Origin of improvement in device performance via the modification role of cesium hydroxide doped tris(8-hydroxyquinoline) aluminum interfacial layer on ITO cathode in inverted bottom-emission organic light-emitting diodes

Abstract

It has been found that cesium hydroxide (CsOH) doped tris(8-hydroxyquinoline) aluminum (Alq 3) as an interfacial modification layer on indium-tin-oxide (ITO) is an effective cathode structure in inverted bottom-emission organic light-emitting diodes (IBOLEDs). The efficiency and high temperature stability of IBOLEDs with CsOH:Alq 3 interfacial layer are greatly improved with respect to the IBOLEDs with the case of Cs 2CO 3:Alq 3. Herein, we have studied the origin of the improvement in efficiency and high temperature stability via the modification role of CsOH:Alq 3 interfacial layer on ITO cathode in IBOLEDs by various characterization methods, including atomic force microscopy (AFM), ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS) and capacitance versus voltage ( C– V). The results clearly demonstrate that the CsOH:Alq 3 interfacial modification layer on ITO cathode not only enhances the stability of the cathode interface and electron-transporting layer above it, which are in favor of the improvement in device stability, but also reduces the electron injection barrier and increases the carrier density for current conduction, leading to higher efficiency.