Magnetoconductance response in unipolar and bipolar organic diodes at ultrasmall fields

We measured magnetoconductance (MC) response in a number of unipolar and bipolar organic diodes based on rr-conjugated polymers and small molecules at fields |B|< 100 mT and various bias voltages and temperatures. Similar to magneto-electroluminescence, the MCðBÞ response in bipolar diodes shows a sign reversal at ultrasmall |B| < 1-2 mT due to interplay f hyperfine and Zeeman interactions in opposite-charge polaron pairs. Surprisingly, similar MCðBÞ response was also measured in unipolar devices, indicating the existence of like-charge polaron pairs, however, with a clear difference between the hyperfine interaction constants of electron polaron and hole polaron.