Single-Carrier Devices for the Understanding of the Voltage Drift in Organic Light Emitting Diodes

The aim of this study is to analyze and mitigate the voltage drift phenomenon observed in top-emitting organic light emitting diodes (OLED) when driven at constant current. An operating device may experience voltage increase over time due to factors such as interface or bulk material degradation, charge accumulation and formation of trap states. Single-carrier devices were fabricated to understand the contribution to voltage drift from each of these causes. Doping in electron injection layer (4, 7-diphenyl-1,10-phenanthroline or Bphen) and hole injection layer (2,2’,7,7’-tetra(N,N-di-tolyl)amino-spiro-bifluorene or Spiro-TTB) were optimized to obtain ohmic injection contacts. Devices with tris(8-hydroxy-quinoline) aluminium (Alq3) degrade significantly with holes injection and undergo high voltage increase in lifetime test measurements. On the contrary, devices with N,N’-di(naphtalen-1-y1)-N,N’-diphenyl-benzidine (NPB) exhibit an ambipolar charge transport behavior and low voltage drift under both hole and electron injection.