In real electronic devices the elevated operating temperature of the active medium with respect to the "standard" room temperature (21-23°C) is a direct result of Joule heating and acts as a limiter to device performance and lifetime. It has been shown for discrete devices that as the active area is reduced the device is less susceptible to Joule heating. Therefore smaller devices may be driven at higher current densities for a longer period of time than similar devices with a larger active area. This is important for electronic display applications where the display brightness, which is proportional to current density and the display lifetime, is critical. We report on how a porous alumina membrane, filled with nickel using a pulsed electro-deposition technique, was used as a nano-structured anode in polymer light-emitting diodes. Devices made using mechanically polished nickel-filled membranes were tested. Electrical data are presented and the uniformly filled porous alumina based devices sustained higher current densities, than equivalent conventional evaporated metallic sheet-electrode devices. It was found that the reproducibility and rectification ratios of the uniformly filled nickel devices represent a significant improvement on similar copper-filled devices.
Organic light emitting diodes were fabricated using a novel electrode structure. The anode structure comprised of a metallic nanowire array and was fabricated by electroplating porous aluminium oxide with copper. These devices were compared with devices with a conventional planar anode structure. The light emitting polymer Poly[(4-methylphenyl) imino-4,4'-diphenylene-(4-methylphenyl)imino-1,4-phenylene-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-ethenylene-1,4-phenylene)] was used as the emissive material in single layer devices of structure Cu: TPD(4M)-MEH-PPV: Al, where the aluminium was negatively biased with respect to the copper. The DC current-voltage characteristics of both device types are presented. The electroluminescence spectra are also presented. We found that due to the reduction in active area in the nanowire device from that of the planar device the current density reached in the nanowire array anode device exceeded that in the planar anode device by a factor of eight. Similarly a relative increase in the electroluminescence intensity was also observed.
Zinc-2, 9,16,23 -tetra- tert-butyl-29H, 31H -phthalocyanines and Zinc-2, 9,16,23-tetrakis-(phenylthio)-29H, 31H-phthalocyanines were recrystallized from an acetone solution to give regular shaped spherical particles of 50nm diameter, confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). A peak broadening of the Q-Band and a shift of the B-Band in the UV-Visible absorption spectrum combined with a significant fluorescence quenching was observed. The z-scan technique was used to investigate the non-linear optical properties and an increase of approximately 200% in the ratio of excited to ground state absorption cross sections in the crystal state was observed indicative of a significant increase in the optical limiting response of the crystals compared to the monomers.