Abstract
We use eleclromodulalion spectroscopy and modeling studies to probe the electric-field distribution in polyfluorene-hascd polymer light-emitting diodes containing poly(3,4-ethylenedioxythiophene) poly(slyrene sulfonate). The hulk internal field is shown to he zero under ordinary operating conditions, with trapped electrons elose to the anode fully screening the hulk semiconductor from the external field. The effect has far-reaching implications for the understanding and optimization of organic devices.