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 (Na 2 CO 3 ) to improve the electro-optical properties of organic light emitting diodes (OLEDs). Na 2 CO 3 layers with various thicknesses were prepared by thermally evaporating Na 2 CO 3 powders. When a 1 nm thick Na 2 CO 3 layer was inserted between the aluminum (Al) cathodes and tris(8-hydroxyquinolinato)aluminum (Alq 3 ) 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 Cs 2 CO 3 buffer layers. The surface of the Alq 3 film became smoother after the Na 2 CO 3 layer was deposited. The reaction mechanisms between Na 2 CO 3 and Alq 3 were also investigated. X-ray and UV photoelectron spectroscopy results show that some electrons transfer from Na 2 CO 3 into Alq 3 , which increases the electron concentration in Alq 3 films and moves the Fermi level close to the lowest unoccupied molecular orbital of Alq 3 . 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. © 2010 The Electrochemical Society.