Dynamics and trap-depth distribution of triplet excited states in thin films of the light-emitting polymer poly(9,9-di(ethylhexyl)fluorene)

We study the behavior of the phosphorescence and delayed fluorescence of the light-emitting polymer poly[9,9-di(ethylhexyl)fluorene] with different excitation intensities at low temperatures. The results indicate a limited number of potentially occupied energy sites (traps) for triplet-excited states. From analysis of temperature-dependent intensity measurements, we demonstrate the Gaussian distribution of these energy traps and determine an average trap depth of $4.4\pm 0.1\mathrm{meV}$ combined with a half-width of $4.5\pm 0.3\mathrm{meV}.$ We also observe a redshift of the phosphorescence spectrum with increasing temperature, but no concomitant spectral narrowing.