Thermal Activation Energy and Semiconducting Properties of Macromolecular Substances

Abstract

SynopsisIn many cases the values of thermal activation energy E , identified with half of the energy gap E, are given for characterization of semiconducting properties of polymers. This is justified only in the case of intrinsically generated current carriers and leads to some misunderstanding when the creation of carriers is more complicated, as when they can be injected from the electrodes. The defects of structure act as carrier traps changing the mechanism of conductivity. To investigate the influence of structure on E , values the dependence of current on temperature, current-voltage characteristics, and thermally stimulated currents (TSC) in poly(p-phenyl) oligomers, i.e. p-terphenyl (p-TPh) and p-quaterphenyl (p-QPh), and in poly(p-phenyl) (p-PPh), were studied. The p-TPh and p-QPh in the form of monocrystals and vacuum-deposited layers were investigated; p-PPh, in the form of tablets obtained from mixtures of it and p-TPh and polystyrene. On the basis of current-voltage characteristics the trap density N T were calculated. The measurements of TSC allow estimation of the energy E T of trap levels. From the relation I = f(l/T), where I is current and T is temperature, the E, values for equilibrium conditions were calculated. The mathematical analysis of the trapping and thermal-release phenomena of current carriers shows that the relation log D = f (UT) gives only the average E value, which depends strongly on trap depths and trap population; thus E, has only a limited meaning for the characterization of the organic semiconductor.The semiconducting properties of macromolecular substances are usually characterized by the thermal activation energy E , and the specific resistance p . Since specific resistance depends on many factors, more importance is given to the E , value, which in many cases is identified with half of the energy gap E, between the conduction and valence bands. This interpretation is justified when current carriers are intrinsically generated and thermally activated from the valence band to conducting band.The physical meaning of E, has been discussed in many monographs devoted to low and high molecular weight organic semiconductors (1-5) 867