Theoretical exploration towards high-efficiency tunnel oxide passivated carrier-selective contacts (TOPCon) solar cells

Solar Energy 155 (2017) 654–660

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Theoretical exploration towards high-efficiency tunnel oxide passivated carrier-selective contacts (TOPCon) solar cells Yuheng Zeng a, Hui Tong a,c, Cheng Quan a,b, Liang Cai a,c, Zhenhai Yang a, Kangmin Chen b, Zhizhong Yuan b, Chung-Han Wu a, Baojie Yan a, Pingqi Gao a,⇑, Jichun Ye a,⇑ a b c

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences (CAS), Ningbo City 315201, PR China School of Material Science & Engineering, Jiangsu University, Zhenjiang City 212013, PR China University of Chinese Academy of Sciences, Beijing City 100049, PR China

a r t i c l e

i n f o

Article history: Received 5 April 2017 Received in revised form 17 June 2017 Accepted 3 July 2017

Keywords: Tunnel oxide Passivated carrier-selective contacts TOPCon Numerical simulation

a b s t r a c t In this work, we used the numerical simulation method to study the tunnel oxide passivated carrierselective contacts (TOPCon) structured solar cells, with the focus especially on the paths towards excellent surface passivation and low contact resistance. The presence of an ultra-thin silicon oxide (SiO2) with high quality (typically low interface-states density, Dit
1  1010 cm2 eV1 and low pinhole density, Dph < 1  104) suppresses the recombination of carriers at the rear surface. As a result, implied open circuit voltage (iVoc) could be promoted by a value of more than 30 mV comparing with the solar cell without oxide layer, which is the primary benefit originated from TOPCon structure. Corresponding, the iVoc and recombination current density (Joe) could reach 745 mV and 9.5 fA/cm2 (Dn = 5  1015 cm3) for the 1-X cm and 200-lm n-type wafer covered with high-quality oxide and n+-Si layers. In addition to passivation, a well-designed SiO2/n+-Si backside structure is also critical for carrier collection. The tunneling current is susceptible to oxide thickness, i.e., a 0.2-nm increase in SiO2 thickness results in the decrease of the tunneling current by more than one magnitude under certain circumstance. Fortunately, raising the doping in n+-Si layer enhances the tunneling possibility of electron, which allows for a thicker oxide that is favorable to a stable mass production. The simulation suggests that to obtain a high fill factor (FF, >84%), a minimum forward-bias saturated tunneling current of about 0.01 A/cm2, more favorable of 0.1 A/cm2, is required for the Si/SiO2/n+-Si structure. Generally, our work offers an improved understanding of tunnel oxide, doping layer and their combined effects on TOPCon solar cells. Besides simulation, we also discuss the practical manufactures of how to control the above mentioned parameters, as well as the problems needed to be solved for further work. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Recently, silicon solar cells with tunnel oxide passivated carrierselective contact (TOPCon) structure (Feldmann et al., 2013, 2014a, 2014b, 2014c; Reichel et al., 2014; Moldovan et al., 2015; Yan et al., 2015, 2016; Tao et al., 2016) or poly-Si on passivating interfacial oxides (POLO) structure (Peibst et al., 2016) have demonstrated the great potential and received significant attention. For these types of solar cells, an ultrathin oxide layer is applied to the rear surface of Si absorber to deliver the high-quality and full-area passivation, as well as the electron tunneling function (Moldovan et al., 2014; Richter et al., 2015). A heavily doped n-type (n+) Si ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Zeng), [email protected] (P. Gao), [email protected] (J. Ye). http://dx.doi.org/10.1016/j.solener.2017.07.014 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.

layer is typically used in between the ultrathin oxide layer and rear metal contact to fulfill the effective full-area carrier collection. As a result, champion efficiency of TOPCon solar cell has been rapidly promoted to 25.1% in laboratory recently (Fraunhofer, 2017), one of the highest efficiencies for crystalline solar cells. More importantly, this high efficiency will not come at the expense of complicated processes. Unlike other advanced cell architectures, such as PERC, IBC (Glunz et al., 2012), TOPCon solar cells have the fullarea contact, eliminating the relatively complicated process for localized contacts. Recently, TOPCon processes, developed quickly by the major PV-equipment suppliers, show great potential for mass production in the near future (Steinkemper et al., 2016). To date, emerging TOPCon solar cells have been studied mainly through the experimental methods. In order to have a more comprehensive understanding of the critical parameters that affect the performance of the devices, a systematic numerical exploration

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is necessary. These critical parameters include the quality and thickness of the silicon oxide (SiO2) layer, the doping concentration and quality of the n+-Si layer, the quality of interface, as well as their combination. By means of numerical simulation, Steinkemper et al. has given a good example to figure out the performances of TOPCon solar cell featuring different n+-Si materials (polycrystalline silicon and amorphous silicon) (Steinkemper et al., 2015a, 2015b). They concentrate on the carrier-selective electron contacts featuring tunnel oxides and point out what kind of structure and materials are suitable for TOPCon solar cell. In this work, we explored the paths to achieve both the excellent surface passivation and carrier collection of TOPCon solar cell using the numerical tool of AFORS-HET (Automat FOR Simulation of HETerostructures) software (AFORS-HET, 1, 2). We agree the main conclusions derived by Steinkemper’s works and our work can be viewed as an add-on to their previous publications. The difference of this work is to consider the effects of detailed parameters on device performances, such as oxide thickness, doping, carrier transport, interface-states density (Dit) and pinhole density through the oxide layer (Dph). Also, our simulation suggested strategies for an optimal design of the oxide and n+-Si layer which leads to the high open circuit voltage (Voc) and low tunneling resistance. In summary, our findings provide a further understanding of limitations and challenges concerning the main factors and will give a guidance to develop the high-efficiency TOPCon solar cells. 2. Simulation setup AFORS-HET V.2.5 developed by HZB (Hahn-Meitner-Institute Berlin) is a very powerful and professional numerical simulation tool for the hetero-junction solar cells. Through this software, a number of simulation papers have been published (AFORS-HET). In this work, we studied exhaustively the solar cells featuring tunnel oxide passivated contact structure, the so-called TOPCon solar cells. Noted that the rear structure of TOPCon solar cell is a hetrojunction. The structure of the TOPCon solar cell used in this work is listed as following: front electrode/SiNx (front pyramid-textured)/p+-Si layer/n-Si wafer absorber/SiOx/n+-Si layer/rear electrode. Carrier transportation through dielectric (SiOx) layer is described by two mechanisms: the thermionic-emission and thermionic-field model and the tunneling model. The optical absorption is described by the multiple reflections and coherence model. Table 1 demonstrates the assumed valued for the parameters of the TOPCon devices used for the numerical simulation. The default parameters of the electrical and optical layers are generally referred to the published work (Feldmann et al., 2014a, 2014b). The graphic representation of the

simulated solar cell structure is given in Fig. 5(a). Noted that the Bandgap Narrowing (BGN) effect is already considered in AFORSHET. Where t is the thickness, SRV the surface recombination velocity, Na and Nd are the acceptor concentration and donor concentration, respectively, Chi the electron affinity energy, Eg the band gap, dk the relative dielectric constant, me and mh the relative effective mass of electron and hole, Dph pinhole density through the insulator layer (dimensionless unit), and w/o means without. Herein, it needs to mention that although AFORS-HET is a 1D simulation tool, it still has the capability to deal with pinholes. As the pinholes are the micro-holes in oxide film, which serve as leak channel for bulk carriers. Through the pinholes, a significant percentage of bulk carrier would leak through dielectric layer. Even the pinholes are distributed in 2-D dielectric, the percentage of leak carrier can be integrated. Hence, the percentage of leak carrier could be deal as 1-D case. 3. Results and discussion 3.1. Path to excellent surface passivation Improving passivation is the primary advantage brought by SiO2/n+-Si structure. To explore the paths towards better passivation, we studied the theoretical lower limit of recombination current density (Joe) (Cuevas et al., 2013) and the upper limit of implied Voc (iVoc) (Sinton and Cuevas, 1996), addressed the underlying physical mechanism of passivation, and examined the negative effects of interface defects on passivation. It should be mentioned that Joe, as another way to represent the effective surface recombination velocity, Seff, is an important parameter to describe the quality of surface passivation. Also, iVoc, derived from implicit current–voltage characteristic curves from quasi-steadystate photo-conductance (QSSPC), is helpful to estimate the potential Voc of solar cell, as well as the surface passivation. Joe and iVoc can be calculated using Eqs. (1) and (2) and Eq. (3) respectively. It should mention that the Ndop and thickness of the wafer are 5  1015 cm3 and 200 lm respectively, and the Joe is calculated under the injection level of 5  1015 cm3.

1

seff



sAuger ¼

Layers

Default parameters

SiNx dielectric Front contact boundary Front contact

t = 70 nm Standard texture surface (54.74°), w/o absorption loss, flat band MS Schottky contact model, SRV = 100 cm/s (assuming good passivation) t = 0.3 lm, Na = 2.6  1019 cm3, lifetime setting: 1 ls t = 200 lm, Nd = 5  1015 cm3, lifetime setting: 1  105 ls (without bulk defects) Chi = 1.0 eV, Eg = 8.9 eV, dk = 3.9, me = 0.98, mh = 0.49, Dph = 0 (Nicollian and Brews, 1982), Dit (at the interface of wafer and oxide) t = 20 nm, Nd = 1  1019 cm3, lifetime setting: 50 ls MS Schottky contact model, SRV = 1  105 cm/s Plane surface, w/o absorption loss, flat band

p+-type Si layer n-type c-Si layer Interface: SiOx

n+-type Si layer Rear contact Rear contact boundary Ag electrode

t = 1 lm

¼

1

sSRH

þ

2J oe ðNdop þ DnÞ qn2i W

ð1Þ

Dn ½C n ðDn þ Ndop Þ þ C p Dn½DnðDn þ Ndop Þ  NC NV expðEg =kTÞ ð2Þ

iV oc ¼ Table 1 Parameters of TOPCon solar cells.

1

sAuger

  DnðDn þ Ndop Þ kT ln q n2i

ð3Þ

where seff, sAuger, sSRH are the effective lifetime, Auger lifetime, SRH recombination lifetime; ni the intrinsic carrier concentration; Ndop the concentration of donor or acceptor atoms; Dn the excess carrier density of electron or hole; Cn and Cp the Auger coefficients; NC and NV the total effective state density of conduction band and valence band; W the Si bulk thickness; Eg the Si band gap; k the Boltzmann constant; q the elementary charge of an electron; T the Kelvin temperature. Fig. 1(a) shows the image of Si(n+)/SiO2/c-Si(n)/SiO2/Si(n+) structure and the corresponding Joe. This symmetrical structure was studied herein in order to estimate the passivation ability of the SiO2/Si(n+) structure for each surface. Joe was as high as 41,200 fA/cm2 for the structure without SiO2 layer nor n+-Si layer, which indicated severe surface recombination. Joe was reduced to a low value of 35 fA/cm2 by adding a n+-Si layer with a high doping concentration of 1  1021 cm3, which is the so-called back-field

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Fig. 1. iVoc of the n+-Si/SiO2/n-Si/SiO2/n+-Si structure with various SiO2 thicknesses (0.0–1.6 nm) and Nd (1  1017–1  1021 cm3). The Ndop and thickness of the n-Si wafer are 5  1015 cm3 and 200 lm.

effect. Similarly, Joe was reduced to about 10 fA/cm2 by using an oxide layer with the thickness of more than 1.6 nm. If a duallayer stack of oxide and n+-Si stack layer was added, Joe was further reduced, as showed in Fig. 1(a). When the thickness is larger than 1.2 nm, Joe shows little sensitivity to the doping level of the n+-Si layer, especially when doping concentration of larger than 1  1018 cm3. It needs to classify that Joe is influenced by lots of factors, including the mathematics model and the adopted parameters, e.g., doping of wafer, lifetime of wafer, doping of n+-Si layer, and SRV, which may lead to different results calculated by various researchers but the physics behind the simulation would not change. iVoc is a more direct parameter to estimate the device performance, as it is calculated under one-sun injection level. Relatively, the same trends were observed in iVoc, as presented in Fig. 1(b). The iVoc was as low as 520 mV for the structure without SiO2 nor n+-Si layer. The iVoc increased to the highest value of about 743 mV by adding a proper oxide and n+-Si stack layer, which suggested TOPCon solar cell has of great potential to achieve a very high Voc if front-sided surface recombination is low. The underlying passivation mechanism of TOPCon-structured can be clearly demonstrated in Fig. 2(a) band structure, (b) electron/hole density, and (c) electron/hole current. An obvious discontinuity of the hole quasi-Fermi level between the n-Si wafer and n+-Si layer appears when a SiO2 layer is inserted. At the open circuit condition, the net recombination equals to the net generation. However, an oxide lyaer stops the flow of minority carriers (holes) toward to the high recombination region (metallized surface), thus more holes present in the substrate and less holes present in the high recombination region. The less recombination at the interface between the SiO2/silicon substrate leads to the discontinuity of the hole quasi-Fermi level, and the discontinuity of hole quasi-Fermi

level is enlarged as an oxide thickness increases, as shown in Fig. 2(a). Also, a thicker oxide layer promotes the accumulated bulk carriers (hole and electron) and enlarges the gap between hole and electron quasi-Fermi levels, which is good to raise the device’s Voc, as shown in Fig. 2(b). Finally, a greatly reduced leak current is observed by inserting an oxide layer, as show in Fig. 2(c). A SiO2 layer of 1.2 nm is thick enough to decrease the electron and hole leak current to almost zero, which explains why a thin SiO2 layer will generate a high Voc. Another advantage of tunneling oxide is that a TOPCon solar cell with a thin oxide shows of great capability to resist a severe rearSRV, as showed in Fig. 3. The rear-SRV is the carrier recombination rate between n+-Si layer and electrode. Significant decay of Voc from 703 mV to 611 mV is caused by the increment of rear-SRV from 1 to 1  105 cm/s in a device without oxide nor doping layer. The Voc in a device with a doping layer decays to about 628 mV with the rear-SRV of 1  107 cm/s, which indicated the decay of Voc could be suppressed but not able to be eliminated by a doping layer. The negative effect of SRV is further suppressed by inserting an oxide layer. The influence of SRV becomes weaker as the growth of oxide thickness. As indicated by simulation, an oxide of 1.0– 1.2 nm is thick enough to keep the Voc above 736 mV even with a severe rear-SRV of 1  107 cm/s, for which the reason could be attributed to the suppression of leak current by oxide. Two key properties of SiO2 layer, the total interface states (Dit) and the pinhole density through dielectric layer (Dph), were studied to assess the effects of SiO2 quality on surface passivation. Noted that Dit is set at the interface of wafer and oxide, while Dph is set in oxide layer. A high iVoc of 742 mV, very close to the highest simulated value of 743 mV, is achieved when Dit has a low value of about 1  109 cm2, as presented in Fig. 4(a). The iVoc keeps above 735 mV at a Dit value of 1  1010 cm2, whereas it falls to 714 mV when Dit increases to 5  1010 cm2, after which iVoc accelerates to decay. A theoretical iVoc of 743 mV is achieved when Dph is lower than 1  108, as presented in Fig. 4(b), indicating that a very lowdensity pinhole density has a negligible effect on iVoc. There is a minor decrease when Dph is less than 1  104. However, the iVoc accelerates to decay when Dph is more than 1  104, and it further decays to 665 mV when Dph is higher than 1  101, which is very close to the lower limit value of 663 mV for the structure without SiO2. A dense and intact SiO2 layer with low pinhole density is essential for the high performance of devices. 3.1.1. Path to excellent carrier collection A trade-off in the thickness of SiOx layer is needed to ensure an excellent passivation and a high tunneling efficiency of electrons. Fortunately, the carrier tunneling efficiency can be improved when the doping concentration in n+-Si goes higher. Thus, a proper match of oxide thickness and doping concentration is a key effort for TOPCon solar cells to achieve high performance in both passivation and tunneling current. A direct way to decide a proper design of oxide and doping is to examine the FF of TOPCon solar cell, as showed in Fig. 5. When Nd is 1  1019 cm3, FF remains at a high level of 84% and the influence of oxide is nearly ignorable, when the oxide thickness is less than 1.0 nm, as indicated in Fig. 5(b). However, the FF begins to decay with the oxide thickness and quickly falls to a low value with several angstrom increase in thickness. For example, FF reduces to 71% and 22% with a 1.4-nm and 1.6-nm oxide layer, respectively. Fortunately, higher doping in n+-Si layer would enhance the tunneling possibility of electrons, which allows for a thicker oxide. For example, a FF of 83% is obtained with a 1.4-nm oxide layer by increasing the doping concentration to 1  1020 cm3. The FF of 83% is still kept with a 1.6nm oxide layer if the doping concentration increases to 1  1021 cm3. Actually, a small fluctuation of thickness

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Fig. 2. (a) Band structure, (b) electron/hole density, and (c) electron/hole leak current of the TOPCon solar cell with various SiO2 thicknesses (0.0, 1.2 and 1.6 nm) under the Voc-operation point.

Fig. 3. Effects of the rear-SRV on Voc of the TOPCon solar cell with various rear surface structures.

(±0.2 nm) is normal for growing such a thin oxide layer. Raising doping concentration makes the manufacture become more tolerant to a thicker oxide layer and more favorable to a stable mass production. Herein, it should be noted that to investigate the contact resistance is better than the FF. However, AFORS-HET is designed only for the 1-D simulation and cannot deal with TLM or Cox measurements, which are aimed to study contact resistance. A detailed study of contact resistance needs to be carried out through experiments. A dark I-V curve of the wafer/SiO2/n+Si structure can be used as a simple indication of the tunnel efficiency of current, as demonstrated in Fig. 5(c–f). For all the doping concentration from 1  1019 to 1  1021 cm3, a typical decrease of current density, about 15–16 times, is observed with every 0.2-nm increment of oxide thickness, which thickness is smaller than one oxide layer (0.26 nm/each layer) (Khalilov et al., 2011). The tunnel current is sensitive to SiO2 thickness even if an increment of less than one layer. Non-linear growth of current density is observed with the increment of doping concentration, indicating that the modulation of tunneling current can be realized by doping. Noted that if a FF was more than 80% that the corresponding forward-bias saturated current density is 0.088, 0.026, and 0.008 A/cm2 with the combination of oxide and doping (1.2 nm & 1  1019 cm3, 1.4 nm & 1  1020 cm3, 1.6 nm & 1  1021 cm3) respectively, in Fig. 5(d–f). All the FFs decay rapidly as the forward-saturated current density become as low as to about 0.001 A/cm2. Through considering the link between FFs and current densities, we suggested that to obtain a reasonably high FF (>84%), a minimum tunneling current of about 0.01 A/cm2, more favorable of 0.1 A/cm2, is essential for the Si/SiO2/n+-Si structure.

Fig. 4. Effects of (a) Dit and (b) Dph on iVoc of the n+-Si/SiO2/n-Si/SiO2/n+-Si structure with various SiO2 thicknesses (0.0, 0.8, 1.2, and 1.6 nm).

The overall performances of the TOPCon solar cells with various oxide thicknesses and doping concentrations are given in Fig. 6. The default parameters of the TOPCon solar cells are listed in Table 1. Noted that the maximum Jsc is about 37 mA/cm2, much lower than a typical value, 40 mA/cm2, leading to a smaller efficiency than references. This is because that optical absorption is not optimized herein, as absorption is not the key concern in this work. It is believed that the Jsc would approach its the highest value calculated by AFORS-HET if each functional layer is optimized carefully in further work. Depending on the simulation, Voc approaches its upper-limited value with the increment of oxide thickness and doping concentration, whereas Jsc decreases as the oxide exceeds a certain thickness. Comparing with Jsc, FF is more susceptible to the oxide thickness, i.e., FF starts to decay at a smaller oxide thickness. That the outline of the efficiency curve is parallel with the FF one

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Fig. 5. (a) Graphic representation of the simulated TOPCon solar cell and (b) the corresponding FF influenced by the doping concentration and oxide thickness. (c) Image of the n-wafer/SiO2/n + -Si structure, and (d–f) the corresponding forward-bias I-V curves with different doping concentrations and oxide thicknesses.

suggests the impact of FF on device performance is more important. A proper design for a reasonable high FF is the crucial step towards a high-efficiency TOPCon solar cell. Besides simulation, we would like to discuss the art-of-state techniques to manufacture the high-quality oxide layer. First, growing a dense, thickness-controllable, and high-oxidation-state oxide layer is the primary step towards excellent passivation. Fraunhofer and Hannover have indicated that the wet-chemical HNO3 oxide, dry-grown UV/O3 oxide, ozonized DI-H2O (DIO3) oxide, and thermal-anneal-grown oxide show comparable passivation effects (Moldovan et al., 2014, 2015; Peibst et al., 2016). The champion iVoc or Joe is very close to the highest simulated value predicted in this work, suggesting that a ‘‘perfect” oxide can be manufactured in practice. Noted that the differences between simulation and experiment are possible from the different wafers. Second, anneal with proper temperature and duration is another key step towards excellent passivation. A high-temperature anneal, typically around 900 °C, helps to eliminate the interfacial crystalline defects. A well design process could lead to a lower limit of Dit lying around 1  1010 cm2 eV1 (Schroder, 2006). Third, a trade-off between the reduction of Dit and Dph is needed in practical manufacture, because an extending high-temperature process will induce significant pinholes (Lancaster et al., 2016). Finally, we would like to discuss several problems that block the further insight into the tunnel oxide and n+-Si layer. First, Dit cannot be characterized directly through the traditional capacityvoltage (C-V) measurement, because the tunnel oxide layer is so thin that it is no longer be electrical insulation. Till now, Dit of TOPCon structure is still unclear. Surface photovoltage (SPV) as an accepted method to characterize Dit of HIT interface (Sark et al., 2011) should be also effective to evaluate TOPCon structure. Second, as far as the author’s knowledge, there is only one report concerning the characterization of pinholes (Lancaster et al., 2016),

which work suggests the pinholes highly relative with tunnel current and uses the conductive atomic force microscopy (cAFM) to assess the pinhole density. The cAFM measurement is reasonable and useful, but the link between the tunnel current and pinhole density needs to be built in future. We believed that to establish a whole set of characterization methods for the tunnel oxide layer is the primary assignment for a further understanding of TOPCon solar cell and its derivate devices.

4. Conclusions In this work, we studied TOPCon solar cell using a numerical simulation method. Our work provides an improved comprehension of tunnel oxide, doping layer and the design of TOPCon solar cell. The primary of TOPCon structure is to promote the Voc of significant, for which the underlying physical mechanism was that an ultra-thin silicon oxide layer suppressed the recombination of carriers at the rear surface. Our simulation indicates that the upperlimited Joe and iVoc could be 9.5 fA/cm2 (injection level: 5  1015 cm3) and 745 mV respectively for the 1-X cm and 200lm n-type wafer. Corresponding, a high-quality oxide layer with low Dit of about 1  1010 cm2 eV1 and low Dph of less than 1  104 is necessary to approach the highest simulated values. In practice, a well-designed process could lead to a lower limit of Dit of around 1  1010 cm2 eV1. Theoretically, it is possible to control the density of pinholes through a well-controlled anneal process. Higher doping concentration in n+-Si layer is useful to improve carrier-collection efficiency, which is typically useful because it helps the manufacture become more tolerant for a thicker oxide and more favorable for a stable mass production. In summary, the above mentioned parameters, such as oxide thickness, doping concentration, Dit, and Dph, are able to be controlled, so we believed that it was possible to fabricate a high-efficiency

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Fig. 6. Effects of oxide thickness (0.0–1.8 nm) and doping concentration (1  1019–1  1021 cm3) on (a) Voc, (b) Jsc, (c) FF, and (d) efficiency of the TOPCon-structured solar cells.

TOPCon solar cell approaching to its theoretical predictive efficiency with a well-designed manufacture.

Acknowledgement The author would like to thank the financial supports by Zhejiang Provincial Natural Science Foundation of China (Grant No. LR16F040002), Natural Science Foundation of China (Grant Nos. 61106096 and 51502315), Natural Science Foundation of Zhejiang Province (Grant No. LY15F040003), Major Project and Key S&T Program of Ningbo (No. 2016B10004), Natural Science Foundation of Ningbo City (Grant Nos. 2015A610033 and 2015A610040), International Cooperation Project of Ningbo (Grant No. 2016D10011), International S&T Cooperation Program of Ningbo (Grant No. 2015D10021), the Key S&T Research Program of Ningbo (Grant No. 2014B10026).

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