Electronic Transport in Organic Materials: Comparison of Band Theory with Percolation/(Variable Range) Hopping Theory

摘要

Percolation theory or hopping theory, used to describe the elec tronic behavior of devices, is very popular in the chemistry dominated research area of organic electronics. The reason is that chemists think in units ("moieties") such as the phenyl ring in a polymer, with specific energy levels (for instance the lowest unoccupied molecular orbital (LUMO) and highest occu pied molecular orbital (HOMO)), and conduction of charge is considered a perturbation to the molecular electronic levels. A typical organic semiconductor consists of a one-dimensional chain — the conjugation backbone — along which current is visualized. Thinking in terms of conduction paths seems very natural and three-dimensional conduction in terms of "hopping" to neighboring conjugation segments seems an obvious approach. Charges reside on specific sites and only occasionally make a jump to neighboring units. This way of thinking has successfully described the low-conductivity end of the spectrum of organic materials, those materials that people imagine when they think of "plastics".