Internal Field Screening in Polymer Light-Emitting Diodes


  • The authors are grateful to Cambridge Display Technology Ltd. for provision of the devices used in this work, and to Drs. Clare Foden, Jeremy Burroughes, and Jonathan Halls for useful discussions.


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