Macroscopic modeling of spin injection and spin transport in organic semiconductors

We describe device models for spin injection, transport, and magneto-resistance in structures consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts. Carrier transport in the organic semiconductor is modeled with spin-dependent transport equations in drift-diffusion approximation. The effectiveness of spin-selective tunnel contacts on spin-polarized injection and magneto-resistance is examined on the basis of a simple analytical model. In agreement with earlier results, we find that spin injection from ferromagnetic metallic contacts into organic semiconductors can be greatly enhanced if (spin-selective) tunneling is the limiting process for carrier injection. We then explore the effects of the injected space charge and of spin relaxation in the semiconductor by comparing the results of a numerical calculation with the analytical model. For relatively thick organic semiconductor layers the injected space charge has strong effects on charge injection and, hence, on spin injection at high bias. Lastly, we consider a simple model for the bias dependence of the tunnel contacts and find that this effect may limit spin injection to relatively low currents. (C) 2009 Elsevier B.V. All rights reserved.