A m -terphenyl-modifed sulfone derivative as a host material for high-efficiency blue and green phosphorescent OLEDs

Article abstract of DOI:10.1021/cm3006748

A m-terphenyl-modified sulfone derivative, BTPS was designed and prepared as a host material for phosphorescent OLEDs. By using BTPS, high power efficiencies of over 46 lm W¹ (EQE 22%) for blue and 105 lm W ¹ (EQE 28%) for green were

Full text of DOI:10.1021/cm3006748

Communication
pubs.acs.org/cm
A m-Terphenyl-Modifed Sulfone Derivative as a Host Material for
High-Efficiency Blue and Green Phosphorescent OLEDs
Hisahiro Sasabe,*,†,‡ Yuki Seino,† Masato Kimura,† and Junji Kido*,†,‡
†Department of Organic Device Engineering and ‡Research Center for Organic Electronics (ROEL), Graduate School of Science and
Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
S
* Supporting Information
KEYWORDS: organic light-emitting devices, phosphorescent material, host material, sulfone, high-efficiency
igh-efficiency organic light-emitting devices (OLEDs)
H
have received considerable attention for energy-saving
solid-state lighting and eco-friendly flat panel display
applications.1 For an energy-saving illumination light source
beyond the fluorescent tube, the use of phosphorescent OLED
technology is imperative because a phosphorescent emitter can
yield an internal quantum efficiency as high as 100%.2 In
phosphorescent OLEDs, a host material plays a key role in
determining OLED performances.3 Among the known host
materials, a host material with an electron-accepting moiety,
such as pyridine, pyrimidine, and phosphine-oxide, can realize
high performances in blue and green OLEDs.4 This type of
host can promote electron injection as well as electron
transport, creating an improved carrier balance of holes and
electrons in an emissive layer (EML).5
Figure 1. Absorption and photoluminescence (PL) spectra of BTPS
film.
the ground state. The single-point energy was calculated at the
corresponding RB3LYP 6-311+G(d,p) level. The calculated
highest occupied molecular orbital (HOMO) energy of BTPS
was estimated to be 6.58 eV, and the lowest unoccupied
molecular orbital (LUMO) energy was 1.88 eV. On the other
hand, as we reported previously, the corresponding HOMO/
LUMO energies of a common host material, N,N′-dicarbazolyl-
3,5-benzene (mCP), were estimated to be 5.80 and 1.26 eV,
respectively.4e The LUMO energy of BTPS was evaluated to be
0.62 eV deeper than that of mCP, probably because of the
strong electron-accepting nature of a sulfone moiety. This
result suggests that BTPS can promote electron injection,
creating superior carrier balance of holes and electrons in an
EML. We also calculated an ET from TD-DFT calculation at
the RB3LYP 6-31G(d) level using an optimized structure
mentioned above. An ET was evaluated to be 3.11 eV, which is
slightly lower than that of mCP (3.18 eV). These results
suggest an effective confinement of phosphorescent blue
emitter in BTPS.
In this regard, a sulfone-containing material is an attractive
candidate to present a great opportunity for a phosphorescent
host material, because a sulfone moiety possesses a strong
electron-accepting nature. Some sulfone-containing materials
have been used as an emitter6 and an electron transporter7 to
realize high-performance fluorescent OLEDs so far. On the
other hand, Hsu and co-workers have reported triphenylamine/
bisphenylsulfonyl-substituted fluorene material for red phos-
phorescent OLEDs.8 Most recently, Kim and co-workers have
developed a solution-processed blue phosphorescent OLED
with an external quantum efficiency (ηext) of 6.9% and a current
efficiency (ηc) of 12.9 cd A−1 at 100 cd m−2 using 4,4′-
bis(phenylsulfonyl)biphenyl (SO1) and a common blue
phosphorescent emitter iridium(III) bis[(4,6-difluorophenyl)-
pyridinate-N,C2′]picolinate (FIrpic).9 Even though sulfone-
containing derivatives can hold tremendous promises as a
host material in phosphorescent OLEDs, their performances
are limited and the full potential is yet to be explored. We have
already reported that m-terphenyl-modified carbazole derivative
(CzTP) has a high thermal stability and a triplet energy (ET) of
2.70 eV to realize high-performance blue and green OLEDs.10
In this communication, we introduce a m-terphenyl-modified
sulfone derivative, 5′,5⁗-sulfonyl-di-1,1′:3′,1″-terphenyl (BTPS,
Figure 1) as a host material for phosphorescent blue and green
OLEDs.
BTPS was prepared via a Suzuki coupling reaction of bis(3,5-
dichlorophenyl)sulfone11 with phenylboronic acid using
Pd2(dba)3/PCy3/K3PO4 catalyst system in 71% yield. The
characterization was established on the basis of 1H NMR, mass
spectrometry, and elemental analysis. The product was purified
by the train sublimation method before device fabrication. The
Prior to material preparation, we conducted the density
functional theory (DFT) calculations.4g,5 An optimized
structure was calculated at the RB3LYP 6-31G(d) level for
Received: March 1, 2012
Revised: April 3, 2012
Published: April 3, 2012
© 2012 American Chemical Society
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purity of BTPS thus obtained was confirmed to be >99.0% by
HPLC analysis.
The thermal properties were measured by differential
scanning calorimetry (DSC). The melting point (Tm) was
observed at 282 °C, and the glass transition temperature (Tg)
was not observed. The decomposition temperature with 5%
loss (Td5) was estimated at 385 °C by using a thermogravi-
metric analysis (TGA). The ionization potential (Ip) was
observed at 6.7 eV by a photoelectron yield spectroscopy
(PYS)12 under the vacuum (∼10−3 Pa). The electron affinity
(Ea) was estimated to be 2.9 eV by subtraction of the optical
energy gap (Eg) from the Ip. The phosphorescent spectrum was
measured by a streak camera at 5 K. The onset phosphor-
escence was observed at 2.79 eV for BTPS (Figure 1).
Therefore, BTPS is considered to be applicable for blue and
green phosphorescent OLEDs.
Prior to the device fabrication, photophysical properties of
5−15 wt % iridium(III) bis[(4,6-difluorophenyl)-pyridinate-
N,C2′]picolinate (FIrpic)-doped host films (30 nm) were
evaluated. The photoluminescent quantum yield (ηPL) was
measured under N2 flow using an integrating sphere, using an
excitation wavelength of 331 nm, and with a multichannel
spectrometer as the optical detector. ηPLs of BTPS/FIrpic films
were estimated to be 81 1% for 5 wt %-doped film, 71 1%
for 10 wt %-doped film, and 50 1% for 15 wt %-doped film,
respectively. The transient PL decay curve of 5 wt % FIrpic-
doped film exhibited almost single-exponential decay with a τp
of 1.4 μs at room temperature (see Supporting Information in
detail). These values are almost similar to that of the mCP/
FIrpic film,14 indicating effective suppression of the triplet
extion of FIrpic.
First, to investigate the function of BTPS as a host material,
several blue phosphorescent OLEDs using FIrpic as an emitter
were fabricated. We used 1,1-bis[4-[N,N-di(p-tolyl)amino]-
phenyl]cyclohexane (TAPC) as a hole-transporting layer
(HTL) and 3,3″,5,5″-tetra(3-pyridyl)-1,1′;3′,1″-terphenyl
(B3PyPB)13 as an electron-transporting layer (ETL); these
materials have higher ET than that of FIrpic. Therefore, the
triplet exciton quenching of FIrpic at the HTL/EML and/or
EML/ETL interface(s) can be minimized. On the other hand,
due to the deep Ip of BTPS (6.7 eV), there can be a large hole-
injection barrier between HTL/EML. For effective hole-
injection from TAPC (5.6 eV) to EML (BTPS and/or FIrpic,
6.2 eV), we used 4,4′,4″-tris(N-carbazolyl)triphenylamine
(TCTA, 5.8 eV), which has higher triplet energy than FIrpic,
between TAPC and EML.15 Then, we fabricated a blue
phosphorescent OLED with a structure of [ITO (130 nm)/
TAPC (15 nm)/TCTA (5 nm)/FIrpic 5−15 wt % doped
BTPS (10 nm)/B3PyPB (50 nm)/Liq (1 nm)/Al (100 nm)].
EL spectra showed an emission only from FIrpic with no
emission from neighboring materials (inset in Figure 2b). The
current density−voltage−luminance (J−V−L) and the power
efficiency−luminance (PE−L) characteristics are shown in
Figure 2a,b, respectively. An OLED with BTPS/15 wt %-FIrpic
showed a much reduced operating voltage of 3.33 V at 100 cd
m−2 and gave an ηp,100 of 46.0 lm W−1 (48.6 cd A−1, ηext 21.8%)
at 100 cd m−2 and 31.4 lm W−1 (39.4 cd A−1, ηext 17.7%) at
1000 cd m−2, respectively. The average performance was 43.7
lm W−1 at 100 cd m−2, which was derived from four devices.
These are among the best performances in FIrpic-based
OLEDs.13,16 In these series of OLEDs, higher doped OLEDs
showed reduced operating voltages. Therefore, electron
Figure 2. (a) J−V−L characteristics and (b) PE−L characteristics of
blue OLEDs using BTPS. 5 wt % (circle), 10 wt % (square), and 15 wt
% (triangle) FIrpic-doped device, respectively. Inset: EL spectra of the
devices.
injection from B3PyPB to EML is greatly enhanced by doping
of FIrpic.
We also tried to estimate green OLED performances using a
common green phosphohrescent emitter fac-tris(2-
phenylpyridyl)iridium(III) [Ir(ppy)3]. Prior to the device
fabrication, photophysical properties of 5−15 wt % Ir(ppy)3-
doped host films (30 nm) were evaluated. ηPLs of BTPS/
Ir(ppy)3 films were estimated to be 72 1% for 5 wt %-doped
film, 70 1% for 10 wt %-doped film, and 56 1% for 15 wt
%-doped film, respectively. The transient PL decay curve of the
5% Ir(ppy)3-doped film exhibited almost single-exponential
decay with a τp of 1.4 μs at room temperature.
A green phosphorescent OLED with a structure of [ITO
(130 nm)/TAPC (30 nm)/5−15 wt % Ir(ppy)3-doped BTPS
(10 nm)/B3PyPB (50 nm)/Liq (1 nm)/Al (100 nm)] was
fabricated. The PE−L characteristics are shown in Figure 3.
Compared with 5 wt %-doped OLED, higher doped OLEDs
showed a much reduced operating voltage of 3.0 V at 100 cd
m−2. Thus, carrier injection to EML is greatly enhanced by
doping of Ir(ppy)3. This result clearly shows that electro-
Figure 3. PE−L characteristics of green OLEDs using BTPS. 5 wt %
(circle), 10 wt % (square), and 15 wt % (triangle) Ir(ppy)3-doped
device, respectively. Inset: EL spectra of the devices.
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phosphorescence is generated by the guest carrier trapping
process in 10−15 wt % Ir(ppy)3-doped green OLEDs. Further,
these operating voltages are much lower than that with mCP
(3.2 V)10 and CzTP (3.6 V)10 and comparable that with mCaP
(2.9 V),4e probably because the electron-accepting nature of
BTPS can promote electron-injection in EML. The 10 wt
RISE) (Creating international research hub for advanced
organic electronics) of Japan Science and Technology Agency
(JST) and by KAKENHI (23750204).
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* Supporting Information
Synthetic procedure and characterization, transient phosphor-
escence decay curves, angular dependence of luminous intensity
in OLEDs, and summary of OLED performances (PDF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: h-sasabe@yz.yamagata-u.ac.jp, kid@yz.yamagata-u.ac.
jp.
Notes
(16) Chopra, N.; Lee, J.; Xue, J.; So, F. IEEE Trans. Electron Devices
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We would like to thank Dr. Yong-Jin Pu and Dr. Masakatsu
Hirasawa at Yamagata University for their helpful comments.
We greatly acknowledge the financial support in part by Japan
Regional Innovation Strategy Program by the Excellence (J-
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