Magnetic field effect spectroscopy of C60-based films and devices

We performed spectroscopy of the magnetic field effect (MFE) including magneto-photoinduced absorption (MPA) and magneto-photoluminescence (MPL) at steady state conditions in annealed and pristine fullerene C-60 thin films, as well as magneto-conductance (MC) in organic diodes based on C-60 interlayer. The hyperfine interaction has been shown to be the primary spin mixing mechanism for the MFE in the organics. In this respect, C-60 is a unique material because 98.9% of the carbon atoms are C-12 isotope, having spinless nucleus and thus lack hyperfine interaction. In spite of this, we obtained substantial MPA (up to similar to 15%) and significant MC and MPL in C-60 films and devices, and thus mechanisms other than the hyperfine interaction are responsible for the MFE in this material. Specifically, we found that the MFE(B) response is composed of narrow (similar to 10mT) and broad (> 100mT) components. The narrow MFE(B) component is due to spin-dependent triplet exciton recombination in C-60, which dominates the MPA(B) response at low pump intensities in films, or the MC response at small current densities in devices. In contrast, the broad MFE(B) component dominates the MPA(B) response at high pump intensities (or large current densities for MC(B)) and is attributed to spin mixing in the polaron pairs spin manifold due to g-factor mismatch between the electron-and hole-polarons in C-60. Our results show that the organic MFE has a much broader scope than believed before. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4800546]