Influence of BCP inserted-layer and working temperature on the sign inversion of magneto-conductance at high magnetic field in organic light-emitting diodes

Organic light-emitting diodes with structure of ITO/CuPc/NPB/Alq 3 /BCP(x)/Al were fabricated in this work. The thickness of BCP is 40, 80 and 120 nm, respectively. The magnetic field effects on the conductance (magneto-conductance, MC) of these diodes were measured at temperature range from 300 to 15 K, respectively. Results show that the MC responses of all the three devices reveal a positive component under low magnetic field at all working temperatures. However, the MC effects under high magnetic field of the BCP(x)-inserted devices show a strong dependence on the temperature and device thickness. In particular, the MC of BCP (120nm) device shows a sign inversion as decreasing the temperature. However, this phenomenon can not be observed in the BCP (40, 80nm) devices. Only negative component of MC is existed in the two devices at low temperatures. We suggest that the positive MC in low magnetic field is caused by hyperfine interaction. Moreover, the sign inversion of MC at high magnetic field is caused by the interaction between the triplet excition with long lifetime at low temperature and holes which are intercalated by the BCP inserted layer.