Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B11: Organic Electronics - Fundamentals of Electronic TransportFocus
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Sponsoring Units: DPOLY DMP Chair: Erin Ratcliff, University of Arizona Room: 270 |
Monday, March 13, 2017 11:15AM - 11:27AM |
B11.00001: Molecular order in MAPLE-deposited conjugated polymer thin films and the implication for carrier transport characteristics Ban Dong, Anton Li, Joseph Strzalka, Gila Stein, Peter Green The morphological structure of poly(3-hexylthiophene) (P3HT) thin films deposited by both Matrix Assisted Pulsed Laser Evaporation (MAPLE) and solution spin-casting methods are investigated. The MAPLE samples possessed a higher degree of disorder, with random orientations of polymer crystallites across the side-chain stacking, $\pi $-$\pi $ stacking, and conjugated backbone directions. Moreover, the average molecular orientations and relative degrees of crystallinity of MAPLE-deposited polymer films are insensitive to the chemistries of the substrates onto which they were deposited; this is in stark contrast to the films prepared by the conventional spin-casting technique. Despite the seemingly unfavorable molecular orientations and the highly disordered morphologies, the in-plane charge carrier transport characteristics of the MAPLE samples are comparable to those of spin-cast samples, exhibiting similar transport activation energies (56 meV versus 54 meV) comparable to those reported in literature for high mobility polymers.This suggests that the film morphology near the buried interface is different from the bulk or that the molecular order measured by GIWAXS and ellipsometry plays only a secondary role in dictating transport in organic thin film transistors. [Preview Abstract] |
Monday, March 13, 2017 11:27AM - 11:39AM |
B11.00002: Carrier-Selective Traps to Control Ambipolar Transport in Conjugated Polymers Michael Ford, John Labram, Ming Wang, Hengbin Wang, Thuc-Quyen Nguyen, Guillermo Bazan Ambipolar conjugated polymers used for organic field-effect transistors exhibit hole and electron mobilities that approach values relevant for commercial applications. Solution deposition and the wide range of chemical structures that can be tailored to fulfill specific application requirements has generated much interest in this class of materials. However, it has been difficult to control hole and electron transport to obtain high on/off ratios, necessary for efficient complementary circuit elements. By introducing carrier-selective traps (i.e., attenuating either p- or n-type transport while keeping the mobility of the opposite charge carrier largely unperturbed), we are able to effectively ``unipolarize'' the ambipolar transport. This simple solution-processable method improves the on/off ratios of organic field effect transistors by up to three orders of magnitude. Different polymer semiconductor/additive combinations are offered as examples of generality. Moreover, we demonstrate how the treatment of a given ambipolar polymer yields tailored blends that can be used to fabricate complementary inverters with excellent gain and low-power characteristics. [Preview Abstract] |
Monday, March 13, 2017 11:39AM - 11:51AM |
B11.00003: Modeling space-charge-limited current transport in spatially disordered organic semiconductors M. Zubair, Y.S. Ang, L.K. Ang Charge transport properties in organic semiconductors are determined by two kinds of microscopic disorder, namely energetic disorder and the spatial disorder. It is demonstrated that the thickness dependence of space-charge limited current (SCLC) can be related to spatial disorder within the framework of fractional-dimensional space. We present a modified Mott-Gurney (MG) law in different regimes to model the varying thickness dependence in such spatially disordered materials. We analyze multiple experimental results from literature where thickness dependence of SCLC shows that the classical MG law might lead to less accurate extraction of mobility parameter, whereas the modified MG law would be a better choice in such devices. Experimental SCLC measurement in a PPV-based structure was previously modeled using a carrier-density dependent model\footnote{Appl. Phys. Lett. 86, 092105 (2005)} which contradicts with a recent experiment\footnote{Adv. Funct. Mater. 26, 21 (2016)} that confirms a carrier-density independent mobility originating from the disordered morphology of the polymer. Here, this is reconciled by the modified MG law which intrinsically takes into account the effect of spatial disorder without the need of using a carrier-density dependent model. [Preview Abstract] |
Monday, March 13, 2017 11:51AM - 12:03PM |
B11.00004: Comparison of spin and charge transport in organic semiconductor P3HT Matthew Groesbeck, Haoliang Liu, Evan Lafalce, Dali Sun, Hans Malissa, Marzieh Kavand, Christoph Boehme, Zeev Valy Vardeny We have investigated spin and charge transport processes in regio-regular poly(3-hexylthiophene) (P3HT) in order to compare spin-transport and charge-transport in organic semiconductors. For the spin transport we measured the spin diffusion length, $\lambda_{s}$ via the inverse spin Hall effect (ISHE) in NiFe/P3HT/Pt trilayer devices, whereby a pure spin-current is generated in the polymer by spin-pumping from the ferromagnetic layer (NiFe), then diffuses to the Pt layer where it is converted into an electrical signal due to the strong spin-orbit coupling of Pt. For the charge transport we measured the carrier mobility of photogenerated charges via the time of flight technique. We also determined the (longitudinal) spin relaxation time, T$_{2}$ by pulsed EPR method, which allows us to calculate the spin diffusion coefficient D$_{s}$ from $\lambda_{s}$. Finally we relate D$_{s}$ to the charge diffusion coefficient D$_{c}$, which is determined from the charge mobility measurements. [Preview Abstract] |
Monday, March 13, 2017 12:03PM - 12:15PM |
B11.00005: Theoretical study of spin Hall effect in conjugated Organic semiconductors. M.R. Mahani, A. Delin The spin Hall effect (SHE), a direct conversion between electronic and spin currents, is a rapidly growing branch of spintronics. The study of SHE in conjugated polymers has gained momentum recently due to the weak spin-orbit couplings and hyperfine interactions in these materials. Our calculations of SHE based on the recent work [1], are the result of the misalignment of pi-orbitals in triads consisting of three molecules. In disordered organics, where the electronic conduction is through hopping of the electrons among randomly oriented molecules, instead of identifying a hopping triad to represent the entire system, we numerically solve the master equations for electrical and spin hall conductivities by summing the contributions from all triads in a sufficiently large system. The interference between the direct and indirect hoppings in these triads leads to SHE proportional to the orientation vector of molecule at the first order of spin-orbit coupling. Hence, our results show, the degree of molecular alignment as well as the strength of the spin-orbit coupling can be used to control the SHE in organics. [1] Z. G. Yu, PRL 115, 026601 (2015). [Preview Abstract] |
Monday, March 13, 2017 12:15PM - 12:27PM |
B11.00006: Organic thin films with charge carrier mobility exceeding that of single crystals Zachary Lamport, Oana Jurchescu, Cynthia Day, William Mitchell, David Sparrowe, Ruipeng Li, Detlef Smilgies, Veaceslav Coropceanu The highest reported mobilities in organic semiconductors are generally derived from single-crystal measurements where the transport is not limited by grain boundaries or mixed crystal orientations found in thin films. Because of this, single crystals are used as the benchmarks for performance of a material. Here we present an example where single crystal performance is inferior to that of thin-films. We evaluate the electrical performance of 7,14-bis-trimethylsilanylethynyl-dibenzo[a,h]anthracene from field-effect transistor measurements and find single crystal mobilities (10$^{-2}$ cm$^{2}$/Vs) two orders of magnitude lower than that obtained from thin films (1 cm$^{2}$/Vs). X-ray diffraction measurements confirm that our single crystals are of high quality and exhibit a pure [001] preferential orientation of the molecules, with $\pi $-$\pi $ stacking parallel to the substrate, whereas thin films display mixed [001] and [02-1] orientations. Density functional theory calculations show that the (100) direction is the main direction for hole transport. Thus, in thin-film devices we partially accessed the direction of fast transport, while in single crystals the reduced mobility is a result of the misalignment of the (100) direct lattice vector and the long axis of crystals, along which measurements were taken. Anisotropy measurements have confirmed that the high-mobility direction is not along the long axis but a different direction in the a-b plane. [Preview Abstract] |
Monday, March 13, 2017 12:27PM - 1:03PM |
B11.00007: Carrier coherence and high-resolution Hall effect measurements in organic semiconductors. Invited Speaker: Vitaly Podzorov Charge conduction in organic semiconductors frequently occurs in a regime at the borderline between a band-like coherent motion of delocalazied carriers in extended states and an incoherent hopping through localized states. Many intrinsic factors are competing for defining the dominant transport mechanism, including the strength of intermolecular interactions represented by the transfer integrals, carrier self-localization due to formation of polarons, electron-phonon coupling, scattering and off-diagonal thermal disorder (see, e.g., [1]). Depending on the interplay between these processes, either band-like or hopping charge transport realizes. Besides these intrinsic factors, a significant role in practical devices is played by the static disorder (chemical impurities and structural defects) that leads to carrier trapping at various energies and time scales. In most of these cases, the charge carrier mobility in OFETs is rather small (0.1 - 20 cm$^{\mathrm{2}}$V$^{\mathrm{-1}}$s$^{\mathrm{-1}})$ \cite{}, and in order to carefully and accurately characterize it, Hall effect measurements are necessary. Conventional Hall measurements are extremely challenging in systems with such low mobilities. Here, we present a novel Hall measurement technique that can be carried out in low magnetic fields with an amazing sensitivity, much greater than that attained in conventional Hall measurements [2]. We apply this method to mobility measurements in a variety of OFETs with mobility as low as \textasciitilde 0.3 cm$^{\mathrm{2}}$V$^{\mathrm{-1}}$s$^{\mathrm{-1}}$ [2] and reveal various peculiarities of Hall effect in low-mobility systems. By taking advantage of this powerful new experimental capability, we have understood several ``mysteries'' of Hall effect observed by various groups in OFETs over the last decade [3]. REFERENCES: [1]. V. Podzorov, ``Organic single crystals - addressing the fundamentals of organic electronics''. \textit{MRS Bulletin} \textbf{38}, 15-24 (2013). [2]. Y. Chen, H. T. Yi and V. Podzorov, ``High-Resolution ac Measurements of the Hall Effect in Organic Field-Effect Transistors'', \textit{Phys. Rev. Applied} \textbf{5}, 034008 (2016). [3]. H. T. Yi, Y. N. Gartstein and V. Podzorov, ``Charge carrier coherence and Hall effect in organic semiconductors'', \textit{Sci. Reports}, srep23650 (2016). [Preview Abstract] |
Monday, March 13, 2017 1:03PM - 1:15PM |
B11.00008: Charge transport physics of single crystal organic semiconductors Emily G. Bittle, Adam J. Biacchi, Andrew A. Herzing, Lisa A. Fredin, Thomas C. Allison, Angela R. Hight Walker, David J. Gundlach Determining the physics of charge transport in organic semiconductors has proven to be a difficult endeavor. The similar energy ranges of the many processes involved in charge transport, including excitonic coupling, charge-phonon coupling, and trap state distributions, result in ambiguity in the interpretation of temperature dependent electrical measurements. In addition, energetic mismatches at electrical interfaces and unique geometries of devices used for measurement often impact the final device characteristics more strongly than the intrinsic transport of the semiconductor. In order to disentangle competing physical effects on device characterization at low temperature, we use TEM and Raman spectroscopy to track changes in the structure and thermal molecular motion in single crystal tetracene, correlated with calculation. We then perform careful DC and AC electrical characterization of single crystal tetracene devices built with a variety of contacting materials in order to fully understand the origin of resulting electrical characteristics. [Preview Abstract] |
Monday, March 13, 2017 1:15PM - 1:27PM |
B11.00009: Lattice phonons of coronene single crystal polymorphs: a theoretical approach Nicola Bannister, Enrico Da Como, Simon Crampin Coronene, a polyaromatic carbon based molecule of disk shape, exhibits a range of peculiar physical properties from room temperature phosphorescence [1] to superconductivity [2]. The fundamental interest in this molecule is linked to its diamagnetism, originating from the delocalized pi electrons. Recently, we reported the discovery of a new crystal structure of coronene, the beta phase, apparently favoured by the presence of an external magnetic field during crystal growth [3]. Ab-initio density functional theory (DFT) calculations of the lattice energy for the two coronene polymorphs, the known gamma and the new beta phase, indicate that the latter has a lower energy minimum and thus should be favoured. Instead experimentally we find that the gamma phase is stable at room temperature and converts into beta at ~150K. This observation calls for a more complete description of the relative energetic stability of the polymorphs including the role of phonons. We present our efforts in describing the lattice phonons of the two structures by performing DFT simulations and comparing them with data from low frequency Raman spectroscopy. [1] Mieno et al. Adv. Opt. Mat. 4, 1015 (2016) [2] Kubozono et al. Phys Chem Chem Phys 13, 16476 (2011) [3] Potticary et al. Nature Comm. 7, 11555 (2016) [Preview Abstract] |
Monday, March 13, 2017 1:27PM - 1:39PM |
B11.00010: Good vibrations in molecularly doped polymers: combining impedance and Raman spectroscopy to elucidate intermolecular interactions Elizabeth von Hauff, Charusheela Ramanan Organic offer many advantages for energy conversion, saving and storage applications. However, the poor electrical properties of organic semiconducting films, particularly low carrier mobilities, trapping and recombination phenomena, are a critical limitation for real applications. Surprisingly, carrier transport is still not well-understood in these systems, making it difficult to develop design strategies for high performance applications. There is an increasing number of reports indicating the importance of considering the effects of molecular dynamics and vibrations on the evolution of excited states in this class of materials. New experimental strategies are urgently needed to correlate dynamic relationships between molecular structure and electrical transport. I will present our current work on developing a new measurement approach which combines Raman and impedance spectroscopies. By monitoring the vibrational fingerprint of the organic semiconductor as a function of electrical perturbation, we investigate the influence of charge transport on molecular vibrations. With these results we aim to correlate molecular interactions with macroscopic electrical properties and device performance. [Preview Abstract] |
Monday, March 13, 2017 1:39PM - 2:15PM |
B11.00011: Visualizing electron dynamics in organic materials: Charge transport through molecules and angular resolved photoemission Invited Speaker: Stephan Kümmel Being able to visualize the dynamics of electrons in organic materials is a fascinating perspective. Simulations based on time-dependent density functional theory allow to realize this hope, as they visualize the flow of charge through molecular structures in real-space and real-time. We here present results on two fundamental processes: Photoemission from organic semiconductor molecules [1] and charge transport through molecular structures [2]. In the first part we demonstrate that angular resolved photoemission intensities - from both theory and experiment - can often be interpreted as a visualization of molecular orbitals. However, counter-intuitive quantum-mechanical electron dynamics such as emission perpendicular to the direction of the electrical field can substantially alter the picture, adding surprising features to the molecular orbital interpretation. In a second study we calculate the flow of charge through conjugated molecules. The calculations show in real time how breaks in the conjugation can lead to a local buildup of charge and the formation of local electrical dipoles. These can interact with neighboring molecular chains. As a consequence, collections of "molecular electrical wires" can show distinctly different characteristics than "classical electrical wires".\\ 1. M. Dauth et al., Physical Review Letters {\bf 117}, 183001 (2016)\\ 2. P. Schaffhauser, S. K\"ummel, Phys. Rev. B {\bf 93}, 035115 (2016) [Preview Abstract] |
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