  We studied spin transport in four organic semiconductors (OSCs) with
different electronic properties, with Fe and Co as the top and bottom
ferromagnetic (FM) contacts, respectively. Magnetoresistance (MR) effects were
observed up to room temperature in junctions based on an electron-carrying OSC,
tris(8-hyroxyquinoline) aluminum (Alq$_3$) and a hole-carrying OSC, copper
phthalocyanine (CuPc). The MR shows similar temperature dependence for these
two OSCs, which suggests that the FM leads rather than the OSCs play a dominant
role on the spin-transport degradation with increasing temperature. We also
investigated junctions based on two high lateral mobility electron-carrying
OSCs, 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 transport
effects. Morphological studies suggest that these high mobility OSC films have
much rougher surfaces than either Alq$_3$ or CuPc, therefore the degradation of
spin transport may originate from enhanced scattering due to the rougher FM/OSC
interfaces. Our study shows that FM/OSC interfaces play an important role for
spin transport in organic devices and need further exploration.
