Effects of carrier mobility and morphology in organic semiconductor spin valves

摘要

We studied spin transport in four organic semiconductors (OSCs) withdifferent electronic properties, with Fe and Co as the top and bottomferromagnetic (FM) contacts, respectively. Magnetoresistance (MR) effects wereobserved up to room temperature in junctions based on an electron-carrying OSC,tris(8-hyroxyquinoline) aluminum (Alq$_3$) and a hole-carrying OSC, copperphthalocyanine (CuPc). The MR shows similar temperature dependence for thesetwo OSCs, which suggests that the FM leads rather than the OSCs play a dominantrole on the spin-transport degradation with increasing temperature. We alsoinvestigated junctions based on two high lateral mobility electron-carryingOSCs, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N'-bis(4-trifluoromethylbenzyl)-1,4,5,8-naphthalenetetracarboxylic diimide(CF$_3$-NTCDI). However, these junctions showed much weaker spin transporteffects. Morphological studies suggest that these high mobility OSC films havemuch rougher surfaces than either Alq$_3$ or CuPc, therefore the degradation ofspin transport may originate from enhanced scattering due to the rougher FM/OSCinterfaces. Our study shows that FM/OSC interfaces play an important role forspin transport in organic devices and need further exploration.