Thermally activated electrical conductivity of thin films of bis(phthalocyaninato)terbium(III) double decker complex

The temperature and field dependence of the electrical conductance of neutral radical bis(phthalocyaninato)terbium(III) (TbPc2) thin films was measured in situ under ultrahigh vacuum. The films behave electrically as narrow gap intrinsic semiconductors, having high conductivity and weakly thermally activated conductance. Taking advantage of the exceptionally high electrical stability of the material and the fast-settling response in the low-field region, precise measurements of the temperature dependence could resolve a linear temperature dependence in the pre-exponential factor of the conduction equation. The activation energy of conductance of the annealed TbPc2 film was determined to be 0.158 eV after adjusting the fitting procedure to take into account the temperature dependent pre-exponential. This new conductance equation, which differs only slightly from the usual Arrhenius expression, arises as the natural consequence of thermal excitation of carriers from a continuous density of deep trap states, or a similar activated process where either the states or the barrier heights distribute over a finite width.