Chemical Composition of Additives That Spontaneously
Form Cathode Interlayers in OPVs
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Abstract
Interlayers
between the active layer and the electrodes in organic
devices are known to modify the electrode work function and enhance
carrier extraction/injection, consequently improving device performance.
It was recently demonstrated that chemical interactions between the
evaporated electrode and interlayer additive can induce additive migration
toward the metal/organic interface to spontaneously form the interlayer.
In this work we used P3HT:PEG blends as a research platform to investigate
the driving force for additive migration to the organic/metal interface
and the source of the work function modification in OPVs. For this
purpose PEG derivatives with different end groups were blended with
P3HT or deposited on top of P3HT layer, topped with Al or Au evaporated
electrodes. The correlation between the additive chemical structure,
the <i>V</i><sub>oc</sub> of corresponding devices, and
the metal/organic interface composition determined by XPS revealed
that the driving force for additive migration toward the blend/metal
interface is the chemical interaction between the additives’
end group and the deposited metal atoms. Replacing the PEG additives
with alkyl additives bearing the same end groups has shown that the
Al work function is actually modulated by the PEG backbone. Hence,
in this work we have identified and separated between structural features
controlling the migration of the interlayer additive to the organic/metal
interface and those responsible for the modification of the metal
work function