Dr. Pavel Anzenbacher, Jr., Advisor
Organic light-emitting diodes (OLEDs) have attracted significant attention for the application in
next generation display technologies and solid state lightning due to their advantages, such as,
the large viewing angle, range of colors, high brightness, low power consumption etc. In OLEDs,
based on the type of emitter fluorescence or phosphorescence can be observed. If the emitter is
florescent then internal quantum efficiency of OLEDs is limited to 25%. Introducing
phosphorescent emitters to the OLEDs allows us to take advantage of spin statistics. The
presence of heavy metal facilitates the intersystem crossing due to the spin-orbit coupling
consequently the internal quantum efficiency can reach up to 100%.6 These type of OLEDs are
classified as Phosphorescent OLED (PhOLED). Because the transition metal complexes are not
the best conductors, in PhOLEDs, the phosphorescent emitter is often dispersed in a host
material, which is usually a better conductor and is capable of energy transfer to phosphorescent
emitters. In order to achieve efficient energy transfer; it is essential that the triplet energy of the
host should be larger than that of guest to prevent reverse energy transfer from guest to the host.
Moreover, efficient charge injection to active layer requires compatible energy levels of frontier
orbitals for adjacent layer since efficiency of an OLED is correlated to recombination efficiency
of electrons and holes. However, in order to obtain high triplet energy π-conjugation of molecule
should be confined which would inversely effect charge transport properties of molecule.
Another requirement for an efficient OLED is materials should be thermally stable to prevent the
degradation of devices.
In this present work, proposed compounds have been designed to display limited conjugation as
a result of large steric congestion and corresponding limited rotations. Introducing bulky
substituents also have improved the morphological properties of molecules, as expected. For
charge transport issue, electron withdrawing and/or electron donating moieties have been
incorporated for possible charge transporting hosts with high triplet energies. For blue
PhOLEDs, new compounds with high triplet energy levels have been synthesized and their
electronic, optical and thermal properties have been studied. Most of these compounds have
revealed that they are possible hosts for blue PhOLEDs. Furthermore, computational results of
some compunds have suggested that they can reveal ambipolar character which benefits charge
transport.