CMDITR Review of Undergraduate Research - Pluto - University of ...

standards? The answer lies in doping
phosphorescent dyes into the organic layer. The
abundant organic material is known as the
“host.” The other much less profuse
phosphorescent dye is the “guest”. These two
parts are made to work together to harvest both
singlet and triplet excitons. An internal quantum
efficiency of 100% is theoretically possible.
The process of luminescence in an OLED
begins when voltage is applied. The cathode
injects electrons into the organic host/guest
mixture as the anode injects holes; excitons are
created as these charge carriers meet. Since the
host compound is more abundant, the majority of
the excitons are formed there. Exciton decay is
an extremely fast process in the fluorescent host.
It is so fast (10—15 seconds) that the guest
materials absorb nearly all of the host’s emitted
photons. The newly formed excitons in the guest
emit light via phosphorescence.
The guest material is specifically designed
to promote phosphorescence. Normally, the
decay of triplets is a spin-forbidden process. To
solve this problem, the guest is made of an
organic heavy metal-ligand complex. Large
atomic weight metals such as Osmium, Iridium,
or Platinum are used in these important
molecules. They possess the ability to give the
excited triplet electrons singlet-like character.
Spin-orbit coupling from the heavy metal ligand
complex produces the effect. This intersystem
crossing makes triplet decay an allowed process.
The guest material can, thus, phosphoresce and
produce very efficient emission.
Why is a host even needed if the guest
material actually produces the useful emission of
the device? Answer: The π-π interaction between
ligands in the guest molecules causes them to
form aggregates as they become more
concentrated. This quenches the light and
decreases device performance. The purpose of a
host is to dilute the guest enough to eliminate
this problem.
The objective of this research in particular,
is to develop host materials that fluoresce in the
UV-blue region and have higher triplet energy
levels than those of the guest. Such a difference
in bandgap will facilitate an exothermic energy
transfer from the host to the guest. The larger
bandgaps of the host will also ensure higher
triplet energy levels to prevent a back energy
transfer. The efficiency of light emission will
decrease if the triplet energy level of the host is
lower than that of the guest. As of now, creating
such host molecules is a very challenging task in
research. Most of the host materials available
now have relatively low triplet energy levels,
which are not suitable for the blue phosphors.
Figure 5. Chemical structure of host materials
A series of materials (Figure 5) have been
synthesized as hosts for the phosphorescent
metal complex. DiCzPF was synthesized by the
Ullman coupling reaction between 2,7-dibromo-
9,9-diphenyl fluorene and carbazole, while
DiCzBPF was synthesized by the Suzuki
coupling between 9,9-diphenyl fluorene
diboronic ester and 3,5-dicarbazole-1-bromo
benzene. Two phenyl groups at the 9 th position
of the fluorene give the molecules a certain
rigidity that keeps them from forming
aggregates.
Methods to determine whether a compound
would be a potentially good host material are to
take UV-Visible and Photoluminescent (PL)
Spectroscopy. UV-Vis indicates the wavelengths
of light a compound absorbs. A PL spectrum
shows the wavelengths that are emitted. The
onset of the UV spectrum can be used to
determine a material’s bandgap. The UV-Vis and
PL spectra comparing the emissions and
absorptions of the two compounds are shown
below (Figure 6).
Although DiCzBPF (tetraphenyl) has a
much longer conjugation length comparing with
DiCzPF (biphenyl), the onsets of their UV
spectra are very similar, indicating that these two
molecules have similar bandgaps (~3.3eV). Both
molecules have emission in the UV-blue region.
52 CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004