A very simple method of constructing efficient inverted top-emitting organic light-emitting diode based on Ag/Al bilayer reflective cathode

A comparative study on top-emitting organic light-emitting diodes (TOLEDs) with normal and inverted structures is briefly investigated. In comparison with the normal TOLED having Ag reflective anode, the inverted device with monolayer Al reflective cathode shows low efficiency as a result of lower reflectance of Al and inferior electron injection. Using Ag/Al bilayer reflective cathode is demonstrated to be a simple and effective method of enhancing efficiency in inverted TOLED. With tris(8-hydroquinoline) aluminum (Alq3) as emitter the luminous efficiency reaches 5.9±0.6 cd/A, which is much higher than those of the corresponding normal TOLED (∼5.1 cd/A) and inverted TOLED with monolayer Al reflective cathode (∼4 cd/A). The improved performance is attributed to the enhanced reflectance and significant microcavity effect. The electron-injection barrier height of ∼0.1 eV from Al to Alq3 via an ultrathin LiF is estimated in the tunneling process for both normal and inverted devices.