The objective of this research is to explore spintronic devices made from conjugated organic semiconductors. The possibility to integrate organic semiconductors with strongly ferromagnetic (even half-metallic) materials is a salient motivation for this project. The approach is to develop predictive physical models for organic semiconductor-based spintronic devices in order to provide a quantitative assessment of the true potential of these materials for that application. The University of Minnesota PI will work closely with researchers at Los Alamos National Laboratory.
Intellectual Merit:
The intellectual merit of the proposed activity includes the advancement of the fundamental understanding of spin injection and transport, and the exploration of new devices and conceivable applications. A recently demonstrated feasibility to integrate organic materials with magnetic materials, together with certain intrinsic properties of organic semiconductors, create exciting opportunities to explore new devices with much greater functionality and performance than expected from conventional future electronics technology. The proposed predictive theoretical models are likely to acquire significant technological impact. However, the work is clearly exploratory and pursues long-term objectives.
Broader Impact:
The project will have broad educational impact through the involvement of graduate students in an exciting interdisciplinary field. Research in this area crosses many boundaries by involving condensed matter physics, organic chemistry, and electronics. Furthermore, the proposed program attains additional educational scope through its ability to recruit women and students from traditionally under-represented groups due to the PI's participation in the University of Minnesota Materials Research Science and Engineering Center and its well-developed outreach network.
Organic semiconductor spintronics is a new but rapidly growing area of research. Semiconductor spintronics refers to ideas aimed at exploiting the spin degree of freedom of (mobile) charge carriers in electronic devices, and the organic semiconductors of interest are polymers or small molecules of conjugated hydrocarbons similar to those already used in organic light emitters. Among the advantages that organic semiconductors possess over conventional inorganic semiconductors is the relative ease with which their electronic structure can be modified through well-developed methods of synthetic chemistry. Furthermore, integration of organic semiconductors with other materials, organic or inorganic, is much less limited by a negative impact on their electronic properties than is the case for inorganic semiconductors. Many organic semiconductors are amenable to device fabrication by solution processes, which can imply low production costs. Of course, the advantages of organic semiconductors come at a price: the relatively weak intermolecular bonding that gives them so much flexibility implies that charge carrier mobilities are very low. Interest in organic semiconductors for spintronics arises from their integrability with a wide range of magnetic materials, in particular ferromagnets. In addition, conjugated organic materials are composed of light elements, primarily carbon and hydrogen, and mobile charge carriers therefore are subject to very weak spin-orbit interaction. As a consequence, the spin direction of mobile charge carriers tends to be preserved over relatively long times, i.e. long travel distances. This recently concluded program explored theoretically two key aspects of organic semiconductor spintronics: (1) the injection of spin-polarized charge carriers from a ferromagnetic contact into an organic semiconductor under suitable applied bias, and (2) the transport and recombination of the injected spin-polarized carriers in the semiconductor. The work performed shed new light on several phenomena: The new model developed for spin-polarized carrier injection through a tunnel process was a significant improvement over prior efforts and was able to account for previously unexplained experimental data, such as the voltage dependence of the magneto-resistance of organic spin valves. The work on bipolar spin transport supports the idea that spin-polarized carrier injection can enhance the efficiency of organic light emitting devices. These advances in the understanding of spin injection and transport phenomena in organic semiconductors constitute the principal intellectual merit of the program outcomes. The program had broad educational impact through the involvement of graduate and undergraduate students in a highly interdisciplinary research effort that spans device physics, electronics, and elements of organic chemistry. Furthermore, elements of the models developed have entered graduate level classes.
