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

Abstract

Organic light-emitting diodes with structure of ITO/CuPc/NPB/Alq 3 /metal cathode (Ca, Al, Cu) were fabricated in this work. The Ca, Al 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, Al electrodes devices increase slowly and become saturated gradually. This magnetic field dependence of MEL in the devices with Ca (or Al) 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, Al 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.