Lattice imperfections in organic solids. Part 3.—A study, using the conductivity glow-curve technique, of trapping centres in crystalline anthracene

Abstract

Previous measurements by Kokado and Schneider have shown that the peak centred at ca. 0°C in the electrical conductivity-temperature plot is an intrinsic property of crystalline anthracene, its occurrence being attributed to the existence of discrete traps—believed to be structural imperfections—for positively charged carriers. This work reports the measurement of conductivity glow-curves in the range –70 to +15°C on vapour-grown, solution-grown, melt-grown, and deformed melt-grown crystals. It is demonstrated that, although a structural fault is probably responsible for the peak—only one peak is obtained with the high-purity material—non-basal dislocations introduced by deformation are not the principal cause for its occurrence. The magnitude of the glow peak, the position of which corresponds to the existence of trapping levels for positive holes 0.7 eV above the valence band, is decreased six-fold (from a value of 3 × 10^–12 A) following a hundred-fold increase in the content of dislocations which glide on (100) and (201) planes in the [010] direction. The significance of these results is discussed.