Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P56: Organic Electronics and Photonics IV: Structure & MorphologyFocus
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Sponsoring Units: DPOLY DMP Chair: Dean DeLongchamp, NIST -Natl Inst of Stds & Tech Room: LACC 515B |
Wednesday, March 7, 2018 2:30PM - 3:06PM |
P56.00001: Sequence-specific placement of defects in pi-conjugated semiconducting polymers Invited Speaker: Christine Luscombe With the realization of the first organic light emitting diode (OLED) where a pi-conjugated semiconducting molecule was used as the active component, organic semiconductors have attracted much interest because of their potential in developing light-weight, low cost, and flexible electronic devices. Within the field of pi-conjugated semiconducting polymers, there is a growing realization that defects such as homocoupling and end-groups can adversely affect the properties, and thus the performance of these polymers in applications. However, what is not clearly understood is why these defects affect the properties in the way that they do. In our group, we have been working on the development of controlled polymerizations that allow us to accurately control the placement of defect in the polymer. By using these polymers, we aim to achieve a level of control over the microstructure that has not been realized before for these classes of materials. In the presentation, our synthetic strategies, as well as structure-property relationship studies using these well-defined polymers, will be discussed. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P56.00002: Quantification of tie-chain content in semicrystalline conjugated polymers and its impact on charge transport Kaichen Gu, Chad Snyder, Jonathan Onorato, Christine Luscombe, Lynn Loo Polymer tie-chains connect adjacent crystalline domains and strongly impact charge transport in semicrystalline conjugated polymers. To assess their role in charge transport, we blended 100% regioregular narrowly-dispersed poly(3-hexylthiophene), or P3HT, having an Mn of 5 kg/mol with a high-MW counterpart having an Mn of 40 kg/mol. Varying the high-MW fraction in the homopolymer blends allows tuning of the tie-chain content, which we quantified with the Primary Structure Parameter (PSP) model, originally developed to correlate crack resistance of polyethylene resins with their microstructure. The charge-carrier mobility of transistors comprising P3HT blends increases with increasing loadings of the high-MW P3HT. We attribute this observation to enhanced interdomain connectivity indicated by higher PSP. Beyond the percolation threshold for macroscopic charge transport, the mobility plateaus because charge transport beyond this point is limited by local structural disorder within P3HT crystalline domains, as evidenced by X-ray paracrystallinity analysis. Our results quantify the role of interdomain connectivity and demonstrate its interplay with local structural disorder in determining charge-transport characteristics in semicrystalline conjugated polymers. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P56.00003: Towards Connecting Fundamental Properties of Conjugated Polymers with Performance in Electronic Devices Renxuan Xie, Ralph Colby, Enrique Gomez Predicting the performance of conjugated polymers in electronic devices requires knowledge of fundamental properties, such as tie chain density, entanglement molecular weight (Me), glass transition temperature (Tg) and liquid-crystal-to-isotropic transition temperature (Tc). Oscillatory shear rheometry can unambiguously yield the glass transition temperature of conjugated polymers, such as poly(3-hexylthiophene-2,5-diyl) (P3HT), to settle a long-standing debate full of conflicting reports in the literature. Furthermore, the molecular weight dependence of Tg is studied with regioregular (RR) and regiorandom (RRa) P3HT, such that Flory−Fox model yields Tg = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a very different molecular weight dependence of Tg is seen below Mn = 14 kg/mol, suggesting this is the typical molecular weight of tie chains. Linear viscoelastic studies in molten and semicrystalline states also reveal that the presence of liquid crystalline phase can affect the effective Me and the tie chain contribution to the semicrystalline modulus. These microstructural parameters connect to both charge transport and mechanical flexibility of conjugated polymers, thereby establishing design rules for flexible electronics. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P56.00004: Morphological, chemical, and electronic changes of the conjugated polymer PTB7 with high temperature annealing Michael Toney, Victoria Savikhin, Lethy Jagadamma, Lafe Purvis, Iain Robertson, Stefan Oosterhout, Christopher J. Douglas, Ifor D. Samuel We study the effect of high temperature annealing on the properties of the semiconducting polymer PTB7, widely used for efficient polymer solar cells. Annealing to moderate temperatures (<260°C) progressively increases morphological order and mobility of the polymer. Annealing to a temperature of 290°C results in a previously unseen polymorph where the π-π stacking distance is decreased by 11%. However, despite the closer packing motif, there is an increase in electronic disorder, decrease in mobility, and decrease in exciton diffusion length. Since the morphology change is accompanied by a mass loss of 10% and changes in FT-IR and UV-Vis , we conclude that thermal cleavage of polymer sidechains leads to the closer packing of molecules, but the chemical reaction may create by-products that act as trap states which decrease mobility. Changes in FT-IR and UV-Vis are similar between 290°C annealing and solar irradiation in air, implying that sidechain cleavage may be an important mechanism in OPV device burn-in. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P56.00005: Out-of-Plane Orientation Control of Solution-Processed Small-Molecule Organic Semiconductor Crystals Kai Zong, Xiaoshen Bai, Jack Ly, Jeremy Mehta, Megan Hand, Kaitlyn Molnar, Sangchul Lee, Bart Kahr, Jeffrey Mativetsky, Alejandro Briseno, Stephanie Lee We demonstrated the ability to control the out-of-plane orientation of triisopropylsilethynyl (TIPS)-derivatized organic semiconductor crystals by tuning the molecular structure and substrate surface energy. For TIPS-pentacene crystals, molecules invariably oriented in a horizontal manner, with the long axis of the conjugated core parallel to the substrate. For TIPS-dibenzopyrene, anthanthrene and pyanthrene crystals, a vertical orientation, with the long axis of the conjugated core perpendicular to substrate, was observed. By varying the surface energy of the substrate, the ratio of the populations of horizontal versus vertical orientations in TIPS-pyranthrene crystals could be tuned. As observed by polarizing optical microscopy, orientation-dependent linear birefringence and linear dichroism were observed. Furthermore, compared to crystals adopting the horizontal orientation, crystals adopting the vertical orientation displayed a 42-fold improvement in out-of-plane hole mobility measured via conductive atomic force microscopy. Because out-of-plane charge transport is necessary for a range of optoelectronic applications, understanding structure-property relationships between molecular structure and orientation is critical to improving device performance. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P56.00006: Using Resonant Soft X-ray Reflectivity (RSoXR) to Probe Vertical Segregation in Organic Semiconducting Thin Films Jacob Thelen, Daniel Sunday, Sebastian Engmann, Lee Richter, Dean DeLongchamp Recent studies have demonstrated the efficacy to blending organic semiconducting polymers and small molecules, such as poly(triarylamine) (PTAA) and 2,8-difluoro-5,11-bis(triethylsilylethynyl) Anthradithiophene (diF-TESADT), in order to facilitate film processing and improve device performance. Segregation of the small molecule to the surface or buried interface(s) can dramatically influence device performance in such systems. In this work, we explore the application of Resonant Soft X-Ray Reflectivity (RSoXR) toward determining vertical composition profiles in polymer/small molecule blend films. The resonance-enhanced soft X-ray contrast facilitates accurate determination of film geometry and layer thicknesses without the need for chemical modification of the blend constituents. We also investigate the feasibility of quantitative layer composition analysis by leveraging Near Edge X-ray Absorption Fine Structure (NEXAFS) data obtained from each of the blend components. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P56.00007: Structure-mechanical property of Stretchable Organic Ultra-thin Film for Electronic Applications Song Zhang, Dakota Ehlenberg, Xiaodan Gu Flexible electronics have gained significant attention due to its potential of achieving large area fabrication and a wide range of application in wearable and implantable devices. A blended system consists of conjugated polymer/elastomer serving as flexible, lightweight, solution-processable, cost-effective, structure tailorable active layer plays a pivotal role in the modern flexible electronics development. To systematically investigate the morphology of the blend system, the different blend ratio of conjugated polymer/elastomer systems were studied by resonant soft x-ray scattering (RSoXs) and correlated to the temperature controlled mechanical tester. We used a pseudo-free standing tensile tester to measure the mechanic property of the ultra-thin films with a thickness of 10 ~ 200nm, which otherwise cannot be obtained using the traditional tensile tester. The full stress-strain curve and the related mechanical property was measured and discussed for the film being floated on water. In the meantime, microstructure morphology, as one of the key variables of electronics performance, was investigated by RSoXs technique to establish the relationship between different blend ratio and phase separation, domain size, and domain spacing in each composite. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P56.00008: Imaging the Nucleation and Growth of Thin-films of Poly(3,4- ethylenedioxythiophene) (PEDOT) using In-situ Liquid Transmission Electron Microscopy Vivek Subramanian, Jinglin Liu, Bin Wei, David Martin Conducting polymer thin films have received considerable scientific and commercial interest for a variety of applications like photovoltaics, organic electronics and neural interfaces. (PEDOT) is particularly important due to its chemical and mechanical stability and high charge transport properties. The morphology of electrochemically-polymerized thin-films can be fine-tuned by controlling the early stage nucleation and growth of the oligomeric clusters. We have used in-situ, low dose Transmission Electron Microscopy to obtain a detailed understanding of the fundamental processes occurring at the electrode-solution interface, especially the evolution of the mobile oligomers that precede solid polymer film formation. The total electron doses for our experiments were in the range of 10-20 mC/cm2 which is well below the critical dose of PEDOT. We have found that the growth rates of individual nuclei were somewhat irregular. Certain droplets were found to gradually decrease in size and mass thickness, indicating some degree of reversibility of the process. We have also found substantial variations when the chemistry of the monomer is changed, including a dramatic increase in the nucleation density when a more hydrophilic, carboxylic acid substituted EDOT was used as a comonomer. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P56.00009: Towards Novel Organic Electronics: Integrating Simulation and Neutron Scattering Thomas Harrelson, Varuni Dantanarayana, Alejandro Bonilla, Allesandro Troisi, Roland Faller, Adam Moule
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Wednesday, March 7, 2018 4:42PM - 4:54PM |
P56.00010: Multiscale Simulation of Conjugated Polymer Aggregation and Optoelectronic Properties Peter Rossky, Thomas Allen We report on progress in using a novel coarse-grain model of poly-3-hexylthiophene to study the aggregation and morphological properties of the neat polymer and bulk heterojunction blends with fullerene, as well as how these properties impact the electronic structure and charge transport behavior of the materials. We make use of a number of simulation strategies, including molecular dynamics and electronic structure methods to capture effects at a range of scales, from single chain conformation to localized chain aggregates to crystalline domains and domain interfaces in order to understand how these structures influence exciton energies and couplings in the material, with a view toward understanding the charge transport and recombination behavior in the bulk material. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P56.00011: Origin of Translational and Dipolar Order in Vapor-Deposited Alq3 glasses Kushal Bagchi, Nicholas Jackson, Ankit Gujral, Juan De Pablo, Mark Ediger Structural anisotropy in vapor-deposited molecular glasses has attracted interest both from a fundamental physics and organic electronics perspective. It has been shown previously that the bulk orientational anisotropy in vapor-deposited glasses is inherited from the free surface of the equilibrium supercooled liquid (surface equilibration mechanism). However, it is unclear if this mechanism can explain other common structural features of vapor-deposited glasses such as a tendency for center of mass ordering and dipolar alignment. To test if surface equilibration can explain these structural features, we perform X-ray scattering studies and molecular dynamics simulations of thin films of common OLED (organic light emitting diode) molecule Alq3. From x-ray scattering we find a substrate temperature dependent tendency for center of mass ordering in vapor-deposited Alq3 glasses. Simulations reveal that the free surface of the equilibrium liquid exhibits a tendency for center of mass ordering and dipolar order. The surface equilibration mechanism can explain our observation of center of mass ordering in vapor-deposited Alq3 glasses and the existence of dipolar alignment reported by other groups. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P56.00012: Variation of Thermal and Optoelectronic Properties Through Compositional Control of Thiophene-Containing Statistical Copolymers Michael Minkler, Bryan Beckingham Conductive polymers are of widespread interest for application to organic-based electronics such as solar panels, thermoelectric generators, and chemical sensors. Polyalkylthiophenes are an important subclass of conjugated polymers that are widely-studied due to their well-controlled synthesis and favorable optoelectronic and solid-state properties. It is known that small changes to the microstructure of 3-substituted polythiophenes can greatly impact material properties, yet a complete understanding of structure-property relationships has remained elusive. By systematically synthesizing a series of statistical copolymers of 3-alkylthiophenes with thiophene, a more complete understanding of the structure-property relationship can be gained. In particular, we synthesize copolymers of 3-hexylthiophene and thiophene of varied comonomer compositions and investigate the resulting material properties. Copolymer molecular weight and dispersity are characterized via 1H NMR spectroscopy and gel-permeation chromatography respectively, the optical absorbance and optical band gaps are characterized with UV/Vis spectroscopy and the Spano Model, and the solid-state properties are evaluated using differential scanning calorimetry and thermogravimetric analysis. |
Wednesday, March 7, 2018 5:18PM - 5:30PM |
P56.00013: Interaction Instabilities in Organic Filling-Enforced Semimetals Benjamin Wieder, Richard Geilhufe, Stanislav Borysov, Alexander Balatsky The identification of experimentally viable nodal semimetals is a crucial step towards the advancement of topological condensed matter physics. Recent efforts have exploited symmetry and electron-counting considerations to narrow this search, focusing on so-called "filling-enforced" (fe-) semimetals. Examining thousands of DFT calculations of previously synthesized organic crystals tabulated in the Organic Materials Database (OMDB) http://omdb.diracmaterials.org, we find over 100 candidate fe-semimetals, and observe that all of them instead develop spontaneous magnetization, a result consistent with available experimental literature. We employ machine-learning to explore this relationship, finding that among thousands of charge-counting- and crystalline-symmetry-based features, minimal insulating filling is the leading predictor of magnetization in the OMDB. We conclude by also discussing possible antiferromagnetic, Mott insulating, and superconducting phases. The band structures and results presented in this talk are freely accessible at http://supmagnetic.diracmaterials.org/. |
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