Pre-exponential Factor in Semiconducting Organic Substances
Barnett Rosenberg, Benoy Brata Bhowmik, Harold C. Harder, Elliot Postow · The Journal of Chemical Physics · 1968
Experimental data on semiconducting organic substances are usually processed by use of the two-constant equation, σ(T) = σ0exp(−E / 2kT). Evidence from a number of cases now indicates that σ0 is dependent on E, when the equation is applied to certain single substances where E varies with preparation. A three-constant equation of the following form applies: σ(T) = σ0′ exp(E / 2kT0) exp(−E / 2kT) where T0 is a new constant, the “characteristic temperature” of the substance. A method of evaluating σ0′ and T0 is to vary the activation energy for semiconduction by any of at least three processes: (1) hydration of the crystals; (2) formation of weak donor–acceptor complexes; (3) using different cis–trans isomers of the substance when possible. One method of evaluating the constants is to plot logσ0vs E. The slope of this line yields (2kT0)−1, and the intercept yields σ0′. Other methods are possible and yield consistent results. These methods are demonstrated for the substances oxidized cholesterol, retinal, and nucleic acids. The implication of these results is that the Meyer–Neldel rule for compound inorganic semiconductors is valid for organic semiconductors as well. No reasonable theoretical explanation has yet been proposed to account for this rule.