Electric field screening in polymer light-emitting diodes

Electromodulation (EM) spectroscopy has been used to probe the electric field distribution in polymer light-emitting diodes. Below the turn-on bias, the EM spectrum is dominated by electroabsorption of the emissive layer. The electroabsorption signal vanishes above the turn-on bias. Under operation, the EM spectrum is composed of bleaching of the emissive layer absorption and excited state absorption from trapped charge with no trace of the electroabsorption signal remaining. We conclude that the internal electric field is effectively screened by accumulation of trapped electrons at the anode.