Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P50: Organic Electronics II: Structure and MorphologyFocus
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Sponsoring Units: DPOLY DMP Chair: Dean DeLongchamp, National Institute of Standards and Technology Room: BCEC 252B |
Wednesday, March 6, 2019 2:30PM - 2:42PM |
P50.00001: Raman Crystallography as a Spectroscopic Probe of Structure in Single Crystal Organic Semiconductors Adam Biacchi, Emily Geraldine Bittle, Lisa A. Fredin, Andrew Herzing, Thomas C. Allison, David James Gundlach, Angela Hight Walker Raman spectroscopy is a powerful technique to nondestructively investigate the vibrational modes of organic crystals. These phonons are spatially localized within the lattice and correlate to the underlying crystallographic structure of the material. This provides a straightforward means to investigate the crystallinity, orientation, composition, and phase with high spatial resolution and no sample preparation. The structure can also be related to important properties such as carrier mobility. Intermolecular motion is theorized to strongly affect the inherent charge transport of organic semiconductors, influencing the anisotropic character. Here we present a polarization-orientation Raman spectroscopy analysis of single-crystalline organic semiconductor tetracene. We find five distinct vibrational modes in the low-energy regime that are ascribed to intermolecular lattice phonons. Polar plots indicate that these motions are Ag modes, in agreement with group theory, directed within the ab plane of tetracene (001), as confirmed by diffraction and density functional theory modeling. Collectively, these data provide a clear picture of the Raman-active lattice motion of tetracene, which we subsequently relate to its anisotropic charge transport behavior. |
Wednesday, March 6, 2019 2:42PM - 2:54PM |
P50.00002: Phonons and anisotropic mobility in a single crystal organic semiconductor Emily Geraldine Bittle, Adam Biacchi, Lisa A. Fredin, Andrew Herzing, Thomas C. Allison, Angela Hight Walker, David James Gundlach Intermolecular phonons are theorized to cause transient localization of charge carriers which is a proposed limiting factor in achieving band-like mobilities in organic semiconductors. In this study we probe the molecular motions associated with phonons in single crystal tetracene within the high-mobility plane correlate with the anisotropy of the field-effect mobility. Phonons measured at low energy (< 25 meV) have associated molecule and lattice distortions that are directionally oriented within the crystal. Calculated changes to the HOMO level show a disruption of orbital overlap between molecules along the high mobility direction for one large amplitude phonon. The measured anisotropic high-to-low mobility ratio in the ab-plane of single crystal tetracene is lower than published static calculations of anisotropy suggesting that transient localization has directionality. |
Wednesday, March 6, 2019 2:54PM - 3:06PM |
P50.00003: Anomalous Pressure Dependence of the Electronic Properties of Molecular Crystals Explained by Changes in Intermolecular Electronic Coupling Maituo Yu, Xiaopeng Wang, Xiong-Fei Du, Bohdan Schatschneider, harald oberhofer, Noa Marom Optimization of the electronic properties of organic semiconductors is important for optoelectronic device applications. One method of tuning the electronic properties is applying external pressure on molecular crystals. A recent dispersion-inclusive density functional theory (DFT) investigation of the effect of pressure up to 20 GPa on herringbone polycyclic aromatic hydrocarbons (PAHs), has revealed anomalous pressure dependence of the electronic properties of six systems. Here, we use intermolecular electronic coupling values (Hab), calculated by fragment orbital DFT (FO-DFT) to elucidate the pressure dependence of the electronic properties. We show that discontinuities in the pressure dependence of the band gap and direct-indirect band gap transitions are correlated with discontinuities in the pressure dependence of the electronic coupling of certain dimers, even when no discontinuities are found in the pressure dependence of the lattice parameters. Hab is therefore a useful descriptor to gain insight into tuning the electronic properties of crystalline organic semiconductors. |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P50.00004: Exploring the impact of atomistic substitution on thin-film structure in a germanyl-ethynyl functionalized pentacene Jeni Sorli, Qianxiang Ai, Devin Granger, Chad Risko, John Anthony, Lynn Loo Functionalization of organic semiconductors through the attachment of bulky side groups to the conjugated core has imparted solution processability to this class of otherwise insoluble materials. A consequence of this functionalization is that the bulky side groups impact the solid-state packing of these materials. To examine the importance of side group electronic character on accessing the structural phase space of functionalized materials, germanium was substituted for silicon in triisopropylsilylethynyl-pentacene (TIPS-Pn) to produce triisopropylgermanylethynyl-pentacene (TIPGe-Pn), with the TIPGe side group comparable in size to TIPS, but higher in electron density. We find TIPGe-Pn single crystals to exhibit slip-stack, herringbone and brickwork packing depending on growth conditions, a stark contrast to TIPS-Pn, which only accesses the brickwork packing motif in both single crystals and thin films. Polycrystalline thin films of TIPGe-Pn exhibit two new, unidentified polymorphs from spin-coating and post-deposition annealing. Our experiments suggest that access to the structural phase space is not solely guided by the size of the side group. Its electronic character appears to play a significant role in dictating the accessible solid structures. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P50.00005: Investigation on charge transport properties of Cyclopentadithiophene-based D-A type semiconducting copolymers Jiyoul Lee, Jisang Hong We have studied the charge transporting properties in Cyclopentadithiophene (CDT)-based donor (D) – acceptor (A) type semiconducting copolymer, which is known to have high performance, as an active layer of a field-effect transistor. In order to investigate the relationship between the charge-carrier mobility and the structural property of each polymer, the CDT-based polymer thin-films were analyzed by ultraviolet-visible spectroscopy, Raman spectroscopy, and density function theory (DFT) simulation. From our experimental results, it was found that the charge transports in the CDT-based polymeric thin-film is more dependent on the efficient intramolecular transport than the intermolecular charge transports. These results suggest that improving the planarity and rigidity of the polymer backbone is essential for efficient intramolecular charge transfer in order to design a high performance D-A type semiconducting copolymer. |
Wednesday, March 6, 2019 3:30PM - 3:42PM |
P50.00006: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 3:42PM - 4:18PM |
P50.00007: Printing Conjugated Polymers to Order via Non-Equilibrium Assembly Invited Speaker: Ying Diao Controlled morphology evolution via directed assembly has played a central role in the development of modern electronic, optical and clean energy materials. In comparison to conventional ‘hard’ materials, polymer-based functional materials can be easily processed into diverse form factors by low-cost, high-throughput methods such as roll-to-roll printing and 3D printing. The printing conditions intimately couple with the assembly process and sensitively modulate the solid-state properties in the fabricated devices. We are particularly interested in semiconducting polymers which have demonstrated potential uses in a diverse range of applications from transistors, thermoelectrics, sensors, light-emitting diodes to solar cells etc. However, major challenges remain, in controlling the nucleation, growth and aggregation of conjugated polymers during solution printing and coating, which critically impact the printed device performance by orders of magnitude. The rapid printing process creates a complex environment with coupled physics that drive the polymer assembly far from equilibrium. |
Wednesday, March 6, 2019 4:18PM - 4:30PM |
P50.00008: Molecular Orientation in Thin Films of Poly(3-hexylthiophene) And Poly(3-(6-bromohexyl)-thiophene) Crystallized on Graphene Oleksandr Dolynchuk, Philip Schmode, Paul M. Reichstein, Matthias Fischer, Mukundan Thelakkat, Thomas Thurn-Albrecht Recent findings have shown that crystallization of polymers on a substrate is advantageous for inducing molecular orientation, especially for semicrystalline conjugated polymers due to their anisotropic charge transport properties. Although graphite induces face-one crystal orientation in monolayers of poly(3-hexylthiophene) (P3HT), a full face-one orientation in thicker P3HT films was not realized so far. The latter is assumed to be a result of two competing interfacial orientations due to different interactions of P3HT side chains with vacuum and the graphite substrate. Here we show that modification of the chemical structure of P3HT side chains can alter the interfacial interactions and result in completely face-on oriented crystals. Specifically, we present a comparative study of the substrate induced molecular orientation of poly(3-(6-bromohexyl)-thiophene) (P3BrHT) and P3HT. The crystal orientation in ultrathin films of both polymers crystallized on a single layer graphene was explored by grazing incidence XRD. The results indicated that P3BrHT on graphene had solely face-on oriented crystals, whereas P3HT on graphene showed mixed crystal orientation with edge-on crystals formed at the top surface. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P50.00009: A Study on Intrinsic Mechanical Properties of n-type Conjugated Polymer via Controlling the Molecular Weight: The Importance of Critical Molecular Weight for Stretchable Organic Electronics Jonnhyeong Choi, Wansun Kim, Taek-Soo Kim, Bumjoon Kim The understanding on the mechanical properties of semicrystalline n-type conjugated polymers is very important for developing the stretchable electronics. In this work, we investigated the intrinsic mechanical properties of naphthalene diimide (NDI) based n-type conjugated polymer, P(NDI2OD-T2) via controlling the number-average molcular weight (Mn), varing from low to very high Mn = 15, 20, 48, 103 and 163 kg mol-1. While we observed the general increasing trend of tensile properties as a function of Mn, a sharp transition in the strain at fracture and toughness values was observed between 48 and 103 kg mol-1 with an increase by a factor of 26 and 160, indicating the presence of the critical molecular weight. This distinct transition from brittle to ductile is mainly attributed to large fraction of amorphous regions including tie molecules and interchain entanglements, which can effectively dissipate a substantial strain energy. The molecular weight dependence of mechanical behavior coincide well with thermal, viscoelastic and microstructural and thin film morphological properties. Therefore, our work suggests design rule for n-type conjugated polymer having good compromise between mechanical reliability and electrical performance for producing stretchable electronics. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P50.00010: Critical Role of Electron-donating Thiophene Group on the Thermomechanical Property of Donor-Acceptor Semiconducting Polymers Song Zhang, Xiaodan Gu Organic semiconducting polymers are promising candidates for stretchable electronics for their mechanical compliance. Donor-Acceptor type conjugated polymers have been the key drive for recent boost in device performance. Up to date, the effect of the backbone structure on the thermomechanical property of conjugated polymers has not been carefully studied. This paper investigated the structure and thin film mechanical property relationship for donor-acceptor polymers with systematically varied backbone structures. The pseudo-free standing tensile test was used to obtain the full stress-strain curve, the glass transition temperature was measured for both thin and bulk films. In the meantime, detailed morphology was detected using AFM, UV-vis, and GIWAXS for further understanding. A general trend was observed and applied to the design of new stretchable conjugated polymers. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P50.00011: Polymer light-emitting diodes with an emitting layer based on a nano-confined semiconducting polymer blend Anielen Ribeiro, Paul Blom, Jasper Michels Blending a visible light-emitting organic semiconductor with an insulator alleviates the trap-limited nature of the electron current. Organic light emitting diodes (OLEDs) comprising such a blend as emissive layer exhibit a two-fold increase in luminous efficiency with only 10% semiconductor. Due to this low content of semiconductor, polymer-LEDs are more attractive than small molecule-based devices. However, polymers impose the difficulty of an inherently low miscibility. In a plain blend macro-phase separation can be avoided if the molecular weight is kept low, which, in case of the semiconductor, is a disadvantage as it suppresses charge carrier mobility. An alternative strategy is to impose a nano-confinement. We prepare aqueous nanodispersions of red (PPV) and blue (polyfluorene) emitting polymers, blended with polystyrene as insulator. We seem to fully suppress macro-phase separation in both cases. For the latter, the combination of nano-confinement and blending influences the phase morphology of the semiconductor in an unprecedented way. Fabricating OLEDs with an emitting layer consisting of nanoparticles poses a considerable challenge due to high operational current densities. We now succeed in fabricating such devices in a reproducible way at very decent efficiencies. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P50.00012: Complexation of a Conjugated Polyelectrolyte and Impact on Optoelectronic Properties Scott Danielsen, Thuc-Quyen Nguyen, Glenn Fredrickson, Rachel Segalman Electrostatic assembly of conjugated polyelectrolytes (CPEs), which combine a π-conjugated polymer backbone with pendant ionic groups, offer an opportunity for overcoming the limited solubility of most semiconducting polymers to make concentrated inks with tunable materials properties and device performance. Complex coacervation, a liquid–liquid phase separation upon complexation of oppositely charged polyelectrolytes in solution, is used to form dense suspensions of π-conjugated material. A model system was used to investigate this complexation behavior of conjugated polyelectrolytes in terms of electrostatic strength, solvent quality, and polymer concentration. The balance of electrostatic interaction between the oppositely charged polyelectrolytes together with their charge compensating counter-ions and solvent quality for the hydrophobic π-conjugated backbone leads to a rich phase diagram of soluble complexes, precipitates, and complex coacervates. The CPE in the polyelectrolyte complexes has an increased π-conjugation length and enhanced emissivity, with ideal chain configurations due to the reduction of kink sites and torsional disorder. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P50.00013: Atomistic modeling of conjugated PEDOT:PSS complexes Wesley Michaels, Jian Qin The performance of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) complexes as conductive or thermoelectric coatings is sensitive to film microstructure and molecular packing details inaccessible to experiment. We combine ab initio calculation, DFT, and atomistic simulation to investigate the effects of ion doping, molecular polarizability, and cosolvent addition on PEDOT-PSS complexation. The results suggest that ionic liquids which bind more strongly to PEDOT are more likely to induce morphological rearragements. Conformational properties of PEDOT:PSS under varying salinity and processing conditions are characterized, providing the basis for multi-scale morphological modeling of this conjugated polyelectrolyte complex. |
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