Effect of charge carrier behaviours in LiF-doped bathophenanthroline (LiF:Bphen) on the performance of organic light-emitting diodes

• Charge carrier behaviours of LiF-doped bathophenanthroline (LiF:Bphen) were studied.
• Electron injection and transport of Bphen films are enhanced by LiF dopant.
• More holes are captured by the trapping sites in LiF:Bphen layers.
• Device properties are enhanced by better electron injection at low LiF doping level.
• The device performance at high LiF doping level is enhanced due to trapped charges.

Charge carrier behaviours of LiF-doped bathophenanthroline (LiF:Bphen) were investigated by single carrier devices: electron-only and hole-only devices. The results indicate that LiF doped with Bphen has dual roles: increasing current density due to enhanced electron injection and transport; decreasing current density owing to capture of holes. In electroluminescent (EL) devices, all doped devices show higher luminance and efficiency than the undoped device. Current density and luminance of doped devices are decreased because hole leakage current decreases with LiF doping level. Compared with the undoped device, 5:100 LiF:Bphen-based EL device exhibits an improved current density. For 10:100 LiF:Bphen-based EL device, current density is smaller at lower electrical field but larger at higher electrical field than that of the undoped device. At higher doping level (15:100 LiF:Bphen), smaller current density than the undoped device is observed. The operation voltage can be lowered by optimizing LiF doping level in electron transport layers.