Absence of spin transport in the organic semiconductor Alq3

There have been differing interpretations regarding the magnetoresistance (MR) reported in spin-valve structures containing thick layers of the organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq(3)). While some attribute it to spin injection and transport in Alq(3), others suggest tunneling through locally thin regions of the Alq(3) layer as the mechanism. We present results of magnetotransport and charge transport measurements on Alq(3)-based spin valves and unipolar devices where the Alq(3) thickness is beyond the tunneling limit. We observe no measurable MR in the Fe/Alq(3)/Co spin valve structures. Measurements of temperature-dependent current-voltage characteristics and comparisons with unipolar devices show that charge transport in Fe/Alq(3)/Co spin valves is by holes only and is injection-limited. The hole-only transport in Alq(3) is stable only at low current densities. This supports the tunneling interpretation of the earlier reported MR. Similar to inorganic semiconductors, the large conductivity mismatch between the metal electrodes and the organic semiconductor prevents spin injection. However, inserting a tunnel barrier between the magnetic electrode and the organic semiconductor did not improve spin injection.