With rapid advances in material synthesis and device performance, the long‐term stability of organic solar cells has become the main remaining challenge toward commercialization. An investigation of photodegradation in blend films of the donor polymer poly(3‐hexylthiophene) (P3HT) and the rhodanine‐flanked small molecule acceptor 5,5′‐[(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(2,1,3‐benzothiadiazole‐7,4‐diylmethylidyne)]bis[3‐ethyl‐2‐thioxo‐4‐thiazolidinone] (FBR) is presented in an ambient atmosphere. The photobleaching kinetics of the pure materials and their blends is correlated with the generation of radicals and triplet excitons using optical and magnetic resonance techniques. In addition, spin‐trapping methods are employed to identify reactive oxygen species (ROS). In films of P3HT, FBR, and the P3HT:FBR blend, superoxide is generated by electron transfer to molecular oxygen. However, it is found that the generation of singlet oxygen by energy transfer from the FBR triplet state is responsible for the poor stability of FBR and for the accelerated photodegradation at later times of the P3HT:FBR blend. In the early stage of degradation of the neat blend, it is protected from singlet oxygen by the fast donor–acceptor charge transfer, which competes with triplet exciton formation. These results provide initial input for a rational design of donor and acceptor materials through tuning the molecular singlet and triplet energy levels to prevent ROS‐related photodegradation.
The excitonic structure of single-wall carbon nanotubes
(SWCNTs)
is chirality dependent and consists of multiple singlet and triplet
excitons (TEs) of which only one singlet exciton (SE) is optically
bright. In particular, the dark TEs have a large impact on the integration
of SWCNTs in optoelectronic devices, where excitons are created electrically,
such as in infrared light-emitting diodes, thereby strongly limiting
their quantum efficiency. Here, we report the characterization of
TEs in chirality-purified samples of (6,5) and (7,5) SWCNTs, either
randomly oriented in a frozen solution or with in-plane preferential
orientation in a film, by means of optically detected magnetic resonance
(ODMR) spectroscopy. In both chiral structures, the nanotubes are
shown to sustain three types of TEs. One TE exhibits axial symmetry
with zero-field splitting (ZFS) parameters depending on SWCNT diameter,
in good agreement with the tighter confinement expected in narrower-diameter
nanotubes. The ZFS of this TE also depends on nanotube environment,
pointing to slightly weaker confinement for surfactant-coated than
for polymer-wrapped SWCNTs. A second TE type, with much smaller ZFS,
does not show the same systematic trends with diameter and environment
and has a less well-defined axial symmetry. This most likely corresponds
to TEs trapped at defect sites at low temperature, as exemplified
by comparing SWCNT samples from different origins and after different
treatments. A third triplet has unresolved ZFS, implying it originates
from weakly interacting spin pairs. Aside from the diameter dependence,
ODMR thus provides insights in both the symmetry, confinement, and
nature of TEs on semiconducting SWCNTs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.