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It is generally believed that spin–orbit coupling (SOC) strength and the associated spin relaxation can be enhanced by introducing heavy metal ions in organic semiconductors. Here, we systematically study the spin transport in two organic semiconductors, tris(2-phenylpyridine)iridium (Ir(ppy)3) and tris-(8-hydroxyquinoline) aluminum (Alq3), which have similar chemical structures except that Ir(ppy)3 contains a heavy metal ion Ir. As expected, the photoluminescence spectroscopy measurements show that the SOC strength in Ir(ppy)3 is several orders of magnitude larger than in Alq3. Surprisingly, the spin diffusion length in Ir(ppy)3, deduced from magnetoresistance measurements in Ir(ppy)3-based organic spin valves, is longer than in Alq3. Considering the lower carrier mobility in Ir(ppy)3, the spin relaxation time in Ir(ppy)3 is much longer than in Alq3, implying that the SOC strength in Ir(ppy)3 is weaker than in Alq3. The seemingly contradictory results of photoluminescence spectroscopy and magneto-transport can be explained by the SOC strength depending on the electronic states of a material. The weak SOC strength in Ir(ppy)3 observed in magneto-transport measurements is due to the strong ligand field induced orbital moment quenching for Ir3+ and the polarons transporting in the ligands. However, the excitons involved in photoluminescence spectroscopy overlap with the Ir ion and transforms Ir3+ to Ir4+, which has non-zero spin and orbital moments and hence results in high SOC strength.

The role of heavy metal ions on spin transport in organic semiconductors