Photoexcitation donor:acceptor materials

Polysilanes are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

In addition photoexcitation can also result in the transfer of an excited state electron to a distant acceptor group resulting in charge separation. This process can be understood within the framework of Marcus theory and subsequent more sophisticated theoretical treatments.2,5 The rate of electron transfer (ke]) drops with distance according to an attenuation factor / el ke °c exp(—/ el /yB) where /Xb is the distance between donor and acceptor components A and B. When the donor and acceptor components are separated by a vacuum J3el is estimated to be ca. 2-5 A-1. However when some kind of material substance is involved such as a bridge L the electron transfer process can be... 

The spectacular success of the semiconductor industry is based on the production of materials selectively designed for specialized applications in electronic and optical devices. By carefully controlled doping of semiconductors with selected impurities—electron donors or electron acceptors—the conductivity and other properties can be modulated with great precision. Fig. 12.8 shows schematically how doped semiconductors work. In an intrinsic semiconductor (a), conducting electron-hole pairs can only by produced by thermal or photoexcitation across the band gap. In (b), addition of a small concentration of an electron donor creates an impurity band just below the conduction band. Electrons can then Jump across a much-reduced gap to the conduction band and act as negatively-charged current carriers. This produces a n-type semiconductor. In (c), an electron acceptor creates an empty impurity band just above the valence band. In this case electrons can jump from the valence band to leave positive holes. These can also conduct electricity, since electrons falling into positive holes create new holes, a sequence... 

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