10 research outputs found
Core-Fluorinated Naphthalene Diimides: Synthesis, Characterization, and Application in n‑Type Organic Field-Effect Transistors
A series
of difluoro- and tetrafluoro-substituted naphthalene diimides
(NDIs) were synthesized by halogen exchange reactions of corresponding
bromo-NDIs with CsF in dioxane. Two strong electron acceptor molecules <b>6</b> and <b>8</b> with low-lying LUMO energy levels of
−4.27 and −4.54 eV were obtained, starting from tetrafluoro-NDI.
Organic field-effect transistors (OFETs) based on these fluorinated
NDIs were fabricated by vapor deposition, exhibiting n-channel field-effect
character under ambient conditions with the highest mobility of 0.1
cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>
Threshold-Voltage Shifts in Organic Transistors Due to Self-Assembled Monolayers at the Dielectric: Evidence for Electronic Coupling and Dipolar Effects
The mechanisms behind the threshold-voltage
shift in organic transistors due to functionalizing of the gate dielectric
with self-assembled monolayers (SAMs) are still under debate. We address
the mechanisms by which SAMs determine the threshold voltage, by analyzing
whether the threshold voltage depends on the gate-dielectric capacitance.
We have investigated transistors based on five oxide thicknesses and
two SAMs with rather diverse chemical properties, using the benchmark
organic semiconductor dinaphthoÂ[2,3-b:2′,3′-<i>f</i>]ÂthienoÂ[3,2-<i>b</i>]Âthiophene. Unlike several
previous studies, we have found that the dependence of the threshold
voltage on the gate-dielectric capacitance is completely different
for the two SAMs. In transistors with an alkyl SAM, the threshold
voltage does not depend on the gate-dielectric capacitance and is
determined mainly by the dipolar character of the SAM, whereas in
transistors with a fluoroalkyl SAM the threshold voltages exhibit
a linear dependence on the inverse of the gate-dielectric capacitance.
Kelvin probe force microscopy measurements indicate this behavior
is attributed to an electronic coupling between the fluoroalkyl SAM
and the organic semiconductor
Microstructural Evolution of the Thin Films of a Donor–Acceptor Semiconducting Polymer Deposited by Meniscus-Guided Coating
Crucial to the development
and refinement of organic electronics
is a fundamental understanding of how deposition processes affect
the active material’s resulting microstructure in the thin
film. Meniscus-guided coating (MGC) methods are attractive because
of their amenability to high-throughput, industrially relevant continuous
processes like roll-to-roll deposition, but the mechanism of solid
film formation has not been elucidated and is valuable for the precise
control of thin-film morphology and thus ultimate device performance.
In this work, we investigate the microstructural evolution of thin
films of a diketopyrrolopyrrole–terthiophene donor–acceptor
polymer semiconductor using both <i>in situ</i> and <i>ex situ</i> X-ray diffraction methods. On the basis of a comparison
of disorder between the film bulk and the top surface and a depth
profiling of the out-of-plane orientation of crystallites, we find
that faster coating speeds introduce more disorder into the resulting
films because the stochastic nucleation of disordered crystallites
at the meniscus air–liquid interface becomes more dominant
than substrate-mediated nucleation. Our results suggest that there
exist three separate deposition regimesî—¸namely the shear-dominate,
disorder-dominate, and Landau–Levich–Derjaguin regimesrevealed
by observing both polymer alignment via dry film thickness and optical
dichroism, a property sensitive to the flow and shear fields. At low
coating speeds, the shear strain imparted upon the solution directly
induces polymer alignment, causing an increase in dichroism as a function
of coating speed. When solvent evaporation becomes too rapid as coating
speeds increase, a decrease in the dichroic ratio is observed before
the classical Landau–Levich–Derjaguin regime occurs
at the highest coating speeds, resulting in isotropic films. The preservation
of out-of-plane crystalline texture throughout the thickness of the
films is seen only for lower coating speeds, and a study of different
deposition temperatures similarly indicates that the lower overall
solvent evaporation is conducive to this process. Increased paracrystalline
disorder (i.e., peak broadening) is observed by grazing-incidence
wide-angle X-ray diffraction at the top interface of the dry films
and at higher coating speeds. Together, these results indicate that
more rapid solvent evaporation at higher coating speeds causes increased
disorder, which can cause the nucleation of misaligned crystallites,
affect the dichroic ratio, and may frustrate the alignment of polymer
molecules in the amorphous regions of the film. Because the polymer
studied and the deposition technique used are representative models,
these results are likely general for aggregating, semicrystalline
donor–acceptor polymers deposited with MGC
Chemical Vapor Deposition of High Quality Graphene Films from Carbon Dioxide Atmospheres
The realization of graphene-based, next-generation electronic applications essentially depends on a reproducible, large-scale production of graphene films <i>via</i> chemical vapor deposition (CVD). We demonstrate how key challenges such as uniformity and homogeneity of the copper metal substrate as well as the growth chemistry can be improved by the use of carbon dioxide and carbon dioxide enriched gas atmospheres. Our approach enables graphene film production protocols free of elemental hydrogen and provides graphene layers of superior quality compared to samples produced by conventional hydrogen/methane based CVD processes. The substrates and resulting graphene films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Raman microscopy, sheet resistance and transport measurements. The superior quality of the as-grown graphene films on copper is indicated by Raman maps revealing average G band widths as low as 18 ± 8 cm<sup>–1</sup> at 514.5 nm excitation. In addition, high charge carrier mobilities of up to 1975 cm<sup>2</sup>/(V s) were observed for electrons in transferred films obtained from a carbon dioxide based growth protocol. The enhanced graphene film quality can be explained by the mild oxidation properties of carbon dioxide, which at high temperatures enables an uniform conditioning of the substrates by an efficient removal of pre-existing and emerging carbon impurities and a continuous suppression and <i>in situ</i> etching of carbon of lesser quality being co-deposited during the CVD growth
Photo-Cross-Linkable Polymeric Optoelectronics Based on the [2 + 2] Cycloaddition Reaction of Cinnamic Acid
We report the synthesis of cinnamic
acid-functionalized conjugated
polymers, which are cross-linked via [2 + 2] cycloaddition by UV illumination,
reducing their solubility. The cross-linking reaction was investigated
by a combination of FTIR and optical spectroscopy, and an optimum
condition for the solubility modulation of thin films, a major challenge
in the solution-phase fabrication of layered optoelectronic devices,
was reached. As proof of concept, OLEDs were fabricated, using these
conjugated polymers as emissive layers
Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs
Polymorphism is pervasive
in molecular solids. While computational
predictions of the molecular polymorphic landscape have improved significantly,
identifying which polymorphs are preferentially accessed and experimentally
stable remains a challenge. We report a framework that correlates
short intermolecular contacts with polymorphic stability. The presence
of short contacts between neighboring molecules prevents structural
rearrangement and stabilizes the packing arrangement, even when the
stabilized polymorph is not enthalpically favored. In the absence
of such intermolecular short contacts, the molecules have added degrees
of freedom for structural rearrangement, and solid–solid polymorphic
transformations occur readily. Starting with a series of core-halogenated
naphthalene tetracarboxylic diimides, we establish this framework
with the packing polymorphs of more than 20 compounds, ranging from
molecular semiconductors to pharmaceutics and biological building
blocks. This framework, widely applicable across molecular solids,
can help refine computational predictions by identifying the polymorphs
that are kinetically stable
Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs
Polymorphism is pervasive
in molecular solids. While computational
predictions of the molecular polymorphic landscape have improved significantly,
identifying which polymorphs are preferentially accessed and experimentally
stable remains a challenge. We report a framework that correlates
short intermolecular contacts with polymorphic stability. The presence
of short contacts between neighboring molecules prevents structural
rearrangement and stabilizes the packing arrangement, even when the
stabilized polymorph is not enthalpically favored. In the absence
of such intermolecular short contacts, the molecules have added degrees
of freedom for structural rearrangement, and solid–solid polymorphic
transformations occur readily. Starting with a series of core-halogenated
naphthalene tetracarboxylic diimides, we establish this framework
with the packing polymorphs of more than 20 compounds, ranging from
molecular semiconductors to pharmaceutics and biological building
blocks. This framework, widely applicable across molecular solids,
can help refine computational predictions by identifying the polymorphs
that are kinetically stable
Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs
Polymorphism is pervasive
in molecular solids. While computational
predictions of the molecular polymorphic landscape have improved significantly,
identifying which polymorphs are preferentially accessed and experimentally
stable remains a challenge. We report a framework that correlates
short intermolecular contacts with polymorphic stability. The presence
of short contacts between neighboring molecules prevents structural
rearrangement and stabilizes the packing arrangement, even when the
stabilized polymorph is not enthalpically favored. In the absence
of such intermolecular short contacts, the molecules have added degrees
of freedom for structural rearrangement, and solid–solid polymorphic
transformations occur readily. Starting with a series of core-halogenated
naphthalene tetracarboxylic diimides, we establish this framework
with the packing polymorphs of more than 20 compounds, ranging from
molecular semiconductors to pharmaceutics and biological building
blocks. This framework, widely applicable across molecular solids,
can help refine computational predictions by identifying the polymorphs
that are kinetically stable
Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs
Polymorphism is pervasive
in molecular solids. While computational
predictions of the molecular polymorphic landscape have improved significantly,
identifying which polymorphs are preferentially accessed and experimentally
stable remains a challenge. We report a framework that correlates
short intermolecular contacts with polymorphic stability. The presence
of short contacts between neighboring molecules prevents structural
rearrangement and stabilizes the packing arrangement, even when the
stabilized polymorph is not enthalpically favored. In the absence
of such intermolecular short contacts, the molecules have added degrees
of freedom for structural rearrangement, and solid–solid polymorphic
transformations occur readily. Starting with a series of core-halogenated
naphthalene tetracarboxylic diimides, we establish this framework
with the packing polymorphs of more than 20 compounds, ranging from
molecular semiconductors to pharmaceutics and biological building
blocks. This framework, widely applicable across molecular solids,
can help refine computational predictions by identifying the polymorphs
that are kinetically stable
Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs
Polymorphism is pervasive
in molecular solids. While computational
predictions of the molecular polymorphic landscape have improved significantly,
identifying which polymorphs are preferentially accessed and experimentally
stable remains a challenge. We report a framework that correlates
short intermolecular contacts with polymorphic stability. The presence
of short contacts between neighboring molecules prevents structural
rearrangement and stabilizes the packing arrangement, even when the
stabilized polymorph is not enthalpically favored. In the absence
of such intermolecular short contacts, the molecules have added degrees
of freedom for structural rearrangement, and solid–solid polymorphic
transformations occur readily. Starting with a series of core-halogenated
naphthalene tetracarboxylic diimides, we establish this framework
with the packing polymorphs of more than 20 compounds, ranging from
molecular semiconductors to pharmaceutics and biological building
blocks. This framework, widely applicable across molecular solids,
can help refine computational predictions by identifying the polymorphs
that are kinetically stable