Influence of doping concentration on the dominant injection and transport mechanisms of electrons within Alq3 doped NPB films

In this work, we separately investigated the dependence of electron injection and transport processes on the doping concentration by comparing the current density-voltage (J-V) characteristics of a series of electron-only devices with tris(8-hydroxyquinoline) aluminum (Alq(3)) doped into N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'dipheny1-4,4'-diamine (NPB) as transport layer. Here, Alq(3) molecules function as electron injection sensitive centers. Experimental results revealed that 0.2 wt% is the optimal doping concentration to facilitate electron injection from hole-block layer into host molecules. With increasing doping concentration, more and more electrons inject directly onto Alq(3) molecules while the average distance of Alga molecules decreases gradually, thus which accelerate electron hopping between Alga molecules. With the help of electroluminescent (EL) spectra, recombination centers of the 02 wt% and 0.6 wt% doped EL devices were probed to shift toward cathode and anode, respectively. These results suggested that electron hopping between Alga molecules becomes the dominant electron transport mechanism when the doping concentration is higher than 0.6 wt%. (C) 2016 Elsevier B.V. All rights reserved.