20 research outputs found
“Fibonacci’s Route” to Regioregular Oligo(3-hexylthiophene)s
We
describe a new synthetic approach to regioregular monodisperse
oligo(3-alkylthiophene)s allowing for simple separation of regioregular
material in gram quantities. The number of repeat units follows the
Fibonacci numbers up to a length of 21. In a small adaption of this
approach, introduction of a protecting group was used to synthesize
an oligo(3-hexylthiophene) with 36 repeating units, the longest regioregular
3-hexylthiophene oligomer synthesized to date
Thioalkyl-Substituted Benzothiadiazole Acceptors: Copolymerization with Carbazole Affords Polymers with Large Stokes Shifts and High Solar Cell Voltages
Copolymers of carbazole and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole
(dTBT) incorporating thioalkyl (−SR) and alkoxy (−OR)
solubilizing groups on the 2,1,3-benzothiazdiazole (BT) unit are synthesized
and compared. The introduction of −SR and −OR groups
onto the BT unit of the polymer was found to have different effects
on the electronic properties of the polymers as well as the conformation
of the polymer backbone. Large conformational changes between the
ground state (GS) and excited state (ES) geometries of the polymers
with −SR groups led to very large Stokes shifts of up to 224
nm. The polymer with −OR groups was found to have approximately
double the photovoltaic efficiency at ∼4% compared to the polymers
with −SR groups (PCE ∼ 2%). However, polymers with −SR
groups were found to give very high open circuit voltages (<i>V</i><sub>OC</sub>) of over 1 V. Changing the −SR chain
length from ethyl to dodecyl was found to have little influence on
the solar cell performance of the polymer or the magnitude of the
Stokes shift
[2.2.2.2Para]cyclophanetetraenes (PCTs): cyclic structural analogues of poly(p‑phenylene vinylene)s (PPVs) [version 2; peer review: 2 approved]
Background: Poly(p-phenylene vinylene)s (PPVs) and [2.2.2.2]paracyclophanetetraene (PCT) are both composed of alternating π-conjugated para-phenylene and vinylene units. However, while the former constitute a class of π-conjugated polymers that has been used in organic electronics for decades, the latter is a macrocycle that only recently revealed its potential for applications such as organic battery electrodes. The cyclic structure endows PCT with unusual properties, and further tuning of these may be required for specific applications. Methods: In this article, we adopt an approach often used for tuning the properties of PPVs, the introduction of alkoxy (or alkylthio) substituents at the phenylene units, for tuning the optoelectronic properties of PCT. The resulting methoxy- and methylthio-substituted PCTs, obtained by Wittig cyclisation reactions, are studied by UV-vis absorption, photoluminescence, and cyclic voltammetry measurements, and investigated computationally using the visualisation of chemical shielding tensors (VIST) method. Results: The measurements show that substitution leads to slight changes in terms of absorption/emission energies and redox potentials while having a pronounced effect on the photoluminescence intensity. The computations show the effect of the substituents on the ring currents and chemical shielding and on the associated local and global (anti)aromaticity of the macrocycles, highlighting the interplay of local and global aromaticity in various electronic states. Conclusions: The study offers interesting insights into the tuneability of the properties of this versatile class of π-conjugated macrocycles
Role of Molecular Weight Distribution on Charge Transport in Semiconducting Polymers
Model
semiconducting polymer blends of well-controlled molecular
weight distributions are fabricated and demonstrated to be a simple
method to control intermolecular disorder without affecting intramolecular
order or degree of aggregation. Mobility measurements exhibit that
even small amounts of low molecular weight material are detrimental
to charge transport. Trends in charge carrier mobility can be reproduced
by a simple analytical model which indicates that carriers have no
preference for high or low molecular weight chains and that charge
transport is limited by interchain hopping. These results quantify
the role of long polymer tie-chains and demonstrate the need for controlled
polydispersity for achieving high carrier mobilities
Electrooptical Spectroscopy of Uniaxially Aligned Polythiophene Films in Field-Effect Transistors
Charge carriers induced in field-effect
transistors based on a
uniaxially aligned polythiophene polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene)
(PBTTT), are investigated by electrooptical charge modulation spectroscopy
(CMS). We used a zone-casting deposition method for uniaxial alignment
of the nanoribbon crystalline domains of the polymer and examined
the optical anisotropy of neutral and charge induced absorptions in
FETs. We find that the charge-induced optical absorptions of the polaronic
carriers at the interface exhibit a similar degree of uniaxial anisotropy
as the absorption spectrum of the neutral polymer bulk suggesting
similar degree of polymer alignment at the interface compared with
the bulk. We observe subtle differences in the spectral shape of the
polaron absorption polarized along and perpendicular to the polymer
chain direction. We also detect an additional charge-induced absorption
peak appearing at high charge carrier concentrations, which is similar
to the charge-induced absorption that is typical for chemically doped
films. These observations provide important insight into the interplay
between polaron transport and polymer microstructure
Droplet Flow Techniques for the Scalable Production of Electronic Materials
<p>Poster presented by James H Bannock and Thomas W Phillips at 2nd SCI/RSC Symposium on Continuous Processing and Flow Chemistry, Novartis, Horsham, UK on 24th September 2013.</p
Conjugated Copolymers of Vinylene Flanked Naphthalene Diimide
We report the synthesis of a novel
naphthalene diimide (NDI) monomer
containing two (tributylstannyl)vinyl groups. The utility of
this building block is demonstrated by its copolymerization with five
different electron-rich comonomers under Stille conditions. The resulting
high molecular weight polymers show red-shifted optical absorptions
in comparison to the analogous polymers without the vinylene spacer
and a significant increase in the intensity of the low-energy intramolecular
charge transfer band. The polymers all exhibit ambipolar behavior
in bottom-gate, top-contact organic thin-film transistors. The insertion
of a solution-processed barium hydroxide layer between the polymer
and the gold electrode led to unipolar behavior with improved electron
mobilities
Influence of Side-Chain Regiochemistry on the Transistor Performance of High-Mobility, All-Donor Polymers
Three novel polythiophene isomers
are reported whereby the only
difference in structure relates to the regiochemistry of the solubilizing
side chains on the backbone. This is demonstrated to have a significant
impact on the optoelectronic properties of the polymers and their
propensity to aggregate in solution. These differences are rationalized
on the basis of differences in backbone torsion. The polymer with
the largest effective conjugation length is demonstrated to exhibit
the highest field-effect mobility, with peak values up to 4.6 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>
Activated Singlet Exciton Fission in a Semiconducting Polymer
Singlet exciton fission is a spin-allowed
process to generate two
triplet excitons from a single absorbed photon. This phenomenon offers
great potential in organic photovoltaics, but the mechanism remains
poorly understood. Most reports to date have addressed intermolecular
fission within small-molecular crystals. However, through appropriate
chemical design chromophores capable of intramolecular fission can
also be produced. Here we directly observe sub-100 fs activated singlet
fission in a semiconducting poly(thienylenevinylene). We demonstrate
that fission proceeds directly from the initial 1B<sub>u</sub> exciton,
contrary to current models that involve the lower-lying 2A<sub>g</sub> exciton. In solution, the generated triplet pairs rapidly recombine
and decay through the 2A<sub>g</sub> state. In films, exciton diffusion
breaks this symmetry and we observe long-lived triplets which form
charge-transfer states in photovoltaic blends
Comparison of Methods for Determining the Mechanical Properties of Semiconducting Polymer Films for Stretchable Electronics
This
paper describes a comparison of two characterization techniques
for determining the mechanical properties of thin-film organic semiconductors
for applications in soft electronics. In the first method, the film
is supported by water (film-on-water, FOW), and a stress–strain
curve is obtained using a direct tensile test. In the second method,
the film is supported by an elastomer (film-on-elastomer, FOE), and
is subjected to three tests to reconstruct the key features of the
stress–strain curve: the buckling test (tensile modulus), the
onset of buckling (yield point), and the crack-onset strain (strain
at fracture). The specimens used for the comparison are four poly(3-hexylthiophene)
(P3HT) samples of increasing molecular weight (<i>M</i><sub>n</sub> = 15, 40, 63, and 80 kDa). The methods produced qualitatively
similar results for mechanical properties including the tensile modulus,
the yield point, and the strain at fracture. The agreement was not
quantitative because of differences in mode of loading (tension vs
compression), strain rate, and processing between the two methods.
Experimental results are corroborated by coarse-grained molecular
dynamics simulations, which lead to the conclusion that in low molecular
weight samples (<i>M</i><sub>n</sub> = 15 kDa), fracture
occurs by chain pullout. Conversely, in high molecular weight samples
(<i>M</i><sub>n</sub> > 25 kDa), entanglements concentrate
the stress to few chains; this concentration is consistent with chain
scission as the dominant mode of fracture. Our results provide a basis
for comparing mechanical properties that have been measured by these
two techniques, and provide mechanistic insight into fracture modes
in this class of materials