Improved electron injection of organic light-emitting diodes by using an ultrathin lithium-hydroxide buffer layer

Abstract

A cathode buffer layer of lithium hydroxide (LiOH) was used to improve the electro-optical properties of organic light-emitting diodes (OLEDs). LiOH layers with various thicknesses were prepared by thermally evaporating LiOH powders. When a 1-nm-thick LiOH layer was inserted between the aluminum (Al) cathodes and the tris(8-hydroxyquinolinato)aluminum (Alq3) electrontransporting layers, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved, becoming better than those of the device with a LiF buffer layer. The surface of the Alq3 film became smoother after the LiOH layer had been deposited. Ultraviolet photoelectron spectroscopy results show that the reaction between LiOH and Alq3 results in an n-type doping effect, which 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 improved the charge carrier balance in the OLEDs and led to a better device efficiency.