A new tricarbazole phosphine oxide bipolar host for efficient single-layer blue PhOLED

Article abstract of DOI:10.1016/j.orgel.2011.08.030

A novel tricarbazole phosphine oxide (POCz3) with high triplet energy and promising physical properties serves as a bipolar host material of blue-emitting phosphor (FIrpic) to realize highly efficient single-layer blue PhOLED which achieves a maximum exte

Full text of DOI:10.1016/j.orgel.2011.08.030

Organic Electronics 12 (2011) 2025–2032
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A new tricarbazole phosphine oxide bipolar host for efficient
single-layer blue PhOLED
Hsin-Hua Chang a, , Wan-Shan Tsai b, Chien-Ping Chang a,c, Nien-Po Chen c,
⇑
d
d
Ken-Tsung Wong b, , Wen-Yi Hung , Shou-Wei Chen
⇑
a Department of Electro-Optical Engineering, Vanung University, Taoyuan 320, Taiwan
b Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
c Department of Electro-Optical Engineering, Yuanze University, Taoyuan 320, Taiwan
d Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2011
Received in revised form 29 August 2011
Accepted 29 August 2011
Available online 15 September 2011
A novel tricarbazole phosphine oxide (POCz3) with high triplet energy and promising phys-
ical properties serves as a bipolar host material of blue-emitting phosphor (FIrpic) to realize
highly efficient single-layer blue PhOLED which achieves a maximum external quantum effi-
ciency up to 9% (gp = 10.4 lm/W, gc = 21.3 cd/A) and shows low efficiency roll-off effect.
POCz3 was also utilized in a three-layer device with a double confinement effect exhibiting
maximum luminance 60098 cd/m2 and maximum EQE (power efficiency) of 14.5%
(31.3 lm/W).
Keywords:
Electrophosphorescence
Bipolar host material
Phosphine oxide
Carbazole
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
goal is the single layer device, which can perform compa-
rable efficiency as compared to their multilayer counter-
Recent progresses on phosphorescence organic light-
emitting diodes (PhOLEDs) receive considerable research
attentions owing to their capability of harvesting both sin-
glet and triplet excitons to realize internal quantum effi-
ciencies up to 100% [1]. In order to efficiently extract the
electro-generated emissive excitons in typical PhOLEDs de-
vice, various functional layers were required for their indi-
vidual role-play such as charge carrier injection, transport,
and blocking as well as the confinement of exciton diffu-
sion. Such sophisticated device configurations inevitably
increase the manufacture complexity and the production
cost. Therefore, it is highly desirable to fabricate PhOLEDs
with simplified device structures. Particularly potential
parts. There are several important factors required for
efficient single layer PhOLEDs: (1) host material with suit-
able energy levels match to cathode/anode for easy charge
injection, (2) host material with balanced electron and hole
mobilities for large charge flux and efficient charge recom-
bination, (3) high triplet energy relative to emissive phos-
phors for exciton confinement within the emissive layer.
More recently, reports of adopting direct charge injection
and transport onto the triplet dopants dispersed in an
appropriate host matrix pave the way to realize efficient
single-layer PhOLEDs. The host materials incorporated in
these single-layer devices include mixed host materials
[2–4], bipolar host materials [5], and electron-transporting
host material [6]. The judicious choice of hosts with appro-
priate energy levels combining with hole-/electron
-transporting characters in mixed systems and/or the sin-
gle molecular bipolar host play the crucial role for the suc-
cess of single layer device. Among these elegant works,
efficient green and red single-layer PhOLEDs have been
⇑
Corresponding authors. Tel.: +886 3 4515811x73607; fax: +886 3
4515811 334 (H.-H. Chang), tel.: +886 2 33661665; fax: +886 2 33661667
(K.-T. Wong).
E-mail addresses: hhua3@mail.vnu.edu.tw (H.-H. Chang), ken
wong@ntu.edu.tw (K.-T. Wong).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.08.030

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H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
successfully demonstrated [5]. However, single layer blue
PhOLEDs [2,5a,6] reported as far were not satisfactory
yet, giving challenges for material scientists. The most pop-
ular blue emitter FIrpic has revealed its bipolar charge car-
rier transport behavior by using the time-of-flight (TOF)
transient photocurrent technique [7]. Accordingly, highly
efficient single layer blue PhOLED should be feasible if
one uses FIrpic dispersed into a tailor-made high triplet en-
ergy host with suitable energy levels for direct carriers
injection. In this paper, we report a novel high triplet en-
ergy host material: 3,30,300-phosphoryl tris(9-phenyl-9H-
carbazole) (POCz3) for efficient single-layer blue PhOLEDs.
The device: ITO/PEDOT:PSS/POCz3 (3 nm)/POCz3:FIrpic
20 wt.% (80 nm)/LiF/Al gives a maximum external quan-
the structure of the new bipolar host material POCz3,
which was synthesized in one step with reasonable yield
from
a
known precursor. The 3-bromo-9-phenyl-
carbazole reacted with magnesium turning in THF to give
a Grignard solution, which was then quenched with POCl3
to give the desired product POCz3 in 55% yield. The
structural identity of POCz3 was confirmed by satisfactory
1H, 13C and 31P NMR spectra, high-resolution mass
spectrometry (HRMS).
2.2. Physical properties characterizations
POCz3 exhibits a high thermal tolerance [5% weight loss
occurred at 395 °C, determined by thermogravimetric anal-
ysis (TGA)] and a distinct glass transition temperature
(Tg) of 163 °C analyzed by differential scanning calorimetry
(DSC). Therefore, POCz3 can form homogeneous and stable
amorphous films upon thermal evaporation, which is a
crucial criterion for OLED application.
tum efficiency
(
gext
)
of 9%, power efficiency
(gp) of
10.4 lm/W, and current efficiency (
gc) of 21.3 cd/A. To the
best of our knowledge, this work represents an unprece-
dented example of realizing a single-layer blue PhOLED
with remarkable device characteristics.
The photophysical property of POCz3 as compared to
that of parent compound 9-phenyl carbazole is shown in
Fig. 1. Apparently, the introduction of phosphine oxide
onto the carbazole ring only brings out limited perturba-
tion on the photophysical property of carbazole. In solu-
tion, POCz3 exhibits a strong absorption band range from
2. Results and discussion
2.1. Materials
The most successful host materials used for FIrpic are
260 to 300 nm that is attributed to the typical p–
p⁄ elec-
carbazole derivatives [8]. Without extended
p
-conjuga-
tronic transition of carbazole, whereas the weak absorp-
tion bands (300–350 nm) are ascribed to the n–p⁄
transitions of carbazole. The emission spectrum of POCz3
is almost superimposable to that of 9-phenyl carbazole.
The phosphorescent spectrum of POCz3 was obtained at
77 K (in EtOH). Again, there is no significant difference on
the triplet energy upon introducing the electron deficient
phosphine oxide to the carbazole as compared to that of
9-phenyl carbazole. The difference between the fluores-
cence (351 nm) and phosphorescence (408 nm) spectra
corresponds to an exchange energy of ca. 0.67 eV between
the singlet and triplet of POCz3. The triplet energy (ET) of
3.03 eV (defined as the 0–0 transition in phosphorescent
spectrum) is sufficiently high for POCz3 to confine the trip-
let excitons of the blue phosphorescence emitter FIrpic
(ET = 2.7 eV) within the emitting layer.
tion from the carbazole core, carbazole-based host mate-
rials usually exhibit high triplet energies. Thus, the triplet
excitons can be efficiently confined within the emitting
layer. However, typical FIrpic-based blue PhOLEDs incor-
porated with carbazole-based hosts require multilayer
structures for facilitating the hole/electron injections into
the emitting layer. In order to enhance the electron injec-
tion capability of carbazole, structural modification by
introducing electron deficient moiety onto the carbazole
core is necessary. Recently, triarylphosphine oxides have
been reported to perform facile electron injection and
transport properties [9–11]. The combination of the struc-
tural features of carbazole and phosphine oxide had led to
promising bipolar hosts for blue PhOLEDs with excellent
device characteristics [12]. The HOMO level of carbazole
compound can be modulated by introducing appropriate
substitutions at the C3 and/or C6 positions of carbazole
[13–14], giving the opportunity of developing potential
host materials for single layer device. Scheme 1 depicts
Fig. 2 displays the cyclic voltammograms of POCz3 and
9-phenyl carbazole in CH2Cl2 using a glassy carbon elec-
trode with 0.1 M of nBu4NPF6 as electrolyte. The oxidation
of POCz3 showed typical electrochemical behavior of C3
Scheme 1. The structure and synthesis of bipolar host POCz3.

H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
2027
Fig. 1. A comparison of UV–vis absorption, photoluminescence (PL) (CH2Cl2), and phosphorescence (EtOH, 77 K) spectra of POCz3 and its parent
counterpart N-phenyl carbazole.
[15] to evaluate the carrier mobilities. Fig. 3(a) displays rep-
resentative TOF transient for electrons of POCz3. The transit
time, tT, can be evaluated from the intersection point of two
asymptotes in the double-logarithmic representation of the
TOF transient. It is then used to determine the carrier mobil-
ities, according to the equation
l
= d2/VtT, where d is the
sample thickness and V is the applied voltage. Fig. 3(b) re-
veals that the field dependence of the electron mobility fol-
lows the nearly universal Poole–Frenkel relationship, with
values in the range from 1.4 ꢁ 10ꢀ6 to 6.4 ꢁ 10ꢀ6 cm2/Vs
for fields varying from 4 ꢁ 105 to 6.8 ꢁ 105 V/cm.
In contrast, the TOF transient for holes exhibited weak
photocurrent signals, which could not be used to deter-
mine the hole mobility. From the chemical structure of
POCz3 (Scheme 1), the carbazole unit is a good hole trans-
port unit and is effective in improving the hole transport
property of the organic material. Hence, we also prepared
hole- and electron-only single-carrier devices, to evaluate
the bipolar transport behavior. We used high-LUMO
Fig. 2. The cyclic voltammogram of POCz3 as compared to that of parent
compound 9-phenylcarbazole.
4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl
h of 10ꢀ3 cm2/Vs order) [16] to limit the electron carrier
in hole-only devices and low-HOMO 2,9-dimethyl-4,7-di-
phenyl-1,10-phenanthroline (BCP,
e of 10ꢀ7 cm2/Vs or-
(NPB,
l
and C6 unprotected carbazole derivatives, which usually
performed a lower reduction peak originated from the
more conjugated dimeric species generated from the cou-
pling of carbazole cationic radicals. It is noteworthy that
the oxidation onset of POCz3 is evidently shifted from
1.28 V of 9-phenyl carbazole to 1.44 V. Thus, the highest
occupied molecular orbital (HOMO) energy of POCz3 is
strongly affected by introducing the electron-withdrawing
phosphine oxide. The electron deficient substituent is also
beneficial to the reduction behavior of POCz3, in which one
quasi-reversible reduction potential, onset potential at
ꢀ2.25 V, was observed in dimethylformamide (DMF) using
a glassy carbon electrode with 0.1 M of nBu4NClO4 as elec-
trolyte. For practical evaluation, the HOMO of POCz3 were
measured using UV photoemission spectroscopy (AC-2) to
be ꢀ5.5 eV, and the lowest unoccupied molecular orbital
(LUMO) energy was deduced to be ꢀ1.8 eV by adding the
energy gap of POCz3 to the HOMO energy.
l
der) [17] to limit the hole carrier in electron-only
devices. As shown in Fig. 4, POCz3 shows bipolar transport
property. However, the current density of electron-only
device is larger than that of hole-only device at the same
voltage, indicating the superior capability of POCz3 to
transport electron other than hole-transport. This result
is in agreement with the observed TOF mobility that only
electron mobility can be resolved.
2.3. Device characteristics
The energy levels diagram of single-layer blue PhOLEDs
with host molecule POCz3 and blue emitter FIrpic is de-
picted in Fig. 5. It is obvious that the energy level align-
ment allows energetically favorable electron injection
from cathode directly into FIrpic (energy barrier ꢂ0.1 eV)
instead of POCz3 which gives an energy barrier ꢂ1.2 eV.
To further understand the charge-carrier transport prop-
erties of POCz3, we used the time-of-flight (TOF) technique

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H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
Fig. 3. Typical transient photocurrent signals for POCZ3 (2.8
l
m thick): (a) electron at E = 5.4 ꢁ 105 V/cm. Insets are the double logarithmic plots of (a).
(b) Electron mobility plotted with respect to E1/2 at ambient temperature.
In contrast, the HOMO energy level of POCz3 allows hole
injection from anode and transporting by way of host to
reach to FIrpic. Based on the promising physical properties
of POCz3, a single layer blue PhOLEDs with POCz3 as host
for FIrpic was fabricated with the device configuration as
ITO/PEDOT:PSS/POCz3:FIrpic/LiF/Al.
It is inevitable that the recombination zone should be as
away as possible from both electrodes for avoiding exciton
quenching. Thus, the thickness of the single layer device
needs to be carefully optimized. We used the typical dop-
ing concentration 10 wt.% of FIrpic for devices configured
as ITO/PEDOT:PSS/POCz3:FIrpic (x nm)/LiF/Al, where the
thicknesses x were varied as 70 nm, 80 nm, and 90 nm,
respectively. The J–V and efficiency characteristics of these
devices are shown in Fig. 6.
showed slow efficiency roll-off as the EQE keeps at 6.4%
at the brightness of 1000 cd/m2. As a consequence, we uti-
lized 80 nm as the optimal thickness of the emitting layer
in the following experiments. It should be noted that the
hole-injection material PEDOT:PSS may act as a strong
quencher of blue phosphorescence, rendering low device
efficiency. In order to suppressing the possible triplet exci-
ton diffusion, a neat thin film of POCz3 was introduced be-
tween the PEDOT:PSS and emitting layer comprised of
POCz3 and FIrpic. Thus, we fabricated two devices with de-
vice configuration of ITO/PEDOT:PSS/POCz3 (x nm)/
POCz3:FIrpic 10.0 wt.% (80 nm)/LiF/Al, where the thickness
x of POCz3 is 3 and 5 nm, respectively. Fig. 7 shows the J–V
and external quantum efficiency-brightness characteristics
of these two devices as compared to those of the parent de-
vice. Fig. 7(b) shows the EL spectra of the devices with and
without introducing a thin film of POCz3 as exciton con-
finement layer. As shown, all devices have almost identical
EL spectra which gave blue light originating from the
FIrpic, indicating that the triplet excitons of FIrpic are
It is clear that the devices have higher driving voltages
as the thickness of POCz3:FIrpic layer increases. Among
these three devices, the single-layer device with thickness
of 80 nm gave the optimal maximum external quantum
efficiency (EQE) of 7.1% at brightness of 100 cd/m2, and

H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
2029
Fig. 4. Current density–voltage (I–V) characteristics of carrier-only
device: hole-only device (ITO/NPB (10 nm)/POCz3 (30 nm)/NPB (10 nm)
/LiF/Al and electron-only device: (ITO/BCP (10 nm)/POCz3 (30 nm)/BCP
(10 nm)/LiF/Al.
Fig. 7. (a) Current–voltage and external quantum efficiency–brightness
characteristic of the exciton confined devices, (b) EL spectra of the devices
with different thickness of exciton confinement layer.
Fig. 5. Relative energy levels of a single-layer PhOLED device with blue
emitter FIrpic and host POCz3.
device due to the exciton confinement effect. However, due
to the low hole transport ability of POCz3, the driving volt-
ages are higher with the thickness of interfacial POCz3
layer increases. As a consequence, a 3 nm neat thin film
of POCz3 was utilized as the exciton confinement layer in
the following experiments.
Inthecaseof directinjectionandtransportonthedopant,
the increase of the doping concentration may lead to en-
hanced probability of carrier injection and transport chan-
nels, resulting in higher device efficiency [6]. In order to
improve device efficiency, four devices of varied concentra-
tions of FIrpic were fabricated with a configuration of ITO/
PEDOT:PSS/POCz3(3 nm)/POCz3: FIrpic x wt.% (80 nm)/LiF/
Al, where the doping concentration x was varied as 10, 15,
20, and 25 wt.%, respectively, in order to determine the opti-
mal dopant concentration. J–V–L curve and efficiency char-
acteristics of these devices are shown in Fig. 8. The driving
voltage decreases as the concentration of dopant concentra-
tion increases from 10 to 20 wt.%. There was limited differ-
ence on driving voltage between the doping concentration
of 20 and 25 wt.%. This result indicates that device with
higher FIrpic concentration imparts better carrier injection
and more transport channels. Fig. 8(b) shows that maximum
external quantum efficiency (power efficiency) enhanced
Fig. 6. Current–voltage and external quantum efficiency characteristic of
POCz3:FIrpic based single layer blue PHOLEDs as a function of device
thickness.
efficiently confined within the POCz3:FIrpic layer. The
device with POCz3 as the interfacial layer (5 nm) achieves
higher efficiency of 7.7% as compared to that of the parent

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H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
Fig. 8. (a) Current–voltage–brightness characteristic, (b) external quantum efficiencies and power efficiencies of single layer blue PhOLEDs.
from 7% (6.7 lm/W) to 9% (10.4 lm/W), when the FIrpic con-
centration increases from 10 to 20 wt.%. At higher doping
concentration, more carriers can be directly injected and
transported on FIrpic, leading to better device efficiency.
However, at dopant concentration reaches to 25 wt.%, the
efficiency decreased dramatically due to the triplet–triplet
annihilation and concentration quenching effects. With
the optimized dopant concentration of 20 wt.%, the device
Fig. 9. Current–voltage–brightness and efficiency characteristic of POCz3 hosted three-layer blue PhOLEDs.

H.-H. Chang et al. / Organic Electronics 12 (2011) 2025–2032
2031
achieves the best performance, yielding maximum external
quantum efficiency 9%, power efficiency 10.4 lm/W, and
current efficiency 21.3 cd/A. The efficiencies of this opti-
mized device show low roll-off effect with comparably high
gext (8.1%), gp (5.6 lm/W), and gc (19.6 cd/A) when the
brightness was increased to 1000 cd/m2.
few anhydrous THF was added in the flask. 3-bromo-9-
phenylcarbazole (3.22 g, 10 mmol) and anhydrous THF
(10 mL) in the addition funnel were added dropwise to
the flask and the reaction mixture was refluxed until Mg
is disappeared. Return to room temperature, POCl3
(0.28 mL, 3 mmol) was added in the Grignard reagent.
The resulting pale-yellow solution was stirred for 8 h.
The mixture was quenched with saturated aqueous solu-
tion of NH4Cl (10 mL). The aqueous layer was extracted
with dichloromethane (3 ꢁ 20 mL), and the combined or-
ganic layers were washed with water (2 ꢁ 20 mL), dried
(MgSO4), and concentrated under reduced pressure. The
residue was purified through column chromatography
(SiO2; EtOAc/hexane 1:1) to yield POCz3 as a white yellow
solid (4.3 g, 55%) 1H NMR (400 MHz, CDCl3) d 8.67 (d,
J = 12 Hz, 3H), 8.09 (d, J = 7.6 Hz, 3H), 7.76 (td, J = 10,
1.6 Hz, 3H), 7.62–7.55 (m, 12H), 7.50–7.41 (m, 12H), 7.27
(td, J = 7.2, 2.4 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 142.
5, 141.2, 136.9, 129.8, 129.7, 129.6, 129.4, 127.8, 127.0,
126.5, 125.3, 125.2, 124.7, 123.6, 123.3, 123.1, 122.9,
120.7, 120.5, 110.0, 109.8, 109.6; 31P NMR (161 MHz,
CDCl3) d 33.9; HRMS(M/z, FAB+) Calcd. For C54 H36N3OP
774.2596, found 774.2674.
For evaluate the potential usefulness of POCz3 as a suit-
able host in multilayer OLEDs, a three-layer device using
hole transport material 1,1-bis[4-[N,N-di(4-tolyl)-amino]
phenyl]cyclohexane (TAPC) with high hole mobility (1 ꢁ
10ꢀ3 cm2/Vs) and electron transport material 1,3,5-tri(m-
pyrid-3-yl-phenyl)benzene (TmPyPB) with high electron
mobility (1 ꢁ 10ꢀ3 cm2/Vs) were fabricated. Considering
the triplet exciton confinement within the FIrpic
(ET = 2.65 eV), a thin neat film of POCz3 (ET = 3.03 eV) was
introduced between TmPyPB (ET = 2.78 eV) and POCz3:FIr-
pic. After optimizing charge balance by tuning the thickness
of TAPC and TmPyPB, the J–V–L and efficiency characteris-
tics of a three-layer device was shown in Fig. 9 with device
configuration: ITO/TAPC (40 nm)/POCz3:FIrPic 8.0 wt.%
(25 nm)/POCz3 (5 nm)/TmPyPB (50 nm)/LiF (0.8 nm)/Al.
This device exhibits maximum luminance 60098 cd/m2
and maximum external quantum efficiency and power effi-
ciency of 14.5% and 31.3 lm/W, respectively. This device
shows slow efficiency roll-off as the
gext retains at 12.1% as
the brightness is 1000 cd/m2.
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Mg (243 mg, 10 mmol) in
round-bottomed flask, equipped with a reflux condenser
and an addition funnel, under an atmosphere of argon,
a 50 mL double-necked
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