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 the contact potential difference (CPD) between a reference electrode and various metals acting as charge carrier injecting contacts in organic light-emitting devices (OLEDs). These investigations show that the work function of anode materials for OLEDs such as Pt, Au, and indium tin oxide depends strongly on the surface treatment and can be increased by more than 1 eV via oxygen plasma or UV-ozone cleaning. The device performance of multilayer OLEDs consisting of these anodes, copper-phthalocyanine (CuPc), N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB), tris-(8-hydroxyquinolinato)aluminum (Alq3), 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 functions, other factors such as the surface roughness and the binding energy of oxygen to the metal surface can significantly influence the injection properties and the long-term stability of the devices.