Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F33: Focus Session: Organic Electronics and Photonics - Light Emission and Management |
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Sponsoring Units: DMP Chair: Russell Holmes, University of Minnesota Room: 341 |
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F33.00001: POLYMER PHYSICS PRIZE BREAK
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Tuesday, March 19, 2013 8:36AM - 8:48AM |
F33.00002: Pure Bending Loss in Nanowire Waveguides Jaeyeon Pyo, Ji Tae Kim, Jewon Yoo, Jung Ho Je One of the major concerns in designing waveguides is unavoidable bending that causes energy loss due to the distortion of modal field. Bending loss in nanowire waveguides has been studied while including substrate coupling loss. Pure bending loss unaffected by substrate coupling in nanowire waveguides still remains unclear. A challenging task in study of pure bending loss is to introduce bending on nanowire waveguides in the air and to tune the radius of bending. We report the characterization of pure bending loss in nanowire waveguides by bending a vertical freestanding nanowire in the air. Specifically, vertical freestanding active nanowire waveguides of MEH-PPV have been fabricated by our meniscus-guided method. To characterize pure bending loss, desired bending was remotely introduced by applying electrostatic force near the top end of the waveguide. Finite-difference-time-domain simulation was performed to confirm the experimental result. We show that the bending losses, by conventional experimental approaches of nanowires rested on substrates, were strongly overestimated attributed to the coupling of the enhanced evanescent field to the substrate. We suggest that our system could be also utilized for studying various intrinsic properties of nanowire waveguides. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F33.00003: Molecular shear and the induced massive enhancement of conjugated polymer MEH-PPV photoluminescence by solvent-dewetting Chi-Ching Liu, Tsang-Lang Lin, Gunter Reiter, Arnold C.-M. Yang The molecular flows triggered by dewetting above Tg in ultrathin polymer films were shown previously generating huge photoluminescence (PL) enhancements for conjugated polymers contained within. By means of annealing in solvent vapor at room temperature, MEH-PPV molecules dispersed in inert polystyrene (PS) manifested massive PL enhancements up to $\sim$ 10 folds when flowed into tiny droplets and residual layer. The enhancement was independent of MEH-PPV chain length but, in contrast to thermal dewetting, decreasing with MEH-PPV concentration ($c$). In addition, the blue shift accompanying thermal dewetting was also reduced. As annealing continued on, the blue shift reversed, illustrating the increase of conjugation length under stretching. The transient blue shift increased with PS molecular weight, unveiling the alteration of inter-segmental chain entanglements up to this stage of dewetting. Surprisingly, vapor of poorer solvent induced larger PL enhancements with narrower transient blue shifts, revealing that solvent was effective in inducing molecular flows, even when in the plasticizing Feakean precursor, relaxing the residual stresses and simultaneously stretching polymer chains for dramatically enhanced optoelectronic efficiencies. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F33.00004: Multilayer polymer light emitting devices Zachary Barcikowski, Adam Thomas, Marian Tzolov The interplay of device layers and their interfaces is a major area of study in Polymer Light Emitting Devices (PLEDs). Many factors such as the degradation, efficiency, and overall performance depend on how these layers interact with each other. A fundamental understanding of the interfaces of these layers can lend to better performing devices using a multitude of organic polymers deposited in conjunction with each other in several ways. We have studied basic PLED devices in which we vary the emissive layer used, along with final bake temperatures. Devices include a glass substrate with Indium Tin Oxide anode, Aluminum cathode, and Plexcore Hole Injection layer. The active polymer films were spin casted from solution of MEH-PPV and PFO. Single layer and dual layers of several polymers are studied by examining current-voltage characteristics, film densities, impedance measurements, light emission, and efficiency calculations. We have found that not only do dual layers positively alter the performance of the device in the majority of cases, but the solvents in which each layer is originally in when deposited affects the formation of the interface, thereby altering the device mechanisms. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F33.00005: Color change in organic light-emitting diodes using the magnetic field effect Tek Basel, Dali Sun, Bhoj Gautam, Eitan Ehrenfreund, Z. Valy Vardeny The magnetic field effect has been widely used to generate magneto-electroluminescence (MEL) in organic light-emitting diodes (OLEDs). We have used the MEL effect to change the emission color from OLED in which the active layer is composed of a host polymer with fluorescence (FL) emission and a guest, heavy atom-based molecule with phosphorescence (PH) emission. The color change has been studied as a function of the guest/host weight ratio, and the optimal ratio was determined. The underlying mechanism of the magnetic-field induced color change is the difference that exists between the MEL intensity of the FL emission band respect to that of the PH emission band. The MEL difference between the two types of emission bands will be thoroughly discussed within models used to explain the MEL in organic devices. Research sponsored by National Science Foundation-Material Science {\&} Engineering Center (NSF-MRSEC), University of Utah. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 10:00AM |
F33.00006: Highly efficient organic light-emitting diodes by delayed fluorescence Invited Speaker: Chihaya Adachi Although typical organic molecules only contain carbon (C), hydrogen (H), nitrogen (N) and oxygen (O) atoms, the unique bonding of C involving sp3, sp2 and sp hybrid orbitals enables generation of very complicated molecular architectures that have extensive functions in a wide variety of organisms and industrial products. In the last two decades, the allure of the unlimited freedom of design of organic molecules has shifted a significant proportion of electronics research from inorganic into organic materials. In particular, great advances have been achieved in organic light-emitting diodes (OLEDs). First-generation OLEDs containing fluorescent molecules have progressed to second-generation ones using phosphorescent molecules, which is an attractive design for practical electronics. Herein, new organic electroluminescent (EL) molecules lacking precious metals are presented. The energy gap between the singlet (S1) and triplet (T1) excited states is minimized by strategic design, promoting highly efficient spin up-conversion from T1 to S1 states while maintaining a high radiative decay rate of \textgreater 106/s, leading to a high fluorescence efficiency of \textgreater 90{\%}. Using these unique molecules, a very high external EL efficiency of \textgreater 19{\%} is realised, which is comparable to those of high-efficiency phosphorescence-based OLEDs. These molecules harvest both singlet and triplet excitons for light emission through fluorescence decay channels. We call this new luminescence concept ``hyperfluorescence.'' [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F33.00007: Kinetic Monte Carlo simulation of organic devices Alison Walker, Edward Wright At Bath, we have developed a model of organic devices that links morphology (packing arrangements) to device characteristics. The model, based on the dynamical Monte Carlo approach pioneered in surface physics, allows us to include interaction processes between different species on many different timescales. In this talk I will show how we have used this approach to compare organic solar cells of rod, blend and gyroid morphologies and to model the influence of interlayers, layers added to improve efficiency and lifetime, in organic light emitting devices, OLEDs. We have developed the model to allow it to distinguish between triplet and singlet excitons and allows for the interactions of these species (triplet-triplet annihilation, triplet-singlet annihilation, triplet-polaron quenching). I will show our predictions for current-voltage-illumination characteristics (solar cells) and current-voltage-luminance characteristics (OLEDs). I will also show how through prediction of emission zone profiles in an OLED, we can gain insight into what determines changes in OLED efficiency with current and how in the longer term this approach can be used to address degradation. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F33.00008: Carrier Conduction and Light Emission by Modification of Poly(alkylfluorene) Interface under Vacuum Ultraviolet Light Irradiation Yutaka Ohmori, Hirotake Kajii, Daiki Terashima, Yusuke Kusumoto Organic field effect transistors (OFETs) have been extensively studied for flexible electronics. The characteristics of poly(9,9-dioctylfluorenyl-2,7-dyl) (F8) modified by thermal or light are strongly dependent on the carrier transport and optical characteristics. We investigate all solution-processed OFETs with Ag nano-ink as gate electrodes patterned by Vacuum Ultraviolet (VUV) (172 nm). Bi-layer gate insulators of amorphous fluoro-polymer CYTOP (Asahi Glass Corp.) and poly(methylmethacrylate) (PMMA) were used. Top-gate-type OFETs with ITO source/drain electrode utilizing F8 or poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as an active layer were fabricated, and investigated the carrier conduction and emission characteristic. Without VUV irradiation, both OFETs showed the ambipolar and light-emitting characteristics. On the other hand, F8 devices with VUV exhibited only p-type conduction. The quenching centers were generated in F8 layer by VUV irradiation, which are related to the electron trap sites at the interface. OFETs with F8BT showed both p- and n-type conduction even after VUV. F8BT suffers less damage by VUV and maintain light emission. Light emitting transistors were realized utilizing F8BT patterned by VUV irradiation. [Preview Abstract] |
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