Absence of spin transport in the organic semiconductorAlq3

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 $\left(\mathrm{Al}{\mathrm{q}}_3\right)$. While some attribute it to spin injection and transport in $\mathrm{Al}{\mathrm{q}}_3$, others suggest tunneling through locally thin regions of the $\mathrm{Al}{\mathrm{q}}_3$ layer as the mechanism. We present results of magnetotransport and charge transport measurements on $\mathrm{Al}{\mathrm{q}}_3$-based spin valves and unipolar devices where the $\mathrm{Al}{\mathrm{q}}_3$ thickness is beyond the tunneling limit. We observe no measurable MR in the $\mathrm{Fe}∕\mathrm{Al}{\mathrm{q}}_3∕\mathrm{Co}$ spin valve structures. Measurements of temperature-dependent current-voltage characteristics and comparisons with unipolar devices show that charge transport in $\mathrm{Fe}∕\mathrm{Al}{\mathrm{q}}_3∕\mathrm{Co}$ spin valves is by holes only and is injection-limited. The hole-only transport in $\mathrm{Al}{\mathrm{q}}_3$ 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.