The initial state of dark spots in degradation of polymer lighting-emitting diodes

The initial state of dark spots in polymer light-emitting diodes (PLEDs) with poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) layer as an emissive layer has been investigated by in situ measurement with the interferometer, fluorescence microscope, and other various analytical instruments. Degradation of the device initiates with the dark spot that is the non-emissive portion of the polymer layer. The cross-sectional profile of interferometer image of Al electrode changes with operating time from dip to con-shaped sharp peak. It is clear that the dark spot originates from the pinhole leading to the severe damages on the microstructure of the polymer layer, polymer/metal interface, and metal electrode. Interferometer is a powerful tool for the investigation of the short-term degradation behavior of the polymeric EL devices.