Abstract
We studied the changes in the magneto-conductance (MC) response in bipolar and unipolar organic diodes based on the π-conjugated polymer poly[2-methoxy-5-(2-ethylhexyl-oxy)-1,4-phenylene-vinylene] (MEH-PPV), upon prolonged illumination of light in the Visible–UV range. We found that the MC response in the hole-unipolar device gradually changes sign from negative to positive upon illumination, caused by the photogeneration of metastable donor-like defects in the MEH-PPV layer. This conclusion is supported by photoinduced absorption spectroscopy of pristine MEH-PPV film. In contrast, we found very little change in the MC response upon illumination of electron-unipolar devices; however, the MC response substantially increases in bipolar devices. We interpret the sign change in the MC response of hole-unipolar devices as due to illumination induced change in the spin-pairs that participate in the MC, from spin pairs of same charge to spin pairs of opposite charges.
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