50 research outputs found
Enhanced photogeneration of polaron pairs in neat semicrystalline donor-acceptor copolymer films via direct excitation of interchain aggregates
We investigate the photogeneration
of polaron pairs (PPs) in neat
films of the semicrystalline donor–acceptor semiconducting
copolymer PCPDTBT. Carefully selecting the solution-processing procedures,
we obtain films with different amounts of crystallinity and interchain
aggregation. We compare the photogeneration of PPs between the films
by monitoring their photoinduced absorption in ultrafast pump–probe
experiments, selectively exciting nonaggregated or aggregated polymer
chains. The direct photoexcitation of interchain π-aggregates
results in prompt (<100 fs) charge generation. Compared to the
case where nonaggregated chains are excited, we find an 8-fold increase
in the prompt PP to singlet-exciton ratio. We also show that highly
crystalline lamellar nanostructures not containing π-stacked
or any light-absorbing aggregates do not improve the efficiency of
PP photogeneration. Our results show that light absorption from interchain
aggregates is highly beneficial for charge photogeneration in semiconducting
polymers and should be taken into account when optimizing film morphologies
for photovoltaic devices
Self-Assembly of Supramolecular Triblock Copolymer Complexes
Four different poly(tert-butoxystyrene)-b-polystyrene-b-poly(4-vinylpyridine) (PtBOS-b-PS-b-P4VP) linear triblock copolymers, with the P4VP weight fraction varying from 0.08 to 0.39, were synthesized via sequential anionic polymerization. The values of the unknown interaction parameters between styrene and tert-butoxystyrene and between tert-butoxystyrene and 4-vinylpyridine were determined from random copolymer blend miscibility studies and found to satisfy 0.031<χS,tBOS<0.034 and 0.39<χ4VP,tBOS<0.43, the latter being slightly larger than the known 0.30<χS,4VP≤0.35 value range. All triblock copolymers synthesized adopted a P4VP/PS core/shell cylindrical self-assembled morphology. From these four triblock copolymers supramolecular complexes were prepared by hydrogen bonding a stoichiometric amount of pentadecylphenol (PDP) to the P4VP blocks. Three of these complexes formed a triple lamellar ordered state with additional short length scale ordering inside the P4VP(PDP) layers. The self-assembled state of the supramolecular complex based on the triblock copolymer with the largest fraction of P4VP consisted of alternating layers of PtBOS and P4VP(PDP) layers with PS cylinders inside the latter layers. The difference in morphology between the triblock copolymers and the supramolecular complexes is due to two effects: (i) a change in effective composition and, (ii) a reduction in interfacial tension between the PS and P4VP containing domains. The small angle X-ray scattering patterns of the supramolecules systems are very temperature sensitive. A striking feature is the disappearance of the first order scattering peak of the triple lamellar state in certain temperature intervals, while the higher order peaks (including the third order) remain. This is argued to be due to the thermal sensitivity of the hydrogen bonding and thus directly related to the very nature of these systems.
Effects of ventilatory pattern on exhaled nitric oxide in mechanically ventilated rabbits
Reaction of unsaturated uronic acid residues with mercuric salts. Cleavage of the hyaluronic acid disaccharide 2-acetamido-2-deoxy-3-O-(β-d-gluco-4-enepyranosyluronic acid)-d-glucose
Block copolymer morphologies in dye-sensitized solar cells: Probing the photovoltaic structure-function relation
We integrate mesostructured titania arrays into dye-sensitized solar cells by replicating ordered, oriented one-dimensional (1D) columnar and three-dimensional (3D) bicontinuous gyroid block copolymer phases. The solar cell performance, charge transport, and recombination are investigated. We observe faster charge transport in 1D "wires" than through 3D gyroid arrays. However, owing to their structural instability, the surface area of the wire arrays is low, inhibiting the solar cell performance. The gyroid morphology, on the other hand, outperforms the current state-of-the-art mesoporous nanoparticle films. © 2009 American Chemical Society