In-situ Determination of Interface Dipole Energy between Tris(8-hydroxyquinoline) Aluminum and MgO Coated Al in Inverted Top-Emitting Organic Light-Emitting Diodes

Abstract

The interface dipole energies between tris(8-hydroxyquinoline) aluminum (Alq(3)) and Al/MgO (or bare Al) were in-situ measured using synchrotron radiation photoemission spectroscopy. The work function of Al/MgO is higher by 0.6 eV than that of Al. The interface dipole energies were determined to be -0.3 and 0.2 eV for Al/MgO and bare Al, respectively. Therefore, the barrier height of Al/MgO is higher by 0.3 eV than that of Al. The operating voltage at a current density of 50 mA/cm(2) of inverted top-emitting organic light-emitting diode (ITOLED) using Al/MgO cathode decreased from 14.3 to 11.7 V, and the luminance value at the current density of 222 mA/cm(2) of ITOLED increased from 1830 to 1950 cd/m(2). Therefore, it is considered that MgO interfacial layer played a role in reducing the effective barrier height of electron injection by tunneling mechanism, leading to a reduction in turn-on voltage and luminance enhancement. (C) 2011 The Japan Society of Applied Physics