Magnetic Field-effects in Pi-conjugated Polymer/Fullerene Blends

摘要

Organic magnetoresistance (OMAR) has been recently observed in a variety of light emitting diodes of ?-conjugated polymers and small molecules with non-magnetic electrodes [1-7]. Due to its potential to be used in magnetically controlled optoelectronic devices and magnetic sensors,this OMAR has become a field attractive for basic research and applications. In spite of the surge of interest in OMAR,its underlying mechanism is still under debate. Because of the weak field involved,it is largely agreed that OMAR originates from spin sublevel mixing via the hyperfine interaction (HFI) [1-7] (but see [11]). However two competing basic models have been proposed for explaining how the spin-mixing process that causes OMAR. These are the excitonic model [2],in which the magnetic field modulates the singlet to triplet inter-conversion; and the bipolaron model [3]. The exciton model,which is based on the spin dependent Coulombically bound polaron pairs (P+P-) formation,needs the existence of both P+ and P- in the active layer; whereas the bipolaron model which relies on the spin dependent formation of doubly charged excitations,only needs one type of charge carrier in the device.