Organic light emitting diodes (OLED) are efficient light sources based on
organic semiconductors. Unlike inorganic LEDs which are more or less point
sources, OLED are planar light sources with up to 1 m2 in area.
By using organic materials, they are cheap to produce and economical to use.
The determination of triplet exciton energy levels is of interest for the
development of efficient OLED, based on the fact that electrical excitation
usually creates three times as many triplets as singlets. Additionally, the
knowledge of these energy levels is crucial for the design and choice of
emitter matrix materials and exciton blocking layers. These values are
normally determined by photoluminescence (PL) measurements in solution for
materials which show intersystem crossing (ISC) between singlet and triplet
states. For some materials, the triplet levels cannot be measured this way
because some materials prohibit ISC. In this work, a method is presented
which allows the determination of the energy levels using low-temperature
electroluminescence (EL) spectroscopy. The dependence on ISC is avoided by
creating triplets directly with electrical excitation and this allows to
measure a large class of organic materials. A low-temperature EL spectrum is
N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) in
a 3-phenyl-4-(1‘-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) matrix (TPD/TAZ
1:3) at 77 K. Triplet emission is only observed at very low charge carrier
density (0.5 μA/mm2). Quenching processes are analyzed using
combined EL and PL measurements and unipolar devices. Two factors can be the
cause of the quenching: A strong quenching based on a low concentration of
electrically activated impurities could explain the dependency. The other
explanation points to a quenching based on electrons in the emitting layer.
This might be explained with triplet-polaron quenching (TPQ). TPQ is
proportional to the charge carrier density and contributes the dominant part
to the quenching at low current densities.