Photoprocessed and micropatterned conjugated polymer LEDs

We present a detailed study of the effect of fabricating light-emitting diodes (LEDs) containing conjugated polymers, using photolithographic processing; a technique more conventionally used in inorganic semiconductor device manufacture. It is shown that for the specific poly (2,5-dialkoxy-p-phenylenevinylene) used here, the photoprocessing procedure chemically modifies the polymer, resulting in an increase in the degree of conjugation and a concomitant reduction in photoluminescence quantum efficiency to 35% compared to an unprocessed film. A similar reduction in electroluminescence quantum efficiency indicates that this is also the dominant effect in photoprocessed LEDs. LED device characteristics show an increase in threshold field for the photoprocessed devices suggestive of the formation of a barrier layer at the polymer/cathode interface, resulting in a further partial reduction in device power efficiency. There are however no catastrophic effects on device performance, showing that standard photoprocessing is a viable approach to fabrication of polymer LED structures. It is anticipated that optimization of the procedures will allow much less degradation in device performance. In addition it is shown that photoprocessing can be readily applied to the fabrication of arrays of micron-sized LEDs, demonstrating advanced applications of this combination of technologies.