Core Structures of Electron-Transport Materials
COMMUNICATION
bandgap is decreased in TSPO1, containing the silicon unit.
The LUMO is most stabilized in TSPO1 due to the silicon
unit, leading to a reduced bandgap.
The triplet energy of the four compounds was calculated
from the first emission peak of low-temperature (77 K) PL
spectra. The triplet energies of TSPA and TCPA are 2.90
and 2.92 eV, whereas those of TSPO1 and TCPO1 are 3.36
and 3.38 eV, respectively. Tetraphenylsilane-based, charge-
transport materials exhibit lower triplet energy than the tet-
raphenylmethane-based material due to the extended conju-
gation. All four charge-transport materials show triplet
energy high enough for exciton blocking in deep-blue
PHOLEDs.
The HOMO and LUMO levels of the four materials were
measured by cyclic voltammetry (CV) and are summarized
in Table 1. The HOMO level of TCPA is À5.78 eV, whereas
that of TSPA is À5.82 eV. There is only 0.04 eV difference
between the HOMO levels of the two hole-transport materi-
als. However, the LUMO level is lowered by 0.15 eV in
TSPA due to the LUMO stabilization effect explained
above. The HOMO and LUMO levels of TCPA and TSPA
are suitable for hole injection and electron blocking in deep-
blue PHOLEDs.
TCPO1 and TSPO1 show deep HOMO levels of À6.75
and À6.79 eV, and LUMO levels of À2.29 and À2.52 eV due
to the electron-withdrawing phosphine oxide unit. As the
bandgap of the two electron-transport materials is large, be-
cause the conjugation is isolated by phosphine oxide unit,
the LUMO levels are rather high.
Figure 1. Molecular-simulation results of TCPA, TSPA, TCPO1, and
TSPO1.
To compare tetraphenylmethane and tetraphenylsilane as
core structures of high-triplet-energy, hole-transport materi-
als, hole-only devices of TCPA and TSPA were fabricated.
The structure of the hole-only devices was as follows:
indium tin oxide (ITO, 50 nm)/N,N’-diphenyl-N,N’-bis-[4-
(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4’-diamine
(DNTPD, 60 nm)/N,N’-di(1-naphthyl)-N,N’-diphenylbenzi-
dine (NPB, 5 nm)/TSPA or TCPA (30 nm)/Au. Figure 2a
shows the current density of hole-only devices of TCPA and
TSPA. The hole current density of TCPA was higher than
that of TSPA, indicating that TCPA is better than TSPA as
a hole-transport material. Because of the small differences
(0.04 eV) between the HOMO levels of TCPA and TSPA,
the high hole current density of TCPA can be explained by
the better hole-transport properties of TCPA. In the molec-
ular-orbital diagram, the HOMO is dispersed over the tri-
phenylamine unit in both materials and is little affected by
the linking unit, indicating that the different hole-transport
properties are not caused by the HOMO distribution. In-
stead, the different hole-transport properties can be ex-
plained by the molecular size of TCPA and TSPA. The
length of the carbon–carbon bond is shorter than that of
carbon–silicon and the atomic size of carbon is smaller than
that of silicon, indicating that the tetraphenylmethane core
is smaller than the tetraphenylsilane core. Therefore, the in-
termolecular packing is improved in TCPA, leading to
better hole-transport properties.[23]
Electron-only devices of TCPO1 and TSPO1 were fabri-
cated to compare tetraphenylmethane and tetraphenylsilane
as core structures of high-triplet-energy, electron-transport
materials. The device structure was as follows: ITO (50 nm)/
LiF (1 nm)/TSPO1 or TCPO1 (50 nm)/LiF (1 nm)/Al
(200 nm). Data for the electron-only devices of TCPO1 and
TSPO1 are shown in Figure 2b. The electron current density
was much higher in TSPO1 than in TCPO1. In contrast to
the data of the hole-only device, tetraphenylsilane was
better than tetraphenylmethane regarding electron trans-
The thermal stability of TCPA, TSPA, and TCPO1 was
analyzed by using a thermogravimetric analyzer and all
three compounds were stable up to 3508C (Figure S2 in the
Supporting Information).
Molecular simulation of the four materials was carried
out to correlate the molecular-orbital distribution with pho-
tophysical properties of the materials. A suite of the Gaussi-
an 03 program and the nonlocal density functional of
Beckeꢁs 3-parameters employing the Lee–Yang–Parr func-
tional (B3LYP) with the 6-31G* basis sets were used for the
simulation.[22] Figure 1 shows the HOMO and LUMO distri-
bution of the four materials. The HOMOs of TCPA and
TSPA are localized on the triphenylamine unit, whereas the
LUMOs are dispersed over the whole molecule. There was
little difference of the HOMO and LUMO distribution be-
tween TCPA and TSPA. In the case of TCPO1 and TSPO1,
the LUMOs are distributed over the phenyl unit connected
to the diphenylphosphine oxide, whereas the HOMOs are
dispersed over the phenyl units attached to the carbon or
silicon atoms, due to the strong electron-withdrawing char-
acter of the diphenylphosphine oxide. The HOMO and
LUMO distributions of TCPO1 and TSPO1 are similar. The
carbon and silicon atoms do not induce significant change to
the molecular-orbital distribution. Therefore, the HOMO
and LUMO difference between carbon and silicon com-
pounds originates from the LUMO stabilization effect of sil-
icon.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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