Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q41: Focus Session: Organic Electronics and Photonics - Structure-Property Relationships |
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Sponsoring Units: DPOLY DMP Chair: Jacob Tarver, National Institute of Standards and Technology Room: 214A |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q41.00001: Quantifying Order in Semiconducting Polymers Chad Snyder Semiconducting polymers form the basis for the burgeoning flexible electronics industry. However, quantifying their order can be challenging due to the nanophase separation induced by the side chains which are used to impart solubility, their propensity to form mesophases, and their often high levels of paracrystalline disorder. Recent successes in our laboratory in understanding these materials and quantifying their order will be presented. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q41.00002: Multi-scale Modeling Study of poly(3-hexylthiophene) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester Towards Organic Photovoltaic Cell Application Hanjong Yoo, Ki Chul Kim, Seung Soon Jang To date, organic photovoltaic cells have gained attention due to their promising potential in the industry. Its efficiency needs to be improved through constructing better morphologies. There are three morphological quantities that affect the efficiency. The domain size of the electron donor phase has to be small and the interface-to-volume ratio of the blend must be large. The percolation ratio has to be high. To investigate the morphological properties of the active layer systems, the state-of-the-art multi-scale modeling is employed. In this study, P3HT and PCBM blends have used as our active layer candidates. We have developed our own force field parameters to accurately describe potential energy surfaces in the layer systems. Subsequently, coarse-grained force field for P3HT and PCBM have been developed based on the improved atomistic force field parameters in order to simulate larger systems. The results from coarse-grained models are validated through the comparison with those from the full atomistic models. Using the molecular dynamics simulations, the newly developed coarse-grained models will be further used to study how the crystallinity of P3HT affects the morphological properties in the active layers. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q41.00003: Structure and segmental dynamics in amorphous conjugated polymers Pengfei Zhan, Janna Maranas, Enrique Gomez Although it is well established that the microstructure strongly affects charge transport in organic semiconductors, the role of fluctuations of the structure on charge mobilities is still not well understood. We have examined the dynamics and structure in amorphous conjugated polymers poly(3-alkylthiophene)s (P3ATs) with neutron and x-ray scattering. We measured the segmental dynamics in amorphous P3ATs with quasi-elastic neutron scattering (QENS). The structure of amorphous P3ATs is measured with small-angle neutron scattering (SANS) and grazing incidence X-ray diffraction (GIXRD). Using SANS, we observe phase separation between the backbone and side-chains in all polymer samples for regiorandom P3ATs. Additionally, the analysis of the QENS data shows that longer side-chains relax faster compared with shorter side-chains and our further analysis of the elastic incoherent structure factor (EISF) suggests that the amplitude of proton motion on the thiophene rings increases by a factor of 3 as the side-chain length increases from 6 to 12, demonstrating that longer side chains lead to enhanced motion of conjugated rings. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:42PM |
Q41.00004: Pushing structural limits to reveal fundamental mechanisms of organic solar cell operation Invited Speaker: Barry Rand Organic-based solar cells are beginning to emerge with the potential to compete with other thin film photovoltaic technologies, with efficiencies of 12{\%} recently demonstrated. Unique to the function of organic photovoltaics are the creation of tightly bound excitons that can only be efficiently separated at a donor/acceptor (D/A) interface capable of providing the necessary energetic driving force for dissociation. The consequences of this are the need for long exciton diffusion lengths and the presence of charge transfer (CT) states, ground state complexes that exist at the D/A interface. We have found that charge transfer states are more easily separated into free charge if they are delocalized; an aspect that becomes most feasible for highly ordered systems. I will discuss our recent efforts to template and control film morphology and molecular orientation. These studies allow us to understand the importance of molecular orientation, crystallite size, and crystal phase. We will show templated devices utilizing neat films as well as bulk heterojunctions, with crystallite dimensions spanning from the more standard nano-sized grains to those with highly ordered micron-sized crystalline domains revealing unprecedented thin film exciton diffusion lengths of 100s of nm. In these highly ordered films, owing to significant delocalization, we are able to directly measure photocurrent from multiple CT states, an aspect which has important consequences for the design of more efficient photocurrent generation. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q41.00005: Predicting X-ray absorption spectra of semiconducting polymers for electronic structure and morphology characterization Gregory Su, Shrayesh Patel, C. Das Pemmaraju, Edward Kramer, David Prendergast, Michael Chabinyc Core-level X-ray absorption spectroscopy (XAS) reveals important information on the electronic structure of materials and plays a key role in morphology characterization. Semiconducting polymers are the active component in many organic electronics. Their electronic properties are critically linked to device performance, and a proper understanding of semiconducting polymer XAS is crucial. Techniques such as resonant X-ray scattering rely on core-level transitions to gain materials contrast and probe orientational order. However, it is difficult to identify these transitions based on experiments alone, and complementary simulations are required. We show that first-principles calculations can capture the essential features of experimental XAS of semiconducting polymers, and provide insight into which molecular model, such as oligomers or periodic boundary conditions, are best suited for XAS calculations. Simulated XAS can reveal contributions from individual atoms and be used to visualize molecular orbitals. This allows for improved characterization of molecular orientation and scattering analysis. These predictions lay the groundwork for understanding how chemical makeup is linked to electronic structure, and to properly utilize experiments to characterize semiconducting polymers. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q41.00006: Phase Separation and Development of a Scanning Time of Flight Microscope to Study Charge Transport in Structured Organic Semiconductors Sanjoy Paul, Suvagata Tripathi, Gautam Singh, Robert Twieg, Satyendra Kumar, Brett Ellman A scanning time-of-flight microscope (STOFm) has been developed to study charge transport in liquid crystalline organic semiconductors (LCOSCs). The STOFm combines the well-known pulsed laser time-of-flight technique with simultaneous polarized light transmission measurements, both on length scales of $\sim$ 10 $\mu$m. In parallel, we have fabricated devices \textit{via} photopolymerization and phase separation of a monomer/LCOSC mixture. The resulting structure has the LCOSC confined to small regions separated by an insulating polymer. We will discuss fabrication of these systems, as well as their characterization using the STOFm. Finally, we will show results on position-dependent charge transport in various pure LCOSC samples. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q41.00007: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q41.00008: Nanoscale Orientation Effects on Carrier Transport in a Low-Band-Gap Polymer Ban Dong, Bingyuan Huang, Aaron Tan, Peter Green We show that the out-of-plane hole mobility of the low-band-gap polymer poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno-[3,4-b]thiophene-2,6-diyl] (PBDTTT-C) is film thickness dependence; and this behavior is associated with the morphology. Due to a geometric confinement and to polymer/substrate interactions, the average orientation of the chains in the thinnest films was predominantly parallel to the substrate. In this thickness range, the out-of-plane hole mobilities $\mu$ were necessarily low and $\beta $, a measure of the strength of the field dependence of the mobility, was largest. Within the framework of the Gaussian Disorder model, the relative value of $\beta $ suggests that the largest effect of positional disorder on the carrier transport was most significant in the thinnest films. The hole mobility $\mu$ increased and depended less on the electric field ($\beta $ decreases in magnitude) with increasing thickness, due evidently to the increased degree of orientation of the domains with respect to the direction of the field (normal to the interfaces). These findings demonstrated the profound impact of the substrate on the morphology and of the morphology on the charge carrier mobility. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q41.00009: Directed alignment of conjugated polymers for enhanced long-range photocurrent collection Anton Li, David Bilby, Ban Dong, Jinsang Kim, Peter Green To realize the full potential of conjugated polymers, possessing anisotropic structure and properties, it is often desirable to extend their organization to larger length scales. An epitaxy-directing solvent additive 1,3,5-trichlorobenzene was combined with an off-center spin-casting technique to produce poly(3-hexylthiophene) (P3HT) fibers with uniaxial in-plane alignment on the centimeter scale, which were incorporated into planar heterojunction solar cells with PCBM acceptor. Topography and photocurrent were mapped by photoconductive AFM; in devices with aligned P3HT, local photocurrent measured on fibers was over 4 times higher than in control devices with unaligned polymer. Even at large distances ($>$200 $\mu$m) between laser spot (carrier excitation) and conductive probe (charge extraction), significant long-range photocurrent was measured in the aligned devices, especially when the separation was oriented parallel to the fiber alignment. Complementary TFT measurements of neat P3HT fibers revealed that the anisotropy of in-plane carrier mobilities was greater than a factor of 3. Together, these findings highlight the importance of conjugated polymer alignment for improving carrier transport and ultimately the performance of solar cells and other devices. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q41.00010: Modifying Photoluminescence Emission from Thin Polymer Films through Local Deformation Zones Po-Jui Chen, Xuan Long Ho, Jonathon David White Controlling light extraction is important for applications ranging from LEDs to the weakly emissive thin films used for trace chemical detection. The commercial importance of GaN photodiodes, has resulted in the majority of work being concentrated on increasing light extraction efficiency ($\eta )$ as GaN's high refractive index results in up to 96{\%} of light being trapped and reabsorbed. Various methods, such as embedding photonic crystals and surface texturing, have been proposed and employed to improve $\eta $. Our focus is not on optimizing this quantity but rather on understanding the effect of surface modification on the angular, spatial and spectral characteristics of the emitted radiation. We do this by simulating the effect of a one-dimensional perturbation of thickness on the outputted radiation of a weakly absorbing fluorescent polymer film. While such a perturbation increases the $\eta $ by a factor of two over a wide range of parameters, the film's other emission properties are quite sensitive to the surface structure. For instance, adjusting the spatial period, allows the spectral peak of the emission to be tuned over a 10nm range and the output to be localized in specific regions of the film. Adjusting the edge angle, allows one to fine tune the direction of radiation escaping the film. Finally, we will discuss tradeoff between structural parameters involved in optimizing light emission for specific detector geometries. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q41.00011: Photocatalytically Active Oligomeric Graphitic Carbon Nitride: Conformational Flexibility, Electronic Levels, Carrier Localization Volker Blum, Vincent Lau, Tiago Botari, William Huhn, Bettina V. Lotsch Polymers consisting of bridged heptazine units (often called ``graphitic carbon nitride'' or ``g-C$_3$N$_4$'') show considerable promise as photocatalysts for solar hydrogen evolution. Recent experimental evidence suggests that oligomeric rather than fully polymerized ``g-C$_3$N$_4$'' exhibits increased intrinsic photocatalytic activity. Using density-functional theory (DFT; van der Waals corrected PBE functional for conformers, hybrid DFT and $GW$ for electronic levels), we show that considerable conformational flexibility exists for the heptazine trimers and tetramers. Analysis of HOMO and LUMO locations as well as trends in photocatalytic activity among heptazine oligomers and polymers reveals the NH$_2$ groups of the oligomers as potential charge-transfer sites. We show that conformational variations of the oligomers can lead to significant, electrostatically motivated carrier localization effects. We suggest that NH$_2$ side groups and the intrinsic conformational variations of the oligomeric species lead to the observed enhanced catalytic activity. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q41.00012: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q41.00013: Development and Studies of Nematic Liquid Crystalline Organic Semiconductors Salma Begum, Sanjoy Paul, Suvagata Tripathi, Robert Twieg, Brett Ellman A huge body of knowledge exists on the creation and alignment of films of nematic liquid crystals (LCs), raising the promise of electronic devices (OFETs, LEDs, etc.) where the structure of the semiconductor is simply controlled with major implications for device parameters. Nematic LCs, however, typically possess low mobilities due to disorder, and are also susceptible to effects due to ionic conduction and screening. We will present material classes of more practical nematic organic semiconductors as well as novel characterization techniques to measure the effects of ionic motion in the nematic phase. [Preview Abstract] |
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