Visualization of trap dilution in polyfluorene based light-emitting diodes

The effects of electron trapping and resulting trap-assisted recombination can be strongly suppressed by diluting a semiconducting polymer with a large band gap polymer such as polystyrene. Polyfluorene (PFO)-based light-emitting diodes (PLEDs) are an excellent model system to visualize this trap dilution effect. The blue emission from the pristine PFO backbone is accompanied by a broad, featureless green emission band originating from monomeric ketone defects that act as an electron trap. We demonstrate that the green emission from radiative trap-assisted recombination at these ketone defects is nearly eliminated upon dilution. The ratio between bimolecular blue emission and trap-assisted green emission as a function of dilution is shown to be in quantitative agreement with model predictions.