33 research outputs found
Multi-phase semicrystalline microstructures drive exciton dissociation in neat plastic semiconductors
The optoelectronic properties of macromolecular semiconductors depend
fundamentally on their solid-state microstructure. For example, the
molecular-weight distribution influences polymeric- semiconductor properties
via diverse microstructures; polymers of low weight-average molecular weight
(Mw) form unconnected, extended-chain crystals, usually of a paraffinic
structure. Because of the non-entangled nature of the relatively short-chain
macromolecules, this leads to a polycrystalline, one-phase morphology. In
contrast, with high-Mw materials, where average chain lengths are longer than
the length between entanglements, two-phase morphologies, comprised of
crystalline moieties embedded in largely unordered (amorphous) regions, are
obtained. We investigate charge photogeneration processes in neat regioregular
poly(3-hexylthiophene) (P3HT) of varying Mw by means of time-resolved
photoluminescence (PL) spectroscopy. At 10 K, PL originating from recombination
of long-lived charge pairs decays over microsecond timescales. Both the
amplitude and decay rate distribution depend strongly on Mw. In films with
dominant one-phase chain-extended microstructures, the delayed PL is suppressed
as a result of a diminished yield of photoinduced charges, and its decay is
significantly faster than in two-phase microstructures. However, independent of
Mw, charge recombination regenerates singlet excitons in torsionally disordered
chains forming more strongly coupled photophysical aggregates than those in the
steady-state ensemble, with delayed PL lineshape reminiscent of that in
paraffinic morphologies at steady state. We conclude that highly delocalized
excitons in disordered regions between crystalline and amorphous phases
dissociate extrinsically with yield and spatial distribution that depend
intimately upon microstructure.Comment: 19 pages, 4 figure
Effet de la microstructure sur les propriétés excitoniques des polymères semi-conducteurs semi-cristallins
Les polymères semi-conducteurs semicristallins sont utilisés au sein de diodes
électroluminescentes, transistors ou dispositifs photovoltaïques organiques. Ces matériaux peuvent être traités à partir de solutions ou directement à partir de leur
état solide et forment des agrégats moléculaires dont la morphologie dicte en grande
partie leurs propriétés optoélectroniques. Le poly(3-hexylthiophène) est un des polymères semi-conducteurs les plus étudiés. Lorsque le poids moléculaire (Mw) des
chaînes est inférieur à 50 kg/mol, la microstructure est polycristalline et composée
de chaînes formant des empilements-π. Lorsque Mw>50 kg/mol, la morphologie est
semicristalline et composée de domaines cristallins imbriquées dans une matrice de
chaînes amorphes.
À partir de techniques de spectroscopie en continu et ultrarapide et appuyé
de modèles théoriques, nous démontrons que la cohérence spatiale des excitons
dans ce matériau est légèrement anisotrope et dépend de Mw. Ceci nous permet
d’approfondir la compréhension de la relation intime entre le couplage inter et
intramoléculaire sur la forme spectrale en absorption et photoluminescence. De plus,
nous démontrons que les excitations photogénérées directement aux interfaces entre
les domaines cristallins et les régions amorphes génèrent des paires de polarons liés
qui se recombinent par effet tunnel sur des échelles de temps supérieures à 10ns. Le
taux de photoluminescence à long temps de vie provenant de ces paires de charges
dépend aussi de Mw et varie entre ∼10% et ∼40% pour les faibles et hauts poids
moléculaires respectivement. Nous fournissons un modèle permettant d’expliquer
le processus de photogénération des paires de polarons et nous élucidons le rôle de
la microstructure sur la dynamique de séparation et recombinaison de ces espèces.Microstructure plays a crucial role in defining the optoelectrical properties of
conjugated polymeric semiconductors which can be used in light harvesting and
generating devices such as organic light emitting diodes, field effect transistors
or photovoltaic devices. These polymers can be processed from solution or solidstate
and form photophysical aggregates, consequently providing a complex network
which controls the fate of any photogenerated species. poly(3-hexylthiopene)
is one of the most studied polymeric semiconductor. In this material, the molecular
weight (Mw) of the polymer governs the microstructure and highly impact
the optical and electronic properties. Below Mw≈ 50 kg/mol, the polymer chains
forms polycrystalline domains of π-stacked molecules while high Mw (>50 kg/mol)
consists of a two-phase morphology of molecularly ordered crystallites that are embedded
in amorphous regions. Such morphology provides a bidimensionnal network
hosting both neutral excitations, known as Frenkel excitons, and polarons.
By means of steady-state and ultrafast spectroscopy experiment and backed up
theoretical modeling, we demonstrate that the spatial coherence of such excitations
are anisotropic in the lattice and depends on the Mw of the polymer, providing a
deep understanding of the interplay between interchain (excitonic) and intrachain
coupling in polymer aggregates. Moreover, we show that direct excitation at the
interface between molecularly ordered and amorphous regions generates tightlybound
charge pairs which decay via quantum tunneling over >10 ns. The yield
of delayed photoluminescence arising from the recombination of those charge pairs
varies between ∼10% and ∼40% for low and high Mw films respectively. We provide
a quantitative model that describes the photogeneration process of those geminate
polaron pairs and determine the role of the microstructure in the charge separation
and recombination processes
Recommended from our members
Documentation and analysis of the development of the Camp Greenough Environmental Education Center, an alternative public school at Yarmouth, Massachusetts.
Charge separation by photoexcitation in semicrystalline polymeric semiconductors: An intrinsic or extrinsic mechanism?
We probe charge photogeneration and subsequent recombination dynamics in neat
regioregular poly(3-hexylthiophene) films over six decades in time by means of
time-resolved photoluminescence spectroscopy. Exciton dissociation at 10K
occurs extrinsically at interfaces between molecularly ordered and disordered
domains. Polaron pairs thus produced recombine by tunnelling with distributed
rates governed by the distribution of electron-hole radii. Quantum-chemical
calculations suggest that hot-exciton dissociation at such interfaces results
from a high charge-transfer character.Comment: 10 pages, 3 figure
Pelvis biomechanics during wheelchair propulsion: a state of the art
ESS 2022, 7th European Seating Symposium, Dublin, IRLANDE, 14-/06/2022 - 17/06/2022For wheelchair users, pelvis represents the basis of stability for movement and the soft tissues in this area can sustain pressure sore, it is important tounderstand its behavior