Efficient CsF/Yb/Ag cathodes for organic light-emitting devices

A high-performance cathode consisting of an ultrathin CsF layer and a rare-earth ytterbium (Yb) metal is reported for application in organic electroluminescent devices. Standard tris-(8-hydroxyquinoline) aluminum/α-napthylphenylbiphenyl diamine devices with this bilayer cathode showed dramatically reduced operating voltage and a low turn-on voltage of 2.42 V as compared to 3.75 and 2.95 V in devices using, respectively, the Mg:Ag and single-layer Yb cathodes. At a current density of 200 mA/cm2, devices with the CsF/Yb cathode exhibited high luminance efficiency of 3.45 cd/A and power efficiency of 1.27 lm/W. Analysis by x-ray photoemission spectroscopy suggested that the performance improvement is related to the substantial reduction of electron injection barrier at the cathode/organic interface. It was found that upon Yb deposition, CsF dissociates to liberate low work function Cs metal atoms resulting in a cathode with a lower electron injection barrier and thus a better balance of carriers in the devic...