Organic spintronics & molecular devices
Organic spintronics is a rapidly evolving field, aiming for new spin- and magnetic functionality in organic electronics. Growing interest has been triggered by successful realization of prototype devices as well as by the recognition of challenging scientific puzzles associated to them. In one approach, relevant for organic spin valves, spin polarized carriers are injected from a ferromagnetic electrode into the organic material. The other approach employs the so-called organic magnetoresistance (OMAR), in which intrinsic magnetic field effects (MFEs) affect the carrier mobility in (or light emission of) organic materials. It leads to large (~20%) magnetoresistance at room temperature, applying just small mT magnetic fields and for devices without any ferromagnetic electrodes. The effect has been found to be caused by reactions between spin carrying carriers in the presence of local hyperfine fields – with an appealing link to migratory birds exploiting spin-chemical reactions for their navigation in the Earth’s magnetic field.
By combining novel experimental approaches, engineering molecular structures and theoretical modeling we made a next step in understanding the MFEs in organic semiconductors. A subtle interplay between microscopic mechanism and device physics has been resolved. Furthermore, it was shown that trap states play an essential role in creating large MFE effects.
Devices used in OMAR research are very similar to ordinary organic light-emitting diodes (OLEDs) and photo-voltaic cells (OPVCs). Polymer-fullerene blends, widely used for OPVCs, provide a unique system for resolving different OMAR mechanisms, but in a reverse way, OMAR (related) research can provide new insight into spin-processes that are relevant for the functioning of OLEDs and OPVCs
In earlier work we addressed the transition from spin-polarized tunneling between two ferromagnetic layers through an organic spacer layer, to successive spin-injection and extraction. A loss of magnetoresistance at the onset of two-step tunneling was found. Other activities addressed the role of spin-relaxation due to hyperfine fields. Magnetoresistance due to spin-injection is presently a controversial issue.