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 (MoO₃) 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 MoO₃, with the inverted device with HAT‐CN as the interlayer showing a hole injection efficiency close to 100%. The results from doping NPB with MoO₃ 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.