Hot Wire Chemical Vapor Deposition as a Novel Synthetic Method for Electroactive Organic Thin Films
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Hot Wire Chemical Vapor Deposition as a Novel Synthetic Method for Electroactive Organic Thin Films Gillian A. Zaharias, Helen H. Shi and Stacey F. Bent Dept. of Chemical Engineering, Stanford University, Stanford, CA 94305-5025 ABSTRACT We are exploring the use of hot-wire chemical vapor deposition (HW-CVD) as an alternative, solvent-free technique to produce compact polyconjugated films from vaporized monomers. This paper will focus on our attempts to deposit polyaniline, a polymer with a wide variety of optoelectronic applications. Infrared and X-ray photoelectron spectra have been obtained to characterize HW-CVD films produced using aniline as a precursor. The aromatic ring structure, essential for the polyconjugated network that leads to semiconducting properties, is largely preserved in our method. The bonding structure is analyzed, and the films are found to have significant metal incorporated from the hot wire. The implications of these results for understanding the polymerization and growth mechanism are discussed. INTRODUCTION Thin films of conjugated polymers and oligomers have emerged as exciting new materials for the semiconductor layers in solar cells, light-emitting diodes, transistors and gas sensors [1]. The large majority of electroactive polymer or oligomer films are fabricated in a multistep process, beginning with polymerization in solution. The solvated polymer is then spin- or dipcoated onto a substrate, followed by various drying steps. Conjugated chains are generally insoluble in common solvents. Much research has therefore been devoted to adding alkyl or other groups to the polymer units to increase solubility, while attempting to retain sufficient conductivity and carrier mobility [1]. Loss of mobility can be crucial, as mobility in organic semiconductors typically lags far behind that in inorganics like silicon. Additionally, solvents able to dissolve conjugated polymers often create hazardous waste. Although promising results have been achieved by adding side chains to conjugated polymers, the development of solvent-free synthetic methods will expand the tools available for designing organic optoelectronic devices. Vacuum-based film growth integrates easily into existing semiconductor processing equipment and can produce highly compact films, improving stability in air and possibly improving conductivity. This paper focuses on the development of a novel synthetic method for polyaniline. Polyaniline (PANI) has been investigated extensively by researchers hoping to exploit its stability and reversible oxidation states, which range from insulating to metallic [1]. The general form of undoped PANI is shown in Figure 1. Thin films of PANI have been used as hole material in dye-sensitized solar cells [2], electrochromic windows [3], LEDs, field-effect transistors[4] and chemical sensors [5]. However, commercial development of semiconducting PANI has been slow due to the processing challenges [1]. Figure 1. Polyaniline in its unprotonated, nonconductive form. n
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PANI wet-c
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