Exciton Quenching due to Hole Trap Formation in Aged Polymer Light-Emitting Diodes

Polymer light-emitting diodes based on two poly(p-phenylene vinylene) derivatives are aged at a constant current density, leading to the formation of hole traps. Time-resolved photoluminescence spectroscopy (TRPL) measurements show that the degraded polymer light-emitting diodes (PLEDs) also demonstrate a decrease in exciton lifetime. The amount of nonradiative exciton quenching sites in the aged devices is quantified by Monte Carlo simulations. It is found that the number of hole traps obtained from electrical charge transport measurements matches the number of newly formed nonradiative quenching sites determined from the TRPL experiments. The results reveal the origin for the apparent different behavior of the electroluminescence and photoluminescence upon PLED degradation. The decrease of the electroluminescence is governed by recombination of free electrons with trapped holes, whereas the photoluminescence is reduced by nonradiative quenching processes between excitons and hole traps.