163 research outputs found
An experimental and computational study of donorâlinkerâacceptor block copolymers for organic photovoltaics
Block copolymers with donor and acceptor conjugated polymer blocks provide an approach to dictating the donorâaccepter interfacial structure and understanding its relationship to charge separation and photovoltaic performance. We report the preparation of a series of donorâlinkerâacceptor block copolymers with poly(3âhexylthiophene) (P3HT) donor blocks, poly((9,9âdioctylfluorene)â2,7âdiylâaltâ[4,7âbis(thiophenâ5âyl)â2,1,3âbenzothiadiazole]â2âČ,2âłâdiyl) (PFTBT) acceptor blocks, and varying lengths of oligoâethylene glycol (OEG) chains as the linkers. Morphological analysis shows that the linkers increase polymer crystallinity while a combination of optical and photovoltaic measurements shows that the insertion of a flexible spacer reduces fluorescence quenching and photovoltaic efficiencies of solution processed photovoltaic devices. Density functional theory (DFT) simulations indicate that the linking groups reduce both charge separation and recombination rates, and block copolymers with flexible linkers will likely rotate to assume a nonplanar orientation, resulting in a significant loss of overlap at the donorâlinkerâacceptor interface. This work provides a systematic study of the role of linker length on the photovoltaic performance of donorâlinkerâacceptor block copolymers and indicates that linkers should be designed to control both the electronic properties and relative orientations of conjugated polymers at the interface.
Molecular weight dependent structure and charge transport in MAPLEâdeposited poly(3âhexylthiophene) thin films
In this work, poly(3âhexylthiophene) (P3HT) films prepared using the matrixâassisted pulsed laser evaporation (MAPLE) technique are shown to possess morphological structures that are dependent on molecular weight (MW). Specifically, the structures of low MW samples of MAPLEâdeposited film are composed of crystallites/aggregates embedded within highly disordered environments, whereas those of high MW samples are composed of aggregated domains connected by long polymer chains. Additionally, the crystallite size along the sideâchain (100) direction decreases, whereas the conjugation length increases with increasing molecular weight. This is qualitatively similar to the structure of spinâcast films, though the MAPLEâdeposited films are more disordered. Inâplane carrier mobilities in the MAPLEâdeposited samples increase with MW, consistent with the notion that longer chains bridge adjacent aggregated domains thereby facilitating more effective charge transport. The carrier mobilities in the MAPLEâdeposited simples are consistently lower than those in the solventâcast samples for all molecular weights, consistent with the shorter conjugation length in samples prepared by this deposition technique. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 652â662The structure of MAPLEâdeposited P3HT thin films is shown to possess molecular weight (MW) dependence behavior. The MAPLE films deposited from low MW materials consist of crystallite domains embedded within a highly amorphous environment, whereas those deposited from high MW materials are composed of long polymer chains bridging the aggregate domains. The inâplane mobility is shown to increase with MW, highlighting the importance of domain connectivity in facilitating charge transport in conjugated polymers.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142537/1/polb24588-sup-0001-suppinfo1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142537/2/polb24588_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142537/3/polb24588.pd
Persistent Oscillations of X-ray Speckles: Pt (001) Step Flow
We have performed coherent x-ray scattering experiments on the hexagonally
reconstructed Pt (001) surface to study the temperature-dependent surface
dynamics. By correlating speckle patterns collected at the (001) anti-Bragg
position we are able to measure surface dynamics when the averaged incoherent
x-ray scattering appears static. In the temperature range above the rotational
epitaxy transition and below the roughening transition (1750 K - 1830 K), we
have observed well-defined oscillatory autocorrelations of speckles that
persist for tens of minutes, in addition to the expected thermal decorrelation.
The observed oscillations indicate surface dynamics due to "step-flow" motion.
This is shown with a simple model in which the phase of the scattered x-rays
from the steps within the illumination area is retained in the coherent x-ray
scattering. This demonstrates a possibility that x-ray speckles can be used to
monitor the real-space real-time evolution of surfaces in addition to the
traditional decorrelation measurements.Comment: 12 pages, 3 figure
Side chain engineering in indacenodithiophene- co -benzothiadiazole and its impact on mixed ionicâelectronic transport properties
Organic semiconductors are increasingly being decorated with hydrophilic solubilising chains to create materials that can function as mixed ionicâelectronic conductors, which are promising candidates for interfacing biological systems with organic electronics. While numerous organic semiconductors, including p- and n-type materials, small molecules and polymers, have been successfully tailored to encompass mixed conduction properties, common to all these systems is that they have been semicrystalline materials. Here, we explore how side chain engineering in the nano-crystalline indacenodithiophene-co-benzothiadiazole (IDTBT) polymer can be used to instil ionic transport properties and how this in turn influences the electronic transport properties. This allows us to ultimately assess the mixed ionicâelectronic transport properties of these new IDTBT polymers using the organic electrochemical transistor as the testing platform. Using a complementary experimental and computational approach, we find that polar IDTBT derivatives can be infiltrated by water and solvated ions, they can be electrochemically doped efficiently in aqueous electrolyte with fast doping kinetics, and upon aqueous swelling there is no deterioration of the close interchain contacts that are vital for efficient charge transport in the IDTBT system. Despite these promising attributes, mixed ionicâelectronic charge transport properties are surprisingly poor in all the polar IDTBT derivatives. Albeit a âânegativeââ result, this finding clearly contradicts established side chain engineering rules for mixed ionicâelectronic conductors, which motivated our continued investigation of this system. We eventually find this anomalous behaviour to be caused by increasing energetic disorder in the polymers with increasing polar side chain content. We have investigated computationally how the polar side chain motifs contribute to this detrimental energetic inhomogeneity and ultimately use the learnings to propose new molecular design criteria for side chains that can facilitate ion transport without impeding electronic transport
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Structural and dynamic disorder, not ionic trapping, controls charge transport in highly doped conducting polymers
Doped organic semiconductors are critical to emerging device applications,
including thermoelectrics, bioelectronics, and neuromorphic computing devices.
It is commonly assumed that low conductivities in these materials result
primarily from charge trapping by the Coulomb potentials of the dopant
counter-ions. Here, we present a combined experimental and theoretical study
rebutting this belief. Using a newly developed doping technique, we find the
conductivity of several classes of high-mobility conjugated polymers to be
strongly correlated with paracrystalline disorder but poorly correlated with
ionic size, suggesting that Coulomb traps do not limit transport. A general
model for interacting electrons in highly doped polymers is proposed and
carefully parameterized against atomistic calculations, enabling the
calculation of electrical conductivity within the framework of transient
localisation theory. Theoretical calculations are in excellent agreement with
experimental data, providing insights into the disordered-limited nature of
charge transport and suggesting new strategies to further improve
conductivities
Structural and dynamic disorder, not ionic trapping, controls charge transport in highly doped conducting polymers
Doped organic semiconductors are critical to emerging device applications,
including thermoelectrics, bioelectronics, and neuromorphic computing devices.
It is commonly assumed that low conductivities in these materials result
primarily from charge trapping by the Coulomb potentials of the dopant
counter-ions. Here, we present a combined experimental and theoretical study
rebutting this belief. Using a newly developed doping technique, we find the
conductivity of several classes of high-mobility conjugated polymers to be
strongly correlated with paracrystalline disorder but poorly correlated with
ionic size, suggesting that Coulomb traps do not limit transport. A general
model for interacting electrons in highly doped polymers is proposed and
carefully parameterized against atomistic calculations, enabling the
calculation of electrical conductivity within the framework of transient
localisation theory. Theoretical calculations are in excellent agreement with
experimental data, providing insights into the disordered-limited nature of
charge transport and suggesting new strategies to further improve
conductivities
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