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After fluorescent and phosphorescent organic semiconductors, thermally
activated delayed fluorescence (TADF) emitters are considered to be the third generation
materials for OLED (organic light emitting diode) devices. While the fluorescent materials
rely on singlet recombination, limiting the internal quantum efficiency to 25%,
phosphorescent materials have emissions from the triplet states, thus increasing the
emission efficiencies drastically. However, phosphorescent materials consist of heavy
metals and are very expensive. Hence, TADF materials, with their unique properties and
high emission efficiencies, have drawn considerable interest for OLEDs. TADF molecules are
designed with small singlet-triplet energy separation, resulting in high rates of reverse
intersystem crossing. Thus the triplets can get converted to singlets, which then contribute
to the emission. In this talk I will discuss some fundamental studies on the excitons in TPA-
DCPP molecules which is a red-emitting TADF semiconductor. We used electro-absorption
(EA) measurements which has proved to be a very effective technique in probing excitons in
semiconductors. The EA spectra, as function of the bias parameters, can give various
information about the type of excitons, built in field etc. This requires rigorous analysis and
modelling of the obtained data. The EA spectra of TADF thin films were found to deviate
from the expected dependence on biasing voltages. I will present our inferences and how
that effects the device performances. |