Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A5: Physics of Emerging Organic Displays - OLEDs and PLEDs |
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Sponsoring Units: FIAP Chair: Geoffrey Nunes, Dupont Room: LACC 502B |
Monday, March 21, 2005 8:00AM - 8:36AM |
A5.00001: Polymer Based Light Emitting Diodes (PLEDs) and Displays Fabricated from Arrays of PLEDs Invited Speaker: Semiconducting (conjugated) polymers are of considerable importance as the active materials in electronic and optical devices including polymer-based light-emitting diodes (PLEDs), photodetectors, photovoltaic cells, sensors, field effect transistors, and lasers. Because of the opportunities associated with passive and active matrix display applications, the development of PLEDs that show efficient, stable blue, green, and red emission is an important ongoing research effort. PLEDs which emit white light are of interest for use in high efficiency active matrix displays (with color filters) and because they might eventually be used for solid state lighting. The ability to fabricate large-area white light emitting PLEDs by processing the active materials from solution is an essential advantage (and requirement) for the use of PLEDs in solid state illumination. I will summarize progress in the field of PLEDs from the fundamental science to recent achievements. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 9:12AM |
A5.00002: Achieving High Efficiency OLEDs for Displays and Solid State Lighting Invited Speaker: Recent results suggest that organic light emitting devices can provide the very highest efficiencies of any other active display medium, with the exception of those media using ambient light to provide contrast. In this talk, I will consider methods and recent advances in achieving very high efficiency OLEDs for displays and white light generation. In particular, I will consider the physics and technology of employing phosphors in both small molecular weight and polymer organic systems for obtaining the highest possible efficiencies[1,2]. Further, outcoupling schemes for maximizing high external efficiencies will be discussed. For solid state lighting applications, methods and challenges for generating white light via electrophosphorescence are considered. In particular, very simple and high efficiency sources can be obtained using a single dopant based on planar Pt complexes that forms a spectrally broad emitting states. Using this approach, we have a unique opportunity to use OLEDs not only for the next generation of displays, but also for very efficient, environmentally friendly room illumination applications. \newline \newline $^{1 }$B. D'Andrade and S. R. Forrest, Adv. Mat. \textbf{16}, 1585 (2004). \newline $^{2 }$M. Segal, M. A. Baldo, R. J. Holmes and S. R. Forrest, Phys. Rev. B \textbf{68}, 075211 (2003). [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:48AM |
A5.00003: Electron-hole capture in polymer heterojunction light-emitting diodes Invited Speaker: Polymer light-emitting diodes based on blends of polyfluorene derivatives show very high efficiencies and low drive voltages. Electron-hole capture in these devices directly produces long-lived exciplex states where the electron and hole are predominantly localized on opposite sides of the heterojunction. The exciplex may then be thermally excited to form an intra-chain exciton, which can itself either emit, or be recycled to reform the exciplex. I will review the physics of exciplex formation and emission in these devices, and will show that exciplex formation rates are consistent with low free charge densities at the heterojunction. I will present evidence that the rate of charge transfer at polyfluorene heterojunctions can be modulated with an applied electric field, leading in some cases to an increase in photoluminescence efficiency with applied field. I will also present recent results showing enhanced triplet exciton formation after photoexcitation in polyfluorene blends, and will discuss the implications of the results for polymer light-emitting and photovoltaic devices. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:24AM |
A5.00004: Industrialization of OLEDs for Lighting Applications and Displays Invited Speaker: Organic light emitting diodes (OLEDs) are an extremely versatile technology that can be tailored to specific applications. The flexibility and adaptability of OLED technology is a result of the variety of material systems and fabrication technologies that can be applied. In this contribution we investigate and compare several material systems and fabrication technologies from an application point of view. Applications without the need of micro-scale structuring open a new window of opportunity for evaporated small molecules. Small molecular OLEDs have the potential for high efficiencies at high brightness rendering them ideal for lighting applications . The first part of our contribution will establish the boundary conditions for lighting applications and we will introduces the current status of our industrialization program for OLEDs for lighting and present our perspective of the OLED lighting market. In the second part of the contribution we will focus on alternative OLED technologies that offer interesting perspectives for industrial fabrication. The light-emitting electrochemical cell (LEC) is a type of organic electroluminescent device that has all the attractive features of the OLED but does not have the drawbacks of reactive cathodes and thin active layers. The crucial difference with OLEDs is that the active layer of a LEC contains mobile ions. This results in two very important advantages for large-area lighting applications compared with traditional OLEDs: (i) thick electroactive layers (ii) and matching of the work function of the electrodes with the energy levels of the electroluminescent material is not required. This means that non-reactive metals such as Ag or Au can be used instead of e.g. Ba. We have studied several types of LECs with the aim to assess the above-mentioned benefits for large-area lighting . Finally to show the immense spectrum of production methods for OLEDs we will conclude the contribution with a manufacturing technique for solution processable material systems: inkjet printing. \newline \newline In collaboration with Eric Meulenkamp, Rene Wegh, Steve Klink, Simone Vulto, and Dietrich Bertram. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 11:00AM |
A5.00005: Organic Thin Film Transistors for Electronic Systems Invited Speaker: The surge of interest in organic thin film transistors (TFTs) has been motivated, on the one hand, by fundamental questions concerning the energetics and transport of localized carriers, and, on the other hand, by the practical advantages of electronic systems fabricated at low temperatures on flexible substrates. The overriding consideration for the usefulness of organic thin-film transistors for electronic systems has been the field-effect mobility. In this paper I will discuss materials-related factors other than mobility that influence the usefulness of organic TFTs. The subthreshold slope determines the voltage excursion that must take place below the threshold voltage to fully turn off the transistor. Typical organic TFTs have subthreshold slopes that are small compared to silicon devices, due to strongly localized states in energy gap between the more extended levels. The excursion required below threshold often has about the same magnitude as that required above threshold to reach a given level of on-current, and the speed of the system, as well as the power supply requirements, can be adversely affected by the additional required voltage swing. Organic TFTs use metallic or conducting polymer contacts that overlap the gate region, unlike the doped source and drain regions that are self-aligned to the gate in high-performance silicon technologies. A self-aligned process has not been developed for organic TFTs, and, as a result, in organic TFTs there are large parasitic capacitances that can limit system performance. If the amount of overlap is fixed by registration capabilities and can not be reduced as channel length $L$ is reduced, the well-known silicon scaling law in which the upper frequency limit $f_{max}$ scales as 1/$L^{2}$ is modified to$ f_{max}$ $\sim $ 1/$L$, altering significantly the economics of increased integration. The usefulness of organic TFTs is hindered by the lack of a technology that provides complementary $n$-channel and $p$-channel transistors on the same substrate. A good case can be made that the benefits of a complementary technology outweigh the gains achieved from modest improvements in single-channel device mobility, and that more effort to develop organic CMOS is warranted. [Preview Abstract] |
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