Optical Outcoupling Efficiency in Polymer Light‐Emitting Diodes
Yungui Li, Bas van der Zee, Gert‐Jan A. H. Wetzelaer, Paul W. M. Blom
Abstract
Abstract The electrical properties of polymer‐based light‐emitting diodes (PLEDs) have been extensively studied, resulting in quantitative device models. However, for a complete description of the optoelectronic properties of a PLED, the obtained recombination profiles need to be integrated with an optical model describing the local outcoupling efficiency of the generated light. In this work, combined electrical and optical model calculations are presented, demonstrating that the light‐outcoupling efficiency in PLEDs is governed by the presence of electron traps and the anisotropy factor of optical dipoles. Electron trapping confines the recombination in a region close to cathode, resulting in strong optical trapping in surface plasmon polariton modes. Trap‐filling leads to a voltage‐dependent recombination profile and optical outcoupling efficiency. For a typical electron trap density of ≈10 23 m −3 , the calculated outcoupling efficiency raises from 5.5% at 2 V to 10.4% at 5 V in case of isotropic emitters. For conjugated polymers such as super yellow poly( p ‐phenylene vinylene) with the chains preferentially aligned in the plane of the film, the outcoupling efficiency can reach ≈18% at 5 V. Elimination of electron trapping would allow for a further enhancement up to 24%.