Transport Properties of Stretch-Oriented PPV Films
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Transport Properties of Stretch-Oriented PPV Films Tiffany Wilson1,2, F. Patrick Doty1, Douglas Chinn3, A. Alec Talin1, Michael King1,4, Luke L. Hunter1, Frank E. Jones1, Christine Cuppoletti1, H. Rouchanian1, and C. Munoz3 1 Sandia National Laboratory, 7011 East Ave, Livermore, CA, 94550 2 Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH, 43210 3 Sandia National Laboratory, Albuquerque, NM, 87185 4 Dept. of Nuclear Engineering, University of California, Berkeley, CA, 94720 ABSTRACT Organic semiconductors are under investigation for radiation sensors at Sandia National Laboratories. The wide band gaps, high resistivities, low dielectric constants, and high dielectric strengths of conjugated polymers suggest these materials may be suitable for solid-state particle counting detectors. A range of solution cast materials have been evaluated for this application, including polythiophenes and poly(p-phenylene vinylene)s, or PPVs. Films were prepared by novel solution casting and mechanical stretching methods. Device structures including interdigital metal electrodes on glass and thin film transistors on SiO2 on silicon were fabricated by drop casting from solution and lamination of solid films. Transient and DC responses were recorded and analyzed. Experiments include laser stimulus for photoconductive pulse response, and field effect transistor testing. Mechanical stretching was shown to dramatically alter electrical properties of polymer films. Future work will analyze the feasibility of single particle detection and analyze various geometries for optimization. The effects of traps and methods for reduction of trapping effects will be analyzed. INTRODUCTION We are investigating novel materials for detection of ionizing radiation. Many conjugated polymers show promise for this use because of their high resistivities and low dielectric constants which yield lower noise. They also have high dielectric strengths, which allows the use of a high electric field, allowing farther charge movement through a device. The family of poly(p-phenylene vinylene)s, or PPVs, in particular show high resistivity, but good mobility and air stability, making them good candidates. Papers published on the PPV family report mobilities typically from ~10-5 cm2/Vs to as high as 1x10-2 cm2/Vs, depending on side chain symmetry, solvent, and annealing [1,2]. Neutrons are detected by scattering in hydrogenous materials, so a polymer with a high ratio of hydrogen to carbon is desirable. Further, high mobility is required, so a polymer with symmetric side chains will be chosen, as they generally have higher mobilities and tend toward more elongated conformations than asymmetric chains do [1,3]. Therefore, we will focus on a PPV with symmetric hydrogenous side chains, poly[2,5-bis(3 ,7 dimethyloctyloxy)-1,4-phenylenevinylene], as shown below. This polymer will hereafter be referred to as D1PPV. ′
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Figure 1. Chemical structure of D1PPV backbone The local order and morphology of the polymer is critical for the
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