| Conjugated polymers are a unique category of organic semiconductor. Their photoluminescent, electroluminescent, and photocurrent generating properties make them an important class of materials for potential application in thin film devices. The fraction of excited singlet states formed either by charge injection or photoexcitation has been determined in some conjugated polymers to be significantly higher than the statistical limit of 0.25. The ladder type phenylene polymer, methyl-substituted ladder-type poly(paraphenylene), MeLPPP, is investigated in this study because of its strong fluorescent and phosphorescent emission which provide direct quantitation of the singlet and triplet state populations. By taking advantage of the delayed luminescence and relatively short phosphorescent lifetime in MeLPPP, a measurement of the singlet fraction is made and determined to be ~0.5. Analysis of the delayed fluorescence reveals excimer formation and J-type aggregation, both attributable to significant chromophore overlap in the amorphous solid state. Analysis of the phosphorescent emission reveals that the phosphorescent lifetime in a neat solid film is as short as 1mus. The power law decay of the phosphoresence reveals a pathway to triplet state population via recombination of polaron pair states. Quenching of phosphoresence by dilution leads to the conclusion that triplet-triplet annihilation is not a significant mechanism of delayed fluorescence in MeLPPP. A kinetic model based on polaron pair recombination generates decay curves that can accurately reproduce the decay of both the singlet and triplet states. The model yields a mean value for a recombination barrier of 0.2 eV and a standard deviation in the barrier distribution of 0.2 eV. The model produces a value for the singlet fraction of from 0.45 to 0.53, consistent with the experimental determination. |
