Field Emission from Polymers
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ABSTRACT Carbon based materials are considered to be amongst the most promising field emitters for use in displays. Semiconducting polymer (regioregular poly3 octylthiophene) has been found by the authors to have the lowest threshold field reported to date for any carbon based material. The emission process is, we believe, solvent associated with surface irregularities, voids, possibly resulting from evaporation. Field intensification occurs, and as a consequence there is field emission from the top ridges of the voids. The current flowing through these regions is controlled by space charge in traps in the upper half of the energy gap. There is, in addition, current injected from the base of the voids and the two currents mix to give field emission with current IcV1.8 where V is the anode voltage. Measurements on capacitor samples using Aluminium, Gold and Calcium electrodes have been used to examine the conduction processes in the material and these are linked to results of field emission to further illuminate the mechanisms involved and provide a model for the emission process. INTRODUCTION Field emission displays are bright but consume relatively large amounts of power due to high emission threshold voltage and the lack of a low threshold voltage phosphor. We have recently announced (1) that a thick (54tm), spin-cast, nominally undoped polymer (regioregular poly-3-octylthiophene-P3OT) is capable of high, stable
electron emission. The observed threshold field (0.2MVcm-') is the lowest so far reported on carbon based materials. As with other carbon based materials there is a need to condition the films by applying a high anode voltage before such low thresholds are obtained. This process is discussed later. It may be associated with thermally induced disorder and the creation of a high density of traps in the top half of the polymer energy gap. There is a large range of conjugated polymers available some of which may well give still lower thresholds. Dopants can be introduced, at room temperature, in attempts to further improve efficiency. With such materials very low cost, simple to produce, sandwich structures are potentially viable for use in displays. SAMPLE PREPARATION, RESULTS AND DISCUSSION
"Thepolymer, purchased from Aldrich, was cast on polished n' silicon wafers and was dried under vacuum without heating. The Tauc band gap of our P3OT films is 1.75eV. No special processes were introduced to eliminate the effects of light or air during their preparation. They are invariably, therefore, p type. NMR studies indicate a head to tail concentration of greater than 90%. Such high regularity has only recently become available. It may not be necessary for this application. Field emission 61 Mat. Res. Soc. Symp. Proc. Vol. 558 @2000 Materials Research Society
measurements were carried with an anode cathode spacing of 27, 47 and 130gtm and a vacuum of 2. 10-7Torr. The anode was an indium tin oxide (ITO) layer on glass, and glass fibre spacers were used to separate the cathode and anode. As the pressure was raise
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