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

the source of the light emitted by the device.
When the electron recombines with the hole, it is
simply returning to its ground state. In doing so,
it releases energy in the form of a photon. The
higher the electron’s excited state, the shorter the
wavelength of the photon it emits when it
decays. This difference in energy between the
excited state and the ground state is a material’s
“bandgap”. The bandgap essentially determines
the frequency of a photon that a material will
emit. To create blue emitting materials, for
example, the bandgap must be quite large,
around 3.0eV.
There are two particular ways in which an
exciton decays. These processes are called
fluorescence and phosphorescence, and they
depend on the type of the exciton. The spins of
the ground state and excited state electrons
determine its type. The two states are referred to
as “singlet” and “triplet”. The relationship
between the spins of the electron in the excited
state and the electron in the ground state defines
whether an exciton is a triplet or a singlet. As
shown in Figure 2, if the spin of the excited
electron is symmetrical to that of the ground
state, the exciton is referred to as a triplet. If their
spins are out of phase, the electrons form a
singlet exciton. Triplet excitons are actually
much more abundant than singlets, typically
existing in a 3:1 ratio. This unequal formation
occurs because the wave function of a triplet is
lower in energy than a singlet, thus making
triplets more energetically favorable.
Fluorescence and phosphorescence are
reliant upon whether a singlet or triplet decays.
Fluorescence is the decay of singlet excitons,
occurs instantaneously, and ceases when the
source of excitation is removed.
Phosphorescence is the decay of triplet excitons
and occurs slowly over time- continuing after the
source of excitation has been removed.
Fluorescence is the preferred source for light
emission because the decay of a singlet is an
allowed process, but it has a problem. Since only
25% of the excitons are formed as singlets at
most, the device efficiency is limited.
Unfortunately, the amount of singlets created
cannot be controlled at this time. Emission from
fluorescent materials alone simply lacks the
efficiency to be used in devices that could
compete in today’s world market.
Phosphorescence also has a problem. It was
reported that “Triplet states in organic materials
were considered useless, since the energy of
triplets was believed to dissipate non-radiatively,
as heat” (OIDA, 2002). The decay of a triplet is
referred to as a “spin-forbidden” process. This
spin-forbidden aspect of phosphorescent
emission keeps fluorescent materials from being
efficient in OLEDs.
How can an OLED work if fluorescence by
itself does not measure up to efficiency
CMDITR Review of Undergraduate Research Vol. 1 No. 1 Summer 2004 51