Upper limit of electron effective mass in organic semiconductors

Abstract

A model of electrical conductivity in solid-state organic semiconductors developed in an earlier paper (Fleming, 1970) is used to estimate an upper limit of 25 times its rest mass for the effective mass of an electron moving in the paraffinic hydrocarbon octacosane (C28H58) under the influence of an external electric field. This value, based on drift mobility calculations, is one-quarter of that deduced by Kemeny and Rosenberg from their analysis of the pre-exponential factor and compensation rule in certain organic semiconductors. In the present model, impurity electrons are considered to move within one-dimensional wells of width comparable to the length of the paraffin molecule (35.6 A) and of depth slightly less than its ionization potential (the range 5 eV-8 eV is considered). The electrons tunnel through one-dimensional potential barriers between neighbouring molecules. In agreement with Kemeny and Rosenberg, their effective masses for tunneling and for intramolecular motion are equated. The quoted upper limit of effective mass (25ma) is expected to be valid for most organic semiconductors.