Polymer light-emitting diodes : from materials to devices

In this paper on polymer LEDs we discuss the formation of black spots, surface treatments of the anode, and photochemical degradation of the emissive polymer. We find that small pinholes in the cathode layer are the origin of the black spots. The black spots form when H2O or O2 diffuse through the pinholes and react with the cathode at the polymer-cathode interface.A model is presented that describes the growth of the spots. We find that for both indium-tin-oxide (ITO) and Au anodes, an UV/O3 or an O2 plasma cleaning treatment increases the work function by 0.8-0.9 eV. A higher work function may lead to a better hole injection and a reduction in the operating voltage. We present a method to measure the quantum yield for bleaching, (gamma) equals 1.6$MN4 and (gamma) equals 1.7 10-7 for bleaching of dialkoxy-PPV in air and vacuum, respectively, indicating that the polymer is 1000 times more stable in vacuum than in air