Highly efficient green phosphorescent organic light-emitting diodes based on tetraphenyl silicon derivative host materials

Article abstract of DOI:10.1016/j.orgel.2019.105581

We report herein two novel tetraphenyl silicon derivatives, bis(4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilane (DBTSiDBT) and bis(4- (dibenzo[b,d]furan-4-yl)phenyl)diphenylsilane (DBFSiDBF) achieved by coupling dibenzothiophene (DBT) and dibenzofuran (DBF) moieties with bis(4-bromophenyl)diphenylsilane. These two compounds show high triplet energy states, high glass transition temperature (Tg) and high decomposition temperature (Td). Therefore, they can serve as hosts in organic light-emitting diodes. Consequently, (2-phenylpyridine) iridium(III) (Ir(ppy)₃) based phosphorescence organic light-emitting diodes (PHOLEDs) with green emission were studied with DBTSiDBT and DBFSiDBF as host materials. The champion external quantum efficiency (EQE) and maximum luminance of the devices were 24.5% and over 27000 cd/m² respectively.

Full text of DOI:10.1016/j.orgel.2019.105581

Journal Pre-proof
Highly efficient green phosphorescent organic light-emitting diodes based on
tetraphenyl silicon derivative host materials
Lei Ding, Wan-Zhen Fo, Xiang-Yang Liu, Hua Chen, Femi Igbari, Jun-Hui Wu, Zuo-
Quan Jiang
PII:
S1566-1199(19)30608-1
DOI:
https://doi.org/10.1016/j.orgel.2019.105581
ORGELE 105581
Reference:
To appear in: Organic Electronics
Received Date: 19 August 2019
Revised Date: 27 November 2019
Accepted Date: 7 December 2019
Please cite this article as: L. Ding, W.-Z. Fo, X.-Y. Liu, H. Chen, F. Igbari, J.-H. Wu, Z.-Q. Jiang, Highly
efficient green phosphorescent organic light-emitting diodes based on tetraphenyl silicon derivative host
materials, Organic Electronics (2020), doi: https://doi.org/10.1016/j.orgel.2019.105581.
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Highly Efficient Green Phosphorescent Organic
Light-Emitting Diodes based on Tetraphenyl
Silicon Derivative Host Materials
Lei Ding,a,b,c* Wan-Zhen Fo , Xiang-Yang Liu, Hua Chen, Femi Igbari,
b
a
a
a
d
a
Jun-Hui Wu , Zuo-Quan Jiang
a
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices,
Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou 215123, P.R. China.
b
College of Electrical and Information Engineering, Shaanxi University of Science
and Technology, Xi'an 710021, P.R. China.
c
Jiangsu jitri org Optoelectronics Technology Co., Ltd, Suzhou, 215215, China.
d
Suzhou Supereal Microelectronics Co.,Ltd, Suzhou, 215011, China.
*
Corresponding authors.
E-mail addresses: dinglei@sust.edu.cn；
Tel: +86-029-86168320
Abstract
We
bis(4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilane (DBTSiDBT) and bis(4-
dibenzo[b,d]furan-4-yl)phenyl)diphenylsilane (DBFSiDBF) achieved by coupling
dibenzothiophene (DBT) and dibenzofuran (DBF) moieties with
report
herein
two
novel
tetraphenyl
silicon
derivatives,
(
bis(4-bromophenyl)diphenylsilane. This linking strategy raises the triplet energy,
glass transition temperature (Tg) and thermal stability of DBTSiDBT and DBFSiDBF.

Therefore, they can serve as hosts, producing phosphorescence at high efficiency in
organic light-emitting diodes. Consequently, (2-phenylpyridine) iridium(III) (Ir(ppy)
3
)
based phosphorescence organic light-emitting diodes (PHOLEDs) with green
emission were designed by applying DBTSiDBT and DBFSiDBF as host materials.
The champion external quantum efficiency (EQE) and maximum luminance of the
2
devices were 24.5% and over 27000 cd/m respectively.
Keywords:
Dibenzothiophene, Dibenzofuran, Organosilicon derivatives, Host material, High
triplet energy, Green phosphorescent organic light-emitting diode
Graphical

Highly Efficient Green Phosphorescent Organic Light-Emitting
Diodes based on Tetraphenyl Silicon Derivative Host Materials
a,b,c
b
a,e
a
a
Lei Ding, *, Wan-Zhen Fo , Xiang-Yang Liu, Hua Chen, Femi Igbari, Jun-Hui
d
a
Wu , Zuo-Quan Jiang
a) Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices,
Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou 215123, P.R. China.
b) College of Electrical and Information Engineering, Shaanxi University of Science
and Technology, Xi'an 710021, P.R. China.
c) Jiangsu jitri org Optoelectronics Technology Co., Ltd, Suzhou, 215215, China.
d) Suzhou Supereal Microelectronics Co.,Ltd, Suzhou, 215011, China.
e) Suzhou Ever Display Advanced Materials CO., LTD, Wujiang, Suzhou, Jiangsu
2
15000, China.
*
Corresponding authors.
E-mail addresses: dinglei@sust.edu.cn (L. Ding)
Tel.: +86-029-86168320; fax: +86-029-86168709;
Abstract: We report herein two novel tetraphenyl silicon derivatives,
bis(4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilane (DBTSiDBT) and bis(4-
(dibenzo[b,d]furan-4-yl)phenyl)diphenylsilane (DBFSiDBF) achieved by coupling
dibenzothiophene (DBT) and dibenzofuran (DBF) moieties with
bis(4-bromophenyl)diphenylsilane. These two compounds show high triplet energy
states, high glass transition temperature (Tg) and high decomposition temperature
(Td). Therefore, they can serve as hosts in organic light-emitting diodes. Consequently,
(2-phenylpyridine) iridium(III) (Ir(ppy)3) based phosphorescence organic
light-emitting diodes (PHOLEDs) with green emission were studied with DBTSiDBT
and DBFSiDBF as host materials. The champion external quantum efficiency (EQE)
2
and maximum luminance of the devices were 24.5% and over 27000 cd/m

respectively.
Keywords: Green phosphorescent organic light-emitting diode, High triplet energy,
Host material, Organosilicon derivatives, Dibenzofuran, Dibenzothiophene.
1
. Intruduction
Organic light emitting diodes (OLEDs) emerged about three decades ago as a new
luminescent technology [1], and much progress has been achieved since then. OLEDs
could be applied in display and lightening. Compared with traditional technologies,
OLEDs have many advantages such as thin and flexible structure, wide view angle,
fast response and decent transparency. Therefore, OLEDs can be used in various
applications [2-8]. However, OLEDs are not stable for a prolonged period and this
remains one of the problems that needs to be addressed to achieve a long device
lifetime [9-11]. Within a typical fluorescence OLED, the ratio of singlet (S) and triplet
(T) excitons is 1:3. Therefore, the internal quantum efficiency (IQE) does not exceed
2
5% and the maximum EQE is less than 5% despite its long-term stability. The
discovery of phosphorescent emitters has facilitated the achievement of highly
efficient OLEDs with the utilization of both singlet and triplet excitons [12]. To date,
low energy PHOLEDs (e.g. orange and red PHOLEDs) have exhibited remarkable
performances [13-16]. However, achieving high energy PHOLEDs (e.g. blue
PHOLEDs) is still a challenge. The reasons are given as follows [9-11]: i) the
availability of high efficiency and long lifetime phosphors is limited. The available
conventional phosphors lack long-term stability. In addition, PHOLEDs may change
their EL spectra with diffenrent driving voltage. ii) In order to match the energy levels
of the blue and green phosphors, the host material needs to exhibit wide energy gap.
This makes the preparation of such a material a big challenge. In addition, these kinds
of host materials often possess deep highest occupied molecular orbital (HOMO) as
well as shallow lowest unoccupied molecular orbital (LUMO). This increases
injection barriers for electrons as well as holes. The device therefore needs much
higher driving voltage which will further limit the application of PHOLEDs.
Therefore, a long-lasting and high-efficiency OLED is still on the wish list with no
satisfying solution in sight. Since host materials serve as the dispersion matrix for
phosphor emitters, it is imperative to develop highly effecient host materials for
practical application of PHOLEDs. To meet this requirement, some semi-empirical

principles have to be established. These include good thermal and morphological
stability, appropriate HOMO/LUMO levels, optimal charge transport and high triplet
energy [9-11]. The design protocol to raise the triplet energy of a host material
includes: i) to incorporate the high triplet energy moieties e.g. arylsilane [17-21],
dibenzofuran [22-29], dibenzothiophene [22,30-36], fluorene [37-41], and carbazole
[
42-44] into host material; ii) to restrain the conjugation length in the host material.
3
Silicon [17-21], phosphine oxide [40] and sp carbon [45] effectively interrupt
intramolecular π-conjugation and are commonly adopted for this purpose. Also,
sterically hindered structures as well as ortho or meta linking strategies can be used to
minimize the conjugation length. This design principle has been applied over the
years to develop many highly efficient host materials [22,39-44].
In this work, by linking a tetraphenyl silicon building block with dibenzothiophene
(DBT) and dibenzofuran (DBF) units, two new host materials exhibiting elevated
glass-transition temperature and triplet energy were prepared. The new host materials
are bis(4-(dibenzo[b,d]thiophen-4-yl)phenyl)diphenylsilane (DBTSiDBT) and
bis(4-(dibenzo[b,d]furan-4-yl)phenyl)
diphenylsilane
(DBFSiDBF).
The
silicon-bridge is capable of facilitating transfer of energy from host to guest for
smaller steric hindrance. As result, highly efficient green PHOLEDs with
(
2-phenylpyridine) iridium(III) (Ir(ppy) ) as emitter were realized using the novel host
3
2
materials with maximum EQE of 24.5% and luminance over 27000 cd/m .
3
3
. Results and Discussion
.1 Syntheses and Characterization
Scheme 1 outlines the preparation steps for bis(4-(dibenzo[b,d]thiophen-4-yl)
phenyl)diphenylsilane (DBTSiDBT) and bis(4-(dibenzo[b,d]furan-4-yl)phenyl)
diphenylsilane (DBFSiDBF). 1,4-Dibromobenzene and dichlorodiphenylsilane were
used
as
precursors
for
the
synthesis
of
the
intermediate,
bis(4-bromophenyl)diphenylsilane. Finally, the target products were obtained in good
yield by one step Suzuki−Miyaura coupling reaction with appropriate boronic acids of
dibenzothiophene (DBT) and dibenzofuran (DBF).
3
.2 Thermal Analyses
Thermogravimetric analysis (TGA) as well as differential scanning calorimetry
(DSC) were conducted to determine the thermal characteristics of DBTSiDBT and

DBFSiDBF. Figure 1 shows that the two compounds possess high thermal stability.
o
The T was measured as 112 C for DBTSiDBT, while DBFSiDBF shows slightly
g
o
lower T of 102 C. The T data for these materials show a slight difference which
g
g
may be ascribed to the similarities in their molecular weight and molecular structure.
o
DBTSiDBT and DBFSiDBF also showed T of 474 and 429 C respectively at 5%
d
weight loss. The T s exhibit a similar trend to their T data.
d
g
3
.3 Optoelectronic Properties
Figure 3 presents the room temperature UV-vis absorption and PL spectra of
DBTSiDBT and DBFSiDBF in dichloromethane as well as their phosphorescence
Phos) spectra at 77 K in 2-methyltetrahydrofuran (2-MeTHF). The detailed
(
parameters are presented in Table 1. DBTSiDBT and DBFSiDBF exhibit similar
absorption feature. Nonetheless, the maximum absorption wavelengths (Abs λmax) as
well as the absorption onset of DBT-based compounds are red-shifted by 16 nm with
respect to those of the DBF-based analogue. Therefore, the band gap (E ) of
g
DBTSiDBT (3.59 eV) is narrower than that of DBFSiDBF (3.77 eV). On the other
hand, DBTSiDBT and DBFSiDBF exhibited very similar PL characteristics. The
similar and structureless PL curves might be due to their similar molecular
configuration [22]. As shown in their Phos spectra, DBTSiDBT and DBFSiDBF
exhibit well defined vibronic bands with the highest values at 464 nm and 460 nm and
triplet energies (E ) of 2.67 and 2.70 eV respectively. Due to their suitable and high
T
levels of E , these compounds could serve as host materials in phosphorescent
T
OLEDs.
3
.4 DFT Simulation.
The electronic distributions of DBTSiDBT and DBFSiDBF were further elucidated
by simulating their FMO spatial distributions through DFT calculations at a
B3LYP/6-31G(d) level. Figure 3 presents the spatial distributions of the highest and
lowest frontier molecular orbitals of these host materials. These two materials exhibit
similar distributions of LUMO and HOMO. The HOMOs of DBTSiDBT and
DBFSiDBF are localized on the DBF and DBT unit, respectively, and the LUMOs
spread across the molecular backbone.
3
.5 FMO Energy Levels
To obtain energy levels of these materials, their HOMO levels were evaluated via

UPS analyses. Figure 4 presents the UPS spectra of DBTSiDBT and DBFSiDBF.
The HOMOs can be extracted using the equation; EHOMO = hν – E , where, hν is
k
photon energy (He I, hν = 21.22 eV) and E is kinetic energy of photo-electrons.
k
LUMOs can be evaluated using equation; ELUMO = E + EHOMO, E is the optical
g
g
energy gap. Therefore, levels of HOMO/LUMO of DBTSiDBT and DBFSiDBF are
calculated to be -6.24/-2.65 and -6.45/-2.68 eV respectively.
3
.6 Electroluminescent Properties
The electroluminescent (EL) characteristics of DBTSiDBT and DBFSiDBF were
investigated by fabricating green phosphorescent OLEDs based on Ir(ppy) with a
3
structure of ITO/HAT-CN (10 nm)/TAPC (55 nm)/Host: 9 vol% Ir(ppy) (20
3
nm)/TmPyPB (35 nm)/Liq (2 nm)/Al (120 nm). 9 wt% Ir(ppy) were doped into
3
DBTSiDBT and DBFSiDBF to achieve the emitting layer (EML). The corresponding
molecular structures and energy diagrams are shown in Figure 5. The current
density–voltage–luminance (J–V–L) characteristics; current efficiency (CE), power
efficiency (PE) and external quantum efficiency (EQE) as a function of luminance
and the EL spectra are shown in Figure 6(a), 6(b) and 6(c), respectively. Table 2
presents the EL parameters.
As revealed in Figure 6(a), the devices hosted by DBTSiDBT and DBFSiDBF
2
exhibit relatively low turn-on voltages of 4.4 and 4.7 V at 1000 cd/m , respectively.
The DBTSiDBT and DBFSiDBF based devices can achieve the maximum luminance
2
of 19000 and 27000 cd/m , respectively. Figure 6(b) presents the efficiencies of green
devices based on Ir(ppy) . For DBFSiDBF hosted device, high efficiency of 83.5
3
cd/A, 61.7 lm/W and 24.5% were realized for CE, PE and EQE respectively.
DBTSiDBT based devices show relatively inferior maximum CE, PE and EQE of
5
2.0 cd/A, 45.8 lm/W and 14.4% respectively. Moreover, it is noteworthy that both
2
devices exhibited flat efficiency roll-offs. At 1000 cd/m , devices based on
DBTSiDBT showed an increase in EQE from 14.4% to a maximum value of 14.5%.
2
2
At 10000 cd/m , the EQE retained a high value of 13.7%. At 1000 cd/m , the device
based on DBFSiDBF demonstrated a reduction in EQE from the highest value 24.5%
2
2
to 24.4%, and was able to retain 23.6% at 5000 cd/m . Notably, at 10000 cd/m , the
EQE was only slightly reduced to 23.3%, which was still higher than the maximum
EQE of 14.4% for devices based on DBTSiDBT. It is worth noting that the relatively
good hole and electron transport properties of DBF-based host material (Figure 7) and

the higher E will favor the balanced charge transport and efficient energy transfer
T
from host to dopant, which may account in part for its better EL performance. The
green devices exhibit typical CIE coordinates of Ir(ppy) based devices are (0.28, 0.64)
3
2
at 5 mA/cm . It means that emission is originated from Ir(ppy) and not from
3
DBTSiDBT or DBFSiDBF. This could be attributed to the high E s exhibited by
T
these hosts which effectively confines emission within the dopant molecule.
4
. Conclusion
In conclusion, two novel organosilicon derivatives, DBTSiDBT and DBFSiDBF,
achieved by the attachment of DBT or DBF to bis(4-bromophenyl)diphenylsilane
were designed, synthesized and characterized. Both compounds exhibited high triplet
energy and good thermal stability due to the suitable linking strategy. The materials
were applied as hosts for Ir(ppy) emitter in green phosphorescence OLEDs with
3
simple device structures. Green PHOLEDs based on DBFSiDBF achieved a
maximum EQE of 24.5% with low efficiency roll-off. These results reveal the great
potentials of tetraphenyl silicon derivatives and the possibility of subjecting them to
extended study for high efficiency OLEDs.
Ackonwledgement
We acknowledge financial supports from the National Natural Science Foundation of
China under Grant No 61076066, 21572152, 61575136. Key research and
development plan of Shaanxi Province (2017ZDCXL-GY-06-03), Special scientific
research plan of Shaanxi Provincial Education Department (17JK0095), The Nature
Science Foundation of Jiangsu Province(BK20180288), Sixty-second grants from
China Post Doctoral Science Fund(7131700918).
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Scheme 1. Synthetic routes to DBTSiDBT and DBFSiDBF.
100
o
o
4
29 C
474 C
80
60
40
20
DBFSiDBF
DBTSiDBT
0
1
00
200
300
400
500
600
700
o
Temperature ( C)
Figure 1. Differential scanning calorimetry (DSC, left) and thermogravimetric analyses (TGA,
right) of DBTSiDBT and DBFSiDBF.
UV-vis
1.0
DBTSiDBT
DBFSiDBF
0.8
0
.6
.4
.2
.0
PL
Phos
0
0
0
300
400
500
600
700
Wavelength (nm)
Figure 2. UV-Vis absorption, PL and Phos spectra of DBTSiDBT and DBFSiDBF.

Table 1. Physical Properties of DBTSiDBT and DBFSiDBF.
a
a
b
c
d
e
Abs λmax
PL λmax
T / T / E /
E /
g
d
g
T
f
g
Host
HOMO /eV LUMO /eV
o
o
/nm
/nm
C
C
eV
eV
DBTSiDBT
DBFSiDBF
285
254
355
112 474 3.61 2.67
102 429 3.50 2.70
-6.24
-6.45
-2.65
-2.68
351
a
b
c
Measured in toluene solution at room temperature. T : Glass transition temperature. T :
g
d
d
Decomposition temperature. E : Band gaps, calculated from the corresponding absorption
g
e
f
onset. E : Measured in 2-MeTHF glass matrix at 77 K. HOMO levels, calculated from UPS
T
g
data. LUMO levels, calculated from the HOMO and E .
g
Figure 3. HOMO/LUMO distributions and molecular structure of DBTSiDBT and DBFSiDBF.

Figure 4. UPS spectra of DBTSiDBT and DBFSiDBF.
Figure 5. Molecular structures and representative energy level diagram of the materials.

Figure 6. Current density–voltage–luminance (J–V–L) characteristics (a); current efficiency (CE),
Current density–voltage–luminance (J–V–L) characteristics (a); current efficiency (CE), power
efficiency (PE) and external quantum efficiency (EQE) versus luminance curves for
2
Ir(ppy) -based devices (b) and EL spectra at 5 mA/cm (c).
3

Figure 7. J–V characteristics of hole- and electron-only devices. Hole-only devices:
ITO/ MoO (10 nm)/ Host (100 nm)/ MoO (10 nm)/ Al (100 nm); electron-only
3
3
devices: ITO/ TmPyPB (20 nm)/ Host (100 nm)/ TmPyPB (20 nm)/ Liq (2 nm)/ Al
(100 nm).
Table 2. Electroluminescence characteristics of the devices
b
c
c
c
d
V
ηCE
ηPE
EQE
[%]
CIE
a
Device
Host
-
1
-1
[
V]
[cd A ]
[lm W ]
[x, y]
1
2
DBTSiDBT 4.4 53.4, 53.4, 50.0 45.8, 37.7, 28.4
DBFSiDBF 4.7 83.5, 82.9, 78.0 61.7, 56.1, 40.9
14.5, 14.5, 13.7 0.28, 0.64
24.5, 24.4, 23.2 0.28, 0.64
a
The notations 1 and 2 in devices indicate the devices fabricated with DBTSiDBT and DBFSiDBF
as the hosts respectively. Device configuration: ITO/HAT-CN (10 nm)/TAPC (55 nm)/Host: 9%
b
2 c
Ir(ppy) (20 nm)/TmPyPB (35 nm)/Liq (2 nm)/Al (120 nm). Voltages at 1000 cd/m , Efficiencies
3
2
2 d
in the order of the maxima at 1000 cd/m and at 10000 cd/m , Commission International de
2
Ieelaiage coordinates measured at 5 mA/cm .

Scheme 1. Synthetic routes to DBTSiDBT and DBFSiDBF.
Figure 1. Differential scanning calorimetry (DSC, left) and thermogravimetric analyses (TGA,
right) of DBTSiDBT and DBFSiDBF.
Figure 2. UV-Vis absorption, PL and Phos spectra of DBTSiDBT and DBFSiDBF.
Figure 3. HOMO/LUMO distributions and molecular structure of DBTSiDBT and DBFSiDBF.
Figure 4. UPS spectra of DBTSiDBT and DBFSiDBF.
Figure 5. Molecular structures and representative energy level diagram of the materials.
Figure 6. Current density–voltage–luminance (J–V–L) characteristics (a); current efficiency (CE),
power efficiency (PE) and external quantum efficiency (EQE) versus luminance curves for
2
Ir(ppy) -based devices (b) and EL spectra at 5 mA/cm (c).
3
Table 1. Physical Properties of DBTSiDBT and DBFSiDBF.
Table 2. Electroluminescence characteristics of the devices

ò
Two novel organosilicon derivatives, DBTSiDBT and DBFSiDBF, were designed,
synthesized, and applied as host materials in green PHOLEDs.
ò
ò
Both materials showed good thermal stability and high triplet energy.
The Ir(ppy) based PHOLED hosted by DBFSiDBF exhibited relatively high EQE
3
of 24.5% with lower efficiency roll-off.

Declaration of interests
✓
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: