Intrinsic magnetic field effects in organic semiconductors
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tion: Organic electronics and spintronics The two thriving technologies of organic light-emitting diodes (OLEDs)1 and magnetoelectronics/spintronics2 until recently had no overlap. However, over the last 10 years or so, there has been a growing realization that the class of materials that OLEDs are made from (i.e., organic semiconductors [OSCs]) show a number of interesting spin-dependent phenomena. This opens up the possibility that magnetoelectronic sensors and/or non-volatile logic gates could be fabricated from inexpensive plastic materials. A basic OLED is composed of an OSC thin film sandwiched between an anode and a cathode. The feature that distinguishes OSCs from “ordinary” non-conductive plastics is the delocalization (“conjugation”) of π electrons over the entire molecule, a feature familiar from the benzene molecule. As a result, a relatively small “forbidden energy gap” on the order of an eV separates the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively). This is similar to the forbidden gap that separates the valence and conduction bands in inorganic semiconductors. However, disorder in the usually amorphous OSC layer prevents band transport. Instead, carriers hop incoherently from molecule to molecule. Electrostatic forces pull holes and electrons—injected by the anode and cathode, respectively—together to form a
“radical pair” (i.e., two spins located on neighboring molecules). By hopping onto the same molecule, the pair can form an exciton, either a spin-0 singlet (S, opposite spins) or a spin-1 triplet (T, parallel spins). T excitons typically have much lower energy than S excitons. The last statement follows from the Pauli exclusion principle: two spin-parallel electrons tend to avoid each other, therefore a hole (missing electron) is attracted to an electron with parallel spin. Electroluminescence occurs when the exciton decays radiatively. In addition to electrostatic forces, charge carriers in OSCs also exert forces on each other mediated by vibrational fields. These forces are always attractive, even in electron-electron and hole-hole pairs. A single charge together with its vibrational field is called a “polaron” and a double charge a “bipolaron.” The attractive force is usually not strong enough to lead to stable bipolarons, but bipolarons may occur as transient states. Like excitons, bipolarons can occur as singlets and triplets. However, different from excitons, the singlet bipolaron state is energetically favored.3 “Spintronics” deals with controlling and utilizing the electron spin degree of freedom (see the Introductory article in this issue). Giant magnetoresistive (GMR) devices use a change in the relative magnetizations of two ferromagnetic electrodes to control the current through a non-magnetic material.2,4,5 The non-magnetic spacer layer can be a conductor6–8 or a thin
Markus Wohlgenannt, Department of Physics and Astronomy, University of Iowa, USA; [email protected] Peter A. Bobbert, Eindhoven University of Technology, The Netherland
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