Spin Response in Organic Light Emitting Diodes*

To understand the origin of the magnetic field effect in OLEDs, we studied the large magnetoresistance (MR) and magnto-electroluminescence (MEL) of OLEDs based on pristine MEH-PPV polymer, as well as MEH-PPV doped with various concentrations of radical impurities and C$_{60}$ molecules. In contrast to OLED based on pristine MEH-PPV that show MR and MEL up to 12{\%} at room temperature, we found in MEH-PPV:C$_{60}$ based OLED the MR and MEL decrease substantially with increasing C$_{60}$ molecule concentration. For MEH-PPV:C$_{60}$ devices with C$_{60}$ concentration of 50{\%}, the MR effect is less than 0.3{\%} at room temperature. In MEH-PPV: radical devices the MR and MEL effects again differ substantially from those found in pristine and C$_{60}$ doped MEH-PPV devices. At 50{\%} radical concentration the MR and MEL effects are about 1{\%} and 3.5{\%}, respectively. The results are discussed with existing models for the magnetic field effects in OLEDs.