Improved electron injection into Alq3 based OLEDs using a thin lithium carbonate buffer layer

A cathode buffer layer of lithium carbonate (Li2CO3) we used to improve the electro-optical properties of organic light-emitting diodes (OLEDs). Li2CO3 layers with various thicknesses were prepared by thermally evaporating Li2CO3 powders. When a 1-nm-thick Li2CO3 layer was inserted between the aluminum (Al) cathodes and tris(8-hydroxyquinolinato)aluminum (Alq3) electron-transporting layers, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved, becoming better than and comparable to those of devices with LiF and Cs2CO3 buffer layers. The surface of the Alq3 film became smoother after the Li2CO3 layer was deposited. The reaction mechanisms between Li2CO3 and Alq3 were also investigated. X-ray and ultraviolet photoelectron spectroscopy results show that some electrons transfer from Li2CO3 into Alq3, which increases the electron concentration in Alq3 films and moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq3. Thus, the electron injection efficiency was enhanced due to a lower electron injection barrier, which improves the charge carrier balance in OLEDs and leads to better device efficiency.