Published on the web March 19, 2013
383
Triphenylene-based Host Materials for Low-voltage,
Highly Efficient Red Phosphorescent Organic Light-emitting Diodes
Kazunori Togashi,1,4 Takuma Yasuda,1,2,3 and Chihaya Adachi*1,2,3
1Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
2International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
3Department of Applied Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
4Hodogaya Chemical Co., 45 Miyukigaoka, Tsukuba, Ibaraki 305-0841
(Received December 15, 2012; CL-121247; E-mail: adachi@cstf.kyushu-u.ac.jp)
Triphenylene-based host materials BDBF-TP and BDBT-TP
were synthesized for use in red phosphorescent organic light-
emitting diodes (PHOLEDs). The current-voltage characteristics
of hole- and electron-only devices revealed that BDBT-TP
exhibits better bipolar carrier transport properties than 4,4¤-
bis(carbazol-9-yl)-1,1¤-biphenyl and BDBF-TP. Red PHOLEDs
containing BDBF-TP or BDBT-TP as the host showed a lower
driving voltage, higher external quantum efficiency, and lower
efficiency roll-off at high current density.
Figure 1. Chemical structures of BDBF-TP and BDBT-TP.
These novel PH host materials are useful for low-voltage, highly
efficient red PHOLEDs.
BDBF-TP and BDBT-TP were synthesized by the Suzuki-
Miyaura coupling reaction13,14 and purified by sublimation
before characterization, as outlined in Scheme S1.22
Organic light-emitting diodes (OLEDs) are attracting con-
siderable attention because of their great potential for practical
applications in flat panel displays and lighting. Recently,
phosphorescent organic light-emitting diodes (PHOLEDs) have
been intensively investigated for the realization of a high
luminance efficiency, because their internal quantum efficiency
The novel PH host materials were designed to achieve high
carrier transport properties and ET values. Although triphenylene
is a planar condensed polycyclic aromatic unit, it has a high ET
value (2.9 eV). Dibenzofuran and dibenzothiophene also exhibit
high ET values (3.0 eV). In addition, triphenylene has been used
as a core unit in liquid crystals which shows high carrier
mobilities.15 Therefore, two dibenzofuran or dibenzothiophene
units were attached at the 2- and 7-positions of a triphenylene
core. The crude PH host materials were sublimed, and their
thermophysical properties were measured by differential scan-
ning calorimetry. Their optical and photophysical properties
were evaluated by photoemission spectrometer (Riken AC-3),
and ultraviolet-visible (UV-vis) absorption and photolumines-
cence (PL) spectroscopies. The thermophysical and optical prop-
erties of BDBF-TP and BDBT-TP are summarized in Table 1.16
Figure 2 shows the UV-vis absorption and PL spectra of
BDBF-TP and BDBT-TP in solution (THF) and as solid thin
films. The Tg values are 106 °C for BDBF-TP and 125 °C for
BDBT-TP, which are higher than those of the conventional PH
host materials CBP and mCP, indicating that BDBF-TP and
BDBT-TP have excellent thermal and morphological stabilities.
The LUMO levels for BDBF-TP (¹2.8 eV) and BDBT-TP
(¹2.9 eV) were estimated from their HOMO levels and energy
gaps (Eg), and indicate that they have higher electron affinities
than CBP and mCP. Phosphorescence (Phos) spectra of BDBF-
TP and BDBT-TP in THF at 77 K show the highest-energy
emission maxima at 495 and 497 nm, respectively, corresponding
to ET value of ca. 2.5 eV. The ET value is higher than that of the
typical red PH emitter tris(1-phenylisoquinolinolato-C2,N)irid-
ium(III) ([Ir(piq)3]),17 suggesting that BDBF-TP and BDBT-TP
should confine the triplet excitons of red PH emitters effectively.
Next, unipolar carrier transport hole-only devices (HODs)
and electron-only devices (EODs) were fabricated for compar-
©
can reach ca. 100%, corresponding to an external quantum
int
efficiency ©ext of ca. 20%. In contrast, ©int is limited to 25%
in fluorescent OLEDs, corresponding to ©ext of ca. 5%.1
However, efficiency roll-off at high current density caused by
triplet-triplet annihilation (TTA) of excitons is significant in
PHOLEDs.2-4 Improving the carrier balance in the emitting layer
and broadening the carrier recombination zone can suppress
such efficiency roll-off at high current density.5,6 Recently,
bipolar phosphorescent (PH) host materials containing hole-
transporting (donor) and electron-transporting (acceptor) moie-
ties have been developed to improve the carrier balance in the
emitting layer.7-9 PHOLEDs with double-emission layers have
also been developed to broaden the carrier recombination zone.5
For the optimization of device performance, PH host
materials need to possess a higher triplet energy level (ET) than
that of the guest emitters in order to confine triplet excitons in
the emitting layer. Furthermore, the host layers should have
balanced carrier transport properties with appropriate highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) levels. Finally, the emitting layers
should have a high glass-transition temperature (Tg) and show
good thermal stability. The conventional PH host materials 4,4¤-
bis(carbazol-9-yl)-1,1¤-biphenyl (CBP) and 1,3-bis(carbazol-9-
yl)benzene (mCP) have rather low Tg values (<100 °C), and
their thin films can crystallize easily,10-12 which reduces the
device lifetime.
Here, we report novel red PH host materials based on a
triphenylene frame, BDBF-TP and BDBT-TP (Figure 1), which
possess high Tg values and good carrier transport properties.
Chem. Lett. 2013, 42, 383-385
© 2013 The Chemical Society of Japan