有机发光二极管中不同金属电极引起的奇特磁效应

Peculiar magnetic field effects in organic light-emitting diodes with different metal cathodes

采用三类金属阴极材料Ca,Al和Cu(Au)通过分子束沉积和电子束加热方式制备了有机发光二极管ITO/CuPc/NPB/Alq3/金属阴极,并在300,200,150,100,50和15K6个温度下,分别测量了不同电极器件的发光随外加磁场的变化(即Magneto-ElectroLuminescence,MEL).在室温300K下,发现Ca,Al和Cu(Au)电极器件的MEL在低场(0B50mT)均表现为快速上升;但随磁场(B>50mT)的进一步增大,Ca和Al电极器件的MEL缓慢变大并逐渐趋于饱和,且与阴极的制膜方式无关;而采用电子束加热方式制备的Cu(Au)电极器件,其MEL却表现出高场缓慢下降;且温度越低,该类Cu电极器件MEL的高场下降更为显著.实验研究表明,Ca和Al电极器件的MEL主要是由超精细耦合作用随外加磁场变化引起的.但电子束加热方式制备的Cu(Au)电极器件的MEL除了超精细耦合作用引起的低场快速上升外,其高场下降的可能机制则是:Cu(Au)电极器件中电子-空穴对的俘获区(e-hCapture Zone)靠近阴极界面,相比较于热蒸发的方式,电子束蒸发的方式更容易使重金属Cu(Au)原子得到更高的能量,使其渗透进相邻的有机层Alq3中,Cu(Au)原子的强自旋轨道耦合作用导致电子-空穴对发生自旋翻转,此为MEL出现高场下降的原因.

Organic light-emitting diodes with structure of ITO/CuPc/NPB/Alq3/metal cathode （Ca, A1, Cu） were fabricated in this work. The Ca, A1 cathode with low melting point was grown by thermal evaporation deposition, and the Cu cathode with high melting point was fabricated by electron-beam deposition. The magnetic field effects on electrolu- minescence （magneto-electroluminescence, MEL） of these diodes were measured at temperature range from 300 to 15 K, respectively. Results show that the MEL responses of all these three devices present a fast rise in low magnetic field at room temperature. However, with further increasing the magnetic field, the MEL effects of Ca, A1 electrodes devices increase slowly and become saturated gradually. This magnetic field dependence of MEL in the devices with Ca （or A1） electrode was found to be insensitive with the preparation of metal cathode. In contrast, the high field MEL effect of Cu （or Au） electrode device shows a slow decrease. Moreover, as decreasing the temperature, the high field MEL effect of Cu electrode device becomes more obvious. We suggest that the MEL effects in low and high magnetic field effect of Ca, A1 electrodes are caused by the hyperfine mixing. The MEL in low magnetic field effect of Cu, Au cathodes is also caused by the hyperfine coupling. The declining of MEL in Cu （or Au） cathode based device at high magnetic field can be explained as follows： electronic injection is difficult for Cu cathode, so the e-h capture zone is close to the cathode interface. Moreover, the Cu （or Au） atoms deposited by electron-beam evaporation have much higher energy than that deposited by thermal evaporation, which cause the Cu （or Au） atoms penetrating into NPB layer. Then, Cu （or Au） atoms with strong spin-orbit coupling strength have an interaction with the e-h capture zone, which causes the MEL declining at high magnetic.