Electronic Structures and Properties of Silicon-Based Polymers

Abstract

Electronic structures of polysilanes are calculated theoretically and are discussed in the light of such experimental data as UV absorption, photoluminescence, carrier mobility and so on. Conduction and valence bands are formed by the sigma conjugation along a silicon backbone chain. The band structures corresponds to those of direct allowed type semiconductors. Substitution by larger alkyl side chains decreases the band gap slightly. With aryl-side-chain substitution, skeleton-pendant group interaction by sigma-pai mixing results in a decrease in ionization potential and the appearance of efficient visible luminescene. Trans-type polysilanes are more stable and have narrower band gap energies and smaller effective masses compared with gauche-type polysilanes. The existence of a gauche link in a trans-conformation has little disturbance for the sigma conjugation along a backbone chain. Band structures of polysilanes with multiphases correspond to the one dimensional heterojunction structure. Electrons and holes localize in the smaller gap region by the potential barrier. The barrier height for electrons is much larger than that of holes, which is the origin of the large mobility difference between them. Silicon-based polymers form a dimensional hierarchy from oligosilane to crystal silicon, and through polysilanes, ladder polymers, plane polymers, polysilane alloys, clusters, and amorphous silicons. The increase in the network dimensions is accompanied by the increase in sigma conjugated area and result in the reduction of energy gap values. This idea was confirmed by the photo-luminescence spectrum measuremens for four kinds of silicon based polymers with different network dimensions between one and three.