Interfacial charge transfer study of hexacarbonitrile-based intermediate connecter in blue tandem organic light-emitting diodes

Abstract

The charge generation process of hexaazatriphenylene hexacarbonitrile-based intermediate connectors (ICs) in blue tandem organic light-emitting diodes (OLEDs) is investigated via photoelectron spectroscopic and device studies. Though 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) is well-known to be a p-type hole-injection layer, our photoelectron spectroscopic results indicate that HATCN becomes the n-type when it is sandwiched between Li-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) and N,N″-diphenyl-N,N″-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) layers, and the charge generation and separation at the Li:BPhen/HATCN/NPB interfaces become energetically favorable, as depicted by their energy level alignments. This explains the superior device performance when Li:BPhen/HATCN/NPB is used as a connecting unit in tandem devices.