Semiconductivity of organic substances. Part 12.—Electrode injection of charge carriers into crystals of small aromatic molecules

Abstract

Although crystalline samples of aromatic substances of low molecular weight show a temperature variable conductivity which obeys the law σ=σ_o exp (–E_act/kT), the values of E_act and σ_o vary over an extremely wide range and show poor reproducibility. A re-investigation of the problem using zone-refined single crystals and powders of benzene derivatives, naphthalene, acenaphthalene and anthracene (fitted with guard rings) shows that the majority of the current flows along the surface. Furthermore, the exact values of E_act and σ_o observed on a given specimen are strongly influenced by the form of the specimens. Charge carrier (positive hole) injection is discussed as a possible rate determining-step in the conduction process.