Theory of spin injection into conjugated organic semiconductors

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Abstract
We present a theoretical model to describe electrical spin injection from a ferromagnetic contact into a conjugated organic semiconductor. In thermal equilibrium the magnetic contact is spin polarized, whereas the organic semiconductor is unpolarized. The organic semiconductor must be driven far out of local thermal equilibrium by an electric current to achieve significant spin current injection. However, if the injecting contact has metallic conductivity, its electron distribution cannot be driven far from thermal equilibrium by practical current densities. Thus, quasi-equilibration between the conjugated organic semiconductor and the metallic contact must be suppressed to achieve effective spin injection. This requires a spin-dependent barrier to electrical injection that may be due either to tunneling through the depletion region of a large Schottky barrier or to tunneling through a thin, insulating, interface layer. Schottky barrier formation on conjugated organic semiconductors differs from that on inorganic semiconductors inasmuch as contacts made to organic semiconductors often follow near-ideal Schottky behavior, thus permitting the energy barrier to electrical injection to be varied over a wide range by using metals with different work functions. In addition, insulating tunnel barriers to organic semiconductors based on organic molecules can be conveniently fabricated using self-assembly techniques.