25 Oct 2007
Chinese researchers have improved the power efficiency of organic light-emitting diodes by introducing novel n-doping and p-doping layers.
The researchers obtained a power efficiency of 4.44 lm/W, which is the best value reported so far for tris (8- hydroxyquinolinato) aluminium-based emitters. Compared to an undoped control device, OLEDS with n- and p-doped layers showed a 100% increase in power efficiency, while luminous efficiency increased by 54% and the driving voltage was reduced by 32% (2007 J. Phys. D: Appl. Phys. 40 6535)
According to the researchers, the n-doped layer in these devices acts as an electron transport layer and the p-doped layer as a hole transport layer. The team attributed the improvement in performance to the increased conductivity of the transport layers and to a better balance of charge carriers in the emission layer.
Both layers were fabrication from aluminium-based complexes: the n-doping layer was composed of 4'7- diphyenyl-1, 10-phenanthroline: 33 and 8-hydroxy-quinolinato lithium, and the p-doping layer of 4, 4', 4″-tris (3-methylphenylphenylamono) triphenylamine (m-MTDATA) and tetrafluro-tetracyano-quinodimethane (F4-TCNQ).
The team tested a series of hole-only and electron-only devices, while a range of n- and p-doped devices was also compared to an undoped control. Devices with n- and p-doped layers displayed improved conductivity in the transport layers, and displayed substantial improvements in both power efficiency and luminous efficiency at a lower drive voltage.