Carbazolylphosphines and carbazolylphosphine oxides: Facilely synthesized host materials with tunable mobilities and high triplet energy levels for blue phosphorescent OLEDs

Article abstract of DOI:10.1039/c7tc01594a

A high triplet energy level (E_T) and balanced carrier mobility are critical factors for host materials in efficient blue phosphorescent organic light-emitting diodes (PHOLEDs). Herein, we report a facile synthesis of four compounds, dicarbazolylphenylphosphine (DCPP), dicarbazolylphenylphosphine oxide (DCPPO), tricarbazolylphosphine (TCP), and tricarbazolylphosphine oxide (TCPO), and their application as host materials in a classic blue phosphorescent emitter bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(iii) (FIrpic). The four compounds show a high E_T of up to 3.0 eV and tunable mobilities. We fabricated four OLEDs with a device structure of ITO/MoO₃ (1 nm)/N,N′-dicarbazolyl-3,5-benzene (mCP):MoO₃ (20 wt%, 10 nm)/mCP (30 nm)/Host:FIrpic (7 wt%, 20 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB, 40 nm)/LiF (1 nm)/Al (100 nm), all of which display maximum external quantum efficiencies (EQEs) exceeding 20%. The DCPPO-based device reaches the highest EQE of 27.5%, the maximum luminance of 14 070 cd m^-2, and the lowest efficiency roll-off of 22.2% from 1 to 10 mA cm^-2, which is among the best-performing FIrpic-based PHOLEDs without using any light extraction method.

Full text of DOI:10.1039/c7tc01594a

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC01594A
Carbazolylphosphines and Carbazolylphosphine Oxides: Facilely
Synthesized Host Materials with Tunable Mobilities and High Tri-
plet Energy Levels for Blue Phosphorescent Organic Light-Emitting
Diodes
†
†
‡
†
†
†
,†
Zifeng Zhao, Gang Yu, Qiaowen Chang, Xiaochen Liu, Yang Liu, Liding Wang, Zhiwei Liu,*
,
†
,‡
†
Zuqiang Bian,* Weiping Liu,* and Chunhui Huang
†Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applica-
tions, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
‡State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, Kunming Institute of
Precious Metals, Kunming 650106, Yunnan, China
T
ABSTRACT: High triplet energy level (E ) and balanced carrier mobility are critical factors for host materials in efficient blue
phosphorescent organic light-emitting diodes (PHOLEDs). Herein, we report the facile synthesis of four compounds, dicarbazol-
ylphenylphosphine (DCPP), dicarbazolylphenylphosphine oxide (DCPPO), tricarbazolylphosphine (TCP), and tricarbazol-
ylphosphine oxide (TCPO), and their application as host materials for a classic blue phosphorescent emitter bis[2-(4,6-
2
difluorophenyl)pyridinato-C ,N](picolinato)iridium(III) (FIrpic). The four compounds show high E
T
up to 3.0 eV and tunable mo-
(1 nm)/N,N'-dicarbazolyl-3,5-benzene (mCP):MoO (20
wt.%, 10 nm)/mCP (30 nm)/Host:FIrpic (7 wt.%, 20 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB, 40 nm)/LiF (1 nm)/Al
100 nm), all of which display maximum external quantum efficiencies (EQEs) exceeding 20%. The DCPPO-based device reaches
bilities. We fabricated four OLEDs with a device structure of ITO/MoO
3
3
(
-
2
the highest EQE of 27.5%, the maximum luminance of 14070 cd m , and the lowest efficiency roll-off of 22.2% from 1 to 10 mA
cm , which is among the best-performing FIrpic-based PHOLEDs without using any light extraction method.
-
2
1
. INTRODUCTION
els.4, 5, 8-21 In many cases, the phenylene moiety is needed as a
spacer between the donor and acceptor for excellent hosts with
Organic light-emitting diodes (OLEDs) are becoming a new
focus in flat or flexible panel display and solid-state lighting.
Phosphorescent OLEDs (PHOLEDs), which promise an inter-
nal quantum efficiency of up to 100%, have been considered
as the ultimate energy-saving technology. So far the best-
performing PHOLEDs are based on noble transition metal (i.e.
iridium and platinum) complexes. Despite the fact that exten-
sive progress has been achieved in PHOLEDs, the realization
9
8
a classical D-π-A structure, such as MPO12 , BCPO and
1
0
TCTP , with either 1, 2 or 3 carbazole subunits linked to the
phenyl rings of a triphenylphosphine oxide core, respectively.
All of these materials could act as excellent hosts for blue
PHOLEDs, showing high external quantum efficiencies
1
-3
1
7
(
EQEs) of 14.3% , 23.5% and 15.9%, respectively. Intri-
guingly, Adachi group designed a D-A compound by directly
linking the donor (a carbazole moiety) and acceptor (diphe-
nylphosphine oxide) without the phenylene spacer. Compared
to the corresponding D-π-A material (MPO12), this D-A com-
4
-7
of blue ones with high efficiency remains a challenge.
It has been proved that the selection of proper materials for
each layer is very important for achieving highly efficient
PHOLEDs. In particular, the design of host material plays a
T
pound showed similar high E while deeper HOMO energy
8
levels, result in a better-performing blue PHOLED with a
crucial role due to the following urgent demands: i) high tri-
17
maximum EQE of 19.7%. Moreover, another D-A com-
plet energy level (E
ii) suitable energy levels of the frontier molecular orbitals
FMOs), i.e. the highest occupied molecular orbital (HOMO)
T
) for efficient energy transfer to the guest;
pound, dicarbazolylphenylphosphine oxide (DCPPO), was
recently reported as an efficient host material for bis(3,5-
difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium (FIr-
(
and the lowest unoccupied molecular orbital (LUMO), for
better charge injection to lower the driving voltage; iii) bal-
anced electron and hole transport ability to extend the recom-
bination zone for low efficiency roll-off; iv) excellent evapora-
tion ability for the vacuum deposition method for OLEDs’
20
22
pic) either by thermal evaporation or wet-processing meth-
od, with maximum EQEs up to 16.5%.
g
fabrication and high glass transition temperature (T ) to in-
crease morphological stability for lengthening the device life-
time. Moreover, the facile and inexpensive characters of the
host materials are also important for the future commercializa-
tion of PHOLEDs, especially in the next-generation, large-
area solid-state lighting market.
To meet the requirements of excellent host materials, the am-
bipolar donor (D)–acceptor (A) structure host materials are
widely studied. For instance, carbazole (as a donor) and phos-
phine oxide (as an acceptor) are chosen as building blocks for
the host materials since they are reported as efficient hole and
electron transporting moieties with suitable FMO energy lev-
.
Scheme 1. Synthetic routes of DCPP, DCPPO, TCP, and
TCPO.

Journal of Materials Chemistry C
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DOI: 10.1039/C7TC01594A
Based on aforementioned background, two compounds di-
carbazolylphenylphosphine (DCPP) and tricarbazol-
ylphosphine (TCP) were synthesized by direct linking carba-
zole and phosphine groups via a one-step reaction of carbazole
and dichlorophenylphosphine or trichlorophosphine, and an-
other two compounds DCPPO and tricarbazolylphosphine
oxide (TCPO) were obtained through the one-step oxidation of
DCPP and TCP with high yields exceeding 90%. Though the
compounds DCPP and TCP have been reported as the ligands
23
to construct Rh/Pd complexes , or with ultra-long phospho-
2
4
rescence , and DCPPO as an efficient host material for FIrpic,
the performance of these four compounds as correlative host
materials in OLEDs have never been completely studied and
compared, especially considering that the oxidation of phos-
phine to phosphine oxide may easily improve the electron
mobility and tune the bipolar property of the host matrix. After
systematically study, it is demonstrated that the four com-
T
pounds showed high E up to 3.0 eV, tunable carrier transport
ability, suitable HOMO and LUMO energy levels, and excel-
lent thermal stability. Furthermore, PHOLEDs based on the
2
5
classic blue phosphorescent emitter FIrpic were constructed
with a simple structure adopting these four host materials. All
of the devices display the maximum EQEs exceeding 20%,
indicating these compounds could act as efficient host matri-
ces for FIrpic. Especially,TCPO, firstly reported here, is a
novel host material for blue PHOLEDs with maximum EQE
up to 22%. The DCPPO-based device reached the highest
Figure 1. Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawing
of molecular structures and π-π stacking interactions at 50%
probability level. a) The π-π stacking of DCPP, b) 3 types of π-
π stacking of DCPPO, the molecular structures of c) TCP and
d) TCPO. All hydrogen atoms have been omitted for clarity.
-
2
EQE of 27.5%, the highest luminance of 14070 cd m , and the
-
2
lowest efficiency roll-off of 22.2% from 1 to 10 mA cm .
2
. RESULTS AND DISCUSSIONS
2
.1. Synthesis. The synthetic routes and chemical structures of
DCPP, DCPPO, TCP, and TCPO are depicted in Scheme 1.
DCPP and TCP were synthesized through a substitution reac-
tion of carbazole and dichlorophenylphosphine or trichloro-
phosphine with high yields of 80% and 61%, respectively.
DCPPO and TCPO could be obtained through the oxidization
of DCPP and TCP by H O with extremely high yields of 90%
2 2
and 97%, respectively. After recrystallization, all the com-
pounds were subsequently underwent thermal gradient subli-
1554055, 1554056, and 1554057.
For all compounds, the phosphine centers form tetrahedral
geometries and they are surrounded by aromatic groups with
propeller-like conformations (Figure 1). As shown in Figure
1
a, there is only one type of π-π stacking interaction in the
crystal of DCPP, while three types of π-π stacking in DCPPO
crystal are observed (Figure 1b). Unlike DCPP and DCPPO,
π-π stacking is suppressed in the crystals of TCP and TCPO,
which could ascribe to larger intramolecular steric hindrance
caused by three carbazoles. The carbazole groups squeeze in
TCP or TCPO molecules, inhibiting the face-to-face interac-
tions between carbazoles of adjacent molecules.
1
31
mation twice. Their characterization details ( H and P NMR
spectroscopy and elemental analysis) are presented in the Ex-
perimental Section.
2
.2. Crystal Structures and Molecular Interactions. The
crystals of the four compounds were obtained by vacuum
thermal sublimation and the structures were confirmed by
single-crystal X-ray diffraction (XRD) analysis. All of the
crystallographic data are available from the Cambridge Crys-
tallographic Data Centre (CCDC) with numbers of 1554053,
2
.3. Photophysical Properties. The UV-Vis absorption and
photoluminescence spectra of DCPP, DCPPO, TCP, and
TCPO are recorded to study their photophysical properties
(
Figure 2a). The corresponding optical absorption data and
calculated optical band gaps (E ) are listed in Table 1.
g
-
5
-1
Figure 2. a) UV-Vis absorption and photoluminescence spectra in CH
2
Cl
solution (1×10 mol L ) at room temperature, and c) Photo-
luminescence spectra in 2-MeTHF solution (1×10 mol L ) at 77 K of DCPP, DCPPO, TCP, and TCPO.
2
solution (1×10 mol L ) at room temperature, b) Photo-
-
5
-1
2 2
luminescence spectra in solid films and absorption of FIrpic in CH Cl
-
5
-1

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DOI: 10.1039/C7TC01594A
a)
b)
c)
Figure 3. a) Thermal gravimetric analysis, b) heat flow curves, and c) differential scanning calorimetry curves of DCPP, DCPPO,
TCP, and TCPO.
Table 1. Photophysical, electrochemical and thermal properties of the four compounds.
a)
b)
c)
d)
e)
HOMO f)
[eV]
LUMO g)
[eV]
h)
λ
max, abs.
λ
max, fl.
λ
max, phos.
E
g
E
T
T
eva
T
g
Material
[nm]
[nm]
[nm]
[eV]
3.70
3.88
3.70
[eV]
3.03
3.03
2.99
3.01
[°C]
341
366
373
386
[°C]
75
80
89
/
DCPP
DCPPO
TCP
290
354
354
344
340
409
409
415
412
5.93
6.33
5.94
6.10
2.23
2.45
2.24
2.18
286, 256
289
TCPO
284, 257
3.92
a)
-5
-1
b)
The maximum absorption wavelength, measured in CH
2
Cl
2
(1×10 mol L ) at room temperature; The maximum fluorescent emission
-5
-1
c)
peak, measured in CH
2
Cl
2
d)
(1×10 mol L ) at room temperature; The maximum phosphorescent emission peak, measured in 2-MeTHF
-
5
-1
e)
(1×10 mol L ) at 77 K; Deduced from the corresponding absorption edges of the spectra; Deduced from the corresponding 0–0 tran-
f)
g)
sition bands of the phosphorescence spectra; Deduced from the oxidation potentials of the compounds; Calculated from the HOMO
energy levels and the corresponding optical band gaps; Evaporation temperature under nitrogen.
h)
A couple of characteristic absorption peaks were observed in
compounds (the emission peaks around 340–400 nm can be
assigned to fluorescence emission, with lifetimes of 6.6, 6.1,
10.6 and 10.6 ns for DCPP, DCPPO, TCP and TCPO, respec-
tively). According to the 0–0 transition bands around 410 nm
the range of 270–340 nm at room temperature with molar ex-
-
1
-1
tinction coefficients between 3000-16000 M cm , which can
be assigned to the π–π* and n–π* transitions of the carbazole
2
6, 27
moiety of the molecules.
The optical absorption edges of
in their phosphorescent spectra, the E
an identical 3.0 eV (Table 1). The high E
T
T
were calculated to be
can be attributed to
DCPP and TCP are almost the same (335 nm), indicating no
significant electronic interactions between the carbazole
or/and phenyl groups. The strong electron-withdrawing char-
acteristic of the P=O group leads to noticeable blue shifts for
the spectra of DCPPO and TCPO, with absorption edges mov-
ing from 335 nm to 320 nm and 316 nm, respectively.
the insulated carbazole moieties.
2
.4. Thermal Properties. The thermal stability of the host
material is vital for achieving high-performance and long-life
optoelectronic devices. Thermal gravimetric analyses (TGA)
demonstrated that all of the compounds possess good evapora-
tion property and stability, which is important for vapor depo-
sition method in OLED fabrication. As depicted in Figure 3a,
the four compounds can be evaporated in the range of 340-
390 °C without any decomposition. The heat flow curves
(Figure 3b) suggest that these materials melt before evapora-
In the fluorescence spectra (Figure 2a), the main peaks for
DCPP and DCPPO are located at 354 nm with two shoulder
emission peaks around 341 nm and 371 nm, respectively. For
TCP and TCPO, the main peaks are blue-shifted to 344 nm
and 340 nm, respectively. In addition, the emission spectra for
the solid films of the four compounds and the absorption of
FIrpic are presented in Figure 2b. The main peaks of DCPP,
DCPPO, TCP and TCPO films are red-shifted to 397, 385, 375
and 350 nm, respectively, while the fine structure of the peaks
disappears. It should be noted that TCP and TCPO show less
red shift emission maximum in solid compared to DCPP and
DCPPO, which may arise from suppressed π-π interaction in
TCP and TCPO as mentioned in the aforementioned structures
and interactions analysis. The emission peaks of the host mate-
rials could well overlap with the absorption spectrum of FIrpic,
suggesting efficient Förster resonance energy transfer from the
host materials to FIrpic.
tion, and the melting points (T
and 363 °C for DCPP, DCPPO, TCP and TCPO, respectively.
High T is also indispensable for increasing morphological
stability of the host materials to lengthen the device lifetime,
and hence the T of DCPP, DCPPO and TCP were investigated
by differential scanning calorimetry (DSC, Figure 3c) and
described in Table 1. However, the T of TCPO could not be
m
) being 225 °C, 232 °C, 307 °C
g
g
g
observed during several heating-cooling cycles. Based on the
thermal properties, it is found that more rigid carbazole moiety
endows TCP and TCPO with higher molecular weights and
better thermal stability. Moreover, the oxidation of the phos-
phine also improves the thermal stability because of enhanced
molecular polarity and weight. Overall, the thermal stabilities
of all the four compounds are considerably excellent for de-
vice fabrication.
The phosphorescence spectra (Figure 2c) of the four com-
pounds were measured in 2-methyltetrahydrofuran (2-MeTHF)
-
5
-1
solution (1×10 mol L ) at 77 K. Similar phosphorescence
spectra were observed in the range of 400-500 nm for the four

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DOI: 10.1039/C7TC01594A
a)
b)
Figure 4. a) Cyclic voltammetry curves and b) Experimental (blue) and DFT calculated (red) HOMOs, LUMOs of the molecules.
.5. Frontier Molecular Orbital Studies. Cyclic voltamme- their geometry structures and calculate their FMO energy lev-
2
try was employed to investigate the electrochemical properties
of the four compounds and evaluate their HOMO energy lev-
els. The electron density distributions and energy levels of the
FMOs are displayed in Figure 4b. The FMO distributions of
DCPP are partly separated, while those of DCPPO are divided
much more clearly. For these two molecules, the HOMOs are
mainly located on the carbazole groups, and the LUMOs are
centralized on the phenylphosphine or phenylphosphine oxide
groups. In contrast, the HOMOs and LUMOs of TCP and
TCPO are distributed around the whole molecules. These re-
sults indicate that DCPPO is a donor-acceptor molecule with
ambipolar property, which may have more balanced carrier
transport capability.
2 2
els. The oxidation process was conducted in degassed CH Cl
2
8
with ferrocene (Fc) as the internal reference. The HOMO
energy levels were estimated from the oxidation potentials
(
Figure 4a) to be 5.93, 6.33, 5.94, and 6.10 eV for DCPP,
DCPPO, TCP, and TCPO, respectively, using the empirical
2
8
formula reported in literature. The LUMO energy levels
were determined from their HOMO energy levels and optical
band gaps calculated from their absorption spectra, which are
2
.23, 2.45, 2.24, and 2.18 eV for DCPP, DCPPO, TCP, and
TCPO, respectively. The HOMO (~ 5.9 eV) of DCPP and TCP
2
.6. Electroluminescence Properties. To study the potential
1
8
is consistent to that of carbazole (5.8 eV) , while the strong
electron-withdrawing phosphine oxide groups remarkably
lower the FMOs of DCPPO and TCPO, especially for their
HOMOs. These data suggest that the FMOs of the four com-
pounds could fit well with plenty of common charge transport
materials, such as mCP (HOMO = 5.9 eV) and TmPyPB
of the four compounds as host matrices for blue PHOLEDs,
devices were fabricated with a structure (Figure 5a) of
ITO/MoO
3
(1 nm)/N,N'-dicarbazolyl-3,5-benzene (mCP):
MoO (20 wt.%, 10 nm)/mCP (30 nm)/Host:FIrpic (7 wt.%, 20
3
nm)/1,3,5-tri(m-pyrid-3-yl-phenyl) benzene (TmPyPB, 40
nm)/LiF (1 nm)/Al (100 nm). The energy levels and structures
of these materials are exhibited in Figure 5b and Figure 5c,
respectively.
(
LUMO = 2.7 eV).
Density functional theory (DFT) calculations were employed
to investigate these compounds for a deeper insight into their
electronic properties. B3LYP/6-31G* was chosen to optimize
a)
b)
c)
T
Figure 5. a) The schematic structure of fabricated PHOLEDs; b) the HOMOs, LUMOs, E levels, and c) the molecular structures of the
compounds used in the devices.

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-
2
Figure 6. a) Electroluminescence spectra for different OLEDs at 1000 cd m ; b) Current density–voltage–luminance (J–V–L) characteris-
tics of the devices; c) power efficiency-current density-external quantum efficiency (PE-J-EQE) of the devices.
A 1-nm-thick layer of MoO
function of ITO to enhance hole injection. A 10-nm layer of
mCP (E = 2.9 eV) was doped by 20 wt. % MoO to enhance
hole injection, followed by another 30-nm pristine mCP as
the hole transporting layer. TmPyPB (E
the electron transporting layer as well as the hole blocking
layer because of its low-lying HOMO level. As shown in Fig-
ure 5b, the HOMO levels of the hosts are about 0.4-0.8 eV
higher than that of TmPyPB. As a result, holes can be effec-
tively confined within the emitting layer (EML). Moreover,
3
was used to modify the work
mances of DCPPO- and TCPO-based devices are better than
corresponding D-π-A hosts (BCPO and TCTP) based devices.
However, at a higher driving voltage or current density, the
DCPP- and TCP-based devices suffered severe efficiency roll-
off. In contrast, the devices based on phosphine oxide hosts
exhibited better performance. All critical data are summarized
in Table 2.
T
3
6, 29
3
0
T
= 2.8 eV) acted as
T
Considering that the vital parameters, such as E , FMO energy
levels and thermal properties, are similar for the four host ma-
terials, the variation in device performance may arise from the
different ambipolar charge mobilities of the compounds. The
hole or electron-only devices were fabricated to evaluate the
charge mobilities of the four compounds. Electron-only devic-
es ITO/BCP (Bathocuproine, 10 nm)/Host (30 nm)/BCP (10
nm)/LiF (1 nm)/Al (100 nm) were fabricated to characterize
the electron transport abilities of these materials. The current
densities of the DCPPO- and TCPO-based devices are higher
than those based on DCPP and TCP (Figure 7a), which indi-
cates that the electron transporting abilities are significantly
enhanced after the oxidation of the phosphines. Additionally,
the higher current density of the DCPPO-based device com-
parted to the TCPO-based one reveals that the phenyl group is
critical to constructing the acceptor moiety in a molecule to
enhance the electron transportation. The hole mobilities of the
four hosts were characterized by corresponding hole-only de-
T T
the chosen materials all possess higher E than FIrpic (E =
2
.65 eV) to confine triplet excitons within the EML. The per-
formance of the devices based on these host materials is sum-
marized in Figure 6. All PHOLEDs exhibited characteristic
sky-blue emission from FIrpic without any visible emission
from the host materials (Figure 6a), indicating sufficient ener-
gy transfer from the host to the dopant. The current density–
voltage–luminance (J–V–L) characteristics are shown in Fig-
ure 6b. The turn-on voltages (Von) of the devices are around
3
.27-3.47 V. We found that the Von increases as the HOMO
level of the corresponding host matrix decreases. This phe-
nomenon reveals that the Von partly depends on the hole injec-
tion barrier from mCP to the host materials. At the same volt-
age, the devices with tri-substituted carbazole hosts (TCP or
TCPO) gave higher current densities than those of the corre-
sponding bi-substituted carbazole hosts (DCCP or DCCPO).
At relative low driving voltages (lower than 5.5 V), the lumi-
nances are consistent with the current densities of the four
devices, while the maximum luminances of DCPP- and TCP-
based devices are much lower than those of the DCPPO- and
TCPO-based devices. Figure 6c exhibits the power efficiency-
current density-external quantum efficiency (PE-J-EQE)
curves of the devices. The maximum EQEs of all devices ex-
ceeded 20%, demonstrating that all the four hosts are suitable
for FIrpic-based blue PHOLEDs. Additionally, the perfor-
vices with a structure of ITO/MoO
3
(10 nm)/host (60
nm)/MoO (10 nm)/Al (100 nm). As shown in Figure 7b,
3
DCPP showed the best hole transporting ability (ignoring the
hole injection barrier differences among the four compounds).
Moreover, the hole transport abilities are weakened by the
oxidation of phosphines. These results clearly indicate that the
oxidation turn the hole-dominated carbazole phosphine into
the bipolar materials.
Figure 7. The current density-voltage curves of a) electron- and b) hole- only devices.

Journal of Materials Chemistry C
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DOI: 10.1039/C7TC01594A
Table 2. Device performances of the blue PHOLEDs with the four different host materials.
EQE [%]
a)
PEmaxb)
[lm W-1]
CEmaxc)
[cd A-1]
Roll-off d)
CIEe,f)
[x, y]
V
on
L
max
Device
-
2
@1 mA
cm
@10 mA
cm
@20 mA
cm
[V]
[cd m ]
[%]
max
-
2
-2
-2
DCPP
DCPPO
TCP
3.27
3.47
3.27
3.31
1893
14070
4596
32.7
42.7
34.3
38.0
38.5
53.0
39.2
43.6
20.2
27.5
20.4
22.0
15.6
22.5
18.0
17.1
7.7
4.6
14.7
8.5
50.6, 70.5
22.2, 34.7
32.8, 52.8
28.1, 39.2
0.15, 0.32
0.15, 0.32
0.15, 0.32
17.5
12.1
12.3
TCPO
13020
10.4
0.15, 0.33
a)
Maximum luminance; Maximum power efficiency; c) Maximum current efficiency; d) From 1 mA cm to 10 mA cm and 1 mA cm to
b)
-2
-2
-2
0 mA cm ; Value at 1000 cd m-2; f) Commission Internationale de l’Eclairage color coordinates.
-
2 e)
2
Based on the study on electron-only and hole-only devices, it
could be deduced that DCPP and TCP are hole-dominated
materials, thus unbalanced carrier transport is expected to oc-
cur in the EML of their corresponding devices under high cur-
rent densities. The excessive hole transport will push the re-
combination zone close to the interface of emission layer and
Synthesis of dicarbazolylphenylphosphine (DCPP). A 2.5
M hexane solution of n-BuLi (20.0 mL, 50.0 mmol) was add-
ed dropwise to a stirred tetrahydrofuran–hexane solution (1:1,
200 mL) of carbazole (8.35 g, 50.0 mmol) at -78 °C. The reac-
tion mixture was allowed to warm to room temperature and
stirred for two hours. Hexane was added to precipitate a white
powder, which was isolated by filtration, washed with hexane
electron transport layer, leading to strong triplet-triplet annihi-
lation in the narrow exciton recombination zone.3
1, 32
As a
2
and then dissolved in THF (200 mL). PhPCl (4.48 g, 25.0
result, the phosphine-based devices exhibited lower maximum
luminance and much more severe efficiency roll-off as com-
pared to their phosphine oxide-based counterparts. The pro-
moted electron mobilities and diminished hole mobilities of
DCPPO and TCPO can balance carrier transport and thus ex-
pand the exciton recombination zone, resulting in brighter
luminance and lower efficiency roll-off. With the greatest
electron mobility and highly balanced carrier transport of
DCPPO, its corresponding device displayed the best EQE of
mmol) was added dropwise and the reaction mixture was
stirred overnight. The solution was passed through a silica
column and the solvent was evaporated under reduced pres-
sure. The resulting white solid was recrystallized from acetone.
3
1
Yield: 8.82 g (80%). P NMR (500 MHz, CDCl
3
): δ 52.10 (s).
1
H NMR (500 MHz, CDCl
3
): δ 7.99-8.00 (d, 4H, J = 6.5 Hz),
7.56-7.58 (d, 4H, J = 7.5 Hz), 7.46-7.49 (t, 3H, J = 7.0, 8.5
Hz), 7.38-7.41 (t, 2H, J = 7.5, 7.0 Hz), 7.21-7.27 (m, 8H).
Calc. for C30H N P: C, 81.80; H, 4.81; N, 6.36. Found: C,
21 2
2
7.5% and the lowest efficiency roll-off of 22.2%.
81.89; H, 4.80; N, 6.41.
3
. CONCLUSION
Synthesis of dicarbazolylphenylphosphine oxide (DCPPO).
DCPP (4.40 g, 10.0 mmol), CH
2 2 2 2
Cl (100 mL), and H O (30%,
Four host compounds, DCPP, DCPPO, TCP and TCPO, were
synthesized by only one or two steps with high yields. All of
them possess high E , suitable FMO energy levels and excel-
T
lent thermal stabilities, making them favorable for the manu-
facturing of blue PHOLEDs. We found that the charge mobili-
ty of the host materials can be modulated by the oxidation of
phosphine. Compared to DCPP and TCP, DCPPO and TCPO
showed improved electron mobility while retaining the high
2
0 mL) were stirred overnight at room temperature. The or-
ganic layer was separated and washed with water and brine.
The extract was evaporated to dryness affording a white solid,
which was further purified by recrystallization and gradient
3
1
sublimation. Yield: 4.12 g (90%). P NMR (500 MHz,
1
CDCl
3
): δ 8.45 (s). H NMR (500 MHz, CDCl
3
): δ 8.00-8.02
(d, 4H, J = 7.5 Hz), 7.80-7.84 (q, 2H, J = 8.0, 6.5, 8.0 Hz),
7
7
7
.67-7.70 (t, 1H, J = 7.5, 7.5 Hz), 7.47-7.51 (m, 2H), 7.24-
.27 (t, 4H, J = 7.5, 7.5 Hz), 7.20-7.23 (t, 4H, J = 7.0, 8.5 Hz),
.11-7.14 (t, 4H, J = 7.0, 8.5 Hz). Calc. for C30H N OP: C,
21 2
T
E and good hole mobilities, resulting in much more balanced
carrier transportation for decreased efficiency roll-off. DCPPO
provided its FIrpic-based device with the highest efficiencies,
-
1
78.94; H, 4.64; N, 6.14. Found: C, 78.90; H, 4.61; N, 6.15.
i.e. maximum current efficiency of 53.0 cd A , power effi-
-
1
ciency of 42.7 lm W , and EQE of 27.5%, and the lowest effi-
Synthesis of tricarbazolylphosphine (TCP). The synthetic
procedure of TCP is similar to that of DCPP, using carbazole
(8.35 g, 50.0 mmol), n-BuLi (20.0 mL, 50.0 mmol), and PCl
-
2
ciency roll-off of 22.2% from 1 to 10 mA cm , which are
among the best-performing FIrpic-based blue PHOLEDs
3
3
3-36
31
without using any light extraction method.
(2.29 g, 16.7 mmol). Yield: 5.42 g (61%). P NMR (500 MHz,
1
CDCl
3
): δ 76.84 (s). H NMR (500 MHz, CDCl
3
): δ 8.05-8.06
4
. EXPERIMENTAL SECTION
(d, 6H, J = 7.5 Hz), 7.23-7.26 (t, 6H, J = 7.5, 7.5 Hz), 7.16-
General. Chemicals were received from commercial resources
and used as received. Tetrahydrofuran (THF) was additionally
dried over sodium and distilled. Reactions for preparing DCPP
and TCP were carried out using Schlenk-line techniques under
a pure dry nitrogen atmosphere, while these for synthesizing
of DCPPO and TCPO were performed under air. The four
7
.18 (d, 6H, J = 8.0 Hz), 7.10-7.13 (t, 6H, J = 7.0, 8.5 Hz).
P: C, 81.65; H, 4.57; N, 7.93. Found: C,
1.56; H, 4.56; N, 7.95.
24 3
Calc. for C36H N
8
Synthesis of tricarbazolylphosphine oxide (TCPO). The
synthetic procedure of TCPO is similar to that of DCPPO, a
mixture of TCP (2.65 g, 5.0 mmol), CH
-
5
2 2
Cl (50 mL), and
materials underwent gradient sublimation (~ 10 Pa) before
3
1
1
H O (30%, 10 mL) was stirred for 36 h. Yield: 2.65 g (97%).
2 2
any characterization and device fabrication. P and H NMR
spectra were recorded on a Bruker Avance III 500 MHz spec-
trometer. Elemental analyzes were performed on a Vario EL
instrument.
3
1
1
P NMR (500 MHz, CDCl
CDCl ): δ 8.04-8.05 (d, 6H, J = 7.5 Hz), 7.26-7.29 (t, 6H, J =
.0, 7.0 Hz), 7.05-7.12 (m, 12H). Calc. for C36 OP: C,
9.25; H, 4.43; N, 7.70. Found: C, 79.33; H, 4.45; N, 7.76.
3
): δ -13.06 (s). H NMR (500 MHz,
3
7
7
24 3
H N

Page 7 of 9
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC01594A
Photophysical Measurement. UV-Vis absorption spectra
were recorded on a Shimadzu UV-3100 spectrometer. Flo-
rescence and low temperature phosphorescence spectra were
carried out on an Edinburgh Analytical Instruments FLS920
spectrophotometer.
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AUTHOR INFORMATION
Corresponding Authors
5. C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A.
Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett.,
2
001, 79, 2082-2084.
*
*
*
E-mail: zwliu@pku.edu.cn (Zhiwei Liu).
E-mail: bianzq@pku.edu.cn (Zuqiang Bian).
E-mail: liuweiping0917@126.com (Weiping Liu).
2
2
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and L.-S. Liao, J. Mater. Chem. C, 2014, 2, 6387-6394.
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Wong, W.-Y. Hung and S.-W. Chen, Org. Electron., 2011, 12,
Author Contributions
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025-2032.
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manu-
script.
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ACKNOWLEDGMENT
We greatly acknowledge the financial support by the State Key
Laboratory of Advanced Technologies for Comprehensive Utili-
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Basic Research Program of China (No. 2014CB643802).
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Page 9 of 9
Journal of Materials Chemistry C
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DOI: 10.1039/C7TC01594A
Table of contents: Selective modulating the charge mobility of the host materials by the oxidation of
phosphines for high efficient blue phosphorescent OLEDs.
1