High-efficiency blue phosphorescent organic light-emitting diodes using a carbazole and carboline-based host material

Article abstract of DOI:10.1039/c3cc42569j

A novel bipolar host 9-(4-(9H-pyrido[2,3-b]indol-9-yl)phenyl)-9H-3, 9′-bicarbazole (pBCb2Cz) was prepared for high efficiency blue phosphorescent organic light-emitting diodes (PhOLEDs), a high triplet energy (E_T) material, employing electron-deficient α-carboline. pBCb2Cz (E_T = 2.93 eV) was effective as a host material for FIrpic- and FCNIrpic-based blue PhOLEDs, and highest quantum efficiencies of 23.0 and 16.2%, respectively, were achieved.

Full text of DOI:10.1039/c3cc42569j

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High-efficiency blue phosphorescent organic
light-emitting diodes using a carbazole and
carboline-based host material†
Sun Jae Kim,a Young Jae Kim,b Young Hoon Son,b Jung A. Hur,a Hyun Ah Um,a
Jicheol Shin,a Tae Wan Lee,a Min Ju Cho,a Jung Keun Kim,c Sunghoon Joo,c
Joong Hwan Yang,c Gee Sung Chae,c Kihang Choi,a Jang Hyuk Kwon*b and
Dong Hoon Choi*a
Cite this: Chem. Commun., 2013,
49, 6788
Received 9th April 2013,
Accepted 8th June 2013
DOI: 10.1039/c3cc42569j
www.rsc.org/chemcomm
A novel bipolar host 9-(4-(9H-pyrido[2,3-b]indol-9-yl)phenyl)-9H- carbazole is the most widely used hole-transport moiety because
3,90-bicarbazole (pBCb2Cz) was prepared for high efficiency blue of a high ET and rigid molecular framework, and can be easily
phosphorescent organic light-emitting diodes (PhOLEDs), a high substituted with various electron transport units such as diphe-
triplet energy (ET) material, employing electron-deficient a-carboline. nylphosphine oxide,21–23 oxadiazole,24 imidazole,25 and tria-
pBCb2Cz (ET = 2.93 eV) was effective as a host material for FIrpic- and zole26 for conversion to the bipolar host. In addition to
FCNIrpic-based blue PhOLEDs, and highest quantum efficiencies of suitable electronic properties and charge balance of blue PhO-
23.0 and 16.2%, respectively, were achieved.
LEDs, the thermal properties of host materials remain
a key consideration in their design, since long-term device
Phosphorescent organic light-emitting diodes (PhOLEDs) have stability is thought to be dependent on glass transition temper-
attracted considerable attention due to high-efficiency device atures (Tgs) of host materials.
performance in flat-panel display applications.1–4 However, the
In this communication, we demonstrate a new carbazole-
development of high-efficiency blue PhOLEDs still remains a based bipolar host material containing an a-carboline unit for
challenge because a larger energy bandgap and higher triplet the purposes of improving device performance and physical
energy are required for carrier transport materials and blue properties, due to its electron-deficient character. a-Carboline
dopants, when compared to red- and green-emitting layer mate- has a rigid structure and is known to be an excellent electron
rials.5,6 In addition to a large triplet energy (ET), and high carrier transport material in combination with an electron-deficient
mobility with balanced hole and electron transport, the practical pyrrolopyridine moiety in the host material.27 The unit has
use of blue host materials also necessitates thermal and mor- been combined with H-3,90-bicarbazole 2 as a hole-transporting
phological stability to improve efficiency and long-term lifetime. unit to yield 9-(4-(9H-pyrido[2,3-b]indol-9-yl)phenyl)-9H-3,90-
Various types of high ET materials have been synthesized as host bicarbazole (3, pBCb2Cz), a high ET host material. The corre-
materials for blue PhOLEDs. Triplet exciton confinement in the sponding fabricated blue-emitting PhOLEDs exhibited very
emission zone of a PhOLED is recognized as one of the more low-driving voltages, with highest external quantum efficiencies
important factors affecting device efficiency.7–10
In general, large bandgap-promoting moieties such as (ca. 6 wt%), respectively, are doped into pBCb2Cz.
carbazole, arylsilane, fluorene, dibenzothiophene, and dibenzo- The synthetic procedure for obtaining compound 3 is
(Zext) of 23.0 and 16.2% obtained when FIrpic and FCNIrpic
furan have been used in molecular structure design to obtain illustrated in Scheme 1. Detailed synthetic procedures and
high ET materials.11–20 Among the large bandgap materials, characterization data are described in the ESI.† a-Carboline-
containing 9-(4-bromophenyl)-9H-pyrido[2,3-b]indole (1) was
designed as an electron transport unit for charge balancing
in the emission zone and was synthesized by employing
a Department of Chemistry, Research Institute for Natural Sciences,
Korea University, 5 Anam-dong, Sungbuk-Gu, Seoul, 136-701 Republic of Korea.
E-mail: dhchoi8803@korea.ac.kr; Fax: +82-2-925-4284; Tel: +82-2-3290-3140
b Department of Information Display, Kyung Hee University, Dongdaemoon-gu,
Seoul, 130-701 Republic of Korea. E-mail: jhkwon@khu.ac.kr;
Fax: +82-2-961-9154; Tel: +82-2-961-0948
c LG Display, Co., LTD. 1007 Deogeon-ri, Wollong-myeon, Paju-si, Gyeonggi-do,
413-811 Republic of Korea
† Electronic supplementary information (ESI) available: Synthetic procedure,
NMR, CV, device fabrication, SCLC data, performance of deep blue PhOLED,
Scheme 1 Synthesis of pBCb2Cz as a host material for blue PhOLEDs.
etc. See DOI: 10.1039/c3cc42569j
c
6788 Chem. Commun., 2013, 49, 6788--6790
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Fig. 1 HOMO and LUMO distributions of pBCb2Cz calculated by using DFT with
the B3LYP 6-31G(d) level.
Fig. 3 Current density–voltage curves of hole (HOD) and electron (EOD) only
methods reported in the literature.28 9H-3,90-Bicarbazole (2) was
prepared as a hole transport moiety. Finally, 3 (pBCb2Cz) was
obtained through the Ullman coupling reaction of compounds 1
and 2, in which the bicarbazole and carboline subunits were
connected through a phenyl ring spacer via 1,4-substitution; this
particular connectivity imparts molecular rigidity to the host
material, thus inducing a high Tg in pBCb2Cz.
devices made of mCP (a) and pBCb2Cz (b).
Absorption, photoluminescence (PL), and low-temperature PL
emission spectra of the pBCb2Cz host material are shown in Fig. 2
and measured data are summarized in Table S1 (ESI†). The optical
bandgap calculated from the absorption edge of the absorption
spectrum of pBCb2Cz was 3.43 eV. PL emission of pBCb2Cz was
observed at 387 nm. The ET of pBCb2Cz was 2.93 eV, obtained
from the first phosphorescent emission peak (l = 423 nm) of the
low-temperature PL spectrum measured at 77 K.
Hole- and electron-only devices composed of pBCb2Cz and
1,3-bis(N-carbazolyl)benzene (mCP) were fabricated to compare
the hole and electron conductivity of host materials (see ESI†).
Fig. 3 shows the current density–voltage curves of hole- and
electron-only devices bearing pBCb2Cz and mCP. Hole and
electron current densities of pBCb2Cz were much higher than
those of mCP, which indicates that pBCb2Cz shows better
performance than mCP, in terms of charge transportation.
The HOMO and LUMO levels of pBCb2Cz were determined to
be suitable for hole and electron injection (Table S2, ESI†),
which accounted for the high hole and electron current den-
sities. Therefore, hole and electron transport properties of
pBCb2Cz were enhanced, resulting in high hole and electron
current densities in the hole- and electron-only devices.
Charge carrier mobilities of mCP and pBCb2Cz were also
determined by the space charge limited current (SCLC) method.
The hole and electron mobilities of pBCb2Cz were found to be
much higher than those of mCP. In particular, pBCb2Cz exhibited
better balanced mobilities compared to mCP (Fig. S3 in the ESI†).
First, to investigate the function of pBCb2Cz as a host material,
blue phosphorescent OLEDs using FIrpic (sky blue) and FCNIrpic
(deep blue) as emitting dopants were fabricated. The mCP-based
devices were also prepared to function as a control device (ET of
mCP = 2.90 eV). We employed TAPC as a hole-transporting layer
(HTL) and TmPyPB13 as an electron-transporting layer (ETL).
These materials possess a higher ET than FIrpic and FCNIrpic;
therefore, the suppression of triplet exciton quenching of FIrpic
and FCNIrpic at the interfaces between HTL, EML, and ETL could
be achieved. Due to the shallow HOMO level of pBCb2Cz, a small
hole injection barrier exists between HTL and EML, resulting in
effective hole injection to EML. A blue PhOLED was fabricated
with a structure of ITO (100 nm)/TAPC (45 nm)/FIrpic 6 wt%
doped pBCb2Cz or mCP (15 nm)/TmPyPB (40 nm)/LiF (1.5 nm)/Al
(100 nm). The electroluminescent spectra showed an emission
from FIrpic only, with no emission from neighboring materials
(Fig. S4 in the ESI†). The J–V–L and the power efficiency–
luminance (PE–L) characteristics are shown in Fig. 4, respectively.
Calculations of the highest-occupied molecular orbital
(HOMO), lowest-unoccupied molecular orbital (LUMO) energy
levels, and triplet energy were performed using density func-
tional theory (DFT) with the B3LYP functional. The 6-31G(d)
basis set was used for the calculation of molecular orbital
energy levels and total energies of molecules. Triplet energy
was calculated as the energy difference between the first excited
triplet state and the ground singlet state. All calculations were
performed using DMol3 implemented in the Materials Studio
6.0 package (Accelrys, Inc.). The calculated HOMO energy of
pBCb2Cz was estimated to be ꢀ5.17 eV, and the LUMO energy
was ꢀ1.69 eV (Fig. 1). We also calculated the ET to be 2.71 eV
using the same basis set, suggesting an effective triplet exciton
confinement of phosphorescent blue emitters in pBCb2Cz.
Thermal properties were measured using differential scan-
ning calorimetry (DSC). The Tg was clearly observed to be
146 1C, and it is reversible during heating and cooling cycles.
The decomposition temperature with 5% loss (Td) was esti-
mated to be 491 1C using thermogravimetric analysis (TGA)
(Fig. S1 in the ESI†). The molecular energy levels of the thin
films were determined using cyclic voltammetry. The HOMO
energy level of pBCb2Cz, as calculated from its oxidative onset
potential, was measured to be ꢀ5.69 eV versus ferrocene as the
standard (Fig. S2 in the ESI†). The LUMO energy level, as
calculated from the obtained HOMO level and optical band-
gap, was determined to be ꢀ2.26 eV. The LUMO and HOMO
levels are suitable for efficient electron and hole injections,
respectively.
Fig. 2 (A) UV-Vis absorption spectrum of pBCb2Cz in CHCl3 (a); photolumines-
cence (PL) spectrum of pBCb2Cz at 77 K (b); PL spectrum of pBCb2Cz at 298 K (c).
Inset: PL image of pBCb2Cz in CHCl3. (B) DSC thermogram of pBCb2Cz during
heating and cooling cycles.
c
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In conclusion, we have developed a novel high ET and high
Tg host material, pBCb2Cz (3), bearing an electron-deficient
a-carboline unit, for blue PhOLEDs. By using pBCb2Cz as a host
material, we successfully developed high-performance blue
OLEDs both with FIrpic and FCNIrpic. To our knowledge, these
performances are one of the highest levels among the existing
literature values. Our results unambiguously demonstrate that
a synthetic strategy combining carbazole and carboline sub-
units produces an effective molecular framework for the pre-
paration of high ET bipolar host materials, with high quantum
efficiencies above 20% achieved in blue PhOLEDs.
The authors acknowledge the financial support from the National
Research Foundation of Korea (NRF2012R1A2A1A01008797) and by
Key Research Institute Program (NRF20120005860). D. H. Choi is
particularly grateful for the support from LG display Co. Limited
(2012–2013).
Notes and references
Fig. 4 (A) J–V–L curves of PhOLEDs made of pBCb2Cz and mCP; (B) CE–L and
PE–L characteristics of blue PhOLEDs using pBCb2Cz (red spheres) and mCP (blue
spheres); 6 wt% FIrpic-doped devices.
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and electron mobility) of the pBCb2Cz host material could
support
a low roll-off performance of the device with
pBCb2Cz/FIrpic (Fig. 4B and Fig. S3, ESI†).
Regarding the deep blue PhOLED (FCNIrpic dopant
1000
in pBCb2Cz), the device showed an Zp
of 8.2 lm Wꢀ1
(20.7 cd Aꢀ1, Zext 15.6%) and Zpmax of 10.7 lm Wꢀ1 (21.5 cd Aꢀ1
ext 16.2%) (Fig. S6 and Table S3, ESI†). We attributed this great
,
Z
improvement to the enhanced electron and exciton confinement
in the devices as pBCb2Cz possesses a bipolar character with
high ET.
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value. Therefore, the device performance obtained using this new
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c
6790 Chem. Commun., 2013, 49, 6788--6790
This journal is The Royal Society of Chemistry 2013