Electronic transport in silicon backbone polymers

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Abstract
Results of recent time-of-flight transport measurements on the silicon backbone polymer poly(methylphenylsilylene) (PMPS) carried out over broad ranges of fields and temperatures described in much greater detail than heretofore. The relative effect of a series of chemical dopants in reducing the hole drift mobility of PMPS is correlated with their respective ionization potentials. Doping effects can be completely understood on the basis of a simple model which is described. Transport in PMPS manifests the key features observed in a broad class of polysilylenes containing aromatic and/or aliphatic side group substituents including those completely devoid of π electrons. Side-group substituents in saturated carbon backbone polymers such as poly(N-vinylcarbazole) completely dominate the transport process which is hopping between the carbazole chromophores. In contrast with this behaviour, comparative measurements indicate that side-group substituents in polysilylenes play a secondary role. The complex field and temperature dependence of the hole drift mobility (electron transits are not observed) is fully deconvoluted. Results are compared with prototypical disordered molecular solids and the common features are delineated. The results indicate that the microscopic transport process is hopping between backbone-derived localized states. These states could be associated with domain-like suborganization of the Si backbone into all trans segments of approximately 15-30 repeat units separated by conformationally disordered regions. The latter could account for features closely akin to those observed in molecularly doped polymers. Alternatively, intragap states could arise spontaneously, even on a perfectly ordered chain, from bond order polaron formation. The latter is a consequence of the coupling of electrons to the quasi-one-dimensional silicon backbone. A theoretical model for these polarons is reviewed in some detail. Although the Si polymer backbones are electronically saturated structures, the model demonstrates their ability to support polaron and bipolaron states with localized intragap levels similar to those found in conjugated polymers. It is demonstrated that these (backbone-derived) intragap levels could constitute the active sites for interchain hopping.