Spin transport and magnetoresistance in organic semiconductors

Organic semiconductors are interesting materials for spintronics applications because of their long spin lifetimes. In addition, organic spintronics offers the possibility to add magnetic functionality to existing organic electronics. Two main topics of organic spintronics are discussed. First, in organic spin valves, spin transport occurs through an organic spacer layer. As a main source of loss of spin polarization the interactions of the spins with random hyperfine fields originating from surrounding hydrogen nuclei has been identified. Recent progress in the development of organic spin valves and related approaches are discussed, addressing conductivity mismatch and the question whether the observed magnetoresistance is from spin injection or direct tunneling. Second, an intrinsic magnetoresistance is observed in many organic semiconductors. This, so-called, organic magnetoresistance (OMAR) is interesting because it shows large effects at room temperature, making it interesting for applications, and poses fundamental questions about spin transport and spin interactions in these organic materials. An overview of the main characteristics of OMAR is given and the three main models are discussed. These models are the bipolaron model, the electron-hole pair model and the exciton-charge interaction model. Finally, a comparison is made between the proposed models and experimental results, where it is concluded that the exact origin of OMAR is still open for debate. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim