Charge Transport in Polyacetylene

The surface charge concentration in the accumulation and inversion layers is limited by the dielectric strength of the insulator layer in the MIS device through equation 30, and with Si02 as insulator, this limit is set at 2 x lO cm 2. We have not given much 

How much do the results we have obtained here tell us about the fundamental limits to the mobilities of carriers in devices fabricated with polymer that is very much better ordered than the polyacetylene that we have used here There are recent reports of very much improved mobilities for devices based on sexithiophene (the six repeat unit oligomer of polythiophene), with a value of 0.4 cm /Vsec now reported [73], and there is now considerable interest in the development of polymer FETs as large area thin film transistors, with interest in polythiophene derivatives [74] and in poly(arylenevinylenes) such as poly(2,5-thienylene vinylene) [75]. We can see from the optical characterisations of the MIS devices that the surface layer of polyacetylene formed on SiC 2 is very much more disordered than the bulk material, but we have not made FET devices with the polymer insulator layers which give better ordered structures as characterised optically. 

We must presume that the extra disorder in the SiC 2 devices reduces the carrier mobility. If we use equation 11 for the mobility limited by thermally-activated bipolaron hopping, but use the experimentally measured values for the activation energy in the MISFETs (figures 51 and 52), we obtain room temperature mobilities of around 0.1 to 1 cm /Vsec, and we consider that the discrepancy between this and the measured mobilities is indeed due to the disorder present in the surface layer of the polymer. It is clearly of interest to develop improved methods for the fabrication of polymer devices in order to realise such improvements in device perfcnmance. 

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