Electronic Properties of Organic Semiconductors

Abstract

The highest occupied und lowest unoccupied molecular orbitals (HOMO and LUMO) form bands in organic semiconductors corresponding to valence and conduction bands of their inorganic counterparts. Organic crystals have comparatively narrow bands and large bandgaps, and they feature strong anisotropies. The Gaussian density of states of HOMO and LUMO supplemented by defect states produces a substantial tailing of the bands into the bandgap. The strong structural and electronic relaxation occurring when a molecule is charged leads to strongly bound Frenkel excitons and to a pronounced polaron character of mobile carriers. The small polaron radius and the high density of defects lead to a prevailing, thermally activated hopping conductance with a relatively low carrier mobility. Band conductance with an increased mobility at low temperature may be found at reduced scattering rates. Molecules at the interfaces between two organic semiconductors have often weak interaction, yielding a band alignment largely described by matching the vacuum levels. Molecular interaction at interfaces to metals is generally strong, creating surface dipoles and an often observed pinning of the Fermi level.