Origin of enhanced hole injection in inverted organic devices with electron accepting interlayer

Conventional organic light emitting devices have a bottom buffer interlayer placed underneath the hole transporting layer (HTL) to improve hole injection from the indium tin oxide (ITO) electrode. In this work, a substantial enhancement in hole injection efficiency is demonstrated when an electron accepting interlayer is evaporated on top of the HTL in an inverted device along with a top hole injection anode compared with the conventional device with a bottom hole injection anode. Current-voltage and space-charge-limited dark injection (DI-SCLC) measurements were used to characterize the conventional and inverted N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'biphenyl)-4,4'diamine (NPB) hole-only devices with either molybdenum trioxide (MoO ₃) or 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) as the interlayer. Both normal and inverted devices with HAT-CN showed significantly higher injection efficiencies compared to similar devices with MoO ₃, with the inverted device with HAT-CN as the interlayer showing a hole injection efficiency close to 100%. The results from doping NPB with MoO ₃ or HAT-CN confirmed that the injection efficiency enhancements in the inverted devices were due to the enhanced charge transfer at the electron acceptor/NPB interface. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.