PVK-AlQ 3 organic electroluminescent diodes: transport properties and color tuning via PVK doping

In this paper we report on Organic Light Emitting Diodes (OLEDs) based on heterojunctions between a polymer, Poly(N- vinylcarbazole) (PVK) as hole transport layer (HTL) and an evaporated layer of Tris (8-Hydroxy)quinoline) Aluminum (Alq₃) as electron transport layer (ETL) and emissive layer (EM). The electrical properties of ITO/PVK/Alq₃/Mg-Ag were thoroughly investigated and Trapped Charges Limited (TCL) currents were shown to be the main transport process in these devices. In order to assess the pertinence of PVK as HTL, a comparison with other HTL deposited by vacuum evaporation was carried out. The color of the emitted light then could have been tuned by adding DCM in PVK. Light emitted by Alq₃ was partly absorbed by DCM thus exciting the photoluminescence of DCM. Exiting light was thus a combination of green-yellow characteristic of Alq₃ and of red-orange due to DCM. Wavelengths spanning the range 530-620 nm were obtained for DCM concentrations in PVK up to 20% wt. without significant degradation of the external quantum yield.