Spin polarized charge carrier injection, transport, and detection in organic semiconductors.

Abstract

In this thesis we explore spin polarized charge carrier injection, transport, and detection in organic semiconductors. Device structures considered have one or more ferromagnetic contacts to the organic semiconductor, and the condition for which charge carrier injection from ferromagnetic contacts is strongly spin polarized is discussed. Spin injection into semiconductors can be greatly enhanced if the injection mechanism is spin selective, such as is the case for tunnelling from ferromagnetic contacts. By contrast, if the carrier injection is by thermionic emission or another process that does not depend on spin, the injection is only weakly spin polarized. To discuss spin transport and spin detection, we consider a unipolar organic spin valve consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts. The polarizations of the magnetic contacts can be parallel or anti-parallel. Spin and charge carrier transport in the organic semiconductor is described by spin dependent transport equations in drift-diffusion approximation and the spin detection process is through magneto-resistance. We discuss the impact of various degrees of spin relaxation in organic semiconductors on the spatial variation of the spin current and its effect on magneto-resistance. The spatial profile of the spin current inside the organic semiconductor depends not only on the spin diffusion length but also on the alignment of the contact polarizations. However, the magneto-resistance decreases strongly with decreasing spin diffusion length. Electron tunnelling from a ferromagnetic contact can have significant spin dependence because the spatial part of the electron wave function is different for the majority and minority spin states of the ferromagnetic contacts. The tunnelling process occurs from the ferromagnetic contact through an insulating layer into the organic semiconductor. The insulating layer is modeled first as an ohmic layer with spin dependent contact resistances. The effectiveness of spin dependent contact resistances on spin polarized injection and magneto-resistance is examined on the basis of a simple analytical model. We then model the insulating layer as a tunnel barrier with spin dependent rate equations. Both majority and minority spin electrons of the ferromagnetic contact tunnel through the insulating layer into the localized molecular states of the organic semiconductor at the semiconductor/insulator interface. Tunnelling matrix elements and transition rates of the two spin types are calculated using a Transfer Hamiltonian approach. The transition rates are thus spin dependent and used in rate equations to calculate the injected (extracted) current for carriers of either spin direction. We explore the various aspects of the ferromagnetic contacts, the thickness and barrier height of the insulating layer, and the energy of the localized molecular states on spin injection and magneto-resistance. Consistent with the experimental data, the spin injection from ferromagnetic contacts can be either positive or negative, and the magneto-resistance decreases strongly with the applied bias across the device.