The role of surface plasmons in organic light-emitting diodes

Organic light-emitting diodes typically take the form of an optical microcavity in which one layer is a metallic cathode. Coupling between emissive species in the light emitting layer and surface plasmon (SP) modes associated with the metallic cathode result in a loss of efficiency; an aspect often discussed but not so far fully quantified. Here we numerically model the extent of this problem, both for organic light-emitting diodes based on small molecules (Alq₃) and those based on conjugated polymers (MEH-PPV). We show that SP modes can significantly detract from device efficiency, particularly those based on small molecules. We then report measurements of photo- and electroluminescence from organic light-emitting diodes incorporating wavelength scale periodic structure. These data demonstrate the existence of the SP modes in organic light-emitting diodes. Finally we consider ways in which the problems associated with SPs might be overcome, and may even be turned to advantage