Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G41: Focus Session: Organic Electronics and Photonics - Small Molecule Semiconductors |
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Sponsoring Units: DPOLY DMP Chair: Enrique Gomez, Pennsylvania State University Room: 214A |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G41.00001: Using photovoltage to measure device-relevant exciton diffusion in luminescent and non-luminescent organic semiconducting materials Tyler Mullenbach, Russell Holmes Exciton diffusion is a prominent component of many emerging organic optoelectronic devices and is commonly described by the exciton diffusion length (L$_{\mathrm{D}})$. Excitons are commonly tracked by measuring their end-of-life products: photons (from radiative decay) or charge carriers (from exciton dissociation). While tracking luminescence provides an accepted means of measuring L$_{\mathrm{D}}$ for many materials, non-luminescent (dark) materials and states are inaccessible via such techniques. For dark materials, the charge carriers generated from exciton dissociation are tracked to estimate L$_{\mathrm{D}}$ (e.g., by fitting device external quantum efficiency), despite the fact that photogenerated carriers are often subject to recombination events prior to collection as current. Here, we present an alternate method of measuring L$_{\mathrm{D}}$, equally applicable to luminescent and dark materials, that uses photovoltage instead of current to determine the number of excitons reaching the dissociating interface in an organic photovoltaic device. Use of the photovoltage sidesteps charge carrier recombination providing an unobscured measurement of L$_{\mathrm{D}}$. The technique is verified against previous luminescence-based methods, and measurements of L$_{\mathrm{D}}$ are presented for a variety of dark materials including fullerenes. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G41.00002: Using Mass Transport to Guide the Purification of Small Molecule Organic Semiconductors via Sublimation Nathan T. Morgan, Yi Zhang, Matthew L. Grandbois, Bruce M. Bell, Russell J. Holmes, E. L. Cussler Organic electronic materials have garnered considerable commercial attention for next generation display and solid-state lighting applications. Widespread adoption of these technologies is slowed by considerable production costs, partially due to an expensive purification step. This work explores the current method of industrial purification, thermal gradient sublimation, in order to isolate the fundamental mechanisms limiting sublimation rate and controlling product deposition. For the archetypical hole transport materials, N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPD) and 4,4',4''-tris(carbazol-9-yl) triphenylamine (TCTA), a combination of viscous flow and physical vapor deposition are shown to be rate-limiting at constant sublimation temperature. Surprisingly, diffusion within the solid feed, reaction at the feed particle surface, and mass transfer within the bed of feed particles are not rate limiting in the case. This mechanism is different from that which is observed in many industrial sublimation systems. These results can be used to guide the design and operation of future large-scale purification systems, which are critical for the widespread adoption of organic optoelectronic devices. [Preview Abstract] |
(Author Not Attending)
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G41.00003: Separation of Carrier-Transport and Light-Emission Functions in a Light-Emitting Organic Transistor with Bilayer Configuration Hui Shang, Hidekazu Shimotani, Kanagasekaran Thangavel, Katsumi Tanigaki Organic single crystal based ambipolar light-emitting field effect transistors is treated as the candidate to realize laser. However, the active layer should contain both superb luminescent property and high charge-carrier mobility, which are always competing with each other in one material. Our basic concept for solving this problem is divide these two factors into two layers, and the combination of these two layers acts as the active layer of LEFET Bottom layer with high carrier mobility can be assigned as carrier transporter, and top layer with high PL efficiency was assigned as light emitter. After injection, the carriers will have a recombination in the bottom layer and formed exciton will transfer into the top layer with light emission. In this work, we have fabricated bilayer structure device, in which tetracene was used as bottom crystaland 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) doped tetracene was laminated on tetracene as light emitter. We have successfully observed light emission from top crystal, from which our aforementioned hypothesis was preliminary proved. Details will be reported in the presentation. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G41.00004: Tuning the opto-electronic properties of donor-acceptor polymers with molecular doping Invited Speaker: Elizabeth von Hauff Organic semiconductors offer vast potential for low cost, flexible energy production. The photocurrents in organic solar cells, however, are inherently limited by the poor electrical properties of the active layer. In this talk, strategies to increase the power conversion efficiency of polymer:fullerene solar cells by microscopically tuning the transport properties of the donor material are discussed. We observe that molecular doping the active layer of the device leads to increased charge separation efficiency and photocurrents. To investigate the influence of doping on the transport properties, impedance spectroscopy, a powerful, non-destructive technique, was applied. This allows us to probe carrier dynamics at different operational points in the current-voltage characteristics, and thereby correlate material properties with device performance. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G41.00005: Polymorphism in Core-Chlorinated Naphthalene Tetracarboxylic Diimide Thin Films Geoffrey Purdum, Falk May, Nan Yao, Thomas Weitz, Yueh-Lin Loo Polymorphism within organic semiconductors can play a critical role in device performance, as some packing motifs may be more favorable to charge transport than others. As-evaporated polycrystalline thin-films of core-chlorinated naphthalene tetracarboxylic diimides (NTCDI-1) adopt a triclinic polymorph that is not different from those of single crystals grown via physical-vapor transport. Exposing these thin-films to saturated vapors of select organic solvents, such as those of acetone and chloroform, induces structural transformation; thermally evaporated films convert from the triclinic polymorph to a monoclinic polymorph that was reported for solution-grown single crystals. Isothermal transformations are well described by second-order Avrami kinetics; molecular dynamic simulations give us insight into how solvents induce different kinds of favorable molecule-molecule interactions. Interestingly, the surface energy of the underlying substrate also plays a role in determining the rate of transformation; the rate of transformation is 2x and 4x faster on hexamethyldisilazane modified-Si/SiO$_{2}$ compared to on Si/SiO$_{2}$ and octadecyltrichlorosilane modified-Si/SiO$_{2}$, respectively. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G41.00006: Small Conjugated Molecules: Orbital Energy Modeling Using Tuned Range-Separated Functional Ram Bhatta, Mesfin Tsige Small conjugated molecules (SCMs) have potential to be efficient electron donors for organic solar cells because of their structural simplicity, good control over synthetic reproducibility and low purification cost. Density functional theory (DFT) and time dependent DFT (TDDFT) computations can guide for designing high-performing SCMs by modeling their orbital energies. However, the accuracy of computed orbital energies depends on the choice of the level of the theory. We present DFT and TDDFT calculations on 12 different SCMs using range-separated functional, LC-BLYP and the popular hybrid functional, B3LYP. Systematic calculations of the highest occupied molecular orbital (HOMO) energies, the lowest unoccupied molecular orbital (LUMO) energies as well as the singlet and triplet excitation energies are performed. We found that the LC-BLYP results are strongly dependent on the range-separation parameter. The computed results are compared with experimental data. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G41.00007: Low Field Electronic Behavior and Contact Impedance of Organic Single Crystal Transistors Emily Bittle, James Basham, Thomas Jackson, Oana Jurchescu, David Gundlach Organic electronic devices are attractive for a range of existing and emerging electronic applications. Most technological demonstrations of organic transistors rely on their large signal response for pixel control or logic. However, considerable application space requires analog circuits, e.g. distributed signal conditioning in sensor arrays. Charge transport and trapping mechanisms differ significantly in organic as compared to inorganic transistors, and as a result commonly used analogies to inorganic band transport theory can break down in response to small signal stimulus and at high frequencies required in some analog circuit applications. Therefore, a detailed investigation of organic transistor behavior at small signals is needed and is critical to developing design models for analog circuit applications. In this study, we look at the small signal AC impedance of small molecule, single crystal transistors to investigate ``ideal'' low field, high frequency electronic behavior. Using a transmission line model to fit the transistor channel coupled with a parallel resistor-capacitor model of the contact impedance, we are able to observe the behavior of the transistor channel and contacts separately at low field and high frequency. We determine the low field mobility of the device independent of contact resistance and show that rapidly changing contact resistance dominates the current flow at low gate voltage in DC current-voltage measurements. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G41.00008: Site energies and charge transfer rates near pentacene grain boundaries from first-principles calculations Hajime Kobayashi, Yuichi Tokita Charge transfer rates near pentacene grain boundaries are derived by calculating the site energies and transfer integrals of 37 pentacene molecules using first-principles calculations. The site energies decrease considerably near the grain boundaries, and electron traps of up to 300 meV and hole barriers of up to 400 meV are generated. The charge transfer rates across the grain boundaries are found to be reduced by three to five orders of magnitude with a grain boundary gap of 4 {\AA} because of the reduction in the transfer integrals. The electron traps and hole barriers also reduce the electron and hole transfer rates by factors of up to 10 and 50, respectively. It is essential to take the site energies into consideration to determine charge transport near the grain boundaries. We show that the complex site energy distributions near the grain boundaries can be represented by an equivalent site energy difference, which is a constant for any charge transfer pass. When equivalent site energy differences are obtained for various grain boundary structures by first-principles calculations, the effects of the grain boundaries on the charge transfer rates are introduced exactly into charge transport simulations, such as the kinetic Monte Carlo method. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G41.00009: Nanoscale domains in thin-film pentacene seen by mid-infrared near-field spectroscopy Fritz Keilmann, Bert Nickel, Christian Westermeier, Clemens Liewald, Sergiu Amarie, Adrian Cernescu The coexistence of structural phases in thin-film pentacene was known from X-ray diffraction, yet the scale of domain sizes remained unknown due to large-scale averaging. Infrared spectroscopy (classical FTIR) can distinguish different structural phases by slightly shifted molecular vibrational resonances but with spatial resolution not better than about 10 micrometer. When FTIR is paired with near-field microscopy performed by back-scattering infrared radiation from an AFM tip (``nano-FTIR'' allowing 20 nm resolution), Bulk-Phase (BP) domains were readily observed to form \textless 100 to 300 nm wide ellipsoids which significantly grow over months at atmospheric conditions, at the cost of the surrounding Thin-Film-Phase (TFP) pentacene. Both the domain interfaces and their continuing dimensional evolution may point to hidden problems for solar conversion systems development, possibly also with molecular materials beyond pentacene. C. Westermeier, A. Cernescu, S. Amarie, C. Liewald, F. Keilmann, and B. Nickel, \textit{Sub-micron phase coexistence in small-molecule organic thin films revealed by infrared nano-imaging}, Nature Communications 5, 4101, DOI:10.1038/ncomms5101 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G41.00010: Tunable Molecular Orientation and Elevated Thermal Stability of Vapor-Deposited Organic Semiconductors Diane Walters, Shakeel Dalal, Ivan Lyubimov, Juan de Pablo, Mark Ediger Physical vapor deposition is commonly used to prepare organic glasses that serve as active layers in organic electronic devices. Orienting the molecules in such layers can significantly enhance device performance. We apply a high-throughput characterization scheme to investigate the effect of the substrate temperature (T$_{Substrate}$) on glasses of three organic molecules utilized as semiconductors. Using spectroscopic ellipsometry, we find that molecular orientation in these glasses is continuously tunable and controlled by T$_{Substrate}$/T$_{g}$, where T$_{g}$ is the glass transition temperature. All three molecules can produce highly anisotropic glasses; the dependence of molecular orientation upon substrate temperature is remarkably similar and nearly independent of molecular length. All three compounds form ``stable glasses'' with high density and thermal stability similar to stable glasses of model glass formers. Simulations reproduce the experimental trends and explain molecular orientation in the deposited glasses in terms of the surface properties of the equilibrium liquid. By showing that organic semiconductors form highly orientated stable glasses, these results provide an avenue for systematic performance optimization of active layers in organic electronics. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G41.00011: GIWAXS characterization of amorphous, anisotropic, vapor-deposited organic semiconductor films Ankit Gujral, Kathryn O'Hara, Michael Chabinyc, Mark Ediger Vapor-deposited organic glasses can be produced with enhanced thermal stability and tunable molecular packing by controlling deposition conditions, such as the rate of deposition and the substrate temperature. Recent work in organic electronics has also shown improved charge carrier mobility associated with anisotropic molecular packing. In this work, grazing-incidence wide angle x-ray scattering (GIWAXS) is used to characterize the structural anisotropy in glasses of a hole transport material, TPD, prepared by physical vapor deposition. A Hermans' order parameter is used to quantify the changes observed in the scattering patterns of glasses prepared at different substrate temperatures. The order parameter correlates closely with spectroscopic ellipsometry measurements showing different molecular orientations depending upon the substrate temperature during deposition. Additionally, the GIWAXS measurements indicate there is a change in structure at the surface of the film compared with the bulk, providing insight into the formation of stable glasses. These findings may contribute in understanding the enhanced charge carrier mobility observed for anisotropic glasses used in OLEDs. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G41.00012: Device applications and structural and optical properties of Indigo -- A biodegradable, low-cost organic semiconductor Zhengjun Wang, Kelly L. Pisane, Konstantinos Sierros, Mohindar S. Seehra, Dimitris Korakakis Currently, memory devices based on organic materials are attracting great attention due to their simplicity in device structure, mechanical flexibility, potential for scalability, low-cost potential, low-power operation, and large capacity for data storage [1]. In a recent paper from our group, Indigo-based nonvolatile organic write-once-read-many-times (WORM) memory device, consisting of a 100nm layer of indigo sandwiched between an indium tin oxide (ITO) cathode and an Al anode, has been reported [2]. This device is found to be at its low resistance state (ON state) and can be switched to high resistance state (OFF state) by applying a positive bias with ON/OFF current ratio of the device being up to 1.02 $\times$ e6. A summary of these results along with the structural and optical properties of indigo powder will be reported. Analysis of x-ray diffraction shows a monoclinic structure with lattice parameters a(b)[c] = 0.924(0.577)[0.1222]nm and $\beta = 117^{\circ}$. Optical absorption shows a band edge at 1.70 eV with peak of absorption occurring at 1.90 eV. These results will be interpreted in terms of the HOMO-LUMO bands of Indigo.\\[4pt] [1] L. Ma et al, Appl. Phys. Lett. 84, 4908 (2004).\\[0pt] [2] Z. Wang et al, Appl. Phys. Lett. (submitted). [Preview Abstract] |
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