Lattice imperfections in organic solids. Part 4.—A study, using space-charge-limited-currents, of trapping centres in crystalline anthracene

Abstract

The density of carrier traps situated within the entire energy gap of 3.7 eV in anthracene has been evaluated from current-voltage characteristics using the Mark and Helfrich equation, which is applicable to space-charge-limited-currents in organic crystals containing an exponential distribution of trap energies. A modified Kallman and Pope cell, consisting of aqueous iodide saturated with iodine as anode, was employed; and four kinds of anthracene crystal were studied. Trap densities range from 10¹³ cm^–3, for vapour-grown samples, to 10¹⁹ cm^–3, for deformed melt-grown samples; and there is strong evidence that positive holes are trapped at the immediate vicinity of the cores of non-basal dislocations. The apparent conflict between some of these results and those of the conductivity-glow measurements is discussed.