Tuning magnetoresistance and magnetic-field-dependent electroluminescence through mixing a strong-spin-orbital-coupling molecule and a weak-spin-orbital-coupling polymer

We report a tunable magnetoresistance by uniformly mixing strong-spin-orbital-coupling molecule fac-tris (2-phenylpyridinato) iridium $[\mathrm{Ir}(ppy{)}_{3}]$ and weak-spin-orbital-coupling polymer poly($N$-vinyl carbazole) (PVK). Three possible mechanisms, namely charge transport distribution, energy transfer, and intermolecular spin-orbital interaction, are discussed to interpret the $\mathrm{Ir}(ppy{)}_{3}$ concentration-dependent magnetoresistance in the $\mathrm{PVK}+\mathrm{Ir}(ppy{)}_{3}$ composite. The comparison between the magnetic field effects measured from energy-transfer and nonenergy-transfer $\mathrm{Ir}(ppy{)}_{3}$ doped polymer composites indicates that energy transfer and intermolecular spin-orbital interaction lead to rough and fine tuning for the magnetoresistance, respectively. Furthermore, the photocurrent dependence of magnetic field implies that the excited states contribute to the magnetoresistance through dissociation. As a result, the modification of singlet or triplet ratio of excited states through energy transfer and intermolecular spin-orbital interaction form a mechanism to tune the magnetoresistance in organic semiconducting materials.