Observation of the Mott–Gurney law in tris (8-hydroxyquinoline) aluminum films

We show that tris (8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_3)$ thin films produced and characterized under ultrahigh vacuum conditions present a well-defined squared-law dependence of the injected current on the applied voltage at applied electric fields of the order of $\text{0.25–1\hspace{0.17em}}\mathrm{MV/cm}.$ From this, one derives an electric-field-independent electron mobility of the order of ${10}^{\text{−7}}\hspace{0.17em}{\mathrm{cm}}^2\mathrm{/(}\mathrm{V s}\mathrm{),}$ with a variation between different samples of about one order of magnitude. Observations of current–voltage characteristics with clear indications of trap-filling and space-charge-limited conduction at high fields in ${\mathrm{Alq}}_3$ excludes the existence of traps with an exponential distribution of trap energies, as is commonly assumed in amorphous materials.