In fl uence of Heterocyclic Spacer and End Substitution on Hole Transporting Properties Based on Triphenylamine Derivatives:

In fl uence of Heterocyclic Spacer and End Substitution on Hole Transporting Properties Based on Triphenylamine Derivatives:

Theoretical Investigation

Yang Ding,

^†

Yue Jiang,*

^,†

Wenhui Zhang,

^†

Linghai Zhang,

^‡

Xubing Lu,

^†

Qianming Wang,

^§

Guofu Zhou,

^∥

Jun-ming Liu,

^⊥

Krzysztof Kempa,

^†,#

and Jinwei Gao*

^,†

†Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, People’s Republic of China

‡School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong, People’s Republic of China

§School of Chemistry & Environment and^∥Electronic Paper Displays Institute, South China Normal University, Guangzhou 510006, People’s Republic of China

⊥Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

#Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, United States

*^S Supporting Information

ABSTRACT: Hybrid organic−inorganic halide−perovskite based solar cells have achieved outstanding progress, approaching one of the most competitive photovoltaic technologies. One of the hot topics is to develop inexpensive and efficient hole transporting materials to improve the performance of devices for practical applications. In this paper, we theoretically design a series of hole transporting materials based on triphenylamine backbone through varying the spacer and the end substitution. The properties of frontier molecular orbital, ionization potential, reorganization energy, and charge

mobility have been calculated and analyzed. The results show that the spacer and the end functional groups strongly influence the molecular geometry, stacking, electron density distribution, and especially hole mobility. The best hole transporting material with furan as spacer and hydroxyl or methoxyl as substitution is proposed due to its highest hole transporting mobility induced by the planar conformation and tightπ−πstacking, which potentially could enable the highly efficient perovskite solar cells.

1. INTRODUCTION

Perovskite solar cells (PSCs) have attracted much attention due to their high power conversion efficiency (PCE) (more than 22%)^1,2and low-cost processes. Perovskite, as a light absorber, wasfirst used in dye-sensitized solar cells (DSSCs) in 2009 with an efficiency of ∼4%,³ which was limited by the instability caused by the liquid electrolyte. This issue was solved by the introduction of solid-state hole transporting materials (HTMs), which brings the efficiency to as high as 10%.⁴⁻⁶ Currently, except for the chasing of high quality perovskite films,⁷ the investigation of simple and cheap HTMs is also considered as an important solution to push PSCs to commercial application.⁸ Spiro-OMeTAD is one of the widely used HTMs for PSCs with an efficiency of ∼20%.^9,10However, it presents a potential hurdle to further commercialization due to its expensive price induced by its complicated synthesis and purification process. Meanwhile, its intrinsic low electrical conductivity requires additive or salt for doping, which reduces the stability of PSC devices.¹¹

The general strategies to design a dopant-free HTM are as follows: (1) proper highest occupied molecular orbital

(HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for efficient hole transportation and electron blocking; (2) high hole transporting properties to reduce charge recombination; (3) stability against moisture and oxygen. Many new HTMs with typical hole transporting blocks including triphenylamine (TPA),¹²naphthalene,¹³carbazole,¹⁴ quinozino,^15,16 triptycene,¹⁷ etc.¹⁸ have been studied. Among them, TPA blocks, with a good hole transporting property, proper ionized potential, and good solubility and stability, were grafted to various frameworks or as the central core which has been used as HTMs for solar cells.¹⁹ Among them, a new simple and cost-effective material named F101 (shown as A3 in Figure 1), is reported as the HTM in PSCs with a PCE of 13%.²⁰ The same doped molecular system with fluorene− dithiophene (FDT) spacer is conducted and an efficiency of 20.2% has been achieved,²¹ suggesting a significant improve- ment of this system in photovoltaic properties. Therefore, theoretically understanding the effect of the central spacers and

Received: June 28, 2017 Published: July 24, 2017

pubs.acs.org/JPCC

the end functional groups on the performance of TPA backbone based HTMs is of great importance, which provides a clue for the rational design of HTMs for highly efficient solar cells.^22,23

Herein, 18 new HTMs, based on triphenylamine backbone, have been properly designed (Figure 1) by two strategies: (1) varying the central spacer“Ar”with furan (A), thiophene (B), selenophene (C),²⁴ 4H-cyclopenta[2,1-b:3,4-b′]difuran (D), 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (E), and dithieno[3,2-

b:2′,3′-d]furan (F); (2) substituting the end functional group

“R” with methoxyl groups (OMe, 3), hydroxyl (OH, 2) and cyano (CN, 1). The properties of the frontier molecular orbitals (FMOs), ionization potentials (IPs), and reorganization energies (REs) of newly designed molecules were calculated by density functional theory (DFT) methods. The charge mobility is derived from the calculations. In the end, we build a clear structure−properties relationship for the rational design of an efficient organic HTM.

2. THEORETICAL BASIS

The chemical structures are shown inFigure 1. Ar represents spacer (A, B, C, D, E, and F), and R is the end functional group (1, 2, and 3). Combined with Ar and R in the above target molecular structures, there are 18 new molecules, namely A1− A3, B1−B3, C1−C3, D1−D3, E1−E3, and F1−F3. The geometries and FMOs of those monomers were optimized by the B3LYP functional with 6-31g(d,p) basis set. Considering that the B3LYP functional cannot describe the broken- symmetry effect of the bis-triarylamines in radical-cation state,^25,26 the calculations for IP and RE are under the ωB97xD/6-31g(d,p) method.²⁷All optimizing results show no imagery frequency, which means all optimized structures are in energy minima. The above calculations are based on the Gaussian 09W software package.²⁸ Then, the reorganization energy λ can be estimated by the adiabatic potential energy surface approach (eq 1).

λ=λ₀+ λ_±=(E₀* −E₀)+(E_±* −E_±) ₍₁₎ whereE₀andE_±respectively represent the energies of neutral and charged species in their lowest energy geometries, whileE₀* Figure 1. Chemical structures of target compounds. Ar represents

spacer (A, B, C, D, E, and F), and R is the end functional group (1, 2, and 3). Combined with Ar and R in above target molecular structure, there are 18 new molecules, namely A1−A3, B1−B3, C1−C3, D1−

D3, E1−E3, and F1−F3, in which F101 is A3.

Figure 2.General molecular structures for (a) A, B, and C, and (d) D, E, and F. Electron density distributions of (b) HOMO and (c) LUMO of A3, and (e) HOMO and (f) LUMO for D3. X, X₂, and X₃denote heteroatoms. R represents end substitution.

The Journal of Physical Chemistry C

and E_±* are respectively the energies of the neutral molecule with a charged geometry and a charged molecule with the ground state geometry.

The crystal structure prediction was performed based on the Materials Studio platform.²⁹ All molecules were optimized by the DMol3 module to obtain the electrostatic potential charges (ESPs).³⁰The PBE functional and the Dreiding forcefield were employed to calculate the crystal structures of all molecules by performing the polymorph predictor module.^31,32 Besides, in order to achieve a good efficiency and accuracy, space groups were restricted to five common space groups of P1, P1̅, P2₁, P2₁/C, and P2₁2₁2₁.³³ The M06-2X/6-31g(d,p) method was employed to calculate the energy and electronic structures of the dimers.³⁴ Then, the Koopmans theorem (KT) approx- imation was further used to calculate the carrier transfer integral t.³²⁻³⁸The transfer integral t_hof hole is given byeq 2.

= − ₋

t E E

h HOMO 2 HOMO 1

(2) wheret_his the transfer integral of hole; E_HOMOandE_HOMO−1 are the energies of the HOMO and HOMO−1 in the closed- shell configuration of the neutral state, respectively. The charge transfer rate (k) was estimated by Marcus theory,³⁹ and the charge hopping rate (k) is calculated byeq 3.

πλ

= λ

ℏ ⎛−

⎝⎜ ⎞

⎠⎟ k t

k T k T

1

4 exp

4

2 2

B B (3)

wheret_his the transfer integral,λis the reorganization energy, ℏis the Dirac constant,Tis the temperature in kelvin, andk_Bis the Boltzmann constant.

In the end, considering Brownian motion of a charge carrier without an electricfield, the carrier mobility was described from the diffusion coefficient D with the Einstein equation (eq 4).^40,41

μ= eD

k T_B (4)

whereeis the electron charge andDis the diffusion coefficient which can be calculated byeq 5.

= D r k

d 2

2

(5) whererrepresents the charge hopping centroid to the centroid distance; d is taken as 3 for the reason that the diffusion is regarded in three dimensions for the compounds investigated.

3. RESULTS AND DISCUSSION

3.1. Geometric Optimization and Frontier Molecular Orbital Calculation. Two types of chemical structures are shown inFigure 2a (a general molecular structure for A, B, and C) andFigure 2d (a general molecular structure for D, E, and F). Electron density distribution of the HOMOs and LUMOs for two typical molecules A3 and D3 are shown inFigure 2b,c andFigure 2e,f, respectively. Here, X, X₁, and X₂represent the atoms of O, S, or Se in substitutions. Table 1 shows the optimized parameters of inter-ring distance C₁−C₂and dihedral angle X−C₁−C₂−C₃ for all molecules. Inter-ring distance has no change with variation of different end functional groups from cyano to methoxyl on the outer phenyl ring (for example, the distance for A1, A2, and A3 is 1.453 Å), suggesting that the electron donating or withdrawing effect of end substitution has no influence on the C₁−C₂distance. The dihedral angles also show trivial change with different end functional groups. With the same substitution, however, changing the heteroatom of O to S and Se, the C₁−C₂bond length changes as the sequence of B≈ C > A in the range 1.45−1.47 Å. Similarly, the dihedral angle showed the order B (25°) > C (23°)≫A (2°). In all, the molecules with central furan spacer show the best planarity compared with those with thiophene and selenophene.

To investigate the influence of spacer size on the molecular conformation, geometric optimizations of D, E, and F series are conducted and compared to their mono-ring counterparts A and B. First, A, D, B, E, and F series molecules show almost the same C1−C2 bond distance separately. Thus, it is clear that the C1−C2 bond distance is highly related to the character of Table 1. Optimized Inter-Ring Distance and Dihedral Angle (X−C₁−C₂−C₃) at the B3LYP/6-31g(d,p) Level

A3 A2 A1 B3 B2 B1 C3 C2 C1

C1C2 (Å) 1.453 1.453 1.453 1.463 1.463 1.464 1.461 1.461 1.463

dihedral angle (deg) 2.1 2.3 2.1 25 25.1 25.4 20.3 21.8 23.0

D3 D2 D1 E3 E2 E1 F3 F2 F1

C1C2 (Å) 1.449 1.45 1.449 1.461 1.461 1.462 1.461 1.461 1.462

dihedral angle (deg) 2.5 2.1 2.3 24.4 24.6 23.1 25.3 25.4 25.9

Figure 3.Calculated FMO alignment of 18 molecules, as well as experimental values of energy bands of FAPbI₃, MAPbI₃, and MAPbBr₃and spiro- OMeTAD (with three types ofpp-,pm-, andpo-).

The Journal of Physical Chemistry C

nearby rings, not the spacer size. In addition, dihedral angles of D and A series are in the same region (∼2°), while those of E series are slightly smaller than those of B series (23−24° vs 25°) and those of F series are slightly larger than those of B series (25.5°vs 25.1°). Hence the DFT calculations show that the conformation of molecules is mainly controlled by the heteroatom in the X and X₂ positions, while the size of the spacer has negligible influence.

Figure 3 shows the calculated LUMO and HOMO energy levels andE_g(LUMO−HOMO) for 18 molecules. The energy bands of spiro-OMeTAD, FAPbI₃, MAPbI₃, and MAPbBr₃are also shown inFigure 3.^42,43Basically, for molecules with mono- ring spacers, HOMO is localized over the whole molecule, whereas LUMO is mainly localized on the central five- membered ring and the adjacent benzenes (Table S1 in Supporting Information). The electron distribution and HOMO of control molecule A3 are consistent with the previous result (−4.24 eV vs−4.23 eV).¹⁸For the same spacer series containing CN, OH, and OMe, the order of HOMO is OMe > OH > CN due to the decrease of the electron cloud density by increasing the electron withdrawing effect. Carrying the same substitution, HOMO with furan spacer is the highest, followed by thiophene and selenophene. This result is consistent with the above-mentioned conclusion that A series molecules have the best planarity and thus the longest conjugation system, which contributes to the higher HOMO and the stacking properties as well.

By sharp contrast, in these molecules (D, E, and F) with fused cycles, the electron density distribution of HOMO and LUMO are both mainly localized on the central conjugating rings while the four outer phenyl rings contribute less as compared to their counterparts A and B, shown in Table S1.

Also, the HOMOs of molecules with fused cycles are relatively higher than those of molecules with monocycles; for example, the HOMO of D3 (−4.08 eV) is higher than that of A3 (−4.24 eV) and the HOMO of E1 (−5.29 eV) is higher than that of B1 (−5.63 eV). Besides, HOMOs of E series are higher than those of F series due to better planarity between the linker and adjacent phenyl rings (with dihedral angles of 23.1°vs 25.9°).

Overall, except for the CN substituted molecules, the energy difference between the HOMOs of the other molecules and the valence bands of perovskite materials is larger than the difference when compared to spiro-OMeTAD, which contrib- utes to efficient hole extraction. Simultaneously, the calculated LUMOs are all higher than the conduction bands of perovskite, indicating the capability of blocking electrons.

3.2. Structures, Ionization Potential, and Reorganiza- tion Energy.Figure 4shows the general structures of (a) A, B, and C series and (b) D, E, and F series for calculations of IP

and RE. Bα, Bβ, and Bγindicate benzene rings at the different position. The dihedral angles (θ) of neutral molecules, radical cations at theωB97xD/6-31g(d,p) level, are shown inTable 2.

The detailed parameters are summarized in Table S2 in the Supporting Information. Considering neutral molecules as references, θAr−Bα of B1, B2, and B3 decreases from 31.77, 19.85, and 19.30°to 4.45, 4.79, and 5.06°in the radical-cation state. θBα−Bβ and θBα−Bγ of B series show a decrease by half.

Other series of molecules present the same trend in dihedral angles between neutral and radical-cation states. Thus, as expected, the planarity of the molecules is much better in the radical-cation state than in the neutral state. Although with different substitutionsθBα−BβandθBα−Bγvary in the order 1 > 2

≈3 in both neutral and radical-cation states, for instanceθBα−Bβ

of radical-cation A1 (20.49°) > A2 (16.76°) ≈ A3 (16.78°), suggesting that the electron donating effect of OH and OMe groups is beneficial for molecular planarity improvement, especially in the radical-cation state. Regarding the variation of Ar core, the TPA block presents a flatter conformation with θBα−Bβ and θBα−Bγ in the sequence C > B > A, while less difference was found in the D, E, and F series. In all, the molecules combined with the electron donating groups and the furan spacer exhibit a good planarity in the radical-cation state, potentially contributing to a good hole mobility.

Figure 4.General structures for (a) A, B, and C series and (b) D, E, and F series for calculations of IP and RE. Bα, Bβ, and Bγmean benzene rings at different positions.

Table 2. Dihedral Angles (θ) of Neutral Molecules and Radical Cations at theωB97xD/6-31g(d,p) Level

θAr−Bα(deg) θBα−Bβ(deg) θBα−Bγ(deg)

compd neutral

radical

cation neutral

radical

cation neutral

radical cation

A1 6.36 1.17 48.47 20.49 48.75 20.50

A2 1.61 1.39 33.24 16.76 35.13 14.86

A3 1.63 1.38 28.32 16.78 34.95 14.97

B1 31.77 4.45 47.55 21.75 49.66 22.56

B2 19.85 4.79 38.76 18.07 42.05 18.79

B3 19.30 5.06 38.61 17.70 42.04 18.79

C1 36.86 2.01 51.03 22.08 51.08 21.91

C2 30.87 3.95 32.71 19.49 31.04 18.67

C3 31.37 4.18 33.34 19.58 32.41 18.54

D1 2.14 0.51 53.51 25.58 51.90 26.19

D2 0.28 1.87 35.49 15.99 33.42 14.20

D3 5.29 2.05 34.57 16.11 35.24 14.54

E1 30.82 12.56 48.59 24.63 48.79 24.97

E2 31.47 8.35 29.54 18.01 33.38 18.50

E3 32.18 8.19 28.67 17.43 33.50 18.52

F1 33.36 11.74 47.99 22.40 45.67 22.70

F2 31.60 8.94 33.21 16.58 29.87 17.35

F3 34.77 8.56 25.28 16.34 32.13 17.46

The Journal of Physical Chemistry C

Values of the calculated adiabatic IP and the hole RE (λh) are listed inTable 3. The IP, the energy required to remove the

loosely bound electron, is an important criterion for the estimation of the hole mobility. Similar results are observed in each series of molecules. The substitution of cyano clearly increases the IP more than hydroxyl. For example, order of IPs of B series is B3 (5.66 eV) < B2 (5.73 eV) < B1 (6.78 eV). B series and C series with S and Se heterorings show similar IP values, while higher than that of the A series. D, E, and F with fused rings obviously lowered the IP due to the stronger electron donating effect, resulting in the minimal IP of 5.31 eV for D3. Normally, lower IP provides better hole mobility for HTMs. From this point, the hole mobility possibly ranks in the order D > E > F > A > B > C, which will be discussed insection 3.3. In this study, we mainly focus on the hole transportation property of TPA based materials, so electron transportation has not been discussed.

Another important factor to assess the charge transportation is the RE (including external RE and internal RE). It is known that the external RE is much lower than the internal RE based on a polarized forcefield calculation.⁴⁴Therefore, the external part is neglected in this study. The calculated hole REs (λh) are listed in Table 3. From Marcus theory,³⁹ it is rational to conclude that the lower RE favors the higher charge transporting rate. FromTable 3, the order ofλhis Ar−CN >

Ar−OH ≈ Ar−OMe. All cyano substitutions show larger λh

values than hydroxyl and methoxyl substitutions, which means the electron withdrawing group restricts the hole transporting properties, while electron donating groups, e.g., OH and OMe, favor hole transportation. Meanwhile, λh values of molecules with various Ar behave in the same trend due to the similar planarity structures, which is consistent with their geometric variation in the radical-cation state that the molecules with OH

and OMe substituting groups improve molecular planarity, favoring an efficient hole hopping process.

3.3. Charge Mobility. According to the Marcus theory, charge mobility is affected by the transfer integral (t),³⁹which is the electron coupling and highly influenced by the dimer structure. The initial crystal structures of dimers were predicted in the Materials Studio platform. In the calculations, the space groups were restricted tofive common space groups ofP1,P1̅, P2₁, P2₁/C, and P2₁2₁2₁,³³ in which structures with the P1̅ space group are the most stable among the predicted crystals (ΔE is the difference between E_SG (energy of different space groups) andE_P1̅(energy of space groupP1̅), see Figure S1 in the Supporting Information). Thus, all of the hole mobility calculations are based on theP1̅space group (shown inTable 4). The detailed parameters for predicting crystal structures are listed in Table S3 in theSupporting Information. The transfer integrals of all hopping pathways were calculated by employing the M06-2X functional based on the direct coupling approach.⁴⁵ Finally, the Koopmans theorem (KT) approx- imation has been used to calculate the carrier transfer integralt based on FMOs of neutral dimers.³⁸

Table 4 lists the centroid distance between dimers (r), hole transfer integrals (t_h), and hole mobilities (μh). The detailed lattice parameters of all crystals on the P1̅ space group are shown in Table S3 in theSupporting Information. The order of centroid distance with the same substituting group shows as r(A) >r(B) >r(C), andr(D) > r(E) <r(F), whereast_h(A) >

t_h(B) >t_h(C), suggesting that although the bigger heteroatom on thefive-atom rings drags molecules closer, it simultaneously induces a larger stagger distance and reducesπ−πstacking. The same conclusion can be drawn for other molecules listed Table S4 in theSupporting Information. Meanwhile, with the same linker OMe substitution drags molecules closer than CN (r(Ar−OMe) < r(Ar−CN)). The slightly higher t of OH substitution than OMe is probably due to the weaker interaction of O···H between adjacent molecules, shown in Figure 5. The measured O···H distances between two A2 molecules are 2.531, 2.670, and 2.673 Å at three positions, smaller than their van der Waals radius of 2.72 Å. Apparently, the molecules with fused rings substituted with OH or OMe groups show increased stacking distance and decreased transfer integral.

Hence the hole mobility follows the trend of transfer integral and the RE, with the order A > B > C, and Ar−OMe≈Ar−OH

> Ar−CN in the case of A, B, and C series. Especially TPA molecules with furan spacer and OMe or OH substitution show larger hole mobility. When fused rings are used as spacers, the molecules present similar hole transportation behaviors for Table 3. Adiabatic IP and Hole RE of Series A, B, C, D, E,

and F Calculated at theωB97xD/6-31g(d,p) Level

compd IP (eV) λh(eV) compd IP (eV) λh(eV)

A1 6.64 0.88 D1 6.23 0.77

A2 5.61 0.63 D2 5.36 0.67

A3 5.55 0.60 D3 5.31 0.70

B1 6.78 0.96 E1 6.42 0.80

B2 5.73 0.70 E2 5.58 0.72

B3 5.66 0.69 E3 5.52 0.73

C1 6.74 1.07 F1 6.59 0.84

C2 5.74 0.71 F2 5.73 0.70

C3 5.67 0.72 F3 5.67 0.70

Table 4. Centroid-to-Centroid Distances (r, Å), Hole Transfer Integrals (th, meV), and Hole Mobilities (μh, cm²V⁻¹s⁻¹) Based on Predicted Crystalline Structures

compd r t_h μh compd r t_h μh

A3 4.97 10.08 1.01×10⁻² D3 5.01 7.85 2.24×10⁻³

A2 4.94 10.91 9.05×10⁻³ D2 5.97 8.26 4.76×10⁻³

A1 4.96 2.69 3.74×10⁻⁵ D1 4.99 3.01 1.63×10⁻⁴

B3 4.38 6.56 1.30×10⁻³ E3 4.81 6.04 8.99×10⁻⁴

B2 4.46 6.65 1.28×10⁻³ E2 4.86 6.05 1.04×10⁻³

B1 4.48 0.99 1.92×10⁻⁶ E1 4.36 3.88 1.44×10⁻⁴

C3 3.98 4.63 3.98×10⁻⁴ F3 4.95 6.57 1.55×10⁻³

C2 3.45 5.09 3.91×10⁻⁴ F2 5.00 6.05 1.27×10⁻³

C1 4.26 0.08 3.67×10⁻⁹ F1 4.99 2.11 3.56×10⁻⁵

The Journal of Physical Chemistry C

thiophene derivatives at the magnitude of−3 to−4. However, the selenophene linker gives the worst performance, compared with other linkers, which is highly related to its poor π−π stacking due to torsional conformation in the dimer structures.

These trends are generally consistent with the conclusion obtained from the IP data.

4. CONCLUSION

A series of molecules have been designed based on F101 with the variation of furan linker to thiophene, selenophene, and fused rings, and substitution of methyloxyl to hydroxyl and cyano functional groups. The newly designed molecules were theoretically studied in electronic structure, molecular orbital, and hole transportation by using DFT B3LYP and ωB97xD methods. These results suggest that the molecule with a furan as a linker has better conformational planarity andπ−πstacking than those with thiophene and selenophene linkers. Regarding the substitution groups, electron withdrawing groups are obviously not suited for the hole transporting materials in view of both energy level and charge transportation property, whereas OH and OMe, two electron donating groups, enhance the hole mobility. Overall, this study builds a clear relationship between molecular structure and hole transporting property, potentially offering a fundamental clue to designing efficient TPA based HTMs.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acs.jpcc.7b06335.

Electron cloud distribution of all molecules, geometric parameters of neutral molecules and radical cations at the ωB97xD/6-31g(d,p) level, lattice parameters of all crystals on P1̅ space groups, 3D graphs of all dimers, andΔEof each space group toP1̅group of every crystal (E_SG−E_P1̅) (PDF)

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AUTHOR INFORMATION Corresponding Authors

*E-mail: jiangyue871116@gmail.com(Y.J.).

*E-mail: gaojw@scnu.edu.cn(J.G.).

ORCID

Jun-ming Liu:0000-0001-8988-8429

Jinwei Gao:0000-0002-4545-1126 Notes

The authors declare no competingfinancial interest.

■

ACKNOWLEDGMENTS

We are grateful forfinancial support from the National Natural Science Foundation of China (51571094 and 21272080), National Key Research Program of China (2016YFA0201002), Guangdong Province Foundation (2016KCXTD009, 2014B090915005, and 2016A010101023), China Postdoctoral Science Foundation (2016M590795), and Program for Chang Jiang Scholars and Innovative Research Teams in Universities Project (No. IRT13064).

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