Enhanced electron injection into inverted polymer light-emitting diodes by combined solution-processed zinc oxide/polyethylenimine interlayers.

TLDR: A solution-processable combination of ZnO and PEI is reported, that facilitates electron injection and enables efficient and air-stable inverted devices.

Abstract: Due to their planar nature and their mechanical fl exibility, organic light emitting diodes (OLEDs) open pathway to many new applications in display technology, signage and general lighting. While state-of-the-art OLEDs are mostly fabricated by thermal evaporation in vacuum, solution processing (printing and coating) is widely discussed for future cost reduction in OLED fabrication. So far, vacuum processed devices outperform their solution processed counterparts not least because thermal evaporation allows for stacking an arbitrary number of functional layers while solution deposition is ruled by solvent limitations. In vacuum processed OLEDs, charge carrier injection can be facilitated by utilizing electrically doped organic semiconductor layers. [ 1‐3 ] Alternatively, high-work function metal oxides such as molybdenum oxide (MoO 3 ), [ 4,5 ] tungsten oxide (WO 3 ) [ 6,7 ] or vanadium pentoxide (V 2 O 5 ) [ 8,9 ] can be used for the injection of holes. Recently, we have demonstrated that MoO 3 or WO 3 hole injection layers for OLEDs can also be applied via a facile precursor route from molybdenum(V)ethoxide, tungsten(V)ethoxide or tungsten(VI)ethoxide, respectively. [ 10,11 ] On the other hand, electron injection into the lowest unoccupied molecular orbital (LUMO) level of light emitting or adjacent electron transport materials is much more challenging since it requires low-work function and hence reactive electrodes, interface modifi ers or buffer layers such as calcium,