Polymer Photonics
Figure 1 shows some of the proposed applications of polymer photonics. Examples of projects in this area include: Polymer Photovoltaic and Thermophotovoltaic Devices, Compensation Optical Films for Flat Panel Liquid Crystal Displays, Full Reflection Mirrors for Light Transportation, Switchable Beam Steering Devices, Light Emitting Displays, Polymer and Organic Field Effect Devices, Solid State Lighting, Photoinduced Magnetism, and Photonic Digital Micromirrors . Nanoparticles synthesized in the nanocomposite application area are essential for some of these projects, while photonic materials and devices developed in this area can be used as biosensors. This crossover in both the technical and device developments provides for a commercial development synergy that would not otherwise be achieved. We will briefly explain the first five projects.
• Polymer Photovoltaic and Thermophotovoltaic (TPV) Devices Photovoltaic cells that convert sunlight to electrical power are mostly fabricated today using solid-state junction devices made from silicon. However, this technology is being challenged by new photovoltaic cells based on conducting polymer films that offer reduced cost and flexibility. Work on new materials for such devices is currently being carried out at UA under the support of the Air Force / UA Center on Polymer Photonics . New materials under development include electron-transporting, heterocyclic, discotic liquid crystals and light harvesting metallodendrimers coated on coaxial carbon nanowires. The nanowires will be prepared from carbon nanotubes at UD . The objective of this work will be to utilize the self-assembly of these nano-materials in the fabrication of photovoltaic cells. In the case of the dendrimers systems, the coaxial structure should allow the nanotube framework to provide good mechanical stability, high thermal/electrical conductivity, and large surface/interface area necessary for efficient charge separation/collection while self-assembling. A conducting substrate (e.g., Au) supporting the aligned nanotube array will be used to collectively harvest charges from each of the constituent nanotube photovoltaic cells, while a thin layer of sputter-coated ITO will be used as the transparent counter electrode. Surface coating with appropriate transparent polymer films could not only extend the lifetime of these photovoltaic cells made from stable nanotubes and metallomers but also lead to flexible film-type devices, consisting of readily addressable nanoscale photovoltaic cells. The proposed work covers several important areas including: (1) synthesis and microfabrication of aligned carbon nanotubes; (2) synthesis and characterization of light-harvesting metallodendrimers; (3) construction of photovoltaic cells by self-assembly; (4) studies of the self-assembled structures and device performance; and (5) theoretical investigation of the interface structure and photovoltaic effect. Thermophotovoltaics (TPV) is a promising energy conversion technology for the production of electricity from the infrared (IR) light radiated by a heated emitter, such as the hot exhaust system of a car. Waste heat from chemical processes or incinerators can also be used. An idealized TPV system is comprised of a source of thermal energy that heats the emitter and an IR photovoltaic (PV) cell that absorbs the light and produces electricity. Work on new materials for such devices is currently being carried out at UA. New TPV devices will be fabricated using titania nanofibers with a spectral emission tuned to the bandgap of GaSb photovoltaic cells. The nanofibers will be prepared by electrospinning of organometallic prepolymers followed by thermal pyrolyels. The major benefit of nanofibers is that a nanofiber selective emitter is in a form where the surface area (responsible for net light emission to the PV cells) is maximized while the volume (responsible for re-absorption of light and thereby preventing it from reaching the PV cells) is minimized. Available photodiodes will be used in the initial experimental work, but their sensitivity to low energy photons is limited. Improved photodetectors will be sought. For example, nanofibers of semiconducting polymers or nanofibers coated with semiconductors and metals will be used to construct photodetectors with p-n junctions in coaxial or bamboo-like geometries.
(b) Optical Compensation Films Compensation films have been developed at UA that greatly improve the off-angle viewability of liquid crystal displays (LCDs). Although these negative-birefringent, polyimide films are used commercially on avionic displays and on large screen LCD TVs, they are relatively expensive. Therefore, work will be carried out on the development of new, less expensive polymers. Another objective of this work will be to develop multi-functional thin films that contain compensation layers. For example, thin compensation layers will be cast on cellulose triacetate films that are used as substrates for polarizers. This combination will then be used in the construction of a new type of polarizer, which will be evaluated at KSU . In our next-generation optical retardation film technology, CNTs including multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) will be integrated into UA optical film systems at UD . Functionalized CNTs will be dispersed into the polyimide matrix through physical interactions (hydrogen-bonding or p - p stacking) or/and chemically covalent bonding. Our preliminary results in this area indicate that the functionalized CNT-based optical polymer films exhibit superior optical performance in addition to excellent mechanical and thermal properties. In particular, the negative optical retardation is significantly enhanced upon the addition of a small amount of CNTs (~1%), which is consistent with the nature of the planar anisotropic orientation of the CNTs on the substrates. The films will be evaluated at KSU .
(c) Full Reflection Mirrors for Light Transportation The goal of this project is to utilize high refractive index (RI) polymers in place of standard high RI inorganics in the production of full reflection mirrors [3D photonic crystals (PCs)]. A polythiophene with the highest RI value ever reported for an organic polymer (3.3 at 530 nm) has recently been synthesized at UA . This polymer should allow the preparation of a 3D PC with a complete photonic band gap (PBG). Nanofabrication of a 3D FCC structure involves the formation of uniformly-sized polymeric particles into ordered arrays called opals. The movement of an isotropic/nematic interface to organize spheres and produce opals has been utilized at UA . The opals, which will be annealed to eliminate defects, will be made on an electrode to enable the in situ electrochemical synthesis of PT; i.e., the PT will be polymerized in the interstices of the opal. Removing the polymer nano-particles will result in 3D PCs with complete PBGs. This approach may also open up the possibility of producing precisely controlled defects in the photonic structures.
(d) Switchable Beam Steering Devices Beam steering devices based on photonic band gap (PBG) materials have been recently developed at KSU . They are one of the most effective ways to steer light, but the ability to select the steering angle is limited. This problem will be overcome through the development of switchable devices. These devices will be fabricated using an LC/polymer composite consisting of a stack of birefringent polymer hologram films interleaved with LC cells that control the polarization state of light. By setting the state of the LC cells, one of the holograms in the stack will be activated to focus the beam to a point-like lens or to steer it at a specific angle with a phase profile. The polymer hologram films will be prepared using a holographic photo-polymerization technique at UA . Submillisecond electro/optical switching in dual-frequency nematic LCs has also recently been demonstrated at KSU . This technology will be utilized in the development of fast optical modulators, beam steering devices, and polarization switches. Finally, a new approach to all optical switching will be investigated, which is based on readily available and thoroughly understood 1-D PCs and conjugated organics that have fast decaying non-linear optical responses. Simulations have shown that for a given frequency at the band edge of a 1-D PC as little as a 3.5 ° change in the angle of approach to the 1-D PC can cause the angle of propagation through the PC to swing from 90 ° to -90 ° due to its sharp density of modes (DOM) frequency profile. This extreme angular sensitivity can be used to magnify relatively small material responses to light intensity. This would allow the transmission and manipulation of information on the femtosecond scale.
(e) Light Emitting Displays In the mid-1990's OSU developed a new approach to organic- and polymer-based light emission, the Symmetrically Configured Ac Light Emitter (SCALE). The SCALE device architecture has three layers, with a hole/electron transporting layer both above and below the light emitting organic or polymer layer. This results in bipolar operation, and allows the use of air stable cathode and anode materials. This reduces degradation and makes the SCALE architecture particularly suitable for flexible displays. By judicious choice of polymer layers, the SCALE structure can be made to emit two different colors of light for different polarities of applied voltage. This opens the opportunity for achieving two colors with only one type of pixel, potentially eliminating the need for inkjet deposition of some of the layers. OSU has licensed a portfolio of patents related to the SCALE architecture in flexible displays to BTG International, Inc . The objective of this work will be to utilize SCALE technology in the development and commercialization of solid state lighting devices.
(f) Polymer and Organic Electric Field Effect and Spintronic Devices Inorganic field effect devices such as those based on silicon were the basis of the electronics revolution during the past half century. Field effect transistor (FET) devices utilizing organic oligomers and polymers as the active channel have been under intense study for the past decade. OSU 's initial studies of multifunctional FETs based on conducting polymers showed that the performance depended on the choice of dielectric material and the means of deposition. Ion motion was found to play a central role in these devices. The work has been extended beyond the conventional insulating dielectric layer to include an ion conducting dielectric layer, such as poly(ethylene oxide) (PEO) doped with Li + ClO 4 - . The response is a substantial improvement over that of initial conducting polymer based field effect devices. The multifunctional response allows the preparation of polymer coatings that control emissivity and also store and process information. Using the spin as well as the charge of the electrons to control current flow is of increasing importance and forms the basis of the burgeoning field of “spintronics.” The spintronics-based read head enabled a 10 6 -fold increase in hard drive capacity. OSU has created the new field of organic-based spintronics using polymer magnets and carbon nanotubes developed at UD . This technology will be used in the development and commercialization of large area and low cost spintronics applications in read heads and MRAM structures.
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