Polytriphenylamine derivatives as materials for hole transporting
layers in electroluminescent devices
Igor K. Yakushchenko, Mikhail G. Kaplunov, Oleg N. EÐmov, Mikhail Yu. Belov and
Sergei N. Shamaev
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka,
Moscow Region, Russia. E-mail: kaplunov=icp.ac.ru
Received 4th November 1998, Accepted 22nd January 1999
A novel, high T (ca. 185 ¡C) material for hole-transporting layers has been synthesized. Combination with a
g
1
,2,4-triazole derivative yields an e†ective blue light-emitting device.
One of the important tasks in the design of organic electrolu-
minescent (EL) devices is improving the thermal stability of
the employed materials to prevent the degradation due to
morphological changes of the amorphous organic layers.
Morphological changes may be promoted by rapid molecular
rimetry (DSC) analysis revealed glass transition temperatures
T \ 185 ¡C for PTA and 175 ¡C for PTA-Br which is much
g
higher than for known TPA oligomers.6 Thermogravimetric
analysis of PTA revealed its thermal stability at high tem-
peratures. Loss of mass was not observed up to 360 ¡C both in
argon atmosphere and in air. Films of PTA prepared by spin
casting from toluene solutions appeared amorphous under a
polarizing microscope. The Ðlms were transparent. The
absorption maxima are slightly shifted to longer wavelengths
compared to TPD (Fig. 2).
motion near the glass transition temperature (T ). Therefore
g
materials with high T are required for organic EL devices.
g
We report a novel high-T oligomeric material for hole-
transporting layers (HTL) in EL devices based on tri-
g
phenylamine (TPA). Dimeric TPA is
a
well-known
hole-transporting material, TPD, which shows excellent hole
injection and transport capability with good electron-blocking
capability at a HTL boundary but is not very stable due to its
Testing of PTA as hole-transporting layer in real
EL device
A light-emitting diode was fabricated with PTA as hole-
transporting layer and a novel 1,2,4-triazole derivative, DA-
BuTAZ,
low T .1,2 Di†erent derivatives of TPA were proposed as
g
materials for HTL.2h9 Oligomers of TPA with oligomerization
number n \ 2È5 were shown to have good holeÈtransporting
characteristics, and T for these materials is shown to increase
g
with increasing n from 60 ¡C for TPD to 140 ¡C for oligomer
with n \ 5.8,9
We have synthesized a novel material comprising a mixture
of TPA oligomers of the general formula
with oligomerization number n \ 7È11 (PTA). The scheme of
synthesis is shown in Fig. 1. PTA was prepared in two steps.
First, bromine-containing TPA oligomer, PTA-Br was pre-
pared. PTA-Br may also be considered as a material for HTL.
Finally, after debromination of PTA-Br, methyl-containing
oligomer, PTA, was obtained.
Properties of PTA and PTA-Br
The number average molar mass (M ) of PTA measured by
n
GPS against polystyrene reference standards is M \ 2330
n
(
which corresponds to an n value about 9) and polydispersity
is about 1.55. PTA is soluble in toluene and chloroform.
Melting temperature interval is measured to be 206È210 ¡C for
PTA and 183È187 ¡C for PTA-Br. Di†erential scanning calo-
Fig. 1 Synthesis of PTA.
Phys. Chem. Chem. Phys., 1999, 1, 1783È1785
1783
Yakushchenko, Igor K.
