TETRACENE AS A HOLE TRAP IN ANTHRACENE CRYSTALS

Tetracene doped anthracene crystals with molar impurity concentrations ranging from 10-8 to 10-4 were grown from the melt by the Bridgman technique. To describe the impurity distribution in the crystals the equation of normal freezing had to be modified concerning the change in density when the crystals were lowered down into the cooler part of the furnace. As a result the equilibrium distribution coefficient was determined to be 1.3 · 10-2. Photocurrent measurements were performed by a transient photoconductivity technique (Kepler and Le Blanc). The behaviour of the hole photocurrent in doped crystals showed that measuring the drift mobility one gets a very sensitive tool for determining quantitatively tetracene contents down to molar concentrations of 10-8. It was found that tetracene is a shallow hole trap having an activation energy of 0.42 eV and a capture cross section of 3.5 · 10-14 cm2. © 1969 Springer-Verlag.