Magnetoconductance effect in organic light-emitting devices

Organic light-emitting diode (OLED) based on tris-(8-hydroxyquinoline) aluminum(III) (Alq3) is fabricated, and its magnetoconductance (MC) effects are measured at different bias voltages. When the bias voltage is small, the OLED exhibits apparently a negative MC effect. After the bias voltage is increased, the MC value changes from negative to positive, displaying a negative-positive inversion. The MC effects in N, N'-Di(naphthalen-1-yl)-N, N' diphenyl-benzidine (NPB) and Copper phthalocyanine (CuPc) unipolar devices show that the negative MC effect in OLED comes from the CuPc layer in device. The MC effect of bipolar current can be explained using the electron-hole pair model. The MC effect of unipolar current can be attributed to the polaron-bipolaron transition in device. The positive-negative MC inversion in OLED results from the simultaneous contributions of the above two mechanisms during the variation of the injection current.