Excitonic singlet-triplet ratios in molecular and polymeric organic materials

A simple technique employing reverse bias measurements of photoluminescent efficiency is described to determine the excitonic singlet-triplet formation statistics of electroluminescent organic thin films. Using this method, the singlet fractions in thin films of two organic emissive materials commonly used in organic light emitting devices, tris(8-hydroxyquinoline) aluminum $\left({\mathrm{Alq}}_3\right)$ and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), are found to be $(20\mathrm{}\pm 1)\%$ and $(20\mathrm{}\pm 4)\%,$ respectively. Results are confirmed using a sensitive synchronous detection scheme. We discuss other measurements and the current understanding of exciton formation statistics in polymeric and small molecular weight organic electroluminescent materials.