Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L41: Focus Session: Organic Electronics and Photonics - Transport in Polymer Thin Films |
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Sponsoring Units: DPOLY DMP Chair: Barry Rand, Princeton University Room: 214A |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L41.00001: Changes in the Solution Behavior of Conjugated Polymers with Light Absorption Mark Dadmun, Brian Morgan Conjugated polymers are well established as functional materials in a broad range of applications including organic photovoltaics, chemical sensors, and organic light emitting diodes. This functionality is mainly derived from their ability to create electron-hole pair excitons through photoexcitation. The presence of these entities on the polymer chains may alter the chain conformation, solution behavior, and ultimately macroscopic morphology, of the conjugated polymer. Previous studies have shown significant changes in properties such as viscosity and photoluminescence upon exposure of select conjugated polymer films to white light. In order to expand upon these preliminary findings, we have performed small angle neutron scattering experiments on solutions of several semiconducting, conjugated polymers in both the presence and absence of incident light. Substantial differences are observed between the light vs dark samples, the magnitude of which are dependent on polymer dispersion, solvent choice, and solution concentration. Analysis of the neutron curves shows real difference in Kuhn lengths and radius of gyration of the polymer, suggesting possible rearrangement of polymer chain conformation or alteration of polymer chain-solvent interactions. [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L41.00002: Correlating Transport with Nanostructure and Chemical Identity in Radical Polymer Conducting Glasses Bryan Boudouris, Lizbeth Rostro, Aditya Baradwaj, Martha Hay Radical polymers are an emerging class of macromolecules that are composed of non-conjugated backbones which bear stable radical groups at the pendant positions. Because of these stable radical sites, these glassy materials are able to conduct charge in the solid state through a series of oxidation-reduction (redox) reactions. Importantly, the redox-active behavior is controlled by both the local chemical environment of the radical polymer groups and by the nanoscale structure of the materials. Here, we demonstrate that proper control of the pendant group chemical functionality allows for the fabrication of transparent and conducting amorphous thin films which have solid-state hole mobility and electrical conductivity values on the same order as those seen in common conjugated, semicrystalline polymer systems [e.g., poly(3-hexylthiophene) (P3HT)]. Furthermore, we show that control of the nanostructure of the materials aids in facilitating transport in these radical polymer thin films. In turn, we implement simultaneous spectroscopic and electrical characterization measurements in order to elucidate the exact mechanism of charge transport in radical polymers. Finally, we demonstrate that, because there is ready control over the molecular properties of these materials, developing bendable and stretchable transparent conducting thin films is relatively straightforward with this unique class of organic electronic materials. [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L41.00003: Using NEXAFS spectroscopy to probe the Biaxial Orientation of a Pyridal[2,1,3]thiadiazole-containing Donor-Acceptor Polymer Shrayesh Patel, Greg Su, Chan Luo, Ming Wang, Alan Heeger, Guillermo Bazan, Michael Chabinyc, Edward Kramer Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a powerful tool to probe the molecular orientation of conjugated polymer thin films. Here, we report on the biaxial orientation of a high mobility donor-acceptor copolymer coated on uniaxial nanogrooved substrates. The polymer system under investigation is regioregular PCDTPT based on cyclopenta[2,1-b:3,4-b$\prime $]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units. In partial electron yield mode, NEXAFS spectroscopy is a surface sensitive technique ($\sim$2-3 nm). This is particularly useful as we are interested in probing the orientation near the interface between the polymer and the substrate. While the carbon K-edge is commonly used for NEXAFS experiments, we can take advantage of the PT unit and use the nitrogen K-edge to probe the biaxial orientation of our films. We will present the biaxial orientation of films coated on substrates with and without uniaxial nanogrooves. The results indicate that the presence of uniaxial nanogrooves are important in obtaining films with high level of orientation. Lastly, anisotropic field-effect transistor mobility values will be presented for various coating conditions. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 9:12AM |
L41.00004: Percolation, tie-lines, and the microstructural determinants of charge transport in semicrystalline conjugated polymers Invited Speaker: Andrew Spakowitz Semiconducting polymers play an important role in a wide range of optical and electronic material applications. It is widely accepted that the polymer ordering impacts charge transport in such devices. However, the connection between molecular ordering and device performance is difficult to predict due to the current need for a mathematical theory of the physics that dictates charge transport in semiconducting polymers. Here, we present a new analytical and computational description in which the morphology of individual polymer chains is dictated by well-known statistical models and the electronic coupling between units is determined using Marcus theory. This effort combines our research group's modeling efforts in polymer conformational properties and reaction-diffusion phenomena to address the multiscale dynamics of charge transport in a heterogeneous material. The resulting model is capable of bridging molecular-level charge transport mechanisms to large scale transport behavior, thus facilitating direct comparison with experiments. The multiscale transport of charges in these materials (high mobility at short length scales, low mobility at long length scales) is naturally described with our framework. Additionally, the dependence of mobility with electric field and temperature is explained in terms of conformational variability and spatial correlation. Our model offers a predictive approach to connecting processing conditions with transport behavior. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L41.00005: Charge motion in Poly(3-hexylthiophene-2,5-diyl) studied with Scanning Probe Microscopy Jason Moscatello, Chloe Castaneda, Katherine Aidala Organic semiconductors like poly(3-hexylthiophene-2,5-diyl) offer the promise of solution-processable, flexible electronics, but the charge motion in these disordered films is not fully understood. We use Kelvin Probe Force Microscopy (KPFM) to study trapped charges in the channel of inverted field effect transistors and have developed a technique to measure real time screening. The tip of the AFM is placed at a specific location above the sample with grounded source and drain electrodes, and the potential of the surface is recorded using KPFM. When a voltage is applied to the back-gate, charges will move to screen this potential. For materials with relatively low charge density and mobility, it will take some amount of time to fully screen. The tip will initially measure the potential of the voltage applied to the back-gate, which will decrease as charges enter the film. The shape and timescales of this decrease reveal information about injection barriers and traps in the material. Our data suggest that we are observing holes entering and exiting trap states as the gate voltage is turned on and off. Other factors, such as aging, increase the timescale of the screening. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L41.00006: Probing charge delocalization in a semi-crystalline supramolecular polymer Keehoon Kang, Shun Watanabe, Katrina Broch, Daisuke Matsumoto, Kazuhiro Marumoto, Hisaaki Tanaka, Shin-ichi Kuroda, Martin Heeney, Henning Sirringhaus Various doping methods have achieved metallic conductivity in $\pi$-conjugated polymer but most of them suffer from dopant-induced-disorder. We developed a simple and effective method of doping a high mobility semi-crystalline polymer, poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) by forming a bi-layer with a small-molecule acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F$_{4}$-TCNQ). The doping realizes an efficient charge-transfer between pBTTT and F$_{4}$-TCNQ (conductivity over 150 S/cm), while preserving the structural order of a pristine pBTTT. The charges are discovered to be sufficiently delocalized to give rise to a nearly-ideal Hall effect, and therefore, a coherent transport in a wide temperature range, with a high Hall mobility of 1.8 cm$^{2}$/Vs at room temperature. The combination of a Pauli magnetic susceptibility and magnetoconductance signatures strengthen the evidence of weak localization in the supramolecular system. Comparison with other amorphous conducting polymers elucidates the role of structural order as an indicator of the degree of charge delocalization. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L41.00007: A Blend Approach to P3HT Based Field Effect Transistor Performance Enhancement via Inclusion of 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene Ping-Hsun Chu, Lei Zhang, Jung Ok Park, Mohan Srinivasarao, Alejandro L. Brise\~no, Elsa Reichmanis Improved OFET performance through a polymer-small molecule semiconductor blend approach was demonstrated. However, a number of serious issues remain. For example, the threshold voltage (Vth) of the blend OFETs is still at a relatively high value (\textbar Vth\textbar \textgreater 10V), which is incompatible with most of portable electronics. Moreover, electrode treatment or thermal annealing is required to avoid a sacrifice in the device performance. Herein, a small molecule, 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT), is proposed to be incorporated within poly(3-hexylthiophene) (P3HT) polymer thin-films and is demonstrated to lead to overall improvement in transistor performance. The resultant blend OFETs exhibited approximately a 5-fold increase in charge carrier mobility, 10-fold increase in on-off current ratio and concomitantly, controlled the Vth as low as 1.7 V. It is worth noting that no pre- or post-treatment is required during the blend OFET fabrication process. Further, the thin-film deposition was conducted under ambient conditions using a volatile low boiling point solvent, suggesting a promising method for low-cost, high-throughput, large-area flexible device fabrication under non-stringent conditions. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L41.00008: Probing electric fields within organic transistors by nonlinear optics Paulo B. Miranda, Silvia G. Motti, Douglas J. C. Gomes Organic field-effect transistors (OFETs) are important building blocks in many organic devices, but further improvements in their performance will require a detailed knowledge of their operation mechanism. Thus mapping the electric fields in OFETs, both in the active organic layer and inside the gate dielectric, will allow a direct comparison with theoretical OFET models and guide advances in device engineering. The nonlinear optical processes of sum-frequency generation (SFG) and second-harmonic generation (SHG) may be used to probe electric fields in OFETs. With a proper choice of pump wavelength, SHG can selectively probe the field component along the OFET channel, inside the organic semiconductor. In contrast, SFG may probe the field within any organic material by selecting a specific molecular vibration and monitoring the field-enhanced SFG signal. Here we investigate OFETs fabricated with a polythiophene derivative (P3HT) on silicon substrates and with the insulating polymer PMMA for the dielectric layer. Both the strength and sign of the electric field in PMMA can be determined, yielding a direct probe of charge accumulation along the OFET channel. An extension of this technique to map the spatial distribution of accumulated charge along the channel will also be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L41.00009: Sub-threshold charge transport in polymer transistors Seohee Kim, Tae-Jun Ha, Prashant Sonar, Ananth Dodabalapur Research on polymer transistors has taken center stage due to their promise for use in displays, large-area electronics, and sensors. Most transistors with disordered semiconductor active layers such as amorphous silicon and polymers, have a large density of bulk trap states. Sub-threshold conduction in such transistors is very important. In particular, charge transport in the drift-limited sub-threshold regime is important and has not been adequately investigated. In this work, we will present an analysis of sub-threshold charge transport in polymer transistors with active layers based on the diketopyrrolopyrrole (DPP) core. Such transistors possess room temperature field-effect mobilities of over 2 cm$^{\mathrm{2}}$/Vs. We present an analysis of both above threshold and below threshold charge transport and show how the transport mechanisms change with temperature and charge density. We will also discuss a method to correctly calculate the density of trap states by sub-threshold modeling. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L41.00010: High Performance Short-Channel Organic Field-Effect Transistors with Graphene Electrodes Narae Kang, Saiful I. Khondaker Organic Field-Effect Transistors (OFETs) have received a great deal of attention due to their easy-processing, low-cost, flexibility, and transparency that can lead to future electronic applications such as flexible display, solar cell, and sensors. One of the major challenges in fabricating high-performance OFETs is to reduce a large injection barrier formed at metal/organic interface, which results in poor electrical transport performance. In order to overcome this issue, graphene has been suggested as a promising electrode material for OFETs due to its unique electronic properties as well as strong $\pi $-$\pi $ interaction with organic molecule, which can reduce the injection barrier at the electrode/organic interface. In this study, we fabricated short-channel OEFTs using mechanically exfoliated graphene electrodes, and performed temperature dependent transport studies. We will present the detailed temperature dependent data and discuss the charge carrier injection mechanism at graphene/organic interface. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L41.00011: Charge transport in ion-gel gated IDTBT transistors Shun Wang, Bei Bao, Xianyi Shao, Lu Tan, Yueshen Wu, Libin Wen, Xuxu Bai, Xiaojun Guo, Ying Liu Ionic liquids (ion gels) have been employed as the gate dielectric for polymer transistors due to its ultra-high capacitance. At high charge carrier density provided by ionic liquid gating, polymers like P3HT and PBTTT can exhibit very high mobility. We have fabricated ion-gel gated IDTBT transistors and measured its charge transport properties. We found that the mobility of ion-gel gated IDTBT transistors is greatly suppressed compared to the Cytop gated devices. At carrier density on the order of 10$^{21}$/cm$^3$, IDTBT shows mobility of about 0.05 cm$^2$/V/s. Detailed analysis of the temperature dependence of resistivity shows 3D Mott variable range hopping in IDTBT at such carrier density, indicating a different charge transport mechanism from Cytop gated device. [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L41.00012: Conditions for the Formation of P3HT Organogels During Spin-Coating: Tuning Electrical Properties in Thin Films Cameron S. Lee, Wen Yen, Adam Holt, Joshua Sangoro, Alexei Sokolov, Mark D. Dadmun Poly(3-hexyl thiophene) (P3HT) is widely studied as a model conjugated polymer in many electrical and photovoltaic applications, and has become the benchmark polymer when studying the physics of these devices. The assembly and growth of P3HT as organogels offers a structure that can bridge the electrodes, providing more efficient transport throughout the active layer. In this work, we identify and discuss a novel set of conditions for P3HT organogel network formation by controlling the spin-coating process from various solvents. The onset of organogel formation was monitored by in situ static light scattering, which measured both the thinning rate and off-specular scattering during film formation. Optical microscopy and thermal annealing experiments provide ex situ confirmation of organogel fabrication. The role of solution characteristics, including solvent boiling point, P3HT solubility, and initial P3HT solution concentration are examined to correlate these parameters to the rate of film formation, organogel-onset concentration, and overall network size. The properties of the film and their correlation to the fabrication parameters were also analyzed within the context of the hole mobility and density-of-states of the organogel, as measured from impedance spectroscopy. [Preview Abstract] |
Wednesday, March 4, 2015 10:48AM - 11:00AM |
L41.00013: Insights from transport modeling of unusual charge carrier behavior of PDTSiTzTz:PC$_{71}$BM bulk heterojunction materials Oleksiy Slobodyan, Sarah Moench, Kelly Liang, Eric Danielson, Bradley Holliday, Ananth Dodabalapur Development of hole-transporting copolymers for use in bulk heterojunctions (BHJs) has significantly improved organic solar cell performance. Despite advances on the materials side, the physics of charge carrier transport remains unsettled. Intrigued by its ability to maintain high fill factors in thick active layers, we studied the copolymer poly[2-(5-(4,4-dioctyl-4H-silolo[3,2-b:4,5-b']dithiophen-2-yl)-3-tetradecylthiophen-2-yl)- 5-(3-tetradecylthiophen-2-yl)thiazolo[5,4-d]thiazole] (PDTSiTzTz) blended with PC$_{71}$BM. Results show mobilities which are carrier-concentration-dependent and characterized by a negative Poole-Frenkel effect. Such behavior is not described by current carrier transport models. Established transport mechanisms like multiple-trap-and-release or variable range hopping yield dependence of mobility on carrier concentration. However, a more basic model like Gaussian distribution model (GDM) is needed to produce the negative Poole-Frenkel effect, though GDM cannot describe carrier-concentration-dependent mobility. We have combined key aspects of existing models to create a unified transport model capable of describing phenomena observed in PDTSiTzTz:PC$_{71}$BM. This model can be used to address open questions about transport physics of organic BHJ materials. [Preview Abstract] |
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