Tuning of charge balance in bipolar host materials for highly efficient solution-processed phosphorescent devices

Article abstract of DOI:10.1021/ol201039n

A novel bipolar host material, which meets the requirements of high triplet energy, good charge carrier transport properties, high solubility, and film-forming ability at the same time, has been designed and synthesized. Utilizing a new compound as host m

Full text of DOI:10.1021/ol201039n

ORGANIC
LETTERS
2011
Vol. 13, No. 12
3146–3149
Tuning of Charge Balance in
Bipolar Host Materials for Highly
Efficient Solution-Processed
Phosphorescent Devices
Wei Jiang, Lian Duan, Juan Qiao, Guifang Dong, Liduo Wang, and Yong Qiu*
Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education,
Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China
qiuy@mail.tsinghua.edu.cn
Received April 20, 2011
ABSTRACT
A novel bipolar host material, which meets the requirements of high triplet energy, good charge carrier transport properties, high solubility, and
film-forming ability at the same time, has been designed and synthesized. Utilizing a new compound as host material, high-efficiency solution-
processed blue and white phosphorescent organic light-emitting diodes (PHOLEDs) have been achieved.
White organic light-emitting diodes (WOLEDs) have
attracted considerable interest because of their potential
applications in full-color display panels, flexible displays,
and lighting sources.1 The most promising approach to
manufacture highly efficient WOLEDs is phosphorescent
OLED (PHOLED) technology because the phosphors
could harvest both singlet and triplet excitons, yielding an
internal quantum efficiency as high as 100%.2 Most re-
cently, Reineke et al. achieved WOLEDs with a record
efficiency of 120 lm Wꢀ1 by using phosphorescent emitters
with improved out-coupling structures.3 This efficiency was
comparable to that of fluorescent tubes.4 Another challenge
for WOLED lighting is to reduce the fabrication cost. A
solution process based device fabrication is considered to
open the way to true low-cost mass production of large-area
lighting devices.5 Other than polymer-based materials, solu-
tion processable small-molecule materials can be conven-
tionally synthesized and purified. However, fabrication of
high-performance solution-processed WOLEDs based on
small molecules is limited by the relatively low efficiency of
blue PHOLEDs. Hence, promotion of small-molecule host
materials for solution-processed blue PHOLEDs is the key
of power-efficient WOLEDs.
A good host material for solution-processed blue PHO-
LEDs should have the following features: high triplet
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r
10.1021/ol201039n
Published on Web 05/17/2011
2011 American Chemical Society

energy, good charge carrier transport properties, high
solubility, and film-forming ability. Recently, during the
search for the host materials for blue PHOLEDs, a great
deal of attention has been given to bipolar transporting
molecules containing electron-transporting diphenylpho-
sphine oxide6 and hole-transporting carbazole7 units. Uti-
lization of bipolar host materials can balance the charge
carriers and broaden the recombination zone in the emit-
ting layer, therefore reduce the efficiency roll-off. Most
importantly, it could allow the fabrication of efficient
single-layer devices.8
high level.9 (ii) Phosphine oxide units are introduced to
replace the carbazole units in unipolar hole-transport host
material 1,3,5-tri(N-carbazolyl)benzene (3Cz) to achieve
bipolar transporting character. (iii) The polar nature of the
phosphine oxide helps to increase the solubility of the host
materials in the most common solvents. The solution-
processed devices with our new hosts showed excellent
performance, with a luminous efficiency (LE) of 23.6 cd
A
ꢀ1 and an external quantum efficiency (EQE) of 12.2%
forbluePHOLEDsand aLEof 33.8cdAꢀ1 and anEQEof
12.0% for white PHOLEDs, which are among the highest
reported values for small molecule based solution-pro-
cessed blue and white PHOLEDs.
As shown in Scheme 1, the starting materials were treated
with excess n-BuLi at ꢀ78 °C to give lithiated intermediates,
which were subsequently quenched with chlorodiphenyl-
phosphine to give the corresponding phosphine-containing
intermediates. Oxidation of the phosphine-containing inter-
mediates with aqueous H2O2 (30%) led to the desired
CzPO1 and CzPO2 in 60% and 51% yields, respectively.
Additionally, CzPO1 and CzPO2 exhibited excellent ther-
mal stability with the 5% weight-loss decomposition tem-
peratures of 391 and 386 °C, with the glass transition
temperatures of 111 and 96 °C, respectively, which suggests
that they could form morphologically stable and uniform
amorphous films. The phosphine oxide group also increased
the solubility of the host materials in the common solvents,
and the smooth surface roughness less than 0.5 nm was
obtained from the spin-coated films.
Scheme 1. Synthetic Route for CzPO1 and CzPO2
In this paper, we report the use of carbazole and diphenyl-
phosphine oxide as the bulky units, which are connected to
the phenyl core to build novel star-shaped host materials, 9,
90-(5-(diphenylphosphoryl)-1,3-phenylene)bis(9H-carbazole)
(CzPO1) and 9-(3,5-bis(diphenylphosphoryl)phenyl)-9H-
carbazole (CzPO2) (illustrated in Scheme 1). There are
several strategies to the design of bipolar host materials
for solution-processed blue PHOLEDs. (i) The unique
star-shaped structure of molecules greatly enhances the
thermal stability of these materials, and the nonconjuga-
tion linkage mode keeps the triplet energy gap at a very
Figure 1. Absorption and emission spectra of CzPO1, CzPO2,
and FIrpic in CH2Cl2 and phosphorescent spectra of CzPO1 and
CzPO2 in 2-methyltetrahydrofuran at 77 K.
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Figure 1 depicts the UVꢀvis absorption and photolumi-
nescence (PL) spectra of the compounds. The absorption
peaks at around 292 nm could be assigned to the nꢀπ*
transitions of the carbazole moiety, while the longer wave-
length absorption at around 339 nm could be attributed to
πꢀπ* transitions from the electron-donating carbazole
moiety to the electron-accepting diphenylphosphine oxide
(8) (a) Su, S. J.; Gonmori, E.; Sasabe, H.; Kido, J. Adv. Mater. 2008,
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Org. Lett., Vol. 13, No. 12, 2011
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moiety. Their PL spectra exhibit peaks at 418 and 397 nm
for CzPO1 and CzPO2, respectively, which were well over-
lapped with the metal-to-ligand charge-transfer absorption
peaks of the iridium(III) bis(4,6-difluorophenylpyridinato)-
This reversible reductive and quasi-reversible oxidative beha-
viors indicate that CzPO1 and CzPO2 possess both hole- and
electron-transporting characteristics.6a,11 DFT calculations were
performed to understand the physical properties of CzPO1
and CzPO2 at the molecular level (Figure 2b). The HOMO
and LUMO levels of CzPO1 and CzPO2 are mainly located
at the electron-donating carbazole and electron-accepting
diphenylphosphine oxide fragments, respectively, consistent
with the electrochemical studies that the carbazole groups
control the HOMO levels and the diphenylphosphine oxide
groups are responsible for the LUMO levels of CzPO1 and
CzPO2.
€
picolinate (FIrpic), indicating that the Forster energy trans-
fer from the host materials to the FIrpic would be efficient.
The triplet energies of CzPO1 and CzPO2, measured from
the low-temperature photoluminescence spectra, were 2.81
and 2.82 eV, respectively, larger than that of the guest blue-
emitter of FIrpic.
The electrochemical properties of CzPO1 and CzPO2
compared with 3Cz were investigated by cyclic voltammetry
(Figure 2a). CzPO1, CzPO2, and 3Cz exhibited similar quasi-
reversible oxidation process, which can be assigned to the
oxidation of electron-donating carbazole moiety, with onset
potentials of 1.79, 1.72, and 1.59 V, respectively. Upon
cathodic sweeping in THF, CzPO1, and CzPO2 exhibited
reversible reduction waves, arising from their electron-accept-
ing diphenylphosphine oxide moiety, with onset potentials
of ꢀ1.84 and ꢀ1.70 V, respectively. In contrast, the diphe-
nylphosphine oxide-free counterpart 3Cz exhibited no reduc-
tion wave during the cathodic scan, which implies that the
incorporation of the electron-accepting diphenylphosphine
oxide functionalities in CzPO1 and CzPO2 would facilitate
electron-transporting in the emitting layer.5b On the basis of
the onset potentials for oxidation and reduction, we esti-
mated the HOMO and LUMO energy levels of CzPO1 and
CzPO2 to be ꢀ5.78, ꢀ2.20, ꢀ5.85, and ꢀ2.34 eV, respec-
tively, with regard to ferrocene (ꢀ4.8 eV below vacuum).10
Figure 3. Current density versus voltage for the hole-only and
electron only devices of 3Cz, CzPO1, and CzPO2.
Hole-only and electron-only devices were fabricated to
investigate the bipolar character of CzPO1 and CzPO2
(Figure 3). The hole-only device consists of the following
structure: ITO/PEDOT:PSS (40 nm)/hosts (60 nm)/NPB
(20 nm)/Al (100 nm). The electron-only devices contain the
following layers: ITO/hosts (60 nm)/TPBI (30 nm)/
Cs2CO3 (2 nm)/Al (100 nm). According to the JꢀV curves
of hole-only devices, a decrease in the numbers of carba-
zole groups contained in the molecule structures leads to a
decrease in hole-current densities of 3Cz, CzPO1 and
CzPO2, which can be explained by the higher HOMO
energy level and the increased hole mobility of 3Cz com-
pared with those of CzPO1 and CzPO2.6d In contrast, the
CzPO2-based device shows the highest electron-current
density, relative to those in the 3Cz- and CzPO1-based
devices, because of the appropriate LUMO level for better
electron injection and high electron mobility. Further-
more, there was a slight difference between the hole- and
electron-current densities in the CzPO2, which can be
expected since holes and electrons can be balanced by the
incorporation of both donor (carbazole) and acceptor
(diphenylphosphine oxide) units into molecule structures.
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Figure 2. (a) Electrochemical properties of 3Cz, CzPO1, and
CzPO2. (b) Optimized geometries and calculated HOMO and
LUMO density maps for 3Cz, CzPO1, and CzPO2.
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Org. Lett., Vol. 13, No. 12, 2011

These results indicate that the CzPO2 shows the bipolar
charge-transport property.6c
The single-layer devices A with the configuration ITO/
PEDOT/PSS (40 nm)/host/FIrpic (10 wt %) (80 nm)/
Cs2CO3 (2 nm)/Al (100 nm) have been fabricated. Excellent
performance was observed in device using CzPO2 as host,
which was turned on at 5.4 V (corresponding to 1 cd mꢀ2),
with the maximum LE of 10.2 cd Aꢀ1 and the maximum
EQE of 5.4%. In contrast, the single-layer device based on
CzPO1 and 3Cz showed poor performance with the max-
imum LE (3.8 cd Aꢀ1, 0.2 cd Aꢀ1), the maximum EQE
(2.0%, 0.1%), and the turn-on voltage (7.0 V, 13.6 V),
respectively. The increased driving voltage can be attributed
to the low electron mobility of CzPO1 and 3Cz. However,
efficiency of the device based on CzPO2 is much higher than
that of the devices based on CzPO1 and 3Cz, maybe because
of the bipolar transporting nature of CzPO2 that increasing
carrier balance in the emitting layer.
Figure 4. Current efficiency versus current density of device C
and device D. Inset: normalized EL spectra of device D.
To further improve the performance of the device on the
basis of CzPO2, a thin 30 nm TPBI electron-transporting
and exciton-confining layer was inserted between the light-
emitting layer and the cathode.12 Devices B with configura-
tions of ITO/PEDOT/PSS/host/FIrpic(10 wt %) (80 nm)/
TPBI(30 nm)/Cs2CO3 (2 nm)/Al (100 nm) have been
fabricated. After the insertion of the TPBI layer, the max-
imum LE of the CzPO2-based device was largely improved
to 23.6 cd Aꢀ1 and the corresponding maximum EQE to
12.2%, which was comparable to the vacuum-deposited
multilayer devices previously reported.7g,13
Furthermore, a two-color-based single-layer phosphores-
cent WOLED via solution process was fabricated by
codoping the blue phosphorescent dye FIrpic and the or-
ange phosphorescent dye bis[2-(9,9-diethyl-9H-fluoren-2-yl)-
1-phenyl-1H-benzoimidazol-N,C3]-iridium(acetylacetonate)
[(fbi)2Ir(acac)] into the emitting layer with the configuration
ITO/PEDOT/PSS/host/FIrpic (10 wt %)/(fbi)2Ir(acac)
(0.2 wt %)/Cs2CO3 (2 nm)/Al (100 nm) (device C). The
maximum efficiency was achieved in the CzPO2-based single-
layer device C with a LE of 21.4 cd Aꢀ1 and an EQE of 7.4%.
The best solution-processed phosphorescent WOLED per-
formance was achieved in devices D with a configuration of
ITO/PEDOT/PSS/CzPO2/FIrpic (10 wt %)/(fbi)2Ir(acac)
(0.2 wt %)/TPBI (30 nm)/Cs2CO3 (2 nm)/Al (100 nm),
which show the maximum LE of 33.8 cd Aꢀ1 and an EQE
of 12.0%. The performance of CzPO2-based device D is
among the highest for solution-processed phosphorescent
WOLEDs. Additionally, the electroluminescence (EL) spec-
tra of device D are show in Figure 4. The Commission
Internationale de L’Eclairage (CIE) coordinates of device
D remain almost invariable when the driving voltage goes
from 8 V (0.36, 0.45) to 14 V (0.35, 0.45).
In summary, we have designed and synthesized a novel
series of solution processable bipolar host materials by
incorporating carbazole and diphenylphosphine oxide
units into the star-shaped structure. Utilizing these com-
pounds as host materials, high-efficiency solution-pro-
cessed blue and white PHOLEDs have been achieved
because of their high triplet energies, excellent morpholo-
gical stability, and bipolar nature.
Acknowledgment. The research was supported by the
National Key Basic Research and Development Program of
China (Grant No. 2009CB623604) and the National Nat-
ural Science Foundation of China (Grant No.51073089).
Supporting Information Available. Experimental and
characterization details for all new compounds.This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
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