Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N24: Organic LEDS and Light Emission |
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Sponsoring Units: DPOLY DMP Chair: Eric Lin, National Institute of Standards and Technology Room: Colorado Convention Center 201 |
Wednesday, March 7, 2007 8:00AM - 8:12AM |
N24.00001: Green polariton photoluminescence in organic microcavities containing the red-emitting phosphor PtOEP Stephane Kena-Cohen, Stephen R. Forrest Green upper and lower branch polariton photoluminescence (PL) is observed in microcavities containing the red-emitting organic phosphor 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphyrin platinum(II) (PtOEP). This PL is attributed to cavity polariton states formed by coupling to the Q(0,0) transition of PtOEP (fluorescence). The PL spectra mirror the polariton dispersion obtained from angle-resolved reflectivity measurements. The increased fluorescence intensity compared to the neat film case is due to the reduced lifetime of the polariton states. This PL is also accompanied by strong red emission (phosphorescence) due to intersystem crossing (ISC) to the triplet state of PtOEP. [Preview Abstract] |
Wednesday, March 7, 2007 8:12AM - 8:24AM |
N24.00002: Morphology, structure and photoluminescence properties of thin films of a conjugated polymer poly(2,5-dinonyl para phenyleneethynylene) Craig Szymanski, Yunfei Jiang, Jasson McNille, Dvora Perahia, Uwe H. F. Bunz Poly(para phenyleneethynylenes) (PPE), are inherently semiconductor and their electro-optical characteristics depends among other factors on their association mode. PPE molecules were trapped on the surface from molecular solutions, aggregates and gels. When cast from toluene solutions bellow the critical micellar concentration (CMC) of the polymers, small clusters with an average diameter of the molecular length were observed. When cast from gel phases, a supra molecular structure that consists of well defined associating rods were detected. All of these association modes emit at 460nm. Further emissions are detected as the structure evolves from collapsed single molecules to supramolecular structure where for small non interacting rods, feature at 511 in observed and for associating rods it shifts to 550nm. Further studies are currently underway to correlate the fluoresce patterns with the structural features. [Preview Abstract] |
Wednesday, March 7, 2007 8:24AM - 8:36AM |
N24.00003: Photoemission study of tris(8-hydroxyquinoline) aluminum/aluminum oxide/tris(8-hydroxyquinoline) aluminum interface Huanjun Ding, Serkan Zorba, Yongli Gao, Liping Ma, Yang Yang The evolution of the interface electronic structure of a sandwich structure involving aluminum oxide and tris(8-hydroxyquinoline) aluminum (Alq), i.e. (Alq/AlO$_{x}$/Alq), has been investigated with photoemission spectroscopy. Strong chemical reactions have been observed due to aluminum deposition onto the Alq substrate. The subsequent oxygen exposure releases some of the Alq molecules from the interaction with aluminum. Finally, the deposition of the top Alq layer leads to an asymmetry in the electronic energy level alignment with respect to the AlO$_{x}$ interlayer. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N24.00004: Hybrid Organic/Inorganic Semiconductor Structures: Efficient F\"orster Energy Coupling and Prospective Optical Devices Invited Speaker: Hybrid organic/inorganic semiconductor structures offer the prospect of combining the favourable electrical and optical characteristics of each to achieve desirable new device functions. They also provide a test bed to study the exciton state interactions between these distinct semiconductor varieties and the resultant energy transfer processes that occur between them. In this talk I will describe work on conjugated polymer/GaN structures for which efficient non-radiative F\"orster transfer can be achieved from the inorganic quantum well excitons to the organic (polymer) excitons. I will also discuss the prospects for optical devices based on this and related hybrid systems. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N24.00005: ABSTRACT WITHDRAWN |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N24.00006: Luminescence from single colloidal nanocrystals embedded in organic light emitting devices. August Dorn, Hao Huang, Vladimir Bulovic, Moungi Bawendi The photophysical properties of individual CdSe/ZnS (core/shell) nanocrystals embedded in the active layers of electrically driven organic light emitting devices (OLEDs) were investigated at room temperature. Emission from the same nanocrystals was recorded under laser illumination and when the OLED was driven electrically. For both types of excitation we observed blinking and spectral diffusion, key signatures of single quantum dot fluorescence. Enhanced electroluminescence from the organics at the sites of nanocrystals suggests the formation of current channels through the quantum dots. This hypothesis is supported by atomic force microscopy studies of the organic layers. [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N24.00007: Charge injection and Raman scattering studies from polyfluorene-based light-emitting diodes M. Arif, S. Guha Efficient and well balanced injection of charge carriers and transport capabilities are of particular importance for high luminescence efficiency in organic light-emitting diodes. Polyfluorene (PF) conjugated polymers have received widespread attention due to their strong blue emission, high charge mobility and excellent chemical and thermal stability which creates great prospect for optoelectronic device applications. Although ethyl-hexyl substituted PF (PF2/6) has a high level of molecular disorder, charge injection in single layer polymer devices can be described very well by space-charge-limited conduction for a discrete set of trap levels. This is attributed to the nature of ordering in the polymer. PFs are characterized by a number of Raman-active peaks originating from C-H bending and C-C stretching type motion. We further analyze our working devices using Raman scattering in the presence of photogenerated carriers. The Raman intensities in the 1000-1250 cm$^{-1}$ corresponding to a C-H bend-type motion quench in the presence of carriers with increasing fields. This effect most probably arises due to the interaction of phonons and free carriers. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N24.00008: Multi-walled carbon nanotube sheets as transparent anodes in organic light-emitting diodes Christopher Williams, Raquel Ovalle Robles, Mei Zhang, Sergey Li, Ray Baughman, Anvar Zakhidov Carbon nanotubes have emerged as useful components for next-generation electronic devices. We have investigated one such area by producing organic light-emitting which use transparent multi-walled carbon nanotube sheets as a replacement for indium tin oxide (ITO). These sheets offer high optical transparency with the additional advantage of being very flexible with no loss in conductivity, making them ideal candidates for devices built on plastic substrates. We have produced devices on both high quality display glass and plastic substrates and have observed bright emission with efficiencies which are comparable to those obtained from ITO-based devices. We also present results for devices which combine ITO and nanotube sheets. Such devices take advantage of the planar conductivity of the ITO and improved injection from the nanotube sheets. We show improved efficiency in plastic devices which use this bilayer anode structure. Our results also demonstrate the importance of using a planarization layer on top of the carbon nanotube sheet to eliminate sources of leakage current. Lastly, we propose alternative device architectures such as transparent and inverted structures. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N24.00009: Conformations in di-octyl substituted polyfluorene: a combined theoretical and experimental Raman scattering study C. Volz, M. Arif, S. Guha The structural properties of polyfluorenes (PF) are extremely sensitive to the choice of functionalizing side chains. Di- octyl substituted PF (PF8) adopts metastable structures that depend upon the thermal history and choice of solvents used in film forming conditions. We present a detailed study of the changes in the backbone and side chain morphology in PF8, induced by the various crystallographic phases, using Raman scattering techniques. The vibrational frequencies and intensities of fluorene oligomers are calculated using hybrid density-functional theory with a 3-21G* basis set. The alkyl side chains are modeled as limiting conformations: all \emph {anti}, \emph{anti-gauche-gauche}, and end \emph{gauche} representations. The calculated vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the $\beta$ phase, which is known to originate in regions of enhanced chain planarity, as a direct consequence of the alkyl side chain conformation. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N24.00010: Influence of Environment on the Electronic Structure of Polyfuorenes Elizabeth M. Lupton, Feng Liu The influence of the structure of polyfluorene molecules on their emissive characteristics, as utilized in polymer LEDs, can be characterized using first principles methods. Here we concentrate on how factors such as external restrictions, structural and chemical defects, and constraints caused by side groups can affect the electronic structure of polyfluorenes, in particular the extent of conjugation along the backbone. Using Car -- Parrinello Molecular Dynamics simulations, where the electronic structure is calculated according to Density Functional Theory `on the fly' for a molecular dynamics trajectory, we systematically investigate how curving the backbone combined with torsional rotation between repeat units, as well as ketone defects, can affect the electronic structure. This demonstrates the way in which restrictions imposed by the environment could ultimately affect the light emitting properties of the polymer. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N24.00011: NEXAFS measurements of chain alignment in order polyfluorene thin films Xiaosong Liu, Hyeunseok Cheun, Frank Galbrecht, F. J. Himpsel, Ullrich Scherf, Michael Winokur Carbon K-edge near edge X-ray absorption fine structure (NEXAFS) has been used to characterize the uniaxial surface chain alignment within the top surface (2-3 nm) of poly[bis(2-ethyl)hexylfluorene] thin films spin-cast atop rubbed polyimide templating substrates before and after thermal annealing. The film thicknesses range from approximately 25 to 130 nm. In the thinnest films appreciable chain alignment extends through to the top surface prior to annealing. Thermal annealing produces comparatively high levels of surface chain alignment in all film thicknesses despite a drop in the dichroic ratios, as measured by polarized optical absorption spectroscopy, in the thickest films. These data support a model that exhibits a graded morphology in which the top and bottom surfaces exhibit planar, uniaxial alignment while the film interior is less well aligned and includes a proportion of homeotropic alignment. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N24.00012: Optical studies of platinum-containing conjugated polymers Minghong Tong, Alessio Gambetti, Tomer Drori, Zeev Vardeny We have used a variety of steady state and ultrafast spectroscopies for studying the photophysics of platinum-containing conjugated polymers, which have potential applications as the active layer of light-emitting diodes. The intrachain heavy metal Pt atom increases the spin-orbit coupling, and this influences both the intersystem crossing time, T$_{ic}$, and the phosphorescence emission strength. From the ps transient pump-probe photomodulation spectroscopy and emission dynamic measurements using the up-conversion technique, we found that T$_{ic}$ for these polymers is of order of few ps; whereas the phosphorescence lifetime is of order of few microseconds. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N24.00013: Crystalline PTCDA waveguides grown by organic molecular beam deposition V.R. Gangilenka, J. Markus, H. Schmitzer, H.P. Wagner During the past few decades molecular organic semiconductors have become interesting candidates for optical and opto-electronic device applications such as organic light emitting diodes (OLEDs). Of many organic materials 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) is one of the most intensively investigated organic molecule in literature. However, so far studies on PTCDA optical waveguides are very limited. We investigate various PTCDA waveguide structures by m-line spectroscopy. The waveguides are fabricated by organic molecular beam deposition (OMBD) under high vacuum. The waveguides are deposited on a pyrex substrate or on a rutile prism with an index matching film of aluminum-quinoline (Alq3). The effective indices of refraction of observed TE and TM modes are consistent with classical electromagnetic theory. The calculated thicknesses of the waveguides under investigation are compared with reflection measurements on the same samples. [Preview Abstract] |
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