1724 Chem. Mater. 2010, 22, 1724–1731
DOI:10.1021/cm903146x
Properties of Fluorenyl Silanes in Organic Light Emitting Diodes
Wei Wei, Peter I. Djurovich, and Mark E. Thompson*
Department of Chemistry, University of Southern California, Los Angeles 90089-0744
Received October 12, 2009. Revised Manuscript Received December 27, 2009
Multifluorenyl silanes have been studied as potential hosts for organic light emitting diodes. Four
molecules, (9,90-dimethylfluoren-2-yl)nSi(phenyl)4-n (SiFln, n = 1, 2, 3, and 4), with an increasing
number of fluorene units have been synthesized and investigated. These compounds possess high
triplet energies (2.9 eV), large HOMO-LUMO gaps (∼5.2 eV), and high glass transition tempera-
tures. Their glass transition and sublimation temperatures increase linearly as the fluorene ratio
increases, but there are only small changes in their electrochemical or photophysical properties. These
studies suggest that the Si center helps maintain the high singlet and triplet energy levels of these
molecules. These materials exhibit ambipolar transport characteristics in undoped OLED devices,
and the charge conductivity of the devices was enhanced by increasing the fluorene ratios in the host
molecules. Compared with phenylsilanes, the fluorenylsilanes show better hole injecting and charge
transporting abilities. SiFl4 was investigated as a host material for red, green, and blue phosphor-
escent devices, giving peak efficiencies of 8, 8, and 3%, respectively.
Introduction
been shown to be chemically unstable in the devices, which
can ultimately lead to a shortened device lifetimes.7,8 Com-
pounds that have large energy band gaps and contain no
easily oxidized nitrogen donor groups, bis(triphenylsilyl)-
benzenes, have been developed as host materials, particu-
larly for blue phosphorescent emitters.9-13 Unfortunately,
these materials have low glass transition temperatures, and
the deep energy level of their HOMOs hinder hole injection
into heterojunction devices. Therefore, further investiga-
tions are still needed to develop host materials that can
improve the performance of OLEDs.
Ever since the doping strategy has been developed to
prevent self-quenching of the emissive molecules, the techni-
que has been widely utilized to optimize the efficiencies of
organic light emitting diode (OLED) devices.1 Typical
doped OLED devices consist of several discrete molecular
or polymeric layers: an electron transport layer (ETL), an
emissive layer (EML), and a hole transport layer (HTL).
While the ETL and HTL are used to inject and transport
electrons and holes into the EML, the EML is where the
charges recombine and form the dopant (emissive molecule)
excitions. In a doped device, a host material is also employed
in the EML to inhibit self-quenching by the dopant and to
confine the energy on the dopant molecules. The host plays
an important role in an OLED device because its energy
levels and charge mobilities determine 1) whether the holes
and electrons recombine at the emissive layer and 2) whe-
ther the emission can be confined into the dopant exclu-
sively. The host material used in an OLED thus has to be
energetically matched with a specific dopant. The most
common hosts for OLEDs are carbazole derivatives, i.e.
N,N0-dicarbazolyl-3,5-benzene (mCP)2-4 and 4, 40-N,N0-
dicarbazole-biphenyl (CBP).2,5,6 However, carbazoles have
relatively low glass transition temperatures (Tg) and have
Fluorene derivatives have been studied in OLEDs due to
their robust thermal stability, high charge transport mobilities,
large HOMO-LUMO gaps, and relatively high Tgs.14-17
Spirobifluorene oligomers (Fln, Scheme 1) have excellent
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*Corresponding author e-mail: met@usc.edu.
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