Bis(diphenylamino)-benzo[4,5]thieno[3,2-b]benzofuran as hole transport material for highly efficient RGB organic light-emitting diodes with low efficiency roll-off and long lifetime

Article abstract of DOI:10.1016/j.orgel.2020.105793

In this work, a novel hole transport material (HTM) based on benzo[4,5]thieno[3,2-b]benzofuran (BTBF) core with diphenylamine group, BTBF-DPA, was synthesized and systematically studied. BTBF-DPA showed high carrier mobility, good thermal stability and high triplet energy as 2.99 eV. Green phosphorescent organic light-emitting diodes (PhOLEDs) using BTBF-DPA were fabricated which displayed outstanding electroluminescent performance with a low turn-on voltage of 2.4 V, the maximum current efficiency (CE) of 123.6 cd A^?1, power efficiency (PE) of 136.6 lm W^?1, and external quantum efficiency (EQE) of 34.1%. Red PhOLEDs with BTBF-DPA also demonstrated a maximum PE of 40.1 lm W^?1, and EQE of 24.9% with a low turn-on voltage of 2.3 V. Besides, blue fluorescence OLEDs presented a maximum PE of 4.9 lm W^?1, EQE of 5.2% and small efficiency roll-off of 11.5% at 10000 cd m^?2. RGB-OLEDs with BTBF-DPA outperformed the reference devices based on NPB as HTM that has been widely utilized. What's more, life-time of RGB OLEDs were T₉₉ = 70 h at 10000 cd m^?2, T₇₀ = 60 h at 10000 cd m^?2, T₈₀ = 125 h at 5000 cd m^?2, respectively. These results imply that BTBF-DPA is a promising HTM for OLEDs.

Full text of DOI:10.1016/j.orgel.2020.105793

Journal Pre-proof
Bis(diphenylamino)-Benzo[4,5]thieno[3,2-b]benzofuran as hole transport material for
highly efficient RGB organic light-emitting diodes with low efficiency roll-off and long
lifetime
Ming Shi, Yaowu He, Yue Sun, Daqi Fang, Jingsheng Miao, Muhammad Umair Ali,
Tao Wang, Yunrui Wang, Tian Zhang, Hong Meng
PII:
S1566-1199(20)30179-8
DOI:
https://doi.org/10.1016/j.orgel.2020.105793
ORGELE 105793
Reference:
To appear in: Organic Electronics
Received Date: 6 April 2020
Revised Date: 25 April 2020
Accepted Date: 25 April 2020
Please cite this article as: M. Shi, Y. He, Y. Sun, D. Fang, J. Miao, M.U. Ali, T. Wang, Y. Wang, T.
Zhang, H. Meng, Bis(diphenylamino)-Benzo[4,5]thieno[3,2-b]benzofuran as hole transport material for
highly efficient RGB organic light-emitting diodes with low efficiency roll-off and long lifetime, Organic
Electronics (2020), doi: https://doi.org/10.1016/j.orgel.2020.105793.
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Bis(diphenylamino)-Benzo[4,5]thieno[3,2-
b]benzofuran as Hole Transport Material for Highly
E� cient RGB Organic Light-Emitting Diodes with
Low Efficiency Roll-off and Long Lifetime
Ming Shi, Yaowu He, Yue Sun, Daqi Fang, Jingsheng Miao, Muhammad Umair Ali, Tao Wang,
Yunrui Wang, Tian Zhang, Hong Meng*
School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen,
Shenzhen, 518055, P. R. China. E-mail: menghong@pku.edu.cn
KEYWORDS: BTBF, hole transport material, RGB OLEDs, power efficiency, lifetime
ABSTRACT: In this work, a novel hole transport material (HTM) based on
benzo[4,5]thieno[3,2-b]benzofuran (BTBF) core with diphenylamine group, BTBF-DPA, was
synthesized and systematically studied. BTBF-DPA showed high carrier mobility, good thermal
stability and high triplet energy as 2.99 eV. Green phosphorescent organic light-emitting diodes
(PhOLEDs) using BTBF-DPA were fabricated which displayed outstanding electroluminescent
performance with a low turn-on voltage of 2.4 V, the maximum current efficiency (CE) of 123.6
−
1
−1
cd A , power efficiency (PE) of 136.6 lm W , and external quantum efficiency (EQE) of
−
1
3
4.1%. Red PhOLEDs with BTBF-DPA also demonstrated a maximum PE of 40.1 lm W , and
EQE of 24.9% with a low turn-on voltage of 2.3 V. Besides, blue fluorescence OLEDs presented
−
1
a maximum PE of 4.9 lm W , EQE of 5.2% and small efficiency roll-off of 11.5% at 10000 cd
1

−
2
m . RGB-OLEDs with BTBF-DPA outperformed the reference devices based on NPB as HTM
that has been widely utilized. What’s more, life-time of RGB OLEDs were T = 70 h at 10000
9
9
-2
-2
-2
cd m , T = 60 h at 10000 cd m , T = 125 h at 5000 cd m , respectively. These results imply
7
0
80
that BTBF-DPA is a promising HTM for OLEDs.
1
. INTRODUCTION
Organic light-emitting diodes (OLEDs) have attained great attention owing to their numerous
1
-4
applications as solid-state light emitting and flexible displays . There are several kinds of
OLEDs according to the light-emitting mechanism, for example, phosphorescent or normal
fluorescent system, thermal activated delayed fluorescent (TADF) system, triplet-triplet
annihilation (TTA) system, hybridized local and charge-transfer excited state (HLCT) system,
5
-9
exciplex system and so on . Many researches have been developed to optimize the emission
1
0-13
layer to achieve as high as the EQE theoretical maximum
. While for commercial applications,
the power efficiency (PE) of OLEDs is a crucial factor as it provides a direct measurement of the
1
4-15
power consumption considering operating voltage and device life-time
. Typically, the
efficiency of OLEDs will decreases with the increasing of brightness, which is efficiency roll-off.
16
The efficiency roll-off reflect great influence on PE due to additional resistive losses . In order
-2
to obtain high PE values at high brightness levels (> 1000 cd m ), high current densities must be
achieved at low voltage. This can be realized by using proper charge transport materials to
1
7-19
reduce injection barriers, facilitate high carrier mobility and adjust charge balance
.
HTMs could effectively transport the holes while blocking the electrons between the hole
injection and the emitting layers, which are significant to obtain highly efficient OLEDs. An
ideal HTM must possess proper HOMO and LUMO energy levels, outstanding hole mobility,
2

2
0-22
good thermal stability and high triplet energy
. Triarylamine-type compounds have been
generally recognized as excellent hole transporter for OLEDs as well as organic/ perovskite solar
23-25
cells because of their intrinsic potential to meet those requirements
methylphenyl)-N, N’-bis-phenyl-benzidine (TPD) and N, N'-di(naphthalen-1-yl)-N, N'-diphenyl-
1, 1'-biphenyl]-4,4'-diamine (NPB) are among the most widely used HTMs due to their
outstanding hole-transporting properties, although their low T (60 � for TPD and 95 � for
. N, N’-bis-(3-
[
g
2
6-30
NPB), and low E
T
(2.23 eV for NPB) are obvious defects of them
. Many Studies on
developing better hole transport materials to replace TPD or NPB (or modify their structure)
have been reported in the literature. Replacement of biphenyl center in TPD by fluorine
derivatives was found to be effective to improve T as well as reduce the ionization energy and
g
redox potential. Moreover, the hole mobility is also affected by a change in neutral state and
2
9, 31-32
radical cation brought by the torsion angle difference between biphenyl and fused fluorene
.
Adding a spiro fluorene or triphenylamine group annulated to the central biphenyl of NPB was
another strategy to enhance the molecule rigidity and T (140 � ) to attain high performance of
g
2
6, 33-34
related OLEDs
.
In this contribution, we designed and synthesized a simple arylamine compound, BTBF-DPA, as
an effective HTM for OLEDs. In the function of electron acceptors, benzofuran and
benzothiophene groups offer excellent stability and high triplet energy since they can decrease
the oxidation potential, increase the number of delocalized electrons, and subsequently enhance
3
2, 35-37
the HOMO energy level such that the hole carriers are transferred readily
. The integration
of furan and thiophene adjacent to biphenyl ring strengthen the ability of hole transport and more
importantly move the performance of OLEDs to the next level. Green and red PhOLEDs using
BTBF-DPA were fabricated to investigate the device performance. Compared with NPB as
3

reference, green and red PhOLEDs with BTBF-DPA exhibited superior device performance. In
addition, blue FOLEDs based on BTBF-DPA were also developed to estimate the potential
commercialization, which show better EL performance with low driving voltage and low
efficiency roll off at high brightness. What’s more, devices lifetime of RGB OLEDs with BTBF-
DPA were investigated which showed long lifetime at high brightness. These encouraging results
reveal the potential of BTBF core as a promising moiety to develop novel and stable HTMs, and
one successful strategy to develop efficient HTMs. Especially, it is worth mentioning that we
3
8-42
designed an new and effective approach to successfully synthesizes BTBF
. This is the first
demonstration of employing BTBF derivative in OLEDs. Besides, the strong deep blue
fluorescence and good solubility of this compound may provide other potential applications in
organic optical electronic fields.
2
2
. RESULTS AND DISCUSSION
.1. Synthesis
4

Scheme 1. The synthesis route of BTBF-DPA
The molecular structure and synthetic routes of the intermediates and target compound are
demonstrated in Scheme 1. The target compound was prepared by Buchwald-hartwig coupling
reaction, then purified by flash column chromatography, recrystallized from DCM and methanol
-5
and subliming furnace at 280 °C under 1.0 × 10 Pa, subsequently, with a final yield of 82.3%.
1
13
The structures of BTBF-DPA were confirmed by H NMR, C NMR and high-resolution mass
spectrometry (HRMS). BTBF-DAP are readily soluble in common organic solvents with strong
fluorescence.
5

2
.2. Electronic Structure
To get more insights into the potential nature of BTBF-DPA and the difference between BTBF-
DPA and NPB, density functional theory (DFT) calculations were performed to simulate the
frontier orbitals of HOMO/LUMO distributions and energy levels under the B3LYP/6-31G (d, p)
basis set as shown in Figure 1. The HOMO of BTBF-DPA is delocalized 46.87% on the BTBF
core and the rest 53.13% is located on diphenylamine moiety. While the LUMO is mainly
(85.43%) distributed on the BTBF core due to the strong electron negativity of oxygen and
sulfur. For NPB, the LUMO only delocalized 6.3% on the biphenyl core, which is completely
different from BTBF-DPA.
Figure 1. HOMO and LUMO distributions and energy levels of BTBF-DPA and NPB by
B3LYP/6-31G (d, p) calculation.
The higher electron density between two nitrogen atoms could bring higher hole transport ability
in molecule which was proven by the hole only device later. The energy of the frontier orbitals of
6

BTBF-DPA is estimated to be −4.69 and −1.18 eV, which are similar to that of NPB, which
demonstrated the comparability between BTBF-DPA and NPB in the same device structure.
2
.3. Photo-physical Properties
The normalized UV-vis absorption, steady-state photoluminescence (PL) spectra and low
-5
temperature (77 K) phosphorescence spectra of BTBF-DPA in toluene (1.0
solid film (deposited under vacuum, 30 nm) are shown in Figure 2a and 2b, respectively. The
short-wavelength absorption band at 265 nm belongs to the
* transition of benzene. The
absorption band at nearly 300 nm is associated with the * transition and n− * transition of
the triphenylamine-centered moiety. The high intensity absorption band ranging from 350 to 390
nm is attributed to the * transition and n− * of the rigid BTBF unit. The compound indicated
an intramolecular charge transfer (ICT) absorption peak at 390 nm in DCM solution and at
01nm in neat film state. According to the absorption edge, the optical energy gap is calculated
× 10 M) and in
π
−π
π−
π
π
π−
π
π
4
to be 3.03 eV. The compound showed strong deep-blue emission with maximum emission peak
at 402 nm and 417 nm in DCM solution and neat film, respectively. The PL of neat film is
redshifted by 15 nm because of the stronger molecular aggregation effect between molecules,
which could also be seen from the long-tailed peak from 468 nm to 612 nm.
7

Figure 2. (a) UV-vis absorption, fluorescence and phosphorescence spectra of BTBF-DPA
in toluene solution; (b) UV-vis absorption, fluorescence spectra of BTBF-DPA in neat film; (c)
TGA and DSC curves of the material BTBF-DPA. Heating rate: 10 °C/min; (d) The CV curve of
BTBF-DPA in DMF solution.
Table 1 Material properties of BTBF-DPA and NPB
b
c
d
d
d,e
g
f
g
g
Compound Absorption Emission
S
1
T
1
HOMO LUMO E
T
T
d
a
a
o
o
λmax [nm] λmax [nm] [eV] [eV]
[eV]
[eV] [eV] [ C] [ C]
BTBF-DPA
390
345
413
441
3.15 2.99 -5.02 -2.12 2.90/3.03 104 430
2.23 -5.12 -2.05 3.0 95 490
2
6
NPB
-
−
5
a. Measured in toluene in a concentration of 1.0 × 10 M.
b. Estimated from onset of the absorption spectra (E = 1240/λonset).
g
c. Estimated from onset of the low temperature phosphorescent spectra
8

d. The HOMO and LUMO energy level is determined from cyclic voltammetry (EHOMO = −4.8 -
Eox, ELUMO = −4.8 – Ered).
e. The band gap is determined from the film absorption spectra.
f. Measured by DSC.
g. Measured by TGA.
Due to the rigid BTBF core, the structural vibrations in the excited states could be suppressed to
some extent, resulting in a narrower PL emission which full width of half maximum (FWHM) of
4
1 nm and 37 nm in toluene solution and neat film, respectively. The triplet energy of BTBF-
DPA was determined by the short-wavelength onset of its low temperature (77 K)
-5
phosphorescence spectra under nitrogen atmosphere in toluene (1.0×10 M) solution, which is as
1
3
high as 2.99 eV .
2
.4. Thermal Properties
The thermal properties of BTBF-DPA were investigated by thermogravimetric analysis (TGA)
and differential scanning calorimetry analysis (DSC) under nitrogen atmosphere with a heating
-1
rate of 10 °C min . As shown in Figure 2c and Table 1, BTBF-DPA showed good thermal
stability with the decomposition temperatures ( , correspond to 5% weight loss) of 430 °C and
glass transition temperature ( ) of 104°C which is higher than that of NPB ( , 95 °C). The
T
d
T
g
T
g
higher Tg suggesting better morphological stability of BTBF-DPA compared to NPB. Therefore,
BTBF-DPA is expected to be a good HTM owing to its excellent thermal stability behavior.
2
.5. Electrochemical Properties
9

The electrochemical behavior of BTBF-DPA were tested by cyclic voltammograms (CV) using a
standard three-electrode electrochemical cell in an electrolyte solution (0.1 M TBAPF6/DMF,
with ferrocene as an external reference. As shown in Figure 2d and Table 1, the HOMO energy
level of BTBF-DPA were calculated to be -5.02 eV, which is higher than that of NPB (-5.12 eV).
The corresponding LUMO energy level is calculated to be -2.12 eV. The energetically favorable
HOMO energy level of BTBF-DPA is supposed to be further facilitate the hole injection and
transport properties from the anode side. Moreover, the shallow LUMO energy level guarantees
the efficient electron blocking ability.
2
.6. Carrier Transport Ability
To explore the hole mobility of BTBF-DPA, we fabricated hole only devices with a structure of
indium-tin oxide (ITO)/1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN, 10
nm)/BTBF-DPA (100 nm)/HAT-CN (10 nm)/Al (100 nm) and ITO/HAT-CN (10 nm)/NPB (100
nm)/HAT-CN (10 nm)/Al (100 nm). The current density vs voltage (J−V) characteristics of the
1
/2
single-carrier devices. The carrier mobility can be calculated based on the J -V curves
according to the space-charge-limited current (SCLC) model as Figure 3 shows. The calculated
−
5
2
−1 −1
hole mobility of BTBF-DPA (3.1
× 10 cm V s ) is about 7 times higher than that of NPB
−
6
2
−1 −1
(4.5
×
10 cm V s ) at the same device structure and conditions.
.
10

Figure 3. Hole mobility of BTBF-DPA andNPB.
2
.7. Electroluminescence Properties
2
.7.1 Green PhOLEDs
In order to figure out the potential of BTBF-DPA in device application, we fabricated
green PhOLEDs devices using Ir(ppy) acac as an emitter. The key device data are
2
summarized in Table 2. As shown in Figure 4b, the green device structure is ITO/ HAT-
CN (5 nm)/ BTBF-DPA (20 nm) or NPB (20 nm)/ Spiro-3-BPF (15 nm)/ DMIC-TRZ :
DMIC-Cz : Ir(ppy)
2
acac (10:10:1) (15 nm)/ ANT-BIZ (20 nm)/ Liq (1 nm)/ Al
43
(
100nm) . Here in, HAT-CN served as a hole-injection layer and Liq as an electron-
injection layer; BTBF-DPA or NPB are employed as HTMs, ANT-BIZ worked as the
electron-transporting material, Spiro-3-BPF was used as electron blocking layer (EBL) to
facilitate hole injection and block electrons. Ir complex was co-evaporated with DMIC-
TRZ and DMIC-CZ to form the emitting layer (EML). The doping ratio of DMIC-TRZ :
11

DMIC-CZ : Ir(ppy) acac complex was 10:10:1. All the details about materials used here
2
were explained in Supporting Information.
Figure 4. (a) Energy diagram, (b) structure and materials (bottom) used in green
PhOLEDs
.
This is an exciplex system in which H1 is n-type host and H2 is p-type host. The singlet
and triplet energies of exciplex hosts transfer to the singlet and triplet states of iridium
complexes through the Foster and Dexter energy transfer, respectively. Then, the singlet
excitons of iridium complex could transfer to a triplet state for emission through the
intersystem crossing (ISC) process to achieve 100% exciton utilization. The
electroluminescent properties of BTBF-DPA and NPB based devices, including current
efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) as well as
12

the electroluminescence spectra, are demonstrated in Figure 5 and the key parameters are
summarized in Table 2
.
Figure 5. Device characteristics of green PhOLEDs with BTBF-DPA and NPB as HTMs :
a) J-L-P characteristics, (b) EQE-L characteristics, (c) J-V-L characteristics, (d) EL
(
spectra.
The BTBF-DPA based device exhibit stable green emission at the peak of 523 nm with CIE of
(0.34, 0.61), whereas NPB based device emits the same green light characteristics. The green
-2
devices based on BTBF-DPA show an extremely low turn-on voltage (Von voltage at 1 cd m ) as
low as 2.4 V which is the same as that of NPB based green device.
Table 2. Performance of Green PhOLEDs.
-1
-1
Green Von
CE(cd A )
PE(lm W )
EQE(%)
CIE
-2
-2
-2
Pospho- (V) max @1000cd m max @1000cd m max @1000cd m
(x,y)
13

-2
-2
-2
rescence
/10000cd m
/10000cd m
/10000cd m
BTBF 2.4 123.6 102.8/72.1 136.6 77.8/36.8 34.1 28.4/20.0 (0.34, 0.61)
NPB 2.4 89.7 71.2/55.7 96.8 43.4/28.8 24.7 19.6/15.3 (0.34, 0.61)
In detail, as shown in Table 2 and Figure 5, NPB based device showed a peak EQE of 27.4%, a
-1
-1
maximum CE of 89.7 cd A , and a maximum power efficiency (PE) of 96.8 lm W . With
enhanced hole mobility and high T , the device based on BTBF-DPA exhibited a peak EQE of
4.1%, a maximum CE of 123.6 cd A , and a maximum PE of 136.6 lm W . The significant
g
-1
-1
3
enhancement in the efficiency of the BTBF-DPA based green device can be attributed to better
balanced charge recombination at the emitting layer due to the outstanding hole mobility of
BTBF-DPA. Furthermore, devices based on BTBF-DPA could reach very high brightness at low
voltage which means higher luminous efficacy and lower energy consumption than the devices
based on NPB.
2
.7.2 Red PhOLEDs
We also fabricated red PhOLEDs to further demonstrate the universality of BTBF-DPA as HTM
The device structure is the same as green PhOLEDs except the dopant is replaced by Irpql-acac,
demonstrated in Supporting Information S1. The electroluminescent properties of BTBF-DPA
based red phosphorescent device are shown in Figure 6 and the key parameters are summarized
in Table 3.
14

Figure 6. Device characteristics of red PhOLEDs with BTBF-DPA and NPB as HTMs :
a) J-L-P characteristics, (b) EQE-L characteristics, (c) J-V-L characteristics, (d) EL
(
spectra.
Table 3. Performance of Green/red/blue PhOLEDs based on BTBF-DPA as HTM.
-1
-1
Red
Von
CE(cd A )
PE(lm W )
EQE(%)
CIE
-
2
-2
-2
-2
-2
Pospho- (V) max @1000cd m max @1000cd m max @1000cd m
(x,y)
-
2
rescence
BTBF 2.3 32.3 31.8/27.4
NPB 2.3 26.3 24.9/19.7
/10000cd m
/10000cd m
40.1 31.2/17.9
29.1 19.5/10.4
/10000cd m
24.9 24.6/21.1 (0.65, 0.34)
20.4 19.2/15.6 (0.65, 0.34)
15

The BTBF-DPA based red device exhibit deep red emission peaking at 624 nm with CIE
-2
of (0.65, 0.34) and a low turn-on voltage (V voltage at 1 cd m ) as 2.3 V. As shown in
on
Table 3 and Figure 6, the maximum EQE, CE, and PE of BTBF-DPA based red
-1
-1
POLEDs are 24.9%, 32.3 cd A , and 40.1 lm W .respectively. At high brightness of
-2
-1
-1
1
000 cd m , the EQE, CE and PE of them are 24.6%, 31.8 cd A , and 31.2 lm W . The
-
2
efficiency roll-off of BTBF-DAP based device is 1.2% at 1000 cd m which is much
smaller than 5.9% of NPB’s. What matters most is the PE of BTBF-DPA based device is
3
7.8% higher than NPB’s. The results of higher PE and lower efficiency roll-off of
BTBF-DPA based device declare that BTBF-DPA could performs much better than NPB
in commercial applications of red OLEDs.
2
.7.3 Blue FOLEDs
In order to further evaluate the potential of BTBF-DPA as HTM in commercialized blue
fluorescent (FOLEDs) system, we tested the performance of blue FOLEDs based
ITO/HAT-CN(5 nm)/ NPB (80 nm)/ TPN-DPF (10 nm)/ BH: BD (20 nm)/ ATN-BIZ (60
nm)/ Liq (1 nm)/ Al (100 nm). Here, ITO is used as anode; HATCN is the hole-injecting
layer; BTBF-DPA or NPB serves as the HTMs; TPN-DPF used as electron blocking layer;
the emitting layer is composed of BH and BD (ratio 96 : 4); ATN-BIZ works as the
electron-transporting layer, Liq is the electron-injecting layer. The device performance
data are summarized in Table 4
.
16

Figure 7. (a) Energy diagram, (b) Device structure and materials used in blue FOLEDs.
The BTBF-DPA based device exhibits stable blue emission peaking at 463 nm with CIE
of (0.15, 0.09), whereas NPB based device emit the same blue light with the same CIE of
-2
(
0.15, 0.09). The devices show the same turn-on voltage (V voltage at 1 cd m ) as low
on
as 2.8 V. As shown in Table 4 and Figure 8, the maximum EQE, CE, and PE of BTBF-
-
1
-1
DPA based device are 4.8%, 4.0 cd A , and 4.2 lm W , while the device based on NPB
-1
-1
exhibited 5.1%, 4.3 cd A , and 4.0 lm W , respectively. However, the EQE, CE and PE
-
2
-1
-1
of both of these devices at 10000 cd m are 4.1%, 3.4 cd A , and 2.0 lm W and 4.1%,
-
1
-1
3
.4 cd A , and 1.8 lm W , respectively. It is clear to see that BTBF-DPA based blue
17

device possess better performance than the reference one at high luminescence owing to
its higher mobility and super triplet energy.
Figure 8. Device characteristics of blue FOLEDs with BTBF-DPA and NPB as HTMs :
(
a) J-V-L characteristics, (b) EQE-L characteristics, (c) J-L-P characteristics, (d) EL
spectra.
Table 4. Performance of blue FOLEDs
-
1
-1
Blue
Von
CE(cd A )
PE(lm W )
EQE(%)
CIE
-
2
-2
-2
-2
-2
Fluo-
(V) max @1000cd m max @1000cd m max @1000cd m (x,y)
-
2
rescence
/10000cd m
/10000cd m
4.2 3.3/2.0
4.0 3.4/1.8
/10000cd m
BTBF 2.8 4.0 3.7/3.4
NPB 2.8 4.3 4.1/3.4
4.8 4.5/4.1 (0.15, 0.09)
5.1 5.0/4.1 (0.15, 0.09)
In order to enhance the EQE of BTBF-DPA based device to over 5% which is the
limitation of blue fluorescence OLEDs. We further optimized the device structure. The
18

hole transport ability of BTBF-DPA thin film of 80 nm is 7 times higher than that of NPB
according to the hole only device, so it may cause the difference of charge balance in
former blue FOLEDs which can be seen from the device performance. Here, we increased
the thickness of BTBF-DPA in blue FOLEDs from 80 nm to 120 nm to get an improved
performance, owing to more balanced charge transport ability. The blue FOLEDs with
120 nm of BTBF-DPA exhibited better EQE, CE, PE and efficiency roll-off. The
-1
-1
maximum of which are 5.2%, 4.7 cd A , 4.9 lm W and the efficiency roll-off is 3.9% at
-
2
-2
1
000 cd m and 9.8% at 10000 cd m . These results showed in Figure 9 and Table 5
,
44-45
which are comparable to reported results
. The CIE changed from (0.15, 0.09) to (0.15,
0.10) owing to the carriers recombination zone become broaden resulting from the 40 nm
thicker of BTBF-DPA as HTM. The consequences indicate that BTBF-DPA can be
adopted as an efficient HTM in blue FOLEDs.
19

Figure 9. Device performances of blue FOLEDs based on 80nm and 120nm of BTBF-
DPA: (a) J-V-L characteristics, (b) EQE-L characteristics, (c) J-L-P characteristics, (d)
EL spectra.
Table 5. Performance of 80nm and 120 nm BTBF-DPA based blue FOLEDs
-
1
-1
Blue
Von
CE(cd A )
PE(lm W )
EQE(%)
CIE
-2
-2
-2
Fluo- (V) max @1000cd m max @1000cd m max @1000cd m
(x,y)
-2
-2
-2
rescence
/10000cd m
/10000cd m
4.2 3.3/2.0
4.9 3.7/2.1
/10000cd m
8
1
0nm
2.8 4.0 3.7/3.4
4.8 4.5/4.1 (0.15, 0.09)
5.2 4.9/4.6 (0.15, 0.10)
20nm 2.8 4.7 4.5/4.2
2
.7.4 Lifetime of OLEDs
Device stability of BTBF-DPA based RGB devices were evaluated by collecting
-
2
-2
operation time dependent luminance at an initial luminance of 10000 cd m , 10000 cd m
-2
and 5000 cd m as supplementary information, respectively. Constant current mode
lifetime test was carried out for the lifetime evaluation in glovebox without encapsulation.
Lifetime data in Figure 10 verifies that life-time of RGB OLEDs are T99 = 70 h at 10000
-2
-2
-2
cd m , T70 = 60 h at 10000 cd m , T80 = 125 h at 5000 cd m . For red PhOLEDs, the
device is extremely stable owing to the low driving voltage and balanced carrier
transportation. For green PhOLEDs, the green emitter is not as stable as red emitter and
the driving voltage is higher than that of green device, so the stability is not as good as
-2
green device under the condition of 10000 cd m . In blue FOLEDs, the blue emitter is
very stable which was reported by a patent demonstrated in Supporting Information.
-2
Under the condition of 5000 cd m , T80 is 125h which is an excellent performance for
20

blue device with CIEy below 0.1, and BTBF-DPA as a stable and efficient HTM plays a
significant role in this system. These results reveal that BTBF-DAP could perform well as
HTM in RGB OLEDs.
Figure 10. Device performances of RGB OLEDs based on BTBF-DPA: (a) EL
characteristics of RGB OLEDs, (b) Life-time of red PhOLED, (c) Life-time of green
PhOLED, (d) Life-time of blue FOLED with 120nm BTBF-DPA.
3
. CONCLUSION
In conclusion, we developed a novel HTM, BTBF-DPA, with proper HOMO, high
mobility and high by introducing a new and electron rich mioty-BTBF into arylamine
system. Using BTBF-DPA, we successfully accomplished efficient and stable RGB
E
T
-1
-1
-1
OLEDs exhibiting maximum PE of 40.1 lm W , 136.6 lm W and 4.9 lm W , EQE of
-2
2
4.9%, 34.1% and 5.2%, and the efficiency roll-off of them are 16.7% at 1000 cd m ,
21

-2
-2
1
.2% at 1000 cd m and 11.5% at 10000 cd m , respectively. As a reference, NPB based
-
1
-1
-1
RGB PhOLED device only has maximum PE of 29.1 lm W , 96.8 lm W and 4.0 lm W ,
-2
EQE of 20.4%, 24.7%, 5.1%, and the efficiency roll-off of them are 20.6% at 1000 cd m ,
-2
-2
5
.9% at 1000 cd m , 19.6% at 10000 cd m , which are much lower than BTBF-DPA based
OLEDs, respectively. The lifetime of RGB OLEDs with BTBF-DPA were investigated,
-2
-2
-2
which are T = 70 h at 10000 cd m , T = 60 h at 10000 cd m , T = 125 h at 5000 cd m .
9
9
70
80
These results indicate that BTBF-DPA is a promising HTM for improving the PE, EQE and
the efficiency roll-off of RGB OLEDs. In the future, phosphorescent and/or fluorescent
OLEDs with high power efficiency, low driving voltage, low efficiency roll-off and long
lifetime can be achieved by employing our newly developed HTM because of its
appropriate energy level, fine hole mobility and very high ET of 2.99 eV. In addition, our
molecular design strategy and new rigid core BTBF demonstrated in this work will
contribute to promote the commercialization of phosphorescent and/or fluorescent OLEDs. In
the next step, we will try to increase the T of HTMs based on BTBF derivatives by connecting
g
high weight arylamine groups or introducing more heteroatom into aromatic conjugation system.
This will continually contribute to enhance the stability of materials as well as devices. We
believe that BTBF derivatives could also perform well in TADF OLEDs system, organic
photovoltaics (OPVs), peroviskite light-emitting diodes (PLEDs) and qumtuam dots light-
emitting diodes (QLEDs) condisering its outstanding stability and solvability.
4
. EXPERIMENT SECTION
Synthesis information
Synthesis of 3-bromobenzo[b]thiophene
2
22

Benzothiophene (3.35 g, 25 mmol) and N-Bromosuccinimide (NBS) (5.34 g, 30 mmol) were
dissolved in a mixture of CHCl (200 mL) and CH COOH (200 mL) at 0 � and stirred for 2h
3
3
before reacted at room temperature for 12 h. After reaction, saturated sodium thiosulfate solution
100 ml) and saturated sodium carbonate (100 ml) were added to remove excess NBS. The
(
mixture was washed with water (100 mL) and extracted with DCM. The solvent was evaporated
and the residual crude product was purified by column chromatography on silica gel using PE as
1
an eluent to afford 2 (7.69 g, 98.6%) as yellow oil. H NMR (300 MHz, CDCl
3
) δ 7.86 (d, J =
8
.6 Hz, 1H), 7.53 – 7.38 (m, 2H).
Synthesis of compounds of 3, 4 and 5
3
-Bromobenzo[b]thiophene (1.77 g, 8.4 mmol) was added into CHCl (120 mL) and CF COOH
3 3
(120 mL) was put in the mixture followed by stirring for 6 minutes at room temperature. Then
2 2
H O
(10 g) was added in the mixture and stirred for 20 h at room temperature. After reaction,
saturated sodium carbonate (50 mL), saturated sodium bicarbonate (50 mL) and water (50 mL)
were added for washing, and then the mixture was extracted by CHCl (150 ml) and concentrated
3
under reduced pressure. The resulting crude product was purified by silica gel column
chromatography using PE: DCM (1: 1, v/v) as an eluent. A yellow oil (1.83 g, 7.74 mmol) was
obtained, which represents a mixture of 3-bromobenzo[b]thiophene-1-oxide and 3-
bromobenzo[b]thiophene-1,1-dioxide as 3. This product was used in the next reaction without
further separation of these two compounds. The mixture was added into dry DMF (30 ml) with
phenol (1.45 g, 15.5 mmol) and potassium carbonate (2.14 g, 15.5 mmol) and stirred at 70 � for
1
5 h before water (3 × 50 mL) was put into the mixture . Then the mixture was extracted by
CHCl (150mL) and concentrated. The resulting crude product was purified by silica gel column
3
chromatography using DCM as an eluent, and 1.86 g (7.46 mmol) yellow oil which is a mixture
23

of 3-phenoxybenzo[b]thiophene-1-oxide and 3-phenoxybenzo[b]thiophene-1, 1-dioxide were
obtained as 4. Without further purification, the mixture was added into toluene (80 ml) with
diisobutylaluminium hydride (12.4 mL, 18.6 mmol) at 0 � and then stirred at 65 � for 20 h.
After reaction, toluene was removed under reduced pressure, then 2 M NaOH solution (30 mL)
was added at 0 � and extracted with DCM. The organic phase was dried and concentrated to get
crude product as yellow oil which was then passed by silica gel column chromatography using
1
PE as an eluent to afford 5 (1.01 g, 59.9%) as yellow oil. H NMR (300 MHz, CDCl
3
) δ 7.84 (d,
J = 1.3 Hz, 1H), 7.78 (d, J = 7.7 Hz, 1H), 7.35 – 7.48 (m, 5H), 7.19 (dd, J = 4.2, 2.4 Hz, 3H),
1
3
6
1
.72 (s, 1H). C NMR (300 MHz, CDCl
3
) δ 157.74, 148.03, 138.14, 132.37, 129.90, 125.33,
24.26, 123.61, 123.23, 121.21, 118.12, 107.16.
Synthesis of 2-Bromo-3-Phenoxybenzo[b]thiophene
6
3
-Phenoxybenzo[b]thiophene (1.01 g, 4.48 mmol) and NBS (0.96 g, 5.37 mmol) were dissolved
3
into CH COOH (120 mL) stirred at 55 � for 12 h. After reaction, saturated sodium bicarbonate
(
100 mL) was added to react with excess CH COOH and then retracted with DCM. The organic
3
1
phase was dried and concentrated to yield 6 (1.26 g, 92.1%) as a white solid. H NMR (300
MHz, CDCl ) δ 7.72 (d, J = 7.9 Hz, 1H), 7.46 (d, J = 7.9 Hz, 1H), 7.34 (m, J = 10.6, 8.1, 1.1 Hz,
3
3
H), 7.06 (t, J = 7.4 Hz, 1H), 6.96 (dd, J = 8.1, 1.5 Hz, 2H).
Synthesis of Benzo[4,5]thieno[3,2-b]benzofuran (BTBF)
7
2
-Bromo-3-Phenoxybenzo[b]thiophene (1.26 g, 4.13 mmol), NaOAc (0.53 g, 8.3 mmol) and
PdCl (PPh ) (0.14 g, 0.2 mmol) were added into a pressure flask with N,N-dimethylacetamide
2
3 2
(30 mL) under nitrogen atmosphere and stirred at 145 � for 12 h. After reaction, 2 M HCl (12
mL) and saturated NaCl solution (100 mL) were added to the mixture and extracted with EA :
24

hexane (1:1, v/v)(6
× 30 mL). The combined organic layer was dried over MgSO and
4
concentrated to pass the column chromatography on silica gel using PE as an eluent to afford
1
white solid 7 (0.83 g, 89.3%). H NMR (300 MHz, CDCl ) δ 8.02 (d, J = 8.2 Hz, 1H), 7.89 (d, J
3
=
8.0 Hz, 1H), 7.74 (dd, J = 7.3, 1.6 Hz, 1H), 7.66 (dd, J = 7.0, 1.8 Hz, 1H), 7.44 – 7.53 (d, 1H),
7
.44 – 7.32 (m, 3H).
Synthesis of 2,7-Dibromobenzo[4,5]thieno[3,2-b]benzofuran (BTBF-2Br)
8
BTBF (2.26 g, 10 mmol) was dissolved in dry CHCl (100 mL) and stirred at 0 °C. Liquid
3
bromine (4.0 g, 25.0 mmol) in dry CHCl
solution at 0 °C for 30 min. The mixture was stirred at room temperature for 24 h before washed
by saturated sodium thiosulfate solution (50 mL). The product was extracted with DCM (3 30
3
(30 mL) was added dropwise into the above BTBF
×
mL). The combined organic phase was collected, dried over MgSO and concentrated. The crude
4
product was purified by column chromatography on silica gel using PE as an eluent to afford
1
BTBF-2Br 8 (3.45 g, 90%) as white solid. H NMR (400 MHz, CDCl
3
) δ 8.02 (d, J = 1.4 Hz,
1
H), 7.85 (d, J = 8.5 Hz, 1H), 7.81 (d, J = 1.5 Hz, 1H), 7.61 – 7.56 (m, 2H), 7.49 (dd, J = 8.3, 1.7
Hz, 1H).
Synthesis of 2,7-Bis(diphenylanmino)-benzo[4,5]thieno[3,2-b]benzofuran (BTBF-DPA)
9
A mixture of BTBF-2Br (1.92 g ,5 mmol), diphenylamine (2.03 g ,12 mmol), sodium tert-
butoxide (1.44 g, 15 mmol), tris(dibenzylideneacetone) dipalladium (0) (0.18 g, 0.2 mol, 4 mol%)
and tri-tert-butylphosphine tetrafluoroborate (0.17 g, 0.6 mmol) were dissolved in
methylbenzene (50 mL) and heated at 120 °C for 12 h under nitrogen. After cooling to room
temperature, the solvent was removed by rotary evaporation under vacuum, and then cold water
was added to the mixture and extracted with DCM (3
× 30 mL). The combined organic phase
25

4
was collected, filtered and dried over by MgSO . The crude product was purified by column
chromatography on silica gel using PE/DCM (3:1, v/v) as eluent to afforded BTBF-DAP 9 (2.32
1
g, 82.3%). H NMR (500 MHz, DMSO) δ 7.84 (d, J = 8.6 Hz, 1H), 7.77 (d, J = 8.5 Hz, 1H),
7
1
1
1
.65 (d, J = 1.9 Hz, 1H), 7.35 – 7.27 (m, 9H), 7.13 (dd, J = 8.6, 2.0 Hz, 1H), 7.09 – 7.03 (m,
1
3
1H), 7.01 (dd, J = 8.5, 1.9 Hz, 1H). C NMR (500 MHz, CDCl ) δ 159.79, 153.06, 148.02,
3
47.88, 145.61, 145.36, 143.10, 129.53, 129.47, 124.55, 124.42, 123.25, 123.10, 122.41, 120.73,
+
20.61, 119.82, 119.75, 119.39, 119.17, 117.59, 108.43. HRMS (m/z): [M+H] calcd for
+
C38H27N2OS, 559.1799; Found [M+H] : 559.1855.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
*
E-mail: menghong@pku.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was financially supported by the Guangdong Key Research Project (No.
2
019B010924003), Shenzhen Engineering Research Center (Shenzhen development and reform
commission [2018]1410), Shenzhen Peacock Plan (KQTD2014062714543296), Shenzhen
Science and Technology research grant (JCYJ20180302153451987, JCYJ20180302153514509),
26

the Guangdong International Science Collaboration Base (2019A050505003). Thanks for Dr.
Junpeng He’s instruction and revision work about this manuscript.
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