4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine bipolar host for bright, efficient and low efficiency roll-off phosphorescent organic light-emitting devices

Article abstract of DOI:10.1016/j.orgel.2016.09.002

A bipolar host 4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine (DBTPhPm) with small singlet-triplet splitting has been synthesized and confirmed through a series of photophysical and electrochemical properties. Monochromatic phosphorescent organic light-emitting devices (PHOLEDs) based on different hosts [(4,4′-N,N'-dicarbazole) biphenyl, 2,7-bis (diphenylphosphoryl)-9-[4-(N,Ndiphenylamino) phenyl]-9-phenylfluorene, (3,3'-bicarbazole) phenyl and DBTPhPm] and dopants are fabricated. Compared to other hosts, the DBTPhPm-based PHOLEDs exhibited high brightness, high efficiency and low efficiency roll-off. The maximum power efficiency of the DBTPhPm-based red (R), green (G), blue (B), yellow (Y), and orange (O) PHOLEDs are 12.2, 47.2, 17.6, 42.6 and 15.1 lm/W, respectively. The current efficiency roll-off of the R, G, B, Y, and O PHOLEDs are 29.8%, 8.6%, 18.2%, 5.9%, and 22.4% from the maximum current efficiency to the high brightness of 5000?cd/m². The detailed working mechanism of the DBTPhPm-based device is discussed.

Full text of DOI:10.1016/j.orgel.2016.09.002

Organic Electronics 38 (2016) 301e306
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine bipolar host for
bright, efficient and low efficiency roll-off phosphorescent organic
light-emitting devices
Jing Yu a, Chunxiu Zang a, Ziwei Yu a, Hui Wang a, Shihao Liu a, Letian Zhang a,
,
, **
Wenfa Xie a *, Hongyu Zhao b
a State Key Laboratory on Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, Changchun, 130012, People's Republic
of China
b Beijing Tuocai Optoelectronics Technology Co. Ltd, Beijing, 100086, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A bipolar host 4, 6-Bis[3-(dibenzothiophen-2-yl)phenyl] pyrimidine (DBTPhPm) with small singlet-
triplet splitting has been synthesized and confirmed through a series of photophysical and electro-
chemical properties. Monochromatic phosphorescent organic light-emitting devices (PHOLEDs) based on
different hosts [(4,40-N,N'-dicarbazole) biphenyl, 2,7-bis (diphenylphosphoryl)-9-[4-(N,Ndiphenylamino)
phenyl]-9-phenylfluorene, (3,3'-bicarbazole) phenyl and DBTPhPm] and dopants are fabricated.
Compared to other hosts, the DBTPhPm-based PHOLEDs exhibited high brightness, high efficiency and
low efficiency roll-off. The maximum power efficiency of the DBTPhPm-based red (R), green (G), blue (B),
yellow (Y), and orange (O) PHOLEDs are 12.2, 47.2, 17.6, 42.6 and 15.1 lm/W, respectively. The current
efficiency roll-off of the R, G, B, Y, and O PHOLEDs are 29.8%, 8.6%, 18.2%, 5.9%, and 22.4% from the
maximum current efficiency to the high brightness of 5000 cd/m2. The detailed working mechanism of
the DBTPhPm-based device is discussed.
Received 16 June 2016
Received in revised form
26 August 2016
Accepted 3 September 2016
Keywords:
OLED
Bipolar host
Phosphorescent
Roll off
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
orbital; LUMO: lowest unoccupied molecular orbital) energy band
gap (Eg) and triplet energy level (T1) of the host material must be
Phosphorescent organic light-emitting devices (PHOLEDs) have
attracted much attention in recent years because their theoretical
internal quantum efficiency can be, in principle, three times higher
than that of the conventional fluorescent based organic light-
emitting devices (OLEDs) through utilizing both singlet and
triplet excitons for the light emission, and tremendous efforts have
been made in the development of highly efficient PHOLEDs [1e7].
In PHOLEDs, phosphorescent materials are often doped into a
suitable host material, which plays an indispensable role in energy
transfer and carrier transport, in order to reduce aggregation
quenching and triplet-triplet annihilation of guest triplet emitters
[8,9]. Therefore, the development of high-performance host ma-
terials is extremely essential for PHOLEDs. As an efficient host
material, HOMO-LUMO (HOMO: highest occupied molecular
higher than those of the guest to facilitate energy transfer from the
host to guest and to prohibit reverse energy transfer from the guest
back to host, and the energy level of the host material must be
matched with neighboring layers for both effective carrier injection
and carrier confinement [6,10e13]. Besides, a good carrier transport
property is expected to restrict the recombination of carriers in the
emitting layer (EML) and thus reduce the efficiency roll-off. Tradi-
tional host materials usually have good transporting properties for
only a single type of carriers. For instance, N, N'-dicarbazolyl-3,5-
benzene (mCP) exhibits only good hole-transporting properties,
while 3-(4-biphenylyl)- 4-phenyl-5-(4-tert-butylphenyl)-1,2,4-
triazole (TAZ) possesses good electron-transporting abilities
[6,11,14,15]. These unbalanced carrier-transporting properties are
negative to the turn-on voltage and the efficiency roll-off
[11,15e17]. Hence, bipolar host materials that can balance carrier-
transporting properties become the focus of attention recently
[18e28]. Hsu et al. reported a bipolar host material 2,7-bis
* Corresponding author.
(diphenylphosphoryl)-9-[4-(N,Ndiphenylamino)
phenyl]-9-
** Corresponding author.
E-mail addresses: xiewf@jlu.edu.cn (W. Xie), zhy_china@126.com (H. Zhao).
http://dx.doi.org/10.1016/j.orgel.2016.09.002
1566-1199/© 2016 Elsevier B.V. All rights reserved.

302
J. Yu et al. / Organic Electronics 38 (2016) 301e306
S
phenylfluorene (POAPF), and the corresponding blue mono-
chromatic OLEDs own a rather low turn-on voltage of 2.5 V due to
the balance of carriers in the EML [24]. Zheng et al. applied a series
of new synthesis host materials to blue monochromatic OLEDs, and
obtained a low voltage of 2.6 V and a gentle efficiency roll-off. The
device using 9,9-bis(9-methylcarbazol-3-yl)-4,5-diazafluorene
(MCAF) as the host has a maximum current efficiency (CE) of
32.2 cd/A and a maximum power efficiency (PE) of 31.3 lm/W, and
still keep a high CE of 27.6 cd/A and a high PE of 14.5 lm/W at the
brightness of 10000 cd/m2 [29]. Furthermore, Son et al. achieved
bipolar host materials 4-(N-a-carbolinyl)-40,4''-(N-carbazolyl) tri-
phenylamine (ADCTA), 4,40-di(N-a-carbolinyl)-4''-(N-carbazolyl)
triphenylamine (DACTA) and 4,40,4''-(N-acarbolinyl) triphenyl-
amine (TATA) with the nature of small singlet-triplet energy gap
S
S
a
b
OH
B
B
Br
OH
OH
A
N
HO
B
C
N
c
Cl
Cl
S
S
N
N
Scheme 1. Synthetic routes to DBTPhPm. (a) toluene, H2O, K2CO3, PdCl2 (PPh3)2; (b)
THF, -78 ꢂC, n-BuLi, B(OPr)3; (c) toluene, H2O, K2CO3, PdCl2(PPh3)2.
(DES-T: the difference between singlet and triplet energy levels) so
that the dopant may avoid being deep trap sites due to the small
singlet to triplet splitting energy (0.4 eV) [30,31].
stirred to be degassed. Then 0.5 g PdCl2(PPh3)2 was added in this
mixture and the atmosphere was replaced with N2. The mixture
was stirred while reaction container was heated. After heating,
water was added to the mixture, and the mixture was filtered to
give residue. The obtained solid was washed with dichloromethane
and methol. The obtained solid was recrystallized from toluene to
give 25 g white solid (yield 63%). 1H NMR(CDCl3, 400 MHz):
7.41e7.51 (m, 4H), 7.58e7.62 (m, 4H), 7.68e7.79 (m, 4H), 8.73 (dt,
2H), 8.18e8.27 (m, 7H), 8.54 (t, 2H), 9.39 (d, 1H). MODI-TOF: 596.76.
Anal. calcd for C40H24N2S2 (%): C 80.51, H 4.05, N 4.69, S 10.75;
Found: C 80.50, H 4.05, N 4.71, S 10.74.
In this paper, a bipolar host material 4,6-Bis[3-(dibenzothio-
phen-2-yl)phenyl] pyrimidine (DBTPhPm) is reported and a series
of low-voltage, highly efficient, and low efficiency roll-off
DBTPhPm-based PHOLEDs are designed. Iridium (III) Bis [1- (3,5-
dimethylphenyl)- 7- methylisoquinoline] (acetylacetone) [Ir(pmi-
q)2(acac)], tris(2-phenylpyridine) iridium [Ir(ppy)3], [(bis[2-(4,6-
difluorophenyl) pyridyl-N,C2'] iridium (III) [Firpic], Iridium(III)bis-
(2-methyldibenzo- [f,h] quinoxaline) (acetylacetonate) [Ir(MD-
Q)2(acac)] and Iridium(III) bis(4-phenylthieno [3,2-c] pyridinato-
0
N,C2 ) acetylacetonate [PO-01] are used as the red, green, blue, or-
ange, and yellow dopants. For comparison, device using (4,40-N,N'-
dicarbazole) biphenyl (CBP), POAPF and (3,3'-bicarbazole) phenyl
(BCZph) [32] as host were also fabricated. The results indicated that
DBTPhPm is an efficient host for most common phosphorescent
dopants.
DBTPhPm has a simple molecular structure that having two
dibenzothiophene and one pyrimidine moiety. Pyrimidine core
structure was designed as the electron transport type core struc-
ture with high triplet energy and high rigidity for high glass tran-
sition temperature (Tg ¼ 268.9 ꢂC). The Tg of DBTPhPm is much
higher than that of the CBP(62 ꢂC) [34], POAPF(129 ꢂC) [24] and
BCZph(100 ꢂC) [35]. The high Tg would be benefited to the stability
of the devices. The dibenzothiophene unit withdraws electron due
to S atom with high electronegativity. The ultravioletevisible
(UVevis) absorption, PL emission and low-temperature photo-
luminescence (LTPL) emission spectra of DBTPhPm in CH2Cl2 so-
2. Experimental
UVevisible absorption and photoluminescence (PL) studies at
room temperature were carried out using U3010 spectrometer
(Hitachi, Japan) and F-7000 FL spectrophotometer, respectively. The
phosphorescence spectrum was recorded from the delayed emis-
sion of DBTPhPm at 77 K. The cyclic voltammogram (CV) experi-
ments were performed using a BAS 100 W instrument at room
temperature in CH2Cl2 solutions at a scan rate of 100 mV/s. All
devices were fabricated on glasses substrates covered by con-
ducting indium tin oxide (ITO). The substrates were cleaned in
Decon 90 and deionized water, dried in the oven and then treated
in plasma for about 5 min. Finally organic layers and cathode ma-
terials were sequentially deposited on the substrates without
breaking vacuum (~5.0 ꢀ 10ꢁ4 Pa). A shadow mask was used to
define the cathode and to make four 10 mm2 devices on each
substrate. Current-Voltage-Luminance (IeV-L) characteristics of
unpackaged devices were measured with a Keithley 2400 Source
Meter and a Minolta Luminance Meter LS-110. The spectra of the
devices were measured with Ocean Optics Maya 2000-PRO
spectrometer.
lution (1
ꢀ
10ꢁ5 M) are measured, and Fig. 1 shows the
characteristics with an inset figure of the chemical structure of
DBTPhPm. Absorption peaks at 336 nm can be attributed to
p
-
p
p
*
*
transitions of the dibenzothiophene chromoph and * and n-
p-p
transitions of central arylenes, respectively. The optical energy
band gap (Eg) of DBTPhPm is calculated to be 3.69 eV from the onset
of the absorption spectrum (336 nm) according to the UVevis ab-
sorption curve. The fluorescence emission peak of DBTPhPm is
3. Results and discussion
Scheme 1 depicts the synthetic route and structure of the new
host material DBTPhPm. Dibenzothiophene-2-boronic acid was
purchased from Bepharm Chemical (China). The boronic acid C was
synthesized according to literature [33]. Other reactants or reagents
were used as received.10 g (67 mmol) 4, 6-dichloropyrimidine, 60 g
(20 mmol) 3-(dibenzothiophen-2-yl) phenylboronic acid, 80 g
(70 mmol) K2CO3, 500 ml toluene and 200 ml H2O were put in a 1 L
recovery flask. While the pressure was reduced, the mixture was
Fig. 1. The UVevis absorption, photoluminescence (PL) emission (at RT) and low-
temperature photoluminescence (LTPL) emission (at 77 K) spectra of DBTPhPm in
CH2Cl2 solution (1 ꢀ 10ꢁ5 M).

J. Yu et al. / Organic Electronics 38 (2016) 301e306
303
Fig. 2. Cyclic voltammogram of DBTPhPm.
411 nm and its single state (S1) is estimated to be 3.02 eV accord-
ingly. The phosphorescence spectrum is obtained from the delayed
emission of DBTPhPm at 77 K and the triplet energy (T1) is
Fig. 4. The structures of red (R), green (G), blue (B), yellow (Y), and orange (O) PHO-
LEDs with different hosts and dopants. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
Table 1
The physical properties of DBTPhPm.
Absorption: labs (nm)a
l
fluo(nm)a
411
lphos(nm)b
HOMO (eV)c
LUMO (eV)d
Eg (eV)e
3.69
S1 (eV)
3.02
T1 (eV)
2.63
ES-T (eV)f
0.39
288, 336
471
ꢁ5.88
ꢁ2.19
a
Measured in CH2C l2 solution (10ꢁ5 M) at room temperature (RT).
Measured in CH2Cl2 solution (10ꢁ5 M) at 77 K.
HOMO was estimated by CV.
b
c
d
e
f
LUMO was calculated from the difference of HOMO and Eg.
The optical energy band gap was calculated from the equation Eg ¼ hc/
l
¼ 1240/
l, where l is the wavelength of the onset of the absorption spectrum.
Singlet-triplet energy gap: the difference between singlet and triplet energy levels.
Fig. 3. Current density-voltage characteristics of the hole-only and electron-only devices with different hosts.

304
J. Yu et al. / Organic Electronics 38 (2016) 301e306
(3 nm)/TAPC: MoO3 (6:1 by weight, 20 nm)/TAPC (20 nm)/TCTA
(5 nm)/host (20 nm)/TAPC (40 nm)/MoO3 (3 nm)/Ag(120 nm)
[where TAPC is 1,1-Bis[(di-4-toly-lamino)phenyl]cyclohexane,
TCTA is 4,40,400-tris (Ncarbazolyl) triphenylamine], while the
electron-only device has the structure of ITO/LiF (0.5 nm)/1,3,5-tris
(m-pyrid-3-yl-phenyl) benzene (TmPyPB: 60 nm)/host (20 nm)/
4,7-diphenyl-1,10- phenanthroline (Bphen: 40 nm)/Ag (120 nm). It
is supposed that only single carriers are injected and transported in
the devices due to the work function of MoO3 (or LiF) being high (or
low) enough to block electron (or hole) injection. The current
density of the single-carrier devices are plotted against voltage in
Fig. 3. As can be seen, the four hosts have the carrier transporting
ability of both holes and electrons. CBP, POAPF and BCZph perform
better hole-transport properties and there is a huge difference
between the hole- and electron-transport properties for BCZph.
However, DBTPhPm performs balanced hole- and electron-
transport properties.
Fig. 5. The energy level diagrams of devices G1-G4.
To investigate the DBTPhPm as host in PHOLEDs, PHOLEDs with
the structure of ITO/MoO3 (3 nm)/TAPC (35 nm)/TCTA (5 nm)/
emitting layer (EML: 20 nm)/TmPyPB (50 nm)/LiF (0.5 nm)/Mg:Ag
(15:1 by weight, 120 nm) as depicted in Fig. 4 were fabricated. The
EML was host doped with different guests. The hosts were CBP,
POAPF, BCzPh, and DBTPhPm, the guests were Ir(ppy)3 (green),
Ir(pmiq)2(acac) (red), Firpic (blue), PO-01 (yellow), and Ir(MD-
Q)2(acac) (orange), respectively. As examples, the green PHOLEDs
are demonstrated in detail, and the corresponding energy level
diagrams of green PHOLEDs are shown in Fig. 5. TAPC with LUMO
energy level of 1.8 eV (LUMO: 2.6 eV for CBP, 2.4 eV for POAPF,
1.16 eV for BCZph, 2.19 eV for DBTPhPm) and high triplet energy
level (T1) of 2.9 eV (T1: 2.56 eV for CBP, 2.75 eV for POAPF, 2.87 eV
estimated to be 2.63 eV (471 nm) which results in a small
DES-T
(0.39 eV) of DBTPhPm. The electrochemical properties of DBTPhPm
sample are investigated with cyclic voltammetric (CV). The CV
curve of DBTPhPm is shown in Fig. 2. According to the onset po-
tential of its oxidation process, the HOMO energy level of DBTPhPm
is estimated to be approximately ꢁ5.88 eV, and the corresponding
LUMO energy level is calculated to be ꢁ2.19 eV, referring to the
HOMO and absorption spectra [36]. All photophysical data and
electrochemical properties of DBTPhPm are summarized in Table 1.
The electron-/hole-transporting characters of the hosts are
investigated with the fabrication of the hole-only and electron-only
devices. The hole-only device has the structure of ITO/MoO3
Fig. 6. Performances of devices G1-G4. (a) Normalized EL spectra at 8 V. (b) Current density-voltage-luminance characteristics. (c) Current efficiency-luminance-power efficiency
characteristics. (d) Current efficiency roll-off characteristics.

J. Yu et al. / Organic Electronics 38 (2016) 301e306
305
for BCZph, 2.63 eV for DBTPhPm) is used as hole transport layer
(HTL), while TmPyPB with HOMO energy level of 6.68 eV (HOMO:
5.9 eV for CBP, 5.26 eV for POAPF, 5.31 eV for BCZph, 5.88 eV for
DBTPhPm) and triplet energy level of 2.78 eV is used as electron
transport layer (ETL) in the devices for the effective confinement of
carriers and/or excitons within the EML [37].
The electrical properties of the green devices are shown in Fig. 6.
Fig. 6(a) depicts the normalized electroluminescence (EL) spectra of
green devices and undoped DBTPhPm device (device 4 as shown in
Fig. 4) at 8 V. Devices G1-G4 show the EL emission peak at 512 nm
with CIE color coordinate of about (0.32, 0.60) that is originate from
Ir(ppy)3. No extra emission from the host and/or the adjacent car-
rier transporting layers except the green emission indicates that the
confinement of carriers is effective and the energy transfer from
host to guest is complete. Fig. 6 (b) shows the current density-
voltage-luminance (J-V-L) curves of device G1-G4. The turn-on
voltage (driving voltage @ 1 cd/m2) of G2 and G4 is 2.3 V and
Fig. 7. The device structure of the green PHOLEDs with moving EML. (For interpre-
tation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
Fig. 8. Turn-on voltage, maximum current efficiency, and current efficiency roll-off characteristics of the green PHOLEDs with moving EML. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
Table 2
Characteristics based on devices.
Device
V
on (V)
Lmax (cd/m2)
hc.max
,
hc.1000
,
hc.5000 (cd/A)
hp.max
,
hp.1000
,
hp.5000 (lm/W)
CERO (%)a,b
PERO (%)a,b
R1
R2
R3
R4
B1
B2
B3
B4
Y1
Y2
Y3
Y4
O1
O2
O3
O4
3.1
2.5
3.3
2.5
3.1
2.5
3.1
2.7
2.9
2.3
3.1
2.5
3.3
2.7
3
20240
9393
6758
19650
10050
5969
11570
18040
76850
40420
26740
97150
15070
15090
12170
39290
8.6, 5.9, 4.5
8.1, 5.7, 3.8
6.8, 3.5, 1.5
10.4, 9.5, 7.3
22.7, 9.5, 4.3
19.9, 17.4, 6.8
23.8, 16.1, 6.8
17.6, 17.2, 14.4
34.5, 34.5, 32.3
33.9, 31.7, 27.8
33.2, 32, 24
39.3, 39.1, 37
15.6, 10.8, 5.6
12.2, 9.8, 6.4
8.5, 3.1, 1.8
10.5, 3.6, 1.6
6.2, 1.7, 0.5
12.2, 6.8, 3.8
23.7, 6.1, 1.9
21.6, 13, 3.5
31.4, 47.7
29.6, 53.1
48.5, 77.9
8.7, 29.8
58.1, 81.1
12.6, 65.8
32.4, 71.4
2.3, 18.2
/, 6.4
6.5, 18
4, 27.7
0.5, 5.9
30.8, 64.1
19.7, 47.5
20.2, 72.6
8.8, 22.4
63.5, 78.8
65.7, 84.8
72.6, 91.9
44.3, 68.9
74.3, 92
39.8, 83.8
54.3, 86
27.3, 52.3
17, 37
22.1, 10.1, 3.1
17.6, 12.8, 8.4
31.1, 25.8, 19.6
46.9, 27.7, 19.1
29.8, 22.9, 14
42.6, 33, 25.3
14.9, 6.5, 2.6
13.4, 6.6, 3.3
15.1, 8.5, 2.1
15.1, 9.3, 6
41, 59.3
23.2, 53
22.5, 40.6
56.4, 82.6
50.7, 75.4
43.7, 86.1
38.4, 60.3
16.8, 13.4, 4.6
14.7, 13.4, 11.4
2.7
a
Values collected from the maximum to the luminance of 1000 cd/m2.
Values collected from the maximum to the luminance of 5000 cd/m2.
b

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J. Yu et al. / Organic Electronics 38 (2016) 301e306
2.4 V, while that of G1 and G3 is high to 3 V and 3.3 V, respectively.
The maximum brightness of devices G1-G4 is 45700, 51040, 64440
and 114400 cd/m2, respectively. Fig. 6 (c) exhibits the efficiency-
luminance characteristics of devices G1-G4. The maximum current
efficiency (CE) of device G4 is 40.7 cd/A, and as a result of the low
driving voltage, the maximum power efficiency (PE) is 47.2 lm/W,
which are much higher than G1-G3 (CE: 29.1 cd/A, 33.6 cd/A,
37.6 cd/A, PE: 24.4 lm/W, 42.9 lm/W, 31.2 lm/W). At the high
brightness of 1000 cd/m2, the CE of G4 maintains 40.3 cd/A, and the
corresponding PE is 35.2 lm/W. Even at the brightness of 10000 cd/
m2, the CE and PE can still reach 35.1 cd/A and 20.8 lm/W. The CE
roll-off (CERO) of devices G1-G4 are shown in Fig. 6 (d). As can be
seen, the efficiency roll-off of device G4 is much lower than others
at high brightness. For example, at brightness of 20000 cd/m2, the
CERO is 22.8% for device G4, which are 51.4%, 28.6%, and 37.0% for
device G1-G3. Therefore, it can be concluded that device G4 has an
overwhelming superiority on efficiency and efficiency roll-off
comparing with device G1-G3.
To investigate the working mechanism of the green PHOLEDs,
four type devices with structure as depicted in Fig. 7 were fabri-
cated. In the devices with same host, the EML was moved from
anode side to cathode side. The detailed performances of the de-
vices are shown in Fig. 8. The maximum current efficiencies of
BCZph-based devices are decreased when the EML is moved from
cathode side to anode side, and the maximum current efficiencies
of other devices are decreased when the EML is moved from anode
side to cathode side. Besides, the EL spectra of the Type A, B and C
devices with BCZph host show the emission from BCZph, while no
host emission is observed in other devices. From Fig 6 (b), we also
can see that the maximum efficiency decrease with the EML
moving is obviously. Besides, the CERO of the devices based on
DBTPhPm host is independent on the location of the EML, while the
CERO of other devices is dependent on the location of the EML. The
results indicated that the recombination zone of device G3 (host:
BCZph) is closed to EML/ETL interface due to the high LUMO energy
barrier between TmPyPB and BCZph and excellent hole-transport
properties of BCZph, while it would be the entire EML for device
G1 (host: CBP), G2 (host: POAPF) and G4 (host: DBTPhPm). However,
more excitons are generated at HTL/EML interface in device G1,
which are generated at both the HTL/EML and EML/ETL interfaces
in device G2 and G4. Thus, the CERO of device G2 and G4 would be
lower in principle. In fact, the efficiency roll-off of device G2 and G4
is relative low. According to the results, we think bipolar trans-
porting property of DBTPhPm and energy level matching to the
adjacent HTL and ETL are not all of the reason for extremely low
efficiency roll-off in DBTPhPm-based device. Another important
and low efficiency roll-off. It could be attributed to the balanced
bipolar transporting properties and small singlet-triplet energy gap
of DBTPhPm.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 61474054, 61475060, 61177026).
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CBP, POAPF, or BCZph host and the details are listed in Table 2.
4. Conclusions
In summary, a new bipolar host material DBTPhPm has been
synthesized and a series of photophysical and electrochemical
properties of DBTPhPm has been confirmed. A systematic research
on red, green, blue, yellow and orange monochromatic organic
light-emitting devices based on different hosts is demonstrated.
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