Charge-Carrier Mobility in Organic Crystals

Abstract

Organic crystals are usually lacking main valence bonds between their constituent molecular repeat units. Therefore, in organic crystals there is usually no substantial electronic overlap between the molecules1; interactions are based on mere van der Waals forces in the very large class of neutral molecular crystals (or on a combination of van der Waals and local [closed shell] Coulombic interactions in the barely studied class of ionic organic crystals). As a consequence, exchange of a possibly present conduction electron or hole between neighboring molecules is not a very efficient process and, hence, electronic transport is generally slow in organic solids. Charge carriers are to be considered as rather localized (on individual molecules), with the consequence, specific for this class of materials, of considerable local changes of nuclear positions, vibrational frequencies, and electronic wavefunctions by polarization interactions (see, e.g., [1007E3]). This situation is usually described by introducing the concept of a polaron [1,4J as an appropriate quasiparticle, comprising the electronic charge and the induced surrounding polarization based on electronic, vibronic and phononic relaxation. Due to strong intraand intermolecular vibrational fluctuations, no coherent propagation of this quasiparticle is usually possible around room temperature and above, not even in a pure and structurally perfect crystal. The strong localization is reflected macroscopically by strong inertial resistance against acceleration (e.g. by an applied electric field), which can formally be ascribed to a high “effective” mass of the “polaronic” charge carrier [4].