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C: Energy Conversion and Storage; Energy and Charge Transport H

M

Larger V (Hole Distribution Volume)/V (Molecular Volume) Induced Higher Charge Mobility of Group IV A ElementBased Host Materials for Potential Highly Efficient Blue OLEDs Chuang Yao, Cheng Peng, Yezi Yang, Lei Li, MaoLin Bo, and Jinshan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06163 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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The Journal of Physical Chemistry

Larger VH (Hole Distribution Volume)/VM (Molecular Volume) Induced Higher Charge Mobility of Group IV A Element-based Host Materials for Potential Highly Efficient Blue OLEDs Chuang Yao,*,†,‡ Cheng Peng,†,‡ Yezi Yang,†,‡ Lei Li,† Maolin Bo† and Jinshan Wang *,¶

† Key Laboratory of Extraordinary Bond Engineering and Advance Materials Technology (EBEAM) of Chongqing, Yangtze Normal University, Chongqing 408100, P. R. China. ¶

School of Materials Science and Engineering, Yancheng Institute of Technology, Yancheng

224051, P. R. China. ABSTRACT: Host materials have a decisive effect on the optoelectronic properties of organic light-emitting diodes (OLEDs), whether for fluorescent, phosphorescent or thermally activated delayed fluorescence OLEDs. In this work, we first conducted a comprehensive investigation of group IV A element-based small molecular host materials (DCzC, DCzSi, DCzGe, DCzSn, and DCzPb). A multiscale simulation was used to investigate the electronic properties of these materials. The results reflected a novel phenomenon, that is, the holetransport mobility of DCzC is larger than those of the other group IV A element-based hole transport materials, which indicated that compared with the already existing Si-tetraphenyl and Ge-tetraphenyl, the C-tetraphenyl has higher potential to act as the core moiety to construct high-performance host materials. DCzC has the best hole transport properties because it has a smaller molecular volume (VM) while keeping the hole distribution volume (VH) unchanged, compared with the other materials (DCzGe, DCzSi, and DCzSn). This results in larger VH/VM value of DCzC, which favorably increased the charge transfer integral and improved the hole mobility. This result demonstrated that a larger VH/VM is beneficial in

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improving the charge mobility of designed host materials. It will be an effective theoretical basis for designing high performance host materials with excellent charge mobility.

1. INTRODUCTION The organic light-emitting diodes (OLEDs) are the core components for the development of flat-panel displays and solid-state lighting sources1-2. To obtain a high optoelectronic performance, luminescent dyes (fluorescent, phosphorescent, or thermally activated delayed fluorescence (TADF) materials) are usually doped into host materials to reduce the self-quenching process3-6. Therefore, host materials have a great influence on the performance of OLEDs. Moreover, the development of blue OLEDs is lagging far behind that of the red and green ones, and one of the main reasons is the lack of suitable host materials for blue OLEDs. Host materials for blue OLEDs should have a wide energy bandgap to prevent the reversing of excitons and a good carrier transporting ability for high performance, but these are difficult to achieve at the same time. Accordingly, developing new blue host materials for OLEDs is a significant progress on the long evolving way of it. To match the blue for display panels recommended by the National Television Standards Committee, guest luminescent materials need to have an energy bandgap of around 2.9 eV (emitting around 440 nm). To prevent the reversing of excitons in the emitting layer, the host materials should have a higher energy bandgap7. Currently, compounds with functional groups connected by a spacer, such as fluorene6, 8, phenyl/biphenyl9, and tetraphenylsilane10-12, have been investigated to construct host materials with high energy bandgaps, among which tetraphenylsilane with ultrahigh singlet (around 4.5 eV) and triplet (around 3.5 eV) energies have been widely used to


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construct host materials for deep blue OLEDs7, 13-14. By linking tetraphenylsilane to fluorene through a non-conjugated, Chi and colleagues developed a host material with wide singlet and triplet energy bandgaps, which maintain thermal and morphological stabilities10. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are not suitable for charge injection and lead to a relatively low efficiency. Therefore, electron-donating and electron-withdrawing moieties were introduced to tetraphenylsilane to modify the HOMO and LUMO energy levels to facilitate charge injection. Carbazole is one of the most widely used electron-donating moieties and has been used to build many-high performance host materials for blue OLEDs15-16. Diphenylphosphine oxide, a typical electronwithdrawing moiety, combined with the carbazole has been used to construct bipolar host materials for blue OLEDs13, 17. Although the development of tetraphenylsilanebased host materials has made great progress, all show a relatively low carrier mobilities. To develop the host materials for high performance blue OLEDs with appropriate HOMO (LUMO) energy levels and excellent carrier mobilities, finding alternative spacer moiety to the tetraphenylsilane is necessary. We have demonstrate in our previous works that the Ge-based host material DCzGe exhibits a higher maximum current efficiency than that of Si-based one18. Furthermore, the OLED device based on DCzGe exhibits an extremely low current efficiency rolloff. To further improve the device performance, we designed a series of Ge-based bipolar small molecular host materials for the blue OLEDs and the CzGeTpo shows balanced charge-transport properties with a hole/electron ratio of 8 (the electron and hole mobilities were 1.6×10−3 and 1.2×10−2 cm2 V−1 s−1), respectively)19. All these results

demonstrate

that

tetraphenylgermanium

can

be

an

alternative

to

tetraphenylsilane constructing high performance host materials. Although some



progress has been made in constructing Ge-based host materials, two questions still need to be answered. Why do Ge-based host materials exhibit better performance than the Si-based one? How do the other group IV A elements-based host materials compared with the already existing Si- and Ge-based host materials in terms of performance?

Figure 1. Chemical structure of designed small molecular host materials DCzX (X=C, Si, Ge, Sn, Pb ). In this work, we tried to answer the above two questions. We systematically investigated a series of group IV A element-based host materials as shown in Figure 1, wherein C-, Si-, Ge-, Sn-, and Pb-tetraphenyl were used as the core moieties and carbazole with electron-donating property to realize the hole transport properties. A multiscale simulation approach was used to investigate the optoelectronic properties of these materials. We found that the hole mobility of DCzGe (8.4×10−2 cm2 V−1 s−1) is about twice as much as that of DCzSi (4.5×10−2 cm2 V−1 s−1), which is mainly caused by the smaller reorganization energy of DCzGe. This explains why the performance of DCzGe-based OLEDs is better than that of the DCzSi-based ones. The results also reflect a novel phenomenon, that is, the hole-transport mobility of DCzC (9.2×10−2 cm2 V−1 s−1) is larger than those of other group IV A elements-based hole transport materials. The hole mobility of these materials is DCzC > DCzGe > DCzSi > DCzSn.


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Which indicated that compared with the already existing Si-tetraphenyl and Getetraphenyl, the C-tetraphenyl has higher potential to act as the core moiety to construct high-performance host materials. This results from the smaller molecular volume of DCzC, while keeping the spatial distribution of hole almost unchanged compared with the other materials (DCzGe, DCzSi, and DCzSn), making it a more favorable to increase the charge transfer integral between two adjacent molecules and improve the hole mobility.

2. COMPUTATIONAL METHODS 2.1 Quantum chemical calculations The geometry optimization, vibration frequency and spatial distribution of HOMO, LUMO, and the electron density (ED) difference between neutral and cations of DCzX (X=C, Si, Ge, Sn) are calculated at B3lYP/def2-SVP level. There is no imaginary frequency is detected based on the optimized geometry. The adiabatic ionization potential (IP) and electron affinity (EA), absolute hardness ‘η’ and hole internal reorganization energy (λ+) of DCzX are calculated based on B3LYP/def2-SVPD ECP(def2-SD=Sn-Pb, def2-SVP=Sn-Pb, def2-SVP/J) level. The HOMO (LUMO) energy levels and triplet energy are investigated by B3LYP/def2-TZVPD ECP(def2SD=Sn-Pb, def2-tzvp=Sn-Pb, def2-TZVP/J). PBE0 hybrid functional is able to estimate reasonable color of most organic dyes by Time-Dependent Density Functional Theory20. Therefore, PBE0/def2-TZVP is used to evaluate the absorption spectra of the designed host materials DCzX. The all calculations are carried out using ORCA21 Revision 4.0.0 package with the auxiliary basis def2/JK22. The molecular volume and hole distribution volume were calculated using Multiwfn 3.523.

2.2 Molecular dynamics (MD) simulations



Gromacs 5.1.4 molecular dynamic simulation program24 is used to accomplish the MD simulations for all systems. The initial geometries of DCzC, DCzSi, DCzGe and DCzSn are optimized using the method mentioned in the quantum chemical calculation section. As the existence of Ge and Sn elements, we utilize Universal Force Field25 model as the atomistic force field for all simulated systems. The amorphous systems for DCzC, DCzSi, DCzGe and DCzSn are constructed and imitated using the same procedure as described in our early work19.

2.3 Charge mobility calculations As the internal reorganization energies are large and the electronic couplings are not strong for this system, we employed the hopping model to calculate the hole mobility of DCzC, DCzSi, DCzGe and DCzSn. The hopping rate from one molecule to the adjacent molecules can be described by the Marcus formula26, as follows:

=

+ ∆ 1 | |

exp − (1) 4 ℏ

where ħ is the reduced Planck’s constant, is the transfer integral between the ith and jth molecule calculated using B3LYP/3-21G method as mentioned in our previous work19, is the Boltzmann constant, T is the temperature and is set as 300 K in this

work, denotes the reorganization energy and ∆ is the free energy estimated to be the HOMO energy difference of ith and jth molecule. In the hopping model, the charge transport can be described as a diffusion process.

In the low-field limit, the carrier mobility can be well-described by the Einstein formula


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μ=

(2)

where e is the electron charge and D is the charge-diffusion coefficient which is defined as the ratio between the square displacement ("(#)) and the diffusion time (t), as shown:

=

"(#) 1 lim (3) 2$ (→* #

where n represents the dimension of the system. As charge hopping rates are computed by equation (1), we performed a Monte Carlo (MC) simulation approach mentioned in previous works19, 27-29 to simulate the charge diffusion in the DCzC, DCzSi, DCzGe and DCzSn amorphous films built by the MD simulations. To get a linear relationship between square displacement and diffusion time, we performed 2000 times of diffusion processes simulation for every system. The periodic boundary conditions are set in all directions for both MD and MC calculations.

3. RESULTS AND DISCUSSION 3.1 Stability The chemical structures of DCzX (X=C, Si, Ge, Sn, and Pb) are illustrated in Figure 1. As the stability of host material is one of the most important criteria to determine the life span of OLEDs, characterizing the stability of the designed materials is necessary. The absolute chemical hardness (η) based on molecular orbital theory is utilized to evaluate the chemical stability of the host materials and is calculated using operational definitions as follows:



,=

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1 ./ 1 .2 IP − EA - 1 = = (4) 2 .0 2 .0 2

where µ indicates the chemical potential and N represents the total electron number. In this work, we mainly used IP and EA to calculate the η of designed host materials, as shown in Table 1.

Table 1 Calculated η values in eV for investigated molecules at the B3LYP/def2SVPD level of theory.

η

DCzC

DCzSi

DCzGe

DCzSn

DCzPb

3.165

3.170

3.180

3.175

2.860

The results indicate that the η values of DCzC, DCzSi, DCzGe, and DCzSn were almost the same and larger than that of DCzPb, indicating that the DCzPb was less stable than the other ones. Meanwhile, lead is a highly poisonous metal, affecting almost every organ and system in the human body. Accordingly, DCzPb is not an ideal choice for high performance host materials and will not be considered in our following investigations.

3.2 Electronic properties To further explore the electronic properties of DCzC, DCzSi, DCzGe, and DCzSn, we depicted the spatial distribution of the frontier molecular orbitals (HOMO and LUMO) in Figure 2. The spatial distribution of HOMOs was mainly located on the electrondonating moieties and was separated by the group IV A elements (C, Si, Ge, and Sn) in the center of tetraphenyl. The separation distance increased as the atomic number increased, which indicated that the conjugation between the two-armed carbazole weakened with increasing atomic number. The LUMOs were localized on the group


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IV A elements-based tetraphenyl, and the distribution gradually moved to the horizontal direction with the atomic number increasing, which increased the overlap of HOMO and LUMO.

LUMO

HOMO

DCzC

DCzSi

DCzGe

DCzSn

DCzPb

Figure 2. Spatial distribution of HOMO and LUMO of host materials DCzX (X=C, Si, Ge, Sn, Pb) (isovalue = 0.02). Aside from the distribution of HOMO and LUMO, energy levels have a crucial regulatory effect on charge injection. Figure 2 shows that the HOMOs of the designed host materials are located on the carbazole moieties, indicating the HOMO energy levels are dominated by a same moiety of these four materials. The HOMO energy levels are −5.69, −5.71, −5.71, and −5.71 eV for DCzC, DCzSi, DCzGe, and DCzSn, respectively. As density functional theory (DFT) usually overestimates the LUMO energy level, we utilized an indirect method to calculate the LUMO energy level of these materials. Firstly, a time-dependent DFT based on PBE0/def2-TZVP was used to evaluate the absorption spectra of the designed hosts as shown in Figure 3. Then, the optical energy bandgap (∆Eg) was estimated from the onset of the absorption spectra. Finally, combined with the HOMO energy levels and ∆Eg, relatively accurate LUMO energy levels were obtained as −2.23, −2.35, −2.30, and −2.28 eV for DCzC, DCzSi,



DCzGe, and DCzSn, respectively. Combined with the spatial distribution of HOMO, we know that the conjugation of two armed carbazole decreased from DCzC, DCzSi, DCzGe to DCzSn. Therefore, the ∆Eg should increase as the atomic number increases. From Figure 3, the ∆Eg gradually increased from DCzSi, DCzGe to DCzSn, but DCzC has a maximum ∆Eg which is anomalous. This may be due to the highly localized LUMO of DCzC, which reduced the overlap of the HOMO and LUMO to increase the energy bandgap. The different spatial distribution of HOMO and LUMO eventually led to such spectral results.

Figure 3. Absorption spectra of DCzX (X=C, Si, Ge, Sn, Pb) in CH2Cl2 calculated at PBE0/def2-TZVP level. a Estimated from the on-set of the absorption spectra. As the ratio of the singlet and triplet excitons formed in OLED devices is 1:330, harvesting triplet excitons plays a very important role in improving device efficiency, such as for phosphorescent and TADF OLEDs. Therefore, ideal blue host materials should not only have a high ∆Eg, but also a high triplet energy (ET) to prevent the reversing of triplet excitons. Here, we utilized the DFT method to estimate the adiabatic triplet energy (energy difference between the relaxed T1 and S0 state), which


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has been proven to be in good agreement with the experimental31. The calculated ET based on B3LYP/def2-TZVP of DCzC, DCzSi, DCzGe, DCzSn, and DCzPb were 3.11, 3.12, 3.12, 3.10, and 3.78 eV, respectively, which are all higher than the perfect blue emitting guest (2.9 eV). This demonstrates that the designed small molecular host materials based on group IV A elements exhibit and excellent-suitable HOMO/LUMO energy levels and energy bandgap for high performance blue OLEDs. The other electronic properties, such as IP and λ+ are summarized in Table 2.

Table 2 Summary of HOMO and LUMO energy levels, ∆Eg, ET, IP, and λ+ of DCzC, DCzSi, DCzGe, DCzSn, and DCzPb.

a

Calculated from the HOMO and energy

bandgap of the absorption spectra.

HOMO (eV)

LUMO (eV)a

∆Eg (eV)

ET (eV)

IP (eV)

λ+ (eV)

DCzC

-5.69

-2.23

3.46

3.11

6.64

0.064

DCzSi

-5.71

-2.35

3.36

3.12

6.70

0.070

DCzGe

-5.71

-2.30

3.41

3.12

6.68

0.055

DCzSn

-5.71

-2.28

3.43

3.10

6.69

0.059

DCzPb

-5.70

-2.23

3.47

3.78

6.67

0.07

3.3 Charge mobility From Table 2, we know that the electronic properties (energy levels, ∆Eg, ET, and IP) of DCzC, DCzSi, DCzGe, and DCzSn are almost the same. Therefore, DCzGe showed better performance than that of DCzSi not because of the related electronic properties. Aside from static electronic properties, charge mobility also has an important impact on the performance of OLEDs.



Thus, we performed a multiscale simulation method to investigate the hole transport properties of DCzC, DCzSi, DCzGe, and DCzSn in amorphous films. Firstly, we used DFT method to calculate the reorganization energy of these molecules and the λ+s are summarized in Table 2. Then, MD simulations were utilized to construct the amorphous films of these materials. The detailed simulation procedures were described in our early work19. Based on the equilibrated structures obtained from the MD simulations, we built up the adjacent molecular pairs for each molecule under periodic conditions and calculated the transfer integral for each molecular pair. Combined with the calculated λ+s in step one, the hopping ratio between each adjacent molecular pair can be estimated by eq. (1). Finally, a MC method was employed to simulate the hole transport process in the MD-constructed DCzC, DCzSi, DCzGe, and DCzSn amorphous films. Detailed simulation procedures were described in the computation method. As described in eq. (2) and (3), to obtain the charge mobility, we need a linear relationship between the square of diffusion distance and diffusion time. Figure 4 shows this linear relationship and five typical simulation processes in DCzC, DCzSi, DCzGe, and DCzSn amorphous films. The diffusion constant for DCzC, DCzSi, DCzGe, and DCzSn is 2.4×10−3, 1.2×10−3, 2.2×10−3, and 2.0×10−4 cm2 s−1, respectively. The corresponding hole-transport mobilities are 9.2×10−2, 4.5×10−2, 8.4×10−2, and 7.9×10−3 cm2 V−1 s−1, respectively.


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Average over 2000 Times 1st Time 2rd Time 3rd Time 4th Time 5th Time

DCzC

3x103 2x103 1x103 0 0.0 2.0x103 1.5x103

3.0x10-10 6.0x10-10 9.0x10-10 Time (s) Average Over 2000 Times 1st Time 2nd Time 3rd Time 4th Time 5th Time

DCzGe

1.0x103 5.0x10

2

0.0 0.0

Square Displacement (nm2)

4x103

3.0x10-10 6.0x10-10 9.0x10-10 Time (s)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 13 of 23

2x103

Average over 2000 Times 1st Time 2rd Time 3rd Time 4th Time 5th Time

DCzSi

1x103

0 0.0 6.0x102

3.0x10-10 6.0x10-10 9.0x10-10 Time (s) Average over 2000 Times 1st Time 2rd Time 3rd Time 4th Time 5th Time

DCzSn

4.0x10

2

2.0x102

0.0 0.0

3.0x10-10 6.0x10-10 9.0x10-10 Time (s)

Figure 4. Squared displacement of five typical simulations and average over 2, 000 times versus simulation time for DCzX (X=C, Si, Ge, or Sn). From this result, we know that the hole-transport mobility of DCzGe is about twice as much as that of DCzSi. As DCzSi and DCzGe show similar static electronic properties, the hole mobility should be the main reason why the Ge-based host materials exhibit better performance than the Si-based one. At the same time, we noticed a novel phenomenon, that is, the hole-transport mobility of DCzC is larger than that of the other group IV A elements-based hole transport materials, which indicated that compared with the already existing Si-tetraphenyl and Ge-tetraphenyl, C-tetraphenyl will have higher potential for acting as the core moiety to construct



high-performance host materials. This result refreshes our concept of designing high-

8

DCzC DCzSi DCzGe DCzSn

DCzC:19 meV DCzSi/Ge: 15 meV DCzSn: 12 meV

6 4 2 0 0

10

20

30

VT (meV)

40

50

Proportion max network

performance host materials based on tetraphenyl derivatives.

Electronic Connectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DCzC DCzSi DCzGe DCzSn

1.0 0.8 0.6 0.4 0

10

20 30 VT (meV)

40

50

Figure 5. (a) Electronic connectivity and (b) proportion of the largest network as a function of threshold (VT) for DCzX (X=C, Si, Ge, Sn) in the simulated amorphous films. To better understand this phenomenon, we analyzed the electronic connectivity and the proportion of the largest network as a function of the threshold for hole transfer integral as shown in Figure 5. The electronic connectivity is used to explore the charge transport network, described as the average number of neighbors for each molecule with hole transfer integral larger than the given threshold (VT). Generally, a continuous and robust network can be formed only when the electronic connectivity is larger than 229. Figure 5(a) shows the electron connectivity properties of the designed materials. We noticed that when assuring the hole transport network is continuous, the VT for DCzC, DCzSi, DCzGe, and DCzSn should be less than 19, 15, 15, and 12 meV, respectively. This indicates that an abundant hole transmission channels with high hole transfer integral in the DCzC system, and it can be demonstrated by the proportion of


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the largest network as shown in Figure 5(b), which explains why DCzC have the highest hole transport mobility.

DCzC

DCzSi

DCzGe

DCzSn

Figure 6. Electron density (ED) difference between neutral and cation of DCzX (X=C, Si, Ge, or Sn) (Isoval=0.0008). To further investigate the deeper science on this phenomenon, the spatial distribution of the electron density (ED) difference between neutral and cation of DCzX (X=C, Si, Ge, or Sn) is depicted in Figure 6, which can be used to describe the hole distribution of DCzX when it received a hole. Clearly, the hole is mainly distributed on the electron-donating moiety of these four materials. The distribution of ED difference can also be found on the central carbon of C-tetraphenyl for DCzC, but it does not exist on the central atom of DCzSi, DCzGe, and DCzSn. To quantitatively describe the distribution of holes, we calculated the hole distribution volume (VH) of DCzC, DCzSi, DCzGe, and DCzSn, which are 280, 280, 279, and 281 Bohr3, respectively. Simultaneously, we discussed the effect of different group IV A elements on the whole molecular configuration. The difference in bond length compared with



DCzC is depicted in Figure 7(a), which indicated that group IV A elements only affect the bond length connected with them and has no effect on the other bond lengths. As the atomic number of group IVA element increases, the X-C bond length prominently increases especially for Sn-C bond. As shown in Figure 7(b), the molecular volume (VM) turned out to have the same tendency. Combined with the hole distribution discussed above, we obtained the value of VH/VM also depicted in Figure 7(b). DCzC has the largest value of VH/VM, indicating that hole is distributed more widely on the molecule. This can efficiently increase the hole transfer integral between two adjacent molecules causing DCzC to have a higher hole mobility. DCzSi and DCzGe have a similar VH/VM and they show a similar hole transfer integral as discussed in Figure 5(a). As DCzGe has a smaller λ+ than that of DCzSi, it shows better hole transport properties. DCzSn exhibits a significantly lower VH/VM value, which results in a poor hole mobility as discussed in Figure 4.

1.0

(b) 5900

C3

DCzSi-DCzC DCzGe-DCzC DCzSn-DCzC

X

0.8 0.6

∆ Bond-length (Å)

1.0

C1 C2 36: C1‒C2 37: C1‒C3 38: C4‒C2 36: X‒C1

0.4 0.2

0.8 0.6 0.4 0.2 0.0 32

34

36

38

40

5.0

20

40

60

4.9 5700

4.8

42

Bond Number

5600

0.0 0

5800

VH/VM (%)

C4

VM (Bohr3)

(a)

∆Bond-length (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

Bond Number

DCzC

4.7 DCzSi

DCzGe

DCzSn

Figure 7. (a) The difference in bond length compared with DCzC, and (b) Molecular volume of designed materials (iso=0.001).

4. CONCLUSIONS


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In conclusion, we first conducted a comprehensive investigation of group IV A elements-based hole transport materials (DCzC, DCzSi, DCzGe, DCzSn, and DCzPb) and answered two scientific questions. (1) Why do DCzGe-based OLEDs exhibit better performance than the DCzSi? We found DCzGe has a smaller reorganization energy that gives it a higher hole mobility than that of DCzSi. (2) How do the other group IV A element-based host materials compared with the existing Si- or Ge-based ones? The hole mobility of group IV A element-based host materials is DCzC > DCzGe > DCzSi > DCzSn. This indicates that compared with the already existing Sitetraphenyl and Ge-tetraphenyl, C-tetraphenyl will have higher potential to act as core moiety in constructing high-performance host materials. Finally, we demonstrated that DCzC with the highest hole mobility benefits from having the largest value of VH/VM. We propose that the larger the proportion of molecular volume occupied by the spatial distribution of hole (electron) with larger VH/VM, the more favorable increase in the charge transfer integral and improve in the hole (electron) mobility of host materials. This will be an effective theoretical basis for designing high-performance host materials with excellent charge mobility.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [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.

ACKNOWLEDGMENTS



This work was supported by the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1712300) , the Natural Science Foundation of Jiangsu Province (BK20160440), the Foundation and Advanced Research Projects of Chongqing Municipal Science and Technology Commission (cstc2017jcyjA1630), and the Scientific Research Grants of Yangtze Normal University (2017XJQN04, 2016KYQD12).

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for

Short

Wavelength

Organic

Electrophosphorescence:


2,7-


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Hole Distribution Volume - American Chemical Society

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