Energy Level Control via Molecular Planarization and Its Effect on

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C: Energy Conversion and Storage; Energy and Charge Transport

Energy Level Control via Molecular Planarization and Its Effect on Interfacial Charge Transfer Processes in Dye-Sensitized Solar Cells Yifan Liu, Xiaomin Zhang, Chen Li, Yuqi Tian, Fengyu Zhang, Yajun Wang, Wenjun Wu, and Bo Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03986 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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The Journal of Physical Chemistry

Energy Level Control via Molecular Planarization and Its Effect on Interfacial Charge Transfer Processes in Dye-Sensitized Solar Cells Yifan Liu,a,c ‡ Xiaomin Zhang,a ‡ Chen Li,b‡ Yuqi Tian,a Fengyu Zhang,a Yajun Wang,a Wenjun Wu,*b and Bo Liu*a a Hebei

Key Laboratory of Organic Functional Molecules, College of Chemistry and Material

Science, Hebei Normal University, No. 20, East Road of Nan Er Huan, Shijiazhuang 050023, P. R. China. E-mail: [email protected]; b

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, Shanghai Key

Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China. E-mail: [email protected] c School

of Pharmacy, Hebei Medical University, Shijiazhuang 050017, P. R. China.

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ABSTRACT. As the critical property of the organic dye, the energy level determines the thermodynamic possibilities and the efficiencies of multiple interfacial charge transfer processes in DSSCs. Thus, suitable energy level is highly required, and selective energy control becomes a quite important and systemic project. Herein, novel planar carbazole unit, which is synthesized through simple aryl immobilization, is applied as donor segment in D-A-π-A organic dye. The considerable dihedral angle between benzene and carbazole is almost eliminated, thus resulting in effectively improvement of molecular planarity. As the planarity of donor segment enhances, the HOMO level of the dye lifts, while its LUMO level remains around the same value, with respect to the twisted dye. Besides, with good molecular planarity, the interfacial charge transfer processes, including charge injection, charge recombination, and dye regeneration, are efficiently improved. Consequently, the optimization of molecular planarity can selectively control the energy level of the dye, while multiple interfacial charge transfer processes can also be finely optimized, showing us a reasonable strategy to develop efficient organic sensitizer with long-term photostability.


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1. Introduction Along with the development of dye-sensitized solar cells (DSSCs), the interfacial charge transfer processes in cell device have attracted much attetion.1-5 Among all factors affecting the interfacial charge transfer, the energy levels, including HOMO and LUMO levels, are key elements, which determines the thermodynamic possibilities and the efficiencies of multiple interfacial charge transfer processes. Unlike LUMO level, which is generally adjusted only according to the conduction band of semiconductor, the HOMO level of the sensitizer should be varied along with the different HTMs. Thus, with the development of novel HTMs, suitable HOMO energy level is eagerly needed for organic dyes. Generally, the HOMO level can be modulated by changing the donor segments, such as the widely used carbazole,6-8 coumarin,9-11 triphenylamine,12-15 and indoline16-18 units, whose HOMO levels are generally determined to be around 1.40, 1.00, 0.90, and 0.80 V, respectively. However, when the HOMO energy is lifted by strengthening the electron-donating capability or extending the π-conjugation system, the LUMO energy of the dye is lifted simultaneously, indicating the low regulation efficiency. 16, 19-21 Thus, a new strategy for the selective regulation of HOMO level is urgently needed. Herein, two similar carbazole donor segments were designed and applied in D-A-π-A dyes, coded as CS-14 and CS-15, whose π-conjugation and acceptor parts were exactly same (shown in Scheme 1). The only difference between two donors is the molecular planarity, which was adjusted by aryl immobilization method. With this simple modification, the HOMO level of CS-15 has been effectively lifted, and noticeably, its LUMO level stays at the same value with that of CS-14, showing us a promising method to selectively control the energy levels. Meanwhile, the effects of


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donor planarity on the photovoltaic performances, especially for multiple interfacial charge transfer processes of both dyes have also been compared and estimated in detail. C8H17O

C8H17O

N C8H17O

53.5o

o N 3.71

N S N

N S N

C8H17O CS-14

COOH

COOH CS-15

Scheme 1. Chemical structures of dyes CS-14 and CS-15, while the dihedral angles between carbazole and phenyl segments were also marked.22 2. Experimental Section 2.1 Materials THF, toluene, CH2Cl2, and CHCl3 were dried before use. 1-Butyl-3-methylimidiazolium iodide (BMII), LiI, tert-butylpyridine (t-BP) and Pd(PPh3)4 were obtained from Energy Chemical Co., Ltd. All the other chemicals were purchased from Sigma-Aldrich and used as received. The synthetic route, procedure, and all the characterization of the CS dyes and their intermediates were shown in Supporting Information. 2.2 Photophysical, electrochemical, and photovoltaic measurements The UV-visible spectra of CS-14 and CS-15 were examined by Shimadzu UV-2501PC spectrometer. Their steady PL spectra was obtained on Hitachi F-4600 spectrometer. The TCSPC fluorescence signals of CS-14 and CS-15 were characterized by FLS980 Spectrometer (Edinburgh


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Instrumen, excited at 450 nm, detected at 680 nm). CV measurements were tested on CHI660B workstation (CH Instruments) using a standard three-electrodes system. 6 μm nanocrystalline TiO2 electrodes with a 4 μm scattering layer were used as the TiO2 electrodes. All the TiO2 electrodes were prepared according to the published procedure.23 The active area was controlled to be 0.25 cm2. During the measurement, an acetonitrile solution of 0.6 M BMII, 0.10 M LiI, 0.05 M I2, and 0.5 M t-BP was used as the electrolyte. For dummy cell, 3 μm ZrO2 electrodes were prepared by similar procedure as reported method using ZrO2 paste instead.24 The experimental methods and instruments of photovoltaic performance characterization, including J-V curves, the photocurrent action spectra (IPCE), the electrochemical impedance spectroscopy (EIS), the stepped light-induced transient measurements of photocurrent and voltage (SLIM-PCV) of the photocurrent and voltage were all shown in Supporting Information. The detailed methods of theoretical calculations were also described in Supporting Information 3. Results and discussion As we can see in Scheme 1, the only difference between two D-A-π-A dyes is the planarity of the substituted carbazole segment, which was adjusted by aryl immobilization method. When the carbazole unit was immobilized on benzene unit, the dihedral angle between carbazole and benzene planes was sharply decreased from an extraordinary large value of 53.5 o to only 3.71 o. That means this dihedral angle was almost eliminated, which would absolutely affect the optical, electrochemical, and photovoltaic performance to significant extent. 3.1 Optical and electrochemical characterization


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Figure 1. UV-vis and photoluminescence spectra of CS-14 and CS-15 in (a) CHCl3 and (b) on 3 μm TiO2 films. As shown in Figure 1a, over 40 nm redshift of the maximum absorption peak (λmax) could be observed for CS-15 compared with CS-14, due to the relatively stronger ICT process in excellent planar molecule. In addition, the molar extinction coefficients (εmax) of planar dye CS-15 was determined to be 18600 M-1 cm-1, which was even over two times higher than that of CS-14. Combined with the broad absorption band, the light-harvesting capability of CS-15 was obviously higher than that of CS-14. While adsorbed on 3 μm TiO2, the λmax of both dyes bathochromic shift by around 15 nm with respect to their λmax in solvent (Figure 1b), suggesting the existence of J-


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aggregation on TiO2 surface. Moreover, the CS-15-loaded TiO2 electrode was also soaked by Xe light and showed extraordinary photostability (Figure S1), which might be benefit to obtain longterm stable cell device. Table 1. Optical and electrochemical properties of CS-14 and CS-15.

λmaxa

εmaxa

λPLa

τsolutiona

λmaxb

Energy loss

Hc

E0-0d

Le

/V

/V

/V

/ nm

/ M-1 / nm cm-1

/ ns

/ nm

CS-14

408

5300

597

5.68

423

0.96

0.93

2.32

-1.39

CS-15

449

18600

607

7.29

462

0.72

0.82

2.22

-1.40

/ eV

a

Measured in CHCl3. b Measured on 3 μm TiO2. c H = HOMO, L = LUMO. d Estimated from the onset wavelength. e LUMO = HOMO - E0-0. As listed in Table 1, photoluminescence properties of CS-14 and CS-15 were also examined and their λPL were obtained at 597 and 607 nm, respectively. Combining with their λmax data, the energy loss was obtained as 0.96 and 0.72 eV for CS-14 and CS-15, respectively. In our opinion, the larger energy loss of CS-14 arises from the more torsional relaxation, which is one kind of the

excited state relaxations.25 As well known, the planarization is generally happened while the dye is excited by the light. Thus, owning to the better molecular planarity, dye CS-15 will apparently save more energy because of its less energy loss through torsional relaxation. To investigate the effect of planarity on energy levels, CV measurements of two dyes were carried out and the data were listed in Table 1. With exactly same acceptor segment, the LUMO levels of two dyes were determined to be quite similar values of -1.39 and -1.40 V vs NHE for CS14 and CS-15, respectively. On the other hand, although the similar substituted carbazole units were also used as the donor part for both dyes, the HOMO level of CS-15 was obviously lifted to 0.82 V vs NHE compared with that of CS-14 (0.93 V), which could only be attributed to the


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different planarity of carbazole segments. That means the HOMO level of the dye could be effectively lifted only by improving the planarity of donor segment. Moreover, when the HOMO level was lifted, the LUMO level of CS-15 was almost not affected, suggesting a selective HOMO energy control. As a matter of fact, the HOMO level is a critical factor affecting the regeneration process of the dye by accepting charge from the redox couple. Along with the improvement of molecular planarity, the HOMO energy of CS-15 was lifted obviously compared with that of CS-14. That means the driving force of charge transfer from I-/I3- to the HOMO of the dye decreased, which may influence the dye regeneration efficiency (ηreg). Thus, time-correlated single photon counting (TCSPC) measurements were further applied to evaluate the ηreg values of both dyes according to Equation 1:24

𝜂reg = 1 ―

𝜏I ― /I ― 3

𝜏blank

(1)

where the τI-/I3- and τblank are photoluminescence lifetimes of ZrO2-based devices injecting I-/I3couple and blank electrolyte, respectively. The ZrO2 film was used to prepared the dummy cell instead of TiO2 film to prevent the charge injection.24 When the I-/I3- electrolyte was injected, the PL decay of dye-loaded ZrO2 film could be only caused by the charge injection from I-/I3- to the oxidized dye, simulating the regeneration process.


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Figure 2. PL decay signals of CS-14 - 15 sensitized ZrO2 film with I-/I3- couple and blank electrolyte. As shown in Figure 2, without I-/I3-, the PL lifetimes (τblank) of CS-14 and CS-15 based ZrO2 devices were determined as 3.65 and 4.91 ns, respectively. While the I-/I3- electrolyte was injected, the PL lifetimes (τI-/I3-) were efficiently quenched to be 0.67 and 0.75 ns with the ηreg values of 81.64% and 84.72% respectively. Obviously, the improvement of molecular planarity is preferred to selectively lift the HOMO energy and obtain high dye regeneration efficiency. Furthermore, through similar TCSPC technique, the lifetimes (τsolution) of CS-14 and CS-15 were examined to be 5.68 and 7.29 ns, respectively (Figure 3). That means although the LUMO levels of both dyes were not affected by the adjustment of molecular planarity, better planar molecule was obviously benefit for longer lifetime of excited charge, which might obtain high charge injection efficiency (ηinj). Thus, to evaluate the influence of planarity on ηinj, the PL lifetimes of dye-loaded TiO2 electrodes were further examined. As shown in Figure 3, while loaded on TiO2 electrode, the PL lifetime of both dyes showed serious quenching, indicating the charge transfer from LUMO to the conduction band of TiO2. Their PL lifetimes (τTiO2) sharply decreased to be


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0.52 and 0.40 ns, respectively. Therefore, the ηinj values for CS-14 and CS-15 were determined as 90.84% and 94.51%, respectively, according to Equation 2:

𝜂inj = 1 ―

𝜏TiO2 𝜏solution

(2)

where τTiO2 is the PL lifetime of the dye-loaded TiO2 film.

Figure 3. PL decay signals of CS-14 and CS-15 in CHCl3 and on TiO2 surface. Accordingly, the improvement of planarity in donor segment can selectively lift the HOMO level and enhance the dye regeneration efficiency. Moreover, the LUMO level is not affected during the above adjustment, however, better planarity of donor segment still prefers obtaining longer lifetime of excited charge and higher charge injection efficiency. 3.2 Theoretical approach The molecular geometries of CS-14 and CS-15 both at ground state and the first excited singlet state were all simulated by Gaussian 09 program. As listed in Table 2, the DFT calculations can satisfactorily reproduce the experimental trend of energy levels for both dyes.


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Table 2. Optical and electrochemical properties of both dyes calculated by Gaussian 09 program. λmaxa

λPL

Energy loss

Ha

E0-0

La

/ nm

/ nm

/ eV

/V

/V

/V

CS-14

417

544

0.70

0.89

2.48

-1.59

CS-15

451

575

0.59

0.80

2.42

-1.62

a

The electrochemical and optical properties in chloroform were calculated at the optimized geometries of ground states (B3LYP/6-311G (d,p)) and at the first excited singlet state (MPWPW91/6-311G (d,p)), respectively. With improved planarity of donor segment, the HOMO energy of CS-15 shift to be more negative while its LUMO energy remains at the same value. The absorption spectra of CS-14 and CS-15 were also simulated by the TD-DFT calculation. The λmax values of two dyes were obtained at 417 and 451 nm, respectively, which were quite similar as the experimental results. Furthermore, theoretical energy losses of both dyes were also calculated by subtracting the emission energy from the absorbed energy, showing good consistent with the experimental results. To be noticed, at the first excited singlet state, a significant planarization effect can be found in Figure 4 for both dyes. As a result, with more planar structure, CS-15 can save more energy with respect to CS-14, due to the less torsional relaxation cost, which may contribute to obtain better photovoltaic performance.


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Figure 4. Optimized geometries of CS-14 and CS-15 at their ground states and the first excited singlet states. 3.3 Photovoltaic performances To investigate the impact of planarity on photovoltaic properties, the cell devices sensitized by CS-14 and CS-15 were fabricated and examined. Due to the relatively broad absorption spectrum, the onset wavelengths (λonset) of the IPCE spectra for planar dye CS-15 was found at around 800 nm, while λonset of CS-14 based device fell to only around 700 nm. Moreover, a broad IPCE action plateau from 430 to 610 nm was found for CS-15-sensitized device with values of over 70%, although the IPCE values of CS-14-sensitized one was slightly higher in region of 270-430 nm. The maximum IPCE value of CS-15-sensitized device was obtained as 82%@470 nm, which was over 10% higher than that of CS-14 (69%@430 nm).


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Figure 5. (a) IPCE working spectra and (b) J-V curves of CS-14 and CS-15 based cell devices. Actually, besides ηinj and ηreg, the IPCE is also affected by another two factors according to Equation 3:26 IPCE = LHE × 𝜂𝑖𝑛𝑗 × 𝜂𝑟𝑒𝑔 × 𝜂𝑐𝑜𝑙

(3)

where LHE is the light-harvesting efficiency, ηcol is the charge collection efficiency. The LHE values of CS-14 and CS-15 were first evaluated according to Equation 4: LHE = 1 ― 10 ―A

(4)


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where A is the absorbance of dye-sensitized TiO2 electrode. Although the εmax of CS-14 was relatively low, CS-14-sensitized TiO2 electrode still showed high LHE values (over 90%) in the region of 360-470 nm (Figure S2). However, the LHE values in the region of 470-600 nm sharply decreased to around 10%. On the other hand, CS-15-sensitized TiO2 presented high LHE values in relatively broad region of 360-560 nm, thus resulting in better IPCE performance. Electrochemical impedance spectroscopy (EIS) measurements were further performed on complete CS-14 and CS-15 based cells in the dark to reveal the effect of molecular planarity on ηcol, which can be calculated according to Equation 4: 𝜂col =

𝑅CT 𝑅CT + 𝑅T

(4)

where RCT is the charge transfer resistance and RT is the transport resistance. Both resistance values were obtained through fitting the Nyquist plots using by Zsimpwin (Figure S3). The RT values were obtained as similar values of 25.4 and 31.4 ohm for CS-14 and CS-15, respectively. On the contrary, The RCT value of CS-15 based cell was measured as a relatively large value of 140.2 ohm, which was nearly 2 times as high as that of CS-14 based cell (71.3 ohm), suggesting faster recombination in the device with dye CS-14. As a result, the ηcol of CS-14 and CS-15 based cells were calculated to be 73.7% and 81.7%, respectively. Accordingly, taking all factors discussed above into consideration, CS-15, with excellent planarity at donor segment, presented much better IPCE performance than the twist dye CS-14, which would lead to better photocurrent value of CS-15 based cell. Accordingly, the short-circuit photocurrent density (JSC) of CS-15-sensitized DSSC was obtained as 15.31 mA cm-2, which was over 150% as high as that of CS-14-sensitized one (Figure 5b). More interestingly, as the planarity of donor segment was improved, the open-circuit photovoltage (VOC) value of CS-15 sensitized


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cell also increased by 36 mV from 0.705 V of CS-14 to 0.741 V. To gain more insights into the VOC enhancement, SLIM-PCV were further investigated on both DSSCs. Table 3. Photovoltaic performance of DSSCs based on CS-14 and CS-15 including dye-loaded amounts. a

JSC

VOC

/ mA cm-2

/V

CS-14

9.78±0.25

0.705±0.006

0.689±0.014 4.75±0.04

1.03±0.09

CS-15

15.31±0.47

0.741±0.008

0.685±0.011 7.76±0.15

0.92±0.11

ff

PCE

Amount

/%

/×10-7mol cm-2

a The

component of the redox electrolyte: 0.6 M 1-butyl-3-methylimidiazolium iodide, 0.10 M LiI, 0.05 M I2, and 0.5 M tert-butylpyridine in acetonitrile. As shown in Figure 6a, the profile of VOC against the charge density was shown to investigate the conduction band shift (∆ECB). Apparently, the charge density collected from CS-15-sensitized cell device was much higher than that of CS-14, lying in the same order as the trend obtained in J-V measurements. At a given charge density, CS-15 sensitized cell device presented about 15 mV enhancements in ∆ECB, with respect to CS-14. That indicates the conduction band of TiO2 of CS15 would be upshifted to a considerable extent due to the better molecular planarity of the dye. As well known, ∆ECB can be expressed as equation (5):27

∆𝐸𝐶𝐵 = ―

𝑞𝜇𝑛𝑜𝑟𝑚𝑎𝑙 𝛾 𝜀 𝜀0

(5)

where q is the electron charge, μnormal is the vertical dipole moment, γ is the surface concentration of the dye, ε0 and ε are the permittivity values of the vacuum and organic monolayer, respectively.


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Figure 6. The plots of (a) VOC and (b) charge lifetime against charge density for CS-14- and CS15-based cell devices. Therefore, to ferret out the essential reason of the larger ∆ECB of CS-15, the vertical dipole moments of CS-14 and CS-15 were extracted from the theoretic calculation results (Figure S4). With improved molecular planarity, the μnormal of CS-15 was determined to be 3.74 D, which was increased by over 80% compared with that of CS-14. Taking the similar dye-loaded amount values of two dyes into account, the higher ∆ECB value of CS-15 can be easily understood. Furthermore, another key factor affecting VOC is the charge density in conduction band of TiO2.28 As discussed above, compared with CS-14, only 15 mV enhancement was found for CS-15, while the VOC of


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CS-15 was 36 mV higher than that of CS-14. Therefore, the left difference should only attribute to the different charge densities, which is tightly depend on the charge lifetime. Thus, the lifetimes as a function of charge density of CS-14 and CS-15 were further examined (Figure 6b). At a given charge density (e.g. at 4×1017 cm-3), the charge lifetime of CS-15 was obtained as 105 ms, which was almost 2.8 times higher than that of CS-14 (29 ms). That means CS-15 with planar donor segment possesses better effect of preventing charge recombination with respect to twist dye CS14, which is coincide with above results of EIS measurements. Accordingly, taking both the conduction band shift and the charge recombination into account, the VOC of CS-15 based DSSC was obtained as a relatively high value of 0.741 V. Combined with a JSC of 15.31 mA cm-2 and a ff of 0.685, an overall power conversion efficiency (PCE) was obtained as 7.76%, which was over 60% higher than that of CS-14 based DSSC (4.75%). To estimate the influence of planarity on photostability, the aging test was carried out by illuminating CS-14- and CS-15-sensitized cell devices at 60 ºC over a period of 1000 h. As shown in Figure 7, during the first 100 h, the photocurrents of both dye-sensitized solar cells slightly enhanced. At the end of the test, the VOC values of CS-14 sensitized DSSC gradually reduced from 705 to around 676 mV, while the fill factor remained at the same value. By contrast, in case of planar dye CS-15, the decline of its VOC values was relatively smooth. The PCEs of CS-14 and CS-15 sensitized DSSCs increased to around 4.79% and 7.77% at 100 h, and then gradually decreased to 4.46% and 7.46%, respectively, during the followed test. Accordingly, around 6.1% and 3.8% decline in PCEs were found for CS-14 and CS-15 based cell devices, respectively, during 1000 h aging test, which was mainly attributed to the decline of photovoltage. Obviously, the improvement of donor segment planarity will enhance the photostability of CS-15 based DSSC to some extent.


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Figure 7. Photostability evaluation of CS-14 and CS-15 sensitized cell devices by illuminating at 60 ºC over 1000 h.

4. Conclusions In conclusion, a novel planar substituted-carbazole unit was synthesized by aryl immobilization and applied as the donor segment in D-A-π-A dye CS-15, while twisted CS-14 was used as the reference dye. With this simple adjustment, the HOMO level of planar dye CS-15 was effectively lifted, while the LUMO level was not affected, indicating a powerful strategy of selective HOMO level control. Besides, excellent planarity of donor segment is obviously preferred to improve multiple interfacial charge transfer efficiencies, including charge injection efficiency, charge collection efficiency, and dye regeneration efficiency, while the charge recombination is also effectively restrained. Moreover, the improvement of donor segment planarity will also enhance the photostability of the DSSC. Consequently, our finding indicates that the HOMO energy level of the dye can be selectively lifted by improving the planarity of donor segment, while the interfacial charge transfer processes can also be finely optimized, showing us a reasonable strategy to develop efficient organic sensitizer with long-term photostability.


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ASSOCIATED CONTENT Supporting Information. Experimental and synthetic details, photostability tests, LHE spectra, Nyquist plots, and the calculated vertical dipole moments of CS-14 and CS-15. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by NSFC (21576070, 21676087), Excellent Young Scientist Foundation of NSF/Hebei (B2016205075), the Scientific Committee of Shanghai (18160723400) and the Program for the Young Talent of Hebei Province. REFERENCES 1.

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