Spin injection and transport in organic semiconductors

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011.Cataloged from PDF version of thesis. Vita.Includes bibliographical references (p. 141-155).The use of organic semiconductors to enable organic spintronic devices requires the understanding of transport and control of the spin state of the carriers. This thesis deals with the above issue, focusing on the interface between the organic semiconductor and the spin source. The morphology of the organic molecule at the interface is shown to play a dominant role in this study, influencing spin injection. The interface molecule morphology affects the form of interaction between the organic molecule and the ferromagnet. This interaction ranges from a weak Van der Waals' to a chemical interaction leading to charge transfer, hybridization and other interface chemistry. As a result, the spin-dependent density of states is modified, influencing the spin injection process. The first part of the thesis focuses on identifying the interface properties between the ferromagnet and the organic semiconductor, rubrene. This is performed in a vertical organic junction geometry using different interface characterization tools. The growth morphology of the rubrene molecule is shown to influence the electronic coupling between the molecule and the ferromagnet. This has a pronounced effect on the spin injection efficiency and the magnetoresistance signals in the devices. The complex nature of the top interface is dealt with in detail. These studies provide insights on the importance of tailoring the interface properties to control and realize optimum behavior. As an alternative to conventional ferromagnets, a spin-filter material, europium sulphide, is demonstrated as a spin-polarized source. Spin-filter materials have shown the possibility to inject spin-polarized electrons into the organic semiconductor at higher voltage bias. This knowledge encouraged to seek organic spin-filter materials. Understanding the importance of molecule morphology from the work on rubrene, the second part of the thesis involved the study on a new class of organic materials called the phenalenyl compounds. Study using the compound, Zinc methyl phenalenyl, was initiated that showed large magnetoresistance signals of 50% at 4.2 K to 20% at close to room temperature. The origin of this magnetoresistance is attributed to a new interface phenomena described by the spin-filtering effect. The interface molecule morphology is found to play a very important role at the interface inducing an antiferromagnetic state and dominating the device physics. This technique opens up a new approach to engineer the interface and realize new functional devices without worrying about the bulk disorder of the organic film. This thesis thus shows the feasibility of tuning the property of the interface using tailor-made molecules to realize new functional devices at room temperature that can lead to development of the field and technological applications.by Karthik V. Raman.Ph.D