Magnetic field effect in organic light-emitting diodes based on Co spin polarization source

Abstract

<p indent=0mm>In this study, Co thin films with different thicknesses have been successfully prepared through electron injection of Co into Alq₃ by the contact of ferromagnetic metal Co with an organic semiconductor Alq₃. The Co thin films were used as the spin polarization source of organic light-emitting devices (OLEDs) to inject spin-polarized electrons. The basic structure of the devices was ITO/MoO₃(5 nm)/NPB<sc>(60 nm)</sc>/Alq₃(80 nm)/Co(<italic>x</italic> nm, <italic>x</italic> = 0, 1, 2, 3)/CsCl<sc>(0.6 nm)</sc>/Al<sc>(120 nm).</sc> The photoelectric performance and magneto electroluminescence (MEL) of the four devices at room temperature <sc>(295 K)</sc> and the variable temperature magnetic field effect with Co film thickness <sc>1 nm</sc> were measured. The results showed that, under room temperature, MEL curves rapidly rise at a low magnetic field (|<italic>B</italic>|<50 mT) with the increase in Co film thickness; while at a high magnetic field (|<italic>B</italic>|>50 mT), the curves slowly rise, finally becoming saturated. The maximum brightness of the fourth device is <sc>46260 cd/m².</sc> At 95 and <sc>45 K,</sc> the MEL curves show some different phenomena, characterized by rapid rise and fall under low and high magnetic fields, respectively. Based on the analysis of the experimental results by intersystem crossing (ISC), triplet-triplet exciton annihilation (TTA), hyperfine interaction (HFI), and magnetic field influence mechanisms, the results reveal that under room temperature, the MEL line type of OLEDs based on Co is mainly due to the HFI and ISC processes, and the MEL lines at 95 and <sc>45 K</sc> are mainly due to the TTA process and applied magnetic field. This study expands the analysis of the spin-mixing process of TTA and ISC based on Co spin polarization source devices and provides a method and idea for improving the photoelectric performance of OLEDs.