Designing a stable cathode with multiple layers to improve the operational lifetime of polymer light-emitting diodes

The short device lifetime of blue polymer light-emitting diodes (PLEDs) is still a bottleneck for commercialization of self-emissive full-color displays. Since the cathode in the device has a dominant influence on the device lifetime, a systematic design of the cathodestructure isnecessary.Theoperational lifetime of bluePLEDscanbegreatly improved byintroducing a three-layer (BaF ₂/Ca/Al) cathode compared with conventional two-layer cathodes (BaF₂/Al and Ba/Al). Therefore, the roles of the BaF₂ and Ca layers in terms of electron injection, luminous efficiency, and device lifetime are here investigated. For efficient electron injection, the BaF ₂ layer should be deposited to the thickness of at least one monolayer (̃3 nm). However, it is found that the device lifetime does not showastrong relation withtheelectroninjection orluminousefficiency. Inorder to prolong the device lifetime, sufficient reaction between BaF₂ and the overlying Ca layer should take place during the deposition where the thickness of each layer is around that of a monolayer.