Improved Performance of Organic Light-Emitting Diodes with MgF₂ as the Anode Buffer Layer

Organic light-emitting diodes (OLEDs) based on N,N’-bis(1-naphthyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine (NPB) and tris (8-hydroxyquinoline) aluminium (Alq₃) are improved by using a thin MgF₂ buffer layer sandwiched between the indium tin oxide (ITO) anode and hole transporting layer (HTL) of NPB. The current-voltage curves of the OLEDs with MgF₂ buffers shift to lower voltages, which can be explained by the tunnelling effect. Under 10V bias, the current density and brightness for the optimized OLED with a 1.0-nm MgF₂ are 196A/m² and 517cd/m², respectively, while for the OLED without anode buffer layer are only 109A/m² and 156cd/m². The atomic force microscopy shows that the rms roughness of NPB on ITO/MgF₂ is only 1/3 of NPB on bare ITO. The improved morphology of the HTL would lead to more robust OLEDs. The OLED with a 1.0-nm MgF₂ layer has a long lifetime of more than five times of the MgF₂-free reference device due to the combined electrical and morphological effects of the MgF₂ layer.