Magnetic field effects in $\pi $-conjugated polymer/fullerene blends; evidence for multiple components*

We studied magnetoconductance (MC) in organic diodes of blends of $\pi $-conjugated polymers and fullerene molecules at various concentrations, $c$. The MC response is composed of several components that depend on the applied bias voltage and $c$. A dominant positive low-field (LF) component, which also governs the magneto-electroluminescence response dramatically decreases and broadens with $c$, thus unraveling a negative LF and positive high-field (HF) components. Our results attest that the positive LF component is caused by Coulombically bound electron-hole (e-h) pairs in the active layer. This is confirmed by studying MC in electron- and hole-unipolar devices, which shows small negative LF that might be due to e-e and h-h pairs (bipolaron), respectively; but lacks the positive LF component.