Abstract
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 at the turn-on bias. Under operation, the EM spectrum is due to by excited state absorption from injected charge and bleaching of the ground state absorption of the emissive layer. We conclude that the internal electric field is effectively screened by accumulation of trapped electrons at the anode.