30 research outputs found
Thermal Diffusivities of Functionalized Pentacene Semiconductors
We have measured the interlayer and in-plane (needle axis) thermal
diffusivities of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). The
needle axis value is comparable to the phonon thermal conductivities of
quasi-one dimensional organic metals with excellent pi-orbital overlap, and its
value suggests that a significant fraction of heat is carried by optical
phonons. Furthermore, the interlayer (c-axis) thermal diffusivity is at least
an order of magnitude larger, and this unusual anisotropy implies very strong
dispersion of optical modes in the interlayer direction, presumably due to
interactions between the silyl-containing side groups. Similar values for both
in-plane and interlayer diffusivities have been observed for several other
functionalized pentacene semiconductors with related structures.Comment: 9 pages, including 4 figures; submitted to Applied Physics Letter
Suppressing Bias Stress Degradation in High Performance Solution Processed Organic Transistors Operating in Air
Solution processed organic field effect transistors can become ubiquitous in flexible optoelectronics. While progress in material and device design has been astonishing, low environmental and operational stabilities remain longstanding problems obstructing their immediate deployment in real world applications. Here, we introduce a strategy to identify the most probable and severe degradation pathways in organic transistors and then implement a method to eliminate the main sources of instabilities. Real time monitoring of the energetic distribution and transformation of electronic trap states during device operation, in conjunction with simulations, revealed the nature of traps responsible for performance degradation. With this information, we designed the most efficient encapsulation strategy for each device type, which resulted in fabrication of high performance, environmentally and operationally stable small molecule and polymeric transistors with consistent mobility and unparalleled threshold voltage shifts as low as 0.1âV under the application of high bias stress in air
Conversion between Triplet Pair States Is Controlled by Molecular Coupling in Pentadithiophene Thin Films
In singlet fission (SF) the initially formed correlated triplet pair state, 1(TT), may evolve toward independent triplet excitons or higher spin states of the (TT) species. The latter result is often considered undesirable from a light harvesting perspective but may be attractive for quantum information sciences (QIS) applications, as the final exciton pair can be spin-entangled and magnetically active with relatively long room temperature decoherence times. In this study we use ultrafast transient absorption (TA) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to monitor SF and triplet pair evolution in a series of alkyl silyl-functionalized pentadithiophene (PDT) thin films designed with systematically varying pairwise and long-range molecular interactions between PDT chromophores. The lifetime of the (TT) species varies from 40 ns to 1.5 ÎŒs, the latter of which is associated with extremely weak intermolecular coupling, sharp optical spectroscopic features, and complex TR-EPR spectra that are composed of a mixture of triplet and quintet-like features. On the other hand, more tightly coupled films produce broader transient optical spectra but simpler TR-EPR spectra consistent with significant population in 5(TT)0. These distinctions are rationalized through the role of exciton diffusion and predictions of TT state mixing with low exchange coupling J versus pure spin substate population with larger J. The connection between population evolution using electronic and spin spectroscopies enables assignments that provide a more detailed picture of triplet pair evolution than previously presented and provides critical guidance for designing molecular QIS systems based on light-induced spin coherence
Site-selective measurement of coupled spin pairs in an organic semiconductor
From organic electronics to biological systems, understanding the role of
intermolecular interactions between spin pairs is a key challenge. Here we show
how such pairs can be selectively addressed with combined spin and optical
sensitivity. We demonstrate this for bound pairs of spin-triplet excitations
formed by singlet fission, with direct applicability across a wide range of
synthetic and biological systems. We show that the site-sensitivity of exchange
coupling allows distinct triplet pairs to be resonantly addressed at different
magnetic fields, tuning them between optically bright singlet (S=0) and dark
triplet, quintet (S=1,2) configurations: this induces narrow holes in a broad
optical emission spectrum, uncovering exchange-specific luminescence. Using
fields up to 60 T, we identify three distinct triplet-pair sites, with exchange
couplings varying over an order of magnitude (0.3-5 meV), each with its own
luminescence spectrum, coexisting in a single material. Our results reveal how
site-selectivity can be achieved for organic spin pairs in a broad range of
systems.Comment: 8 pages, article, 7 pages, supporting informatio
The effect of ring expansion in thienobenzo[b]indacenodithiophene polymers for organic field-effect transistors
A fused donor, thienobenzo[b]indacenodithiophene (TBIDT), was designed and synthesized using a novel acid-promoted cas-cade ring closure strategy, and copolymerized with a benzothiadiazole (BT) monomer. The backbone of TBIDT is an expan-sion of the well-known indacenodithiophene (IDT) unit and was expected to enhance the charge carrier mobility, by improving backbone planarity and facilitating short-contacts between polymer chains. However, the optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm2 Vâ1sâ1, lower than the performance of IDT-BT (~1.5 cm2 Vâ1sâ1). Mobilities extracted from time-resolved microwave conductivity (TRMC) measurements were consistent with the trend in hole mobilities in OFET devices. Scanning Tunneling Microscopy (STM) measurements and computational modelling illustrated that TBIDT-BT exhibits a less ordered microstructure in comparison to IDT-BT. This reveals that a regular side chain pack-ing density, independent of conformational isomers, is critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations indicated that TBIDT-BT, despite containing a larger, planar unit, showed less stabilization of planar backbone geometries, in comparison to IDT-BT. This is due to the reduced electrostatic stabilizing inter-actions between the peripheral thiophene of the fused core with the BT unit, resulting in a reduction of the barrier to rotation around the single bond. These insights provide a greater understanding of the general structure-property relationships required for semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with IDT-type units, guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect transistors
Strongly exchange-coupled triplet pairs in an organic semiconductor
From biological complexes to devices based on organic semiconductors, spin interactions play a key role in the function of molecular systems. For instance, triplet-pair reactions impact operation of organic light-emitting diodes as well as photovoltaic devices. Conventional models for triplet pairs assume they interact only weakly. Here, using electron spin resonance, we observe long-lived, strongly-interacting triplet pairs in an organic semiconductor, generated via singlet fission. Using coherent spin-manipulation of these two-triplet states, we identify exchange-coupled (spin-2) quintet complexes co-existing with weakly coupled (spin-1) triplets. We measure strongly coupled pairs with a lifetime approaching 3 ”s and a spin coherence time approaching 1 ”s, at 10 K. Our results pave the way for the utilization of high-spin systems in organic semiconductors.Gates-Cambridge Trust, Winton Programme for the Physics of Sustainability, Freie UniversitÀt Berlin within the Excellence Initiative of the German Research Foundation, Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from Nature Publishing Group at http://dx.doi.org/10.1038/nphys3908
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Adaptive responses of animals to climate change are most likely insufficient
Biological responses to climate change have been widely documented across taxa and regions, but it remains unclear whether species are maintaining a good match between phenotype and environment, i.e. whether observed trait changes are adaptive. Here we reviewed 10,090 abstracts and extracted data from 71 studies reported in 58 relevant publications, to assess quantitatively whether phenotypic trait changes associated with climate change are adaptive in animals. A meta-analysis focussing on birds, the taxon best represented in our dataset, suggests that global warming has not systematically affected morphological traits, but has advanced phenological traits. We demonstrate that these advances are adaptive for some species, but imperfect as evidenced by the observed consistent selection for earlier timing. Application of a theoretical model indicates that the evolutionary load imposed by incomplete adaptive responses to ongoing climate change may already be threatening the persistence of species. © 2019, The Author(s)
Adaptive responses of animals to climate change are most likely insufficient
Biological responses to climate change have been widely documented across taxa and regions, but it remains unclear whether species are maintaining a good match between phenotype and environment, i.e. whether observed trait changes are adaptive. Here we reviewed 10,090 abstracts and extracted data from 71 studies reported in 58 relevant publications, to assess quantitatively whether phenotypic trait changes associated with climate change are adaptive in animals. A meta-analysis focussing on birds, the taxon best represented in our dataset, suggests that global warming has not systematically affected morphological traits, but has advanced phenological traits. We demonstrate that these advances are adaptive for some species, but imperfect as evidenced by the observed consistent selection for earlier timing. Application of a theoretical model indicates that the evolutionary load imposed by incomplete adaptive responses to ongoing climate change may already be threatening the persistence of species.Peer reviewe
Synthesis and Properties of a Covalently Linked Angular Perylene Imide Dimer
Utilizing the unexplored chemistry of a monocarbon analog to perylene bisimide, a covalently linked angular perylene dimer was synthesized. On the basis of measured optical properties and molecular modeling, the spectral changes relative to a monomeric reference perylene can be explained by an angle-dependent oblique exciton coupling model. With a roughly trigonal interchromophore arrangement, the dimer building block is promising for larger, cyclic assemblies to mimic naturally occurring light harvesting complexes
Epitaxially Intergrown Conformational Polymorphs and a Mixed Water/Methanol Solvate of 5âČ-Deoxy-5âČ-iodoguanosine
5âČ-Deoxy-5âČ-iodoguanosine
(<b>I</b>) crystals
deposited from mixtures of water and methanol grow as nonsolvated
hybrids of conformational polymorphs (<b>Ia</b>, <b><b>Ib</b></b>) and as a mixed solvate (<b>Ic</b>). Some
solvent-free crystals are purely <b>Ia</b>, while others have
varying amounts of <b>Ib</b> epitaxially intergrown with <b>Ia</b>. In <b>Ia</b> and <b>Ib</b> the conformations
differ primarily by torsion about the C4âČâC5âČ
bond (guanosine numbering scheme), which dramatically affects the
iodine atom position. Powder diffraction and reconstructed reciprocal-lattice-slice
images had small peaks incompatible with <b>Ia</b>. Some solvent-free
crystals required lattices for both <b>Ia</b> and <b>Ib</b> to index all observable reflections. Unit-cell dimensions for <b>Ia</b> and <b>Ib</b> suggest the potential for epitaxial
intergrowth. Hydrogen-bond networks in <b>Ia</b> and <b>Ib</b> are essentially identical and result in double layers of molecules
in the <i>ab</i> plane, with layers of iodine at the layer
surfaces. The iodine layers of <b>Ia</b> and <b>Ib</b> are incompatible: in <b>Ia</b> adjacent iodine atom layers
interdigitate slightly, whereas in <b>Ib</b> they do not. Theoretical
calculations support the conclusion that at room temperature <b>Ia</b> is the thermodynamically more stable polymorph and that <b>Ib</b> represents a kinetic product