Imaging of the ageing on organic electroluminescent diodes, under different atmospheres by impedance spectroscopy, scanning electron microscopy and SIMS depth profiling analysis

We have fabricated single-layer electroluminescent diodes with poly(p-phenylene vinylene) (PPV), using indium-tin oxide (ITO) as hole-injecting anodes and aluminium as electron-injecting cathodes. We continuously monitored the impedance of che diodes, both in inert atmosphere (Ar, vacuum, N-2) and under air. The impedance results reveal a binary role of oxygen. Firstly, the equivalent resistance of the device in air is one order of magnitude larger with respect to that of a device in an inert atmosphere. Secondly, measurements in air under constant d.c.-bias stress reveal a rapid ageing of the device. In an inert atmosphere no change in performance is seen in the same period of time. Optical and scanning electron microscopy (SEM) inspections of devices stressed in air reveal the presence of damage in the electrode region. Device failure is observed in the form of inhomogeneously distributed tight packing of bubbles with blisters randomly dispersed over the electrode surface. For devices run until complete failure, tiny dark spots can be observed, due to local melting. Depth profiling of the structures by secondary ion mass spectrometry (SIMS) confirms the inhomogeneous ageing. Depth profiling shows not only an important change in the electrode structures but also reveals complete oxidation of the polymer layer in the aged parts of the device.