Improved electron injection in polymer light-emitting diodes using anionic conjugated polyelectrolyte

We report improved performance in polymer light-emitting diodes incorporating conjugated polyelectrolytes as an electron injection layer (EIL). When we introduce water soluble conjugated polymers, poly[$9,9′$-bis(4-sulfonatobutyl)fluorene-co-alt-1,4-phenylene] (anionic PFP), between the aluminum (Al) cathode and emissive layer, the devices show an increased electroluminescence efficiency with a lowered turn-on voltage. We believe the mobile $Na+$ ions in the EIL layer directly influences the device efficiency by forming a low work function layer at the interface between the EIL and Al cathode, thereby facilitating the electron injection into the emissive layer.