Newly
synthesized perylene diimide dimers were investigated as
nonfullerene electron acceptors for organic solar cells. In particular,
two analogous positional isomers exhibiting twisted vs planar geometries
were prepared to make a direct comparison of their optical and electronic
properties. These properties were investigated to provide information
regarding the impact of the nonfullerene acceptor geometry on the
ultimate photovoltaic performance. The two isomers exhibited strikingly
different optical and photophysical properties in solution as well
as in film. The ultrafast spectroscopic investigation in solution
revealed the occurrence of charge transfer upon photoexcitation, which
takes place more efficiently in the planar isomer. This is also supported
by theoretical simulations. The planar conformation exhibits higher
aggregation in the neat film as well as in the blend. However, our
results suggest that the dominance of intramolecular charge transfer
in the planar isomer is the crucial factor in determining the improved
power conversion efficiency of organic solar cells.
We
developed a new optical method to determine the rate of reverse
intersystem crossing (k
rISC) in thermally
activated delayed fluorescent (TADF) organic chromophores using time-resolved
transient absorption spectroscopy. We successfully correlated the k
rISC of the TADF-chromophores with device performance.
Specifically, we focused on the external quantum efficiency (ηEQE) and the stability of the device at high brightness levels.
It is believed that by obtaining a large k
rISC one may reduce the possibility of triplet–triplet annihilation
(TTA) and increase the long-term stability of organic light emitting
diodes (OLEDs) devices at high brightness levels (ηEQE roll-off). In this contribution, we investigate the photophysical
mechanism in a series of TADF-chromophores based on carbazole or acridine
derivatives as donor moieties, and triazine or benzonitrile derivatives
as the acceptor moieties. We found a relationship between large k
rISC values and high ηEQE values
at low operating voltages for the TADF-chromophores investigated.
In addition, those chromophores with a larger k
rISC illustrated a smaller ηEQE roll-off (higher
stability) at high operating voltages. These features are beneficial
for superior OLEDs performing devices. Contrarily, we found that if
a chromophore has a k
rISC ≤ 105s–1 its ηEQE is ≤5%.
Such a small k
rISC suggests that there
is no TADF effect operating in these organic systems and the molecule
is not efficient in harvesting triplet excitons. Emission lifetime-based
methodologies for determining the k
rISC were included for comparison but failed to predict the devices performance
of the investigated TADF-chromophores to the same extent of our proposed
methodology.
In this study, two analogous perylene diimide (PDI) trimers, whose structures show rotatable single bond π−bridge connection (twisted) vs. rigid/fused π−bridge connection (planar), were synthesized and investigated. We show via...
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