Interlayer Interaction Enhancement in RuddlesdenâPopper Perovskite

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Letter

Interlayer Interaction Enhancement in Ruddlesden-Popper Perovskite Solar Cells Towards High Efficiency and Phase Stability Mingzhu Long, Tiankai Zhang, Dongcheng Chen, Minchao Qin, Zefeng Chen, Li Gong, Xinhui Lu, Fangyan Xie, Weiguang Xie, Jian Chen, and Jianbin Xu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00351 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Energy Letters

Interlayer Interaction Enhancement in RuddlesdenPopper Perovskite Solar Cells Towards High Efficiency and Phase Stability Mingzhu Long, 1,† Tiankai Zhang, 1,† Dongcheng Chen, 2 Minchao Qin, 3 Zefeng Chen, 1 Li Gong, 4 Xinhui Lu, 3 Fangyan Xie, 4 Weiguang Xie, 5 Jian Chen, 4 and Jianbin Xu 1,* 1

Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New

Territories, 999077, Hong Kong. 2

State Key Laboratory of Luminescent Materials and Devices, South China University of

Technology, Guangzhou, 510640, P. R. China. 3

Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, 999077,

Hong Kong. 4 Instrumental

Analysis and Research Centre, Sun Yat-sen University, Guangzhou, 510275, P.R.

China. 5

Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials,

Department of Physics, Jinan University, Guangzhou, 510632, P.R. China.

ACS Paragon Plus Environment

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†These authors contributed equally to this work. *E-mail: [email protected]


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ABSTRACT

The Ruddlesden-Popper perovskites (RPP) have attracted intense attention owing to the superior stability. However, the interspace and weak interaction between the inorganic layers inhibits direct charge transfer and brings phase segregation issue under illumination. Herein, it is presented that, after the incorporation of a symmetric cation guanidinium (GA+) with three amino groups, the crystallinity, photoelectronic properties and stability can be synchronously ameliorated owning to the enhanced interlayer interaction. The new (BA, GA)2(MA)2Pb3I10 RPP film shows a preferentially vertical orientation and possesses a spatially well-aligned band from the bottom to the top surface, resulting in a fast energy transfer. It contributes to a large Voc of 1.25 V with minimized nonradiative loss of 0.14 V and a boosted efficiency of 14.47%. More importantly, the lattice strain in the RPP film can be eliminated due to the medium size of GA+, protecting it from phase segregation even under laser irradiation.

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Organo-inorganic halide perovskites have revolutionized the field of photovoltaics, with spectacular achievements in power conversion efficiencies (PCE) reaching over 23% recently,1 that already rivals the commercialized thin-film technologies. In addition to the achieved remarkable performance, successful industrialization of perovskite solar cells (PSCs) in the near future must require long-term reliable operation under real working conditions, involving humidity-, photo-, and thermal-stability.2-8 Comparing to the three-dimensional (3D) perovskite ABX3 (A = methylammonium (MA) or formamidinium (FA), B = Pb, X= I, Br or Cl), which is susceptible to decomposition in heat or humidity condition, the two-dimensional (2D) organicinorganic hybrid Ruddlesden-Popper counterpart A′2An−1BnX3n+1 demonstrates superior environmental stability property.9-19 It is attributed to the reason that the hydrophobicity of the bulk aliphatic or aromatic alkylammonium cations, such as n-butylammonium (BA), 1naphthylmethylamine, phenethylammonium, can protect the metal-halide framework from water molecular penetration, amounting to an encapsulation effect.20-28 However, in the RPP structure, the hydrophobic, nanometre-scale organic cations prevent the carrier charge from transferring between the adjacent inorganic octahedra layers.11, 24, 29 The strong difference in dielectric constant between organic and inorganic layers induces excitons with large binding energy and low carrier mobility, which favours recombination before being collected by the electrodes and hinders its photovoltaic applications.30 The carrier mobility and diffusion length limit the current, so the increase of optical thickness of the RPP phase cannot ensure high PCE. It is demonstrated that hot casting technique can drive the PCE improvement for RPP solar cells, arising from the out-of-plane orientation of the inorganic framework. 11 Besides, recent studies show that the orientation of the RPP framework can be effectively tuned through a mixed solvent method or different additives incorporation in the precursor.31-33 Additionally, composition


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tailoring of organic cations in RPP can also significantly improve out-of-plane preferential orientation of RPP films.27,

34

The elaborate orientation enables directional carrier collection

through highly ordered structure and brings the possibility of high-efficiency optoelectronic devices, opening up the academic research about the crystallization dynamics and performance optimization for the RPP-based solar cell devices. On the contrary, the disordered 2D perovskite would cause heavy carrier loss and thus a high price of energy loss of several hundred meV due to high recombination rates. In addition to the crystallization optimization, another point is that the existence of “edge state” in quasi-2D perovskite enables the possibility of lowering the energy loss for exciton separation, resulting in significantly improved performance of optoelectronic devices. 24

As the exciton dissociation and carrier transport in the RPPs dominate the PCE values for

photovoltaic devices, therefore, there is still great potential to improve photophysical properties through tuning the crystal texture as well as exciton-carrier dynamics in the RPP phases. Recently, a relatively large cation candidate guanidinium (C(NH2)3+ (GA+), ionic radius: 2.78 Å) has been employed to boost the photovoltaic performance of 3D PSCs which was attributed to a surface passivation effect.

35-37

It has also been reported that GA incorporation into a mixed-

cation mixed-halide and stable perovskite (FA1‒xCsx)Pb(I1‒yBry)3 demonstrates remarkable further enhanced materials stability.38 Additionally, a new-type layered perovskite (GA)(MA)nPbnI3n+1 was developed with both GA and MA acting as interlayer spacers, of which the uncommon structure is defined as Dion-Jocobson (DJ) perovskite phase. For this n=3 DJ perovskite thin film prepared by normal spinning-coating method, its solar cell device exhibits a high Voc of 1 V, an FF of 80% and a PCE of 7.26%.39 Therefore, the cation GA holds the promise to improve the efficiency of either 3D or 2D layered PSCs. Considering the above impacts and the moderate ion size between BA and MA, the idea of incorporating the GA cation into 2D structure has been


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proposed in this work to explore the crystallographic properties, exciton-carrier dynamics and phase stability of the quasi-2D perovskite BA2MA2Pb3I10. It shows the GA incorporation into RPP BA2MA2Pb3I10 enables to obtain high quality thin film with desired perpendicular orientation, where the GA molecule can intercalate into the quasi-2D framework through occupying BA position. Transient absorption (TA) characterization illustrates an enhanced interlayer electronic interaction and an ultrafast charge transfer (~1 ps) between RPPs from small n RPP phase to large n phase, which favours charge separation and suppresses trapassisted charge recombination, leading to remarkably enhanced photoelectronic properties. The fast energy transfer process can be attributed to the well aligned band diagram between RPPs, highly oriented crystal structure and enhanced interaction between adjacent layers. We demonstrate that, the (BA0.775GA0.225)2(MA)2Pb3I10-based solar cell delivers a large open circuit voltage (Voc) value up to 1.25 V with a minimized nonradiative loss of 0.14 V and a doubled short circuit current density (Jsc) of 16.20 mA cm‒2, leading to a boosted efficiency of 14.47%. This high photovoltage is rarely reported in small n RPP-based solar cells. It expresses different crystal structure and carrier kinetics information in the RPP film after GA incorporation. Furthermore, GA occupying the bulky BA position is favourable for lattice strain relaxation in the BA2MA2Pb3I10 structure, leading to suppressed phase segregation and decomposition under humidity, and remarkably enhanced photo-stability even subjected to laser irradiation. This work reveals that modulation of interlayer interaction through organic cation design in RPP phase enables structure and carrier kinetics tuning, which provides great potential for performance and stability breakthrough in the RPP-based photoelectric devices. The BA2MA2Pb3I10 (n=3) thin films with different molar ratios of GA incorporation were prepared through one-step solution process. Crystallographic orientation of the films was


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characterized using X-ray diffraction (XRD), as shown in Figure 1a. For the pristine BA2MA2Pb3I10, strong diffraction peaks at 14.0° and 28.2° are observed, corresponding to the (111) and (202) planes, respectively. Another high diffraction peak at 2θ=13.5° is presented here, which corresponds to crystal plane (004) of the BA2MAPb2I7 (n=2) perovskite phase,40 reflecting a mixture of different RPP phases in the quasi-2D thin film. In small angle diffraction range as shown in Figure S1, the presence of low diffraction peaks at 6.5° and 10.6°, assigned to (040) and (060) planes of the BA2MA2Pb3I10 phase, indicates the orientation heterogeneity of the crystal structure in the polycrystalline thin film.30 The samples show enhanced diffraction intensity of the (111) and (202) peaks with the increasing amount of GA addition. Notably, with GA content reaching to 15 mol%, the (040) and (060) peaks for n=3 phase, and (004) peak for n=2 phase vanish completely. The diffraction intensity of the (111) and (202) planes increases nearly fivefold compared to that of the pristine thin film. The lack of diffraction features for other phases and other crystal planes reflects a higher phase purity and better crystal orientation. However, when the GA amount further reaches to 20%, it starts to show a decrease of the (111) peak intensity, indicating that too much GA addition causes a deterioration of the perfect crystallinity. Furthermore, the (111) and (202) peaks for the n=3 phase show a small right shift with GA addition, from 14.0° to 14.1° and from 28.2° to 28.4°, respectively. The right-shift reflects a lattice contraction of the RPP phase, which is due to the reason that smaller-size GA cation prefers to occupy the position of larger-size cation BA in the crystal structure, resulting in the formation of new RPP (BA0.775GA0.225)2(MA)2Pb3I10.


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Figure 1. (a) XRD patterns of BA2MA2Pb3I10 films. GIWAXS images of films without (b) and (c) with 15 mol% GA addition. Schematic illustration of crystal structure with random orientation for the pristine thin film (d1) and perpendicular orientation to the substrate after 15 mol% GA incorporation (d2). (e) Steady-state absorption spectra of BA2MA2Pb3I10 films.

The grazing incidence wide-angle X-ray scattering (GIWAXS) was conducted to further check the in-depth crystallography orientation difference of the quasi-2D perovskite thin films as shown in Figure 1b and 1c. The pristine sample shows some Debye-Scherrer rings at specific q values of 0.5, 1.0, 1.7 and 2.0 Å‒1, illustrating a much


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random orientation of the polycrystalline structure. On the contrary, the optimized film with 15 mol% GA addition shows sharp and discrete Bragg spots with diffraction rings disappearing, especially for the crystal planes of (11 1) ,（202） and (311) in GIWAXS image, representing a nearly single crystalline orientation of the RPP thin film. The comparison of diffraction intensity versus qz shown in Figure S2 also indicates improved crystallinity in the presence of GA. Therefore, it is interesting to find that the GA incorporation can greatly enhance the out-of-plane orientation of the RPP phase and the optimum content is demonstrated to be 15 mol% from the crystallization perspective. The crystal structure orientation is schematically illustrated in Figure 1d1 and 1d2. The inorganic framework is randomly oriented for the pristine thin film, while it is well perpendicular to the substrate surface after GA addition, which is presumed to provide an efficient charge-transport channel for the electronic devices due to circumventing the insulting organic barrier.34

The steady-state absorption spectra of the Q-2D BA2MA2Pb3I10 films with different amount of GA addition are displayed in Figure 1e. The onset of spectrum for the sample


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with 15 mol% GA addition shows a very small red shift from 711 nm to 717 nm compared to the pristine film, corresponding to a bandgap shift from 1.74 to 1.73 eV as shown in Figure S3, and the absorption intensity obviously increases in the whole absorption wavelength range. The distinct peak at ca. 604 nm is assigned to excitonic absorption of n=3 phase BA2MA2Pb3I10. To check the spatial distribution of RPP phases with different n values in the quasi2D perovskite thin films, photoluminescence (PL) spectra were conducted with excitation laser wavelength at 532 nm and varied laser intensity, illuminated from both the perovskite side (front side) and substrate side (back side) as shown in Figure 2a-2d. For the pristine thin film with back excitation, there are three dominant PL peaks located at ~618, 650 and 672 nm, which are attributed to n=3, 4 and 5 RPP phases, respectively, and the peaks at 650 and 672 nm possess comparable intensity. From the front excitation, it shows three PL peaks at almost the same position with the n=4 phase owning the highest intensity, while the intensity of other two peaks is obviously attenuated especially for the one of n=5 phase. It suggests a spatially random distribution of the RPP phases


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in the whole thin film. Specifically, the top surface is mainly consisted with n=3 and 4 phases, while the bottom is comprised of n=3, 4 and 5 phases. Besides, PL results also confirm

there

is

not

any

large

n

(n>5)

RPP

phase

formed.

For

the

(BA0.775GA0.225)2(MA)2Pb3I10 film, from the substrate-side excitation, it also demonstrates three main PL peaks at 621, 654 and 678 nm, of which the position is nearly the same as that of the pristine film. Therefore, GA incorporation does not alter the RPP phase composition. However, the relative intensity of peaks at 621 and 654 nm is greatly reduced compared to the one at 678 nm from back excitation, which is different from the pristine one. Furthermore, from the front excitation, there is only one PL peak observed which is located at 678 nm displayed in Figure 2d. The distinct difference of the PL signals suggests that the n=5 RPP phase prefers to locate at the top of the whole thin film with GA addition, while the bottom is comprised with a series of different-n phases. Therefore, it is inferred that the (BA0.775GA0.225)2(MA)2Pb3I10 film possesses a gradient composition distribution with n=5 phase located at the top, and thus a vertically better aligned band diagram between RPP phases from the bottom to the surface.


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Figure 2. PL spectra of the RPP films under excitation intensity of 0.007, 0.07, 0.21, 0.35 and 0.7 W m‒2 with laser illuminated from back (a, c), and front sides (b, d). (e1, e2) Schematic illustration of the exciton transfer and photoemission process in the thin film without

and

with

GA

addition.

Surface

topology

(f1)

and

C-AFM

(f2)

of

(BA0.775GA0.225)2(MA)2Pb3I10 thin film (scale bar: 0.5 µm).

The relation between PL signals (IPL) and excitation intensity (I0) is summarized in Figure S4 and it can be considered as I PL   I 0a . When the exponent α is close to 1 or 2, the recombination is mainly involved with exciton or free carriers, respectively.

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It is

obvious that the PL intensity for the sample after 15 mol% GA addition gets much stronger under different excitation conditions. In addition, the PL intensity for the BA2MA2Pb3I10


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thin film depends linearly on the photoexcitation intensity with α value of ~1.1 from the front-side excitation. It reveals that the emission of the pristine thin film mainly originates from exciton recombination. Notably, the relation turns into nonlinearity for the (BA0.775GA0.225)2(MA)2Pb3I10 film from both front and back excitations, with α value increasing to ~1.45 from the front excitation. The nonlinear relation corresponds to a mixed bimolecular and monomolecular recombination. It reflects a completely different excitonic behavior and partial free-carrier nature of the GA-incorporated thin film under excitation. In other words, photo-induced excitons can be partially dissociated into carriers, leading to a bimolecular-recombination dominant PL for the GA incorporated thin film. The trap density of the thin films is estimated here using space-charge-limited-current (SCLC)

technique.

The

dark

I-V

curve

of

an

electron-only

device

SnO2/perovskite/PCBM/Ag is measured to obtain the electron trap density as shown in Figure S5. The trap density can be calculated based on the equation VTFL  ent L2 / 2 0 , where the L is the thickness of the perovskite film; ε is the relative dielectric constant (taken as 25); and ε0 is the vacuum permittivity.41 The trap density nt in the pristine perovskite is calculated to be 1.4 ×


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1016 cm−3 and is reduced to 7.6×1015 cm−3 after 15 mol% GA addition. This reduced trap density contributes to the dramatically improved radiative recombination, which is probably due to the defect passivation effect with the addition of GA. The schematic illustration of the charge dynamics in the RPP phases is provided in Figure 2e1 and 2e2. Recombination processes are elucidated here to correlate the PL emission with the photo-induced exciton transfer in RPP thin films. A more efficient energy transfer process from n=3 phase to n=4 and n=5 phases, reduces charge accumulation probability in the n=3 phase for the thin film with GA addition. Furthermore, to check the electrical property of the thin films, morphology and the correlated microscale local current mapping are characterized using conductive atomic force microscopy (c-AFM). From the topography in Figure S6, the crystal grain for the pristine thin film is very tiny with the size less than 100 nm, which can be caused by phase competition between different RPPs during crystallization process. The large molecular size difference between MA+ and bulky BA+ tends to bring about lattice distortion, deteriorating the crystallographic orientation and hindering the homogenous crystal


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growth process.

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However, it is interesting that, the crystallization process is greatly

retarded after GA addition, with annealing time increasing from 15 min to 50 min, and the degree of crystallinity becomes higher during the annealing process as shown in Figure S7. The grain size gradually increases with the increase of the GA amount, which is remarkably enlarged up to 0.6 µm with 15 mol% GA incorporation displayed in Figure 2f1. The surface roughness reduces from 12.8 nm to 3.5 nm with the surface roughness profile displayed in Figure S8. The inner crystal grain shows a distinct step-like morphology which corresponds to the layered crystal structure of the RPP phases. However, with further increase of GA amount to 20 mol%, the crystal grain is slightly reduced which is probably due to deterioration of the RPP crystal structure by too much GA occupation. From the correlated local current mapping in Figure S6, it shows that the overall current increases with the increase of GA addition. It is nearly three time larger compared to that of the pristine one when the addition amount is 15 mol% displayed in Figure 2f2, with the current distribution statistics shown in Figure S9. Similar to previous report, higher current collection near the grain boundaries is also observed here. 15, 42 This high current can be attributed to the substantial existence of benign defect sites or large n RPP phase located


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ACS Energy Letters

at the grain boundaries in quasi-2D perovskite, which can do beneficial effects in terms of exciton dissociation and carrier collection.

Figure 3. Time- and wavelength-dependent TA images of the BA2MA2Pb3I10 thin films without (a) and with GA addition (b) under 400 nm laser excitation. TA spectra at selected probe time of the BA2MA2Pb3I10 thin films without (c) and with GA addition (d). (e) TA intensity as a function of pump-probe delay time for different n RPP phases without (the bottom) and with GA addition (the top) obtained from vertical cuts from TA spectra in (a) and (b). (f) Energy transfer dynamics illustration for layered perovskites evolved from the dominant TA signals.


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To understand the photoinduced charge transfer dynamics within the quasi-2D perovskite thin films, transient absorption (TA) measurements were conducted under 400 nm laser pump excitation. Figure 3a and 3b show the TA spectrograms of the samples as a function of delay time, consisting of negative (black) ground state bleach and positive (yellow) excited state absorption. The positive band at short wavelength region from 550 to 580 nm is ascribed to the excited state absorption of the n5), indicating a phase segregation process occurred under irradiation due to phase instability issue. In comparison, the BA2MA2Pb3I10 with GA addition does not show any PL peak shift or splitting during the whole irradiation process as shown in Figure 4h, which well illustrates the superior photostability. It is essentially important for the long-term application of the PSC devices. The stark and interesting difference of the stability is probably caused by the reduced trap states and the key size feature of GA favorable for the lattice strain relaxation in the sandwiched quantum well structure. Furthermore, as shown in Figure S13b, the PCE of the BA2MA2Pb3I10-based solar cells degrades very fast under continuous light illumination. However, after GA incorporation, the PCE of the (BA0.775GA0.225)2(MA)2Pb3I10 solar cell can main 84% after 720 h one-sun illumination. Therefore, the optoelectronic behaviors and stability of the RPP-based devices can be


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further optimized through interlayer interaction enhancement by deliberate structure design and composition management of the bulky cation.

This work demonstrates that the crystallinity, carrier transport and structure stability of the RPP thin film can be remarkably ameliorated through enhancing the interaction between the adjacent perovskite interlayers. Specifically, the introduction of a symmetric cation GA with three amino groups can form a new homologous phase (BA, GA)2(MA)2Pb3I10 with GA occupying the position of BA. With a much slower crystallization process, it exhibits a substantially improved crystallinity with a predominant out-of-plane orientation. The grain size of the thin film is remarkably enlarged from 100 nm to as large as 1 µm. Besides, the thin film shows a spatially better-aligned band structure between different RPP phases from the bottom to the upper surface, resulting in an ultrafast interphases energy transfer process upon light excitation and lowered energy loss for charge separation. The (BA0.775GA0.225)2(MA)2Pb3I10 based solar cell shows a rarely reported high

Voc of 1.25 V with a minimized nonradiative loss of 0.14 V, and a doubled Jsc of 16.20 mA cm‒2, amounting to a PCE as high as 14.47%. Furthermore, it is worth noting that the


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overall stability of the RPP phase is dramatically improved with suppressed phase decomposition and segregation under humidity and illumination condition, which is due to the released lattice strain and enhanced interlayer interaction from the amino groups of GA. The device can sustain 84% of its PCE after 720 h continuous one-sun illumination. In summary, the interaction tuning between interlayers in the RPP layered structure suggests that the optoelectronic properties and overall stability can be greatly optimized through the rational design of organic cation, providing the possibility to explore the physical limits and bring the performance breakthrough of RPP-based optoelectronic devices.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. Experimental methods describing device fabrication and different characterization details, XRD patterns of RPP films, diffraction features from GIWAXS, absorption curves in a small range, PL intensity summary, SCLC curves, AFM and C-AFM results, XRD patterns obtained from different annealing time, surface roughness profile, current distribution from C-AFM results, EQEEL curves, photovoltaic parameters, steady state output, and energy band diagram of the solar cells.


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Author Information

Corresponding Author

E-mail: [email protected].

Notes: The authors declare no conflict of interest. Acknowledgements

The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant

Nos.

AoE/P-03/08,

T23-407/13-N,

AoE/P-02/12,

14204616,

14203018,

N_CUHK438/18 and CUHK Group Research Scheme, CUHK Postdoctoral Fellowship, and ITS/088/17, ITS/390/18 by Innovation and Technology Commission, Hong Kong SAR Government. J.B.X. would like to thank the National Science Foundation of China for the support, particularly, via Grant No. 61229401. W.G.X. would like to thank the National Natural Science Foundation of China (Nos. 11574119, 61674070).

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