Exciton dynamics in electroluminescent polymers studied by femtosecond time-resolved photoluminescence spectroscopy

We report measurements of the time-resolved photoluminescence from unoriented films of poly[2-methoxy,5-(2^'-ethyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(p-phenylenevinylene) (PPV). An extremely rapid luminescence rise is observed over the entire spectral region measured. A broad high-energy luminescence tail can be seen for very short times after excitation. A redshift of the luminescence with time is also observed and is larger in MEH-PPV than in PPV. The measured decay times of the photoluminescence indicate that the nonradiative decay of singlet excitons is more rapid in MEH-PPV than in PPV, lowering its photoluminescence efficiency. We attribute the differences between the materials to the reduced crystallinity of MEH-PPV compared to PPV. This increases the density of conformational defects and thus reduces the effective conjugation length of the polymer chains. These defects also provide a more efficient nonradiative decay channel. Polarized measurements of the luminescence were performed and provide insight into the ultrafast dynamics of the excitons.