Electron injection property at the organic–metal interface in organic light-emitting devices revealed by current–voltage characteristics

Electron injection and transport are key issues in organic light-emitting devices (OLEDs). In this letter, we successfully demonstrated the electron injection property which is usually characterized by electronic barrier height from aluminum cathode to tris(8-hydroquinoline) aluminum (Alq3) or 4,7-diphyenyl-1,10-phenanthroline (BPhen) (the most common two electron-transport materials) by current–voltage (J–V) characteristics. The electronic barrier height of 0.66 eV at the Alq3/Al interface and that of 0.10 eV at the Alq3/LiF/Al interface was obtained, which is in good agreement with the experimental results investigated by ultraviolet photoelectron spectroscopy (UPS) by Mori et al. [Appl. Phys. Lett. 73 (1998) 2763]. The electronic barrier height of 0.83 eV at BPhen/Al interface and that of 0.098 eV at BPhen/Cs2O/Al interface was also demonstrated for the first time, suggesting Cs2O is a highly efficient electron-injection material (EIM) for BPhen. We proposed a simple and yet practical approach to estimate the electron-injection barrier-height at the organic–metal interface in OLEDs. The results were further explained by the dipole effect.