Current-Voltage Characteristics in Phenanthrene Single Crystals

The obtained current-voltage characteristics of phenanthrene single crystals exhibited three distinct types of behavior: (1) at low voltages, up to 2×104 V.per cm., the dependence of the current on the voltage obeyed Ohms’ law, (2) at intermediate voltages, the current increased with the square of the voltage, and (3) at higher voltages, over 5×104 V.per cm., the current rose steeply with the voltage. These results have been analyzed as space-charge-limited currents phenomena, and a value of 1.2×106 cm−2 has been obtained as the concentration of the bulk-free carriers from the transition voltage between the ohmic region and the square-law behavior. The resistivity calculated by applying this value is in good agreement with the resistivity observed for the ohmic region of current-voltage characteristics. The trap density of 1.1×1014cm−3 has been evaluated from the voltage where the current rose steeply. Although this value seems to be rather low for an organic solid, it is comparable to the trap densities for organic molecular crystals ever obtained.