Enhancement of Electron Injection in Organic Light Emitting Diodes Using an Ultrathin Sodium Carbonate Buffer Layer

Abstract

We used a cathode buffer layer of sodium carbonate to improve the electro-optical properties of organic light emitting diodes (OLEDs). layers with various thicknesses were prepared by thermally evaporating powders. When a 1 nm thick layer was inserted between the aluminum (Al) cathodes and tris(8-hydroxyquinolinato)aluminum 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 buffer layers. The surface of the film became smoother after the layer was deposited. The reaction mechanisms between and were also investigated. X-ray and UV photoelectron spectroscopy results show that some electrons transfer from into , which increases the electron concentration in films and moves the Fermi level close to the lowest unoccupied molecular orbital of . 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.