Organic photovoltaics (OPV) are close to reaching a landmark 20%
device efficiency.[1] One of the proposed reasons that OPVs have yet to
attain this milestone is their propensity toward triplet formation.
In this talk,[2] the small molecule donor, DRCN5T, is studied using a
variety of spectroscopy techniques, and blended with both fullerene and
non-fullerene acceptors. Specifically, picosecond and microsecond transient
absorption and Raman spectroscopies are focused on. Despite DRCN5T's ability
to achieve OPV efficiencies of over 10%,[3] it generates an unusually high
population of triplets. These triplets are primarily formed in amorphous
regions via back recombination from a charge transfer state. As such,
triplets have a dual role in DRCN5T device efficiency suppression: they both
hinder free charge carrier formation and annihilate those free charges that
do form.
Using microsecond transient absorption spectroscopy under oxygen conditions,
this triplet-charge annihilation (TCA) is directly observed as a general
phenomenon in a variety of DRCN5T: fullerene and non-fullerene blends. Since
TCA is usually inferred rather than directly observed, it is demonstrated
that this technique is a reliable method to establish the presence of TCA.
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