Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K11: Organic Electronics - Processing, Structure, Function |
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Sponsoring Units: DPOLY DMP Chair: Dean DeLongchamp, NIST Room: 270 |
Wednesday, March 15, 2017 8:00AM - 8:12AM |
K11.00001: Crystalline Stratification in Semiconducting Polymer Thin Film Quantified by Grazing Incidence X-ray Scattering Eliot Gann, Mario Caironi, Yong-Young Noh, Yun-Hi Kim, Christopher R. McNeill The depth dependence of crystalline structure within thin films is critical for many technological applications, but has been impossible to measure directly using common techniques. In this work, by monitoring diffraction peak intensity and location and utilizing the highly angle-dependent waveguiding effects of X-rays near grazing incidence we quantitatively measure the thickness, roughness and orientation of stratified crystalline layers within thin films of a high-performance semiconducting polymer. In particular, this diffractive X-ray waveguiding reveals a self-organized 5-nm-thick crystalline surface layer with crystalline orientation orthogonal to the underlying 65-nm-thick layer. While demonstrated for an organic semiconductor film, this approach is applicable to any thin film material system where stratified crystalline structure and orientation can influence important interfacial processes such as charge injection and field-effect transport. [Preview Abstract] |
Wednesday, March 15, 2017 8:12AM - 8:24AM |
K11.00002: Photostability of vapor-deposited thin films can be significantly modulated by glass packing Yue Qiu, Mark Ediger, Olivier Lebel Photochemically robust materials are desired for organic electronics. In previous research, we demonstrated that organic glasses prepared by physical vapor deposition can be highly stable against photoisomerization reactions if the substrate temperature is chosen correctly. In this work, we further show that the enhanced photostability in vapor-deposited glasses is a general phenomenon exhibited in other molecular systems. For indomethacin (with a carboxylic acid group), we are able to monitor the photo-decarboxylation in situ by measuring the mass loss during the irradiation. For molecule with an azobenzene moiety, we can track photoisomerization by directly measuring the population of trans and cis molecules with the absorption spectrum. For both cases, we show that the rate of photoreaction varies as a function of the substrate temperature during the deposition, and photostability correlates with density of packing. These results provide a molecular level explanation for enhanced photostability in amorphous materials, and they may provide insight in designing organic photovoltaics and light emission devices with longer lifetimes. [Preview Abstract] |
Wednesday, March 15, 2017 8:24AM - 8:36AM |
K11.00003: Roll-to-Roll printed large-area all-polymer solar cells with 5{\%} efficiency based on a low crystallinity conjugated polymer blend Xiaodan Gu, Yan Zhou, Kevin Gu, Tadanori Kurosawa, Hongping Yan, Cheng Wang, Micheal Toney, Zhenan Bao The challenge of continuous printing in high efficiency large-area organic solar cells is a key limiting factor for their widespread adoption. We present a materials design concept for achieving large-area, solution coated all-polymer bulk heterojunction (BHJ) solar cells with stable phase separation morphology between the donor and acceptor. The key concept lies in inhibiting strong crystallization of donor and acceptor polymers, thus forming intermixed, low crystallinity and mostly amorphous blends. Based on experiments using donors and acceptors with different degree of crystallinity, our results showed that microphase separated donor and acceptor domain sizes are inversely proportional to the crystallinity of the conjugated polymers. This methodology of using low crystallinity donors and acceptors has the added benefit of forming a consistent and robust morphology that is insensitive to different processing conditions, allowing one to easily scale up the printing process from a small scale solution shearing coater to a large-scale continuous roll-to-roll (R2R) printer. We were able to continuously roll-to-roll slot die print large area all-polymer solar cells with power conversion efficiencies of 5{\%}, with combined cell area up to 10 cm2. This is among the highest efficiencies realized with R2R coated active layer organic materials on flexible substrate. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K11.00004: Doping of an Ambipolar DPP Polymer with an Organometallic Dopant Erin Perry, Chien-Yang Chiu, Karttikay Moudgil, Craig Hawker, Seth Marder, Michael Chabinyc We report the n-doping of the non-planar ambipolar polymer Poly((E)-3-(5-([8,8'-biindeno[2,1 b] thiophenylidene]-2-yl)thiophen-2-yl)-2,5-bis(2-octyldodecyl)-6(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (P(BTP-DPP)) with the organometallic dimer pentamethylcyclopentadienyl mesitylene (RuCp*mes)$_{\mathrm{2}}$ processed via sequential spin-casting. Maximum n-type conductivities of 0.45 S/cm are observed, which are amongst the highest reported for n-type semiconducting polymers. Using a combination of photoemission, spin resonance and optical spectroscopy we have studied the impact of processing conditions on the resulting electronic structure of the doped polymer. Significantly, we observe evidence of the coexistence of polarons and bipolarons in optimally doped films. The origin of the increased electrical conductivity observed in this system was probed using structural methods such as grazing incidence X-ray scattering and atomic force microscopy. Our results suggest that sequential processing allows for formation of efficient percolation pathways for charge transport. This work provides us with a framework for developing future high conductivity systems using sequential processing of semiconducting polymers with non-planar backbones that enable efficient packing of dopants. [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K11.00005: Understanding morphology-property relationship in chemically doped P3HT Eunhee Lim, Kelly Peterson, Michael Chabinyc Enhancing the electrical conductivity of semiconducting polymers is crucial for high performance organic electronic applications. Chemical doping with dopant molecules can increase the electrical conductivity by increasing charge carrier density through charge transfer between the polymer chain and the dopants. As charge transfer is closely related to the film morphology, studying the impact of doping on morphology is important for finding an efficient doping method. In this work, we studied the morphology-performance relationship in P3HT thin films doped with F4TCNQ by comparing vapor doped and solution doped films using a combination of X-ray scattering and electrical characterization. The maximum electrical conductivity was similar for both doping methods. However, grazing incidence wide-angle scattering showed that vapor doping gives a larger number of heavily doped regions compared to solution doping. Resonant soft x-ray scattering showed that vapor- and solution- doping have similar long-range connectivity between the crystalline regions explaining the similarity in conductivity in the films. This work allows us to better understand morphological influences on chemical doping methods, which opens up new possibilities of exploring efficient doping methods. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K11.00006: Thermally induced texture flip in semiconducting polymer stabilized by epitaxial relationship. Kathryn A. O'Hara, Balaji S.S Pokuri, Christopher J. Takacs, Pierre M. Beaujuge, Baskar Ganapathysubramanian, Michael L. Chabinyc The morphology of semiconducting polymer films has a large effect on the charge transport properties. Charges can move easily along the conjugated backbone and in the pi-pi stacking direction. However, transport through the film is determined by the connectivity between domains, which is not well understood. We previously observed quadrites in the polymer, PSBTBT, and proposed that the preferential overlap between lamellae may improve connectivity and provide an additional conduction pathway. Now, the presence of quadrites is revealed in another successful donor polymer, PBDTTPD, using high resolution transmission electron microscopy (HRTEM). A study of how side-chain substitution affects the epitaxial crossing is conducted by examining several PBDTTPD derivatives. The stability of the film texture with annealing is also examined as a function of quadrite formation. It has been shown that heating some semicrystalline polymers above the melting temperature and slow cooling can flip the lamellar texture from face-on to edge-on. We hypothesize that the orientation of lamellar crystallites in PBDTTPD films is stabilized by the epitaxial overlap between adjacent crystalline domains. This may have important implications for the electronic transport properties. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K11.00007: Resolving Morphology-Dependent Optical Properties with Momentum-Resolved Reflectometry Ryan DeCrescent, Steven Brown, Ruth Schlitz, Michael Chabinyc, Jon Schuller The propensity of organic materials to self-assemble into highly ordered structures leads to strong morphology-dependent optical properties. These optical properties, in turn, reveal important qualities of the underlying electronic excitations and have a significant impact on device performance and design. We present a novel model-blind, momentum-resolved reflectometry technique for determining accurate and precise optical constants, with quantifiable error estimates, for organic thin films. Unlike ellipsometry—where spectral data is fit to unknown multi-oscillator models with dozens of free parameters—we use Fourier imaging techniques and Fresnel models to determine optical constants wavelength-by-wavelength without any input assumptions. We demonstrate this technique on the n-type polymer P(NDI2OD-T2) which is deposited with distinct edge-on and face-on morphologies depending on processing conditions. We show that this approach produces exceptional agreement with UV-Vis-NIR absorption measurements, while simultaneously avoiding the need to construct complicated dispersion models. Finally, we use this procedure to resolve subtle differences in optical anisotropies of different film morphologies that were previously obscured in ellipsometry measurements. [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K11.00008: Polarized soft X-ray scattering reveals differences in chain orientation within block copolymer lamellae Joshua Litofsky, Thinh Le, Melissa Aplan, Youngmin Lee, Enrique Gomez Fully conjugated block copolymers can serve as the active layer in organic photovoltaics (OPV) and other organic electronic devices. The use of Resonant Soft X-Ray Scattering (RSoXS) allows for studies into the molecular orientation and domain spacing of the polymers within lamellae by tuning the X-ray energy and polarization to examine various components of block copolymers. Using the conjugated block copolymer system of poly(3-hexylthiophene)-\textit{block}-poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl), P3HT-b-PFTBT, and PFTBT derivatives, we can examine the effects of various polymer blocks on the differences of morphology between the donor and acceptor. Polarized Soft X-Ray Scattering (PSoXS) allows us to quantify the type and the degree of orientation of chains within block copolymer domains in thin films. Our work suggests that within our conjugated block copolymers, the P3HT chains orient parallel to the block copolymer interface. Furthermore, examining the anisotropy in PSoXS data provides a clear signature of the block copolymer microstructure, confirming the domain spacing extracted from PSoXS scales with the end-to-end distance of the blocks. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K11.00009: Entanglements in Conjugated Polymers Renxuan Xie, Youngmin Lee, Melissa Aplan, Nick Caggiano, Enrique Gomez, Ralph Colby Conjugated polymers, such as poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2',2''-diyl) (PFTBT), are widely used as hole and electron transport materials in a variety of electronic devices. However, fundamental knowledge regarding chain entanglements and nematic-to-isotropic transition is still lacking and are crucial to maximize charge transport properties. A systematic melt rheology study on P3HT with various molecular weights and regio regularities was performed. We find that the entanglement molecular weight Me is 5.0 kg/mol for regiorandom P3HT, but the apparent Me for regioregular P3HT is significantly higher. The difference is postulated to arise from the presence of a nematic phase only in regioregular P3HT. Analogously, PFTBT shows a clear rheological signature of the nematic-to-isotropic transition as a reversible sharp transition at 278 C. Shearing of this nematic phase leads to anisotropic crystalline order in PFTBT. We postulate that aligning the microstructure will impact charge transport and thereby advance the field of conducting polymers. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K11.00010: A bio-derived liquid crystal template for highly ordered semiconducting polymers Bailey Risteen, Jung Ok Park, Mohan Srinivasarao, Paul Russo, Elsa Reichmanis One main challenge in producing flexible organic electronic devices is ensuring adequate performance of the active semiconducting material. Spontaneous self-assembly of semiconducting and semicrystalline polymers yields undesirable amorphous aggregates that prevent effective charge-carrier transport. Polymer alignment by intra and interchain $\pi $-$\pi $ stacking is crucial to achieving superior optoelectronic properties and high charge carrier mobility. In this work, the liquid crystal ordering of bio-derived cellulose nanocrystals (CNCs) was investigated as a means of enforcing long-range order in the semiconducting polymer poly[3-(potassium-4-butanoate) thiophene-2,5-diyl], PPBT. It was found that the inclusion of these renewable particles in PPBT solutions resulted in enhanced polymer chain alignment. Furthermore, the presence of anisotropic polymer aggregates and chain co-planarization was confirmed by UV-Vis and circular dichroism spectroscopy. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K11.00011: Conducting Polymers under Mechanical Strain: Tuning Polyaniline's Piezoresistivity through Structural Control Melda Sezen, Yueh-Lin Loo The use of conducting polymers in various flexible or conformable electronics applications necessitates control over their piezoresistive response. We show that we can tune both the polarity and the magnitude of the piezoresistive response of PANI-PAAMPSA by controlling its thin-film morphology. PANI-PAAMPSA forms electrostatically-stabilized colloidal particles as-synthesized whose size affects the solid-state structure of PANI-PAAMPSA. By altering the molecular weight of PAAMPSA at the onset of synthesis, we change the particle size of PANI-PAAMPSA and the crystallinity of PANI. As the molecular weight of PAAMPSA decreases, PANI-PAAMPSA forms smaller particles and PANI's crystallinity increases. These structural changes cause PANI-PAAMPSA's gauge factor (GF) to decrease linearly and even become negative with decreasing PAAMPSA molecular weight. The tunability of PANI-PAAMPSA's piezoresistive response, especially our access to negative gauge factors, has resulted in design rules with which we can specify a priori, and synthesize accordingly, PANI-PAAMPSA with defined piezoresistive response for strain sensing (high GF) or flexible chemo- or thermo-resistive sensor (zero GF) applications. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K11.00012: Exploration of Solvent Effects On Morphology of Polyaniline {\&} Other Polymer Films Deposited Through RIR-MAPLE Enrique Barraza, Adrienne Stiff-Roberts Through the use of aromatic solvents with varying numbers of hydroxyl and methyl moieties, there is an opportunity to positively impact morphology of polymer films deposited through emulsion-based Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE). These more complex solvents may result in smaller emulsified particles within the target, such that smoother films are achieved. We hypothesize the amphiphilic nature of polymers, like doped Polyaniline, requires a solvent with the same solubility to form a stable emulsion target. Control over the emulsion and resulting film properties can yield beneficial device properties, like low contact resistance. Our hypothesis is also tested against hydrophobic polymers, like P3HT, which have been deposited successfully using RIR-MAPLE with chlorobenzenes as the solvent family. We propose that the addition of hydroxyl moieties to the aromatic ring of the solvent should also yield more control over the film morphology. Atomic force microscopy, UV-Vis absorbance, and dark current density-voltage measurements of the resulting films will be reported, as well as a discussion of how these results relate to previously understood paradigms in RIR-MAPLE deposition. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K11.00013: Characterization of pi-conjugated oligopeptide superstructures created via continuous directed self assembly Lawrence Valverde, Charles Schroeder, William Wilson Development of precise methods for controlling the nanoscale ordering of functional materials is a key challenge to achieving efficient and reliable organic semiconductor-based devices. In this work, we report a new continuous flow method for the directed assembly of oligopeptides with pi-conjugated cores based on tailored flow profile manipulation in microreactors. In particular, we implement 3-D flow focusing of oligopeptide streams combined with a planar extensional flow to achieve supramolecular assembly, alignment, and ordering. We characterize the assembly reaction in real-time using in situ confocal fluorescence microscopy and fluorescence lifetime imaging (FLIM). Through shifts in spectra and fluorescence lifetimes of accompany amyloid formation in oligopeptides with OPV3, 4T, or PDI cores, we demonstrate oligopeptide assembly in a 3D confined volume along the extensional flow axis. Moreover, we characterize assembled materials via the emerging technique of nano-Fourier transform infrared spectroscopy (nano-FTIR), which combines the molecular fingerprinting of traditional FTIR with the spatial resolution of atomic force microscopy (AFM). We further employ conductive probe AFM to characterize the optoelectronic properties of these materials. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K11.00014: Of Devices and Droplets: Evaporative Structuring of Solution-Processed Semiconducting Polymer Blends. Jasper Michels Many organic and hybrid thin film electronic devices (e.g. memory diodes, solar cells, light emitting diodes, transistors, capacitors) contain an active layer based on a blend of multiple polymeric or small-molecular species whose properties cooperatively give rise to a specific function. Depending on the desired functionality, phase separation during film processing is either encouraged or suppressed. It is often observed that during solution-casting droplet-like structures emerge due to liquid-liquid demixing, which may or may not be desirable. This presentation will focus on the role of demixing in thin film electronics by demonstrating how phase composition and morphology affect opto-electronic performance. Calculations based on mixing thermodynamics and results from dynamic phase field simulations will be placed in direct context with electronic processes in the thin film. Strikingly, the theoretical part of this study has led to novel fundamental insight concerning the influence of solvent evaporation on spinodal decomposition. We quantitatively show how both the early and late stages of demixing are affected by the composition drift imparted by evaporation, and why significant deviation from the classical laws governing structure formation is observed. [Preview Abstract] |
Wednesday, March 15, 2017 10:48AM - 11:00AM |
K11.00015: Machine learning and computation for $\pi$-conjugated oligopeptides: toward in silico design of assembled organic electronics Bryce Thurston, Andrew Ferguson Self assembly of peptides has shown to be a promising approach for the fabrication of novel macromolecular materials. Oligopeptides that contain embedded $\pi$-conjugated subunits assemble to form $\beta$-sheet-like 1D ribbons with useful electronic and photophysical properties. Such aggregates have been shown to be able to produce charge separated states and to function as the active layer in an FET. Morphological and spectroscopic properties of the assembled aggregates can vary significantly with the chemistry of the peptides. We selectively study peptides having a peptide-$\Pi$-peptide architecture by employing computer simulations in order to assess the chemical dependence of such properties. We compute free energies of association, inter-peptide hydrogen bonding, and other such attributes. To speed up computation, we calculate molecular descriptors for peptides and apply machine learning techniques to produce a model to predict previously computed properties. This model performs well and lays the basis for a rapid search of chemistry space for peptides with specific attributes. This work assists in understanding the influence of peptide composition on the characteristics of aggregates, and progresses toward computationally driven design of novel bioelectronic materials. [Preview Abstract] |
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