Buffer-enhanced electron injection in organic light-emitting devices with copper cathode

► High-efficiency OLEDs with Cu-cathode are fabricated. ► The optimal thickness of LiF buffer layer for Cu-cathode device is obtained. ► The mechanism of LiF buffer layer in Cu-cathode OLEDs is proposed to be a type of dual effect. ► The conclusion is further demonstrated by UPS and XPS measurements. ► The optimal-thickness-LiF-induced dipole effect is significant for high work function metal cathode.