and δ_H values in the toluene blend solution. The Hansen solubility parameters diagram of P3HT, PC₇₁BM and the related solvents is shown in Fig. 1, and the detailed HSPs of P3HT, PC₇₁BM toluene and EB are summarized in Table 1. After careful calculations, we obtained the RED values of P3HT in toluene and EB as 0.75 and 1.25, respectively, and the RED values of PC₇₁BM in toluene and EB as 0.77 and 0.58, respectively. These results demonstrate that toluene can dissolve P3HT and PC₇₁BM, whereas comparatively speaking, EB is a bad solvent for P3HT and a good solvent for PC₇₁BM. Based on these characteristics, a small amount of EB incorporated in toluene can improve the P3HT selective aggregation, which is propitious for phase separation. Hence, through the control of the incorporated concentration, we could effectively regulate the domain size of every phase. On the other hand, the boiling point of EB is 212.6 °C, which is considerably bigger than that of TL (110.6 °C). The incorporation of EB increases the drying time of the P3HT:PC₇₁BM film, which benefit the self-assembly of P3HT and the aggregation of PC₇₁BM. Moreover, it is worth noting that EB is micro-poisonous to humans, and it exists naturally in peach, pineapple and black tea. Therefore, EB is a type of environment-friendly additive that is not harmful to human health or the environment. To conclude, EB is very suitable as an additive for P3HT:PC₇₁BM toluene solution.

Fig. 1 The HSP diagrams of the solvents and materials mentioned in this study.

Table 1 The HSPs of some solvents and materials

Material/solvents	HSPs [MPa^1/2]				RED with
	δD	δP	δH	Ro	P3HT	PC71BM
P3HT	19.05	3.3	2.8	3.9
PC71BM	20.2	5.4	4.5	8.4
Toluene	18.0	1.4	2.0		0.75	0.77
EB	17.9	6.2	6.0		1.25	0.58

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3 Experimental

3.1 Fabrication of PSCs

In this work, poly(3-hexylthiophene) (P3HT, 98% regioregularity, Luminescence Ch) and phenyl-C71-butyric acid methyl ester (PC71BM, 99.5%, American Dye Sources Inc.) were selected as photovoltaic materials. The device structure of polymer solar cells is shown as the following module: glass/ITO/poly(3,4-ethyl-enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/P3HT:PC71BM/Ca/Al.⁶ Fig. 2 shows the schematic diagram of PSCs. Details of the fabrication processes are as follows: ITO-coated glass substrates with a nominal sheet resistance of 15 Ω sq⁻¹ were ultrasonically cleaned with detergent, de-ionized water, acetone and iso-propyl alcohol for 20 min in each round, and subsequently dried in an oven at 110 °C. The substrates were treated in a Plasma cleaning instrument (Plasmon Preen II-862, Mycro Co.) for 6 min prior to the deposition of the hole transport materials. The PEDOT:PSS solution (Clevios PVP Al4083, HC Starck) was spin-coated at a speed of 4000 rpm for 30 s to form a hole transport layer and then annealed at 150 °C for 20 min. The thickness of the PEDOT:PSS film was about 40 nm. To optimize the concentration effect of EB in the toluene solution, 1 vol%, 2 vol% and 3 vol% of EB were added to 36 mg ml⁻¹ of P3HT : PC₇₁BM (1 : 1) toluene solution. The blend solution was completely mixed before spin-coating. The substrates with PEDOT:PSS and blend solutions were then transferred into a nitrogen-filled glove box, and the P3HT : PCBM (1 : 1) toluene solutions were spin-coated on a PEDOT:PSS layer at 600 rpm for 40 seconds to form ∼330 nm thick active layers. The device fabrication parameters were optimized prior to this preparation of PSC. The fabricated PSC active layer was subjected to thermal annealing at 160 °C for 10 min if needed. Finally, Ca/Al electrodes were thermally evaporated onto the active layer with a thickness of 5 nm and 100 nm, respectively, to form a cathode under vacuum (≤10–6 Torr) conditions, and the active layer area of the device was 0.1 cm² as defined by a shadow mask for all the solar cell devices discussed in this work.²³

Fig. 2 The schematic of the PSC structure, and the structural formulas of TL and EB.

To investigate the hole carrier mobility of P3HT:PC₇₁BM films casted from toluene solution and toluene solutions with EB, we fabricated devices that could only transport holes, and the structure of which was ITO/PEDOT:PSS/P3HT:PC₇₁BM/Au. The charge carrier mobilities of the devices were studied by the trap-free-space-charge-limited-current (SCLC) method.¹² All the fabrication processes before evaporating the metallic electrode were the same with that for fabricating the devices of PSCs; the only difference was that the evaporated electrode was a Au electrode.

3.2 Characterization

The current density–voltage (J–V) characteristics were measured under AM 1.5 solar illumination at an intensity of 100 mW cm⁻² with a Newport solar simulator by a Keithley 2420 source measurement N₂ filled glove-box. The external quantum efficiencies (EQE) of the PSCs were analyzed using a certified Newport incident photon conversion efficiency (IPCE) measurement system. The UV-Vis absorption spectra of pristine P3HT:PC₇₁BM film and P3HT:PC₇₁BM films casted from toluene solution and toluene solutions with EB were taken using a Varian Cary 50 UV-Vis spectrometer. The thickness of the P3HT:PC₇₁BM films were acquired using a Veeco Dektak 150 surface profiler. The X-ray diffraction (XRD) spectra were taken by a Mac Science, equipped with Cu Kα source. The 2-D X-ray scattering in grazing incidence geometry (GIWAXS) measurements were performed by the Shanghai Synchrotron Radiation facility. Atomic force microscopy (AFM) measurements were carried out using an Agilent 5400 AFM at ambient temperature. The transmission electron microscopy (TEM) images of the P3HT:PC₇₁BM film and the P3HT:PC₇₁BM films casted from toluene solution with EB were obtained using a JEM-2000 Ex.

4 Results and discussion

In order to evaluate the effect of EB in toluene, we fabricated devices with P3HT:PC₇₁BM cast from toluene solutions with 0 vol%, 1 vol%, 2 vol% and 3 vol% EB. The J–V curves of these devices are shown in Fig. 3, and the detailed photovoltaic parameters are summarized in Table 2. The device without the EB additive and thermal annealing showed a poor photovoltaic performance, with a low short circuit current density (J_sc) of 2.72 mA cm⁻² and a low fill factor of 35.65%; in addition, the V_oc was distinctly higher than that of the device with the EB additive. All of these representations can be ascribed to the unavailable interpenetrating network of P3HT:PC₇₁BM. It can be easily found that the photovoltaic performances of devices with the EB additive are clearly better than that of the device without the EB additive. For instance, the J_sc and FF are greatly improved and therefore the PCEs of the devices are greatly enhanced. With the incorporation of 2 vol% EB in toluene, we achieved the best photovoltaic performance (PCE = 4.11%) of device without annealing. It is worth noting that with the increasing EB concentration, the V_oc of the devices decreases. Similar changes in V_oc have been reported by Yang et al.¹⁶ for P3HT:PC₇₁BM devices, which had a decreased V_oc after solvent annealing treatment, and they ascribed the decrease of V_oc to improved vertical phase segregation and the upward-shift HOMO level of P3HT when its aggregation was enlarged. Hence, it is reasonable to consider that the incorporation of EB in toluene improved the phase separation of P3HT:PC₇₁BM even without thermal annealing. In addition, the photovoltaic performance of devices fabricated with 2 vol% EB in toluene can be further improved to a maximum PCE of 4.82%, with J_sc and FF also reaching their maximum values of 11.48 mA cm⁻² and 69.97%, respectively.

Fig. 3 The J–V curves of P3HT:PC₇₁BM devices with different EB ratios.

Table 2 The photovoltaic parameters of the devices

	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
0	0.82	2.72	35.65	0.80
1	0.61	9.99	56.03	3.41
2	0.56	11.22	65.42	4.11
3	0.54	11.19	63.92	3.86
0 anneal	0.61	10.64	62.35	4.05
2 anneal	0.60	11.48	69.97	4.82

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External quantum efficiency (EQE) can verify the better photovoltaic performances of PSCs. Based on this concept, the EQE value reaches 100% when all the incident photons generate electron–hole excitons. However, the EQE value is usually less than 100% due to the reflection of incident light, weak absorption of photo-active materials and the recombination of electrons and holes.²³ The EQE spectra of various devices are shown in Fig. 4. From the EQE spectra, we can observe that even without thermal annealing the EQE intensities can be greatly enhanced by incorporating the EB additive. In addition, after thermal annealing, the devices with 2% EB additive show a maximum EQE value, which is in accordance with the maximum photovoltaic performance. All of the results correspond to those of the device photovoltaic performances.

Fig. 4 The EQE spectra of P3HT:PC₇₁BM devices with different EB ratios.

In order to uncover the reasons for the photovoltaic performance discrepancies of the devices casted from toluene solutions with or without EB additive, we carried out an absorption characterization of the P3HT:PC₇₁BM films, and the results are shown in Fig. 5a. It is easy to see that there are significant red-shifts in the absorption spectra of P3HT:PC₇₁BM films casted from toluene solutions with the EB additive, compared with that of P3HT:PC₇₁BM film casted from pure toluene solution. It is worth noting that there are three distinct characteristic absorption peaks in the absorption spectra with the EB incorporation ratio >2%, which are a peak at ∼520 nm, and two shoulders at ∼550 nm and ∼610 nm. The peak at 520 nm corresponds to the π–π* electron transition, and the other two shoulder peaks are a manifestation of the strong interchain–interlayer interaction or high crystallinity of P3HT. Therefore, all of these results of the absorption spectra demonstrate an enhanced π–π stacking of the P3HT by incorporating EB into the toluene solutions.

Fig. 5 (a) The absorption spectra of P3HT:PC₇₁BM films casted from toluene solutions with different EB ratios, and (b) the XRD curves of P3HT:PC₇₁BM films casted from toluene solutions with different EB ratios.

To validate that it is the enhanced crystallinity of P3HT that causes the red shifts of the absorption spectra, we carried out XRD characterization on the films casted from different toluene solutions with various EB ratios, and the results are shown in Fig. 5b. The diffraction peak of 2θ ≈ 5.5° is the characteristic diffraction peak of the P3HT crystal, which corresponds to the (100) lattice plane of P3HT.¹² As we can observe from the XRD spectra, all of the spectra of the films show strong diffraction peak at 2θ ≈ 5.5°, but the intensity of the films increase with increasing EB incorporation ratio. To eliminate the possible influence of film thickness on the XRD diffraction intensity, we measured the films thicknesses and found no big differences as 325 nm, 330 nm, 338 nm and 342 nm. In addition, there are small peaks at 2θ ≈ 11° in the spectra of films with EB additives, which correspond to the (200) lattice plane of P3HT, and the emerging of the (200) lattice plane of P3HT indicates a large crystallinity of P3HT. Therefore, it can be concluded that the incorporation of EB can increase the crystallinity of P3HT.

To further understand the development of the crystal structure of P3HT and PCBM during the film-processing process, we performed 2-D X-ray scattering in grazing incidence geometry (GIWAXS) measurements.^16,24 The measurements were taken with the X-ray incident angle within the characteristic angles for the total angle reflection of P3HT and PCBM but below that of the Si substrate, which can probe the molecular ordering of the entire thickness of the active layer. The packing along the side chains of the P3HT crystal is denoted as the a-axis, (h00), and the π–π stacking direction within the P3HT crystal is denoted as the b-axis, (0k0). From the GIWAXS results (Fig. 6), we can see that the addition of EB in toluene solution greatly increased the diffraction intensity (in the range of 0–5 nm⁻¹) of P3HT, which indicated that the crystallinity of P3HT was greatly enhanced. After treating by the annealing process, the P3HT:PCBM film processed without EB showed a higher diffraction intensity (∼0–7 nm⁻¹), which corresponds to the higher crystallinity of (100) and (200). However, the P3HT:PCBM film processed with 2% EB showed no diffraction intensity difference after annealing treatment. Moreover, the diffraction intensity of the P3HT:PCBM film processed without EB was clearly lower than that of P3HT:PCBM film processed with 2% EB even after thermal annealing. Hence, it can be concluded that the micro-structure regulation effect of EB cannot be achieved by thermal annealing.

Fig. 6 2-D GIWAXS results of P3HT:PCBM processed with different amounts of EB (a–d) and with thermal annealing treatment (e and f).

Previous studies showed that small amounts of additives still have a great influence on the morphology of the films that are formed. Therefore, we used AFM characterization to study the morphology changes of the P3HT:PC₇₁BM casted from toluene solutions with different EB additive ratios. Fig. 7 shows the AFM topographic images of the films. From Fig. 7, we can observe that the P3HT:PC₇₁BM film without EB additive shows a very smooth surface with a root mean square roughness (RMS) of 0.36 nm, and the films casted from solutions with EB additive show greatly enhanced RMS values, which are 2.2 nm, 8.9 nm and 9.2 nm. The larger roughness demonstrated the larger donor/acceptor domain size, which means a better donor/accepter interpenetrating network.¹³ Therefore, we can conclude that the better interpenetrating network benefits the extraction of holes and electrons. After careful analysis, we ascribed the greatly enhanced RMS values to the enhanced phase separation caused by the incorporation of EB. For instance, compared with toluene, EB is a poor solvent for P3HT and a good solvent for PC₇₁BM. Therefore, small amounts of EB incorporation will lead to the aggregation of P3HT, which will be good for the phase separation. However, only appropriate phase separation is needed for the relatively short excitons diffusion length (<20 nm). Excess EB additive (3%) will lead to enlarged phase separation, which is not beneficial for the excitons diffusing to the interface of the donor material and acceptor material, and thus will decrease the photovoltaic performance.

Fig. 7 The AFM images of P3HT:PC₇₁BM films casted from toluene solutions with (a) 0% EB, (b) 1% EB, (c) 2% EB and (d) 3% EB.

To intuitively demonstrate the phase separation differences between films cast from toluene solution with or without EB additive, we utilized TEM characterization. Fig. 8 shows the TEM images of films casted from toluene solution without EB and with 2% EB. From the results, we can observe that a better interpenetrating network is formed after incorporating 2% EB additive, which is caused by its poor solubility for P3HT and good solubility for PC₇₁BM. Fig. 8a demonstrated an intensively mixed P3HT:PC₇₁BM film, and in that the film very small P3HT or PC₇₁BM domains were formed, which goes against the transport of charge carriers. Moreover, there were instinct P3HT or PC₇₁BM rich domains formed, as shown in Fig. 8b, which should definitely improve the charge carrier transportation and prevent the unfavorable electron–hole recombination. However, as mentioned above, only an appropriate domain size is needed for a balance of the excitons diffusion and charge carrier transportation, so only appropriate EB doping concentration is needed.

Fig. 8 The TEM images of P3HT:PC₇₁BM films cast from toluene solutions with (a) 0% EB and (b) 2% EB.

Charge carrier mobility mainly depends on the crystallinity of P3HT and the domain size of PC₇₁BM; hence, according to the results of XRD and TEM, we speculated that the charge carrier mobility of the P3HT:PC₇₁BM films casted from toluene solutions with EB additives are considerably larger than that of films casted from solutions without EB additives. To testify our speculations, we investigated the hole mobility of P3HT:PC₇₁BM films by the trap-free-space-charge-limited-current (SCLC) method. The J–V curves of the devices with the structure of ITO/PEDOT:PSS/P3HT:PC₇₁BM/Au are shown in Fig. 9. The calculated hole mobility (μ_h) of P3HT:PC₇₁BM films casted from toluene solutions with 0%, 1%, 2% and 3% EB additives were 1.26 × 10⁻⁷ cm² V⁻¹ S⁻¹, 3.24 × 10⁻³ cm² V⁻¹ S⁻¹, 4.80 × 10⁻³ cm² V⁻¹ S⁻¹ and 3.91 × 10⁻³ cm² V⁻¹ S⁻¹, respectively. From the results, we can observe that the hole mobility of the films casted from toluene solutions with EB additive was improved more than 10 000 times compared with that of film casted from pristine toluene solution. As mentioned above, we ascribe this enhancement to the improved crystallinity of P3HT and the enlarged P3HT of the PC₇₁BM domain sizes.

Fig. 9 The J–V curves for hole-only devices fabricated with different EB ratios.

5 Conclusions

By studying the HSPs of EB in P3HT:PC₇₁BM, we believe that EB can act as a processing additive in the toluene solution of P3HT:PC₇₁BM for its poor P3HT solubility and good PC₇₁BM solubility. After optimizing the incorporation ratio, we demonstrated a 5.1 times enhanced PCE of PSCs even without thermal annealing, and a 4.82% PCE could be achieved after thermal annealing. By systematically analysing the results of the absorption spectra, XRD spectra, AFM, TEM and SCLC, we concluded that the incorporation of EB in P3HT:PC₇₁BM can increase the phase separation and the crystallinity of P3HT and can achieve an effective balance between excitons diffusion and charge carrier mobility.

Acknowledgements

This work was supported by (1) The National One-Thousand Foreign Expert Program (WQ20123700111), (2) Natural Scientific Foundation of China, Grant #51273096, (3) Natural Scientific Foundation of China, Grant #51373081, (4) Natural Scientific Foundation of China, Grant #51473082, and (5) Shandong Province Project: Tackle Key Problem in Key Technology, #2010GGX10327.

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Footnote

† The authors contribute equally to this work.

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