Kelvin probe investigations of metal work functions and correlation to device performance of organic light-emitting devices

Using the vibrating capacitor Kelvin probe technique, we have determined th e contact potential difference (CPD) between a reference electrode and vari ous metals acting as charge carrier injecting contacts in organic light-emi tting devices (OLEDs). These investigations show that the work function of anode materials for OLEDs such as Pt, Au, and indium tin oxide depends stro ngly on the surface treatment and can be increased by more than 1 eV via ox ygen plasma or UV-ozone cleaning. The device performance of multilayer OLED s consisting of these anodes, copper-phthalocyanine (CuPc), N,N'-di(naphtha lene-1-yl)-N, N'-diphenyl-benzidine (NPB), tris-(8-hydroxyquinolinato)alumi num (Alq(3)), and a low-work-function metal cathode is correlated with the results of the CPD measurements. However, our investigations indicate that, apart from the measured work function, other factors such as the surface r oughness and the binding energy of oxygen to the metal surface can signific antly influence the injection properties and the long-term stability of the devices. (C) 2000 Elsevier Science S.A. Ail rights reserved.