PLED polymers evolve characteristically during operation (Vol. 51, No. 5)

Molecular dynamics simulations have shown that the mysteriously high efficiency of polymer LEDs arises from interactions between triplet excitons in their polymer chains, and unpaired electrons in their molecular impurities.

Polymer LEDs (PLEDs) are devices containing single layers of luminescent polymers, sandwiched between two metal electrodes. They produce light as the metal layers inject electrons and holes into the polymer, creating distortions which can combine to form two different types of electron-hole pair: either light-emitting ‘singlets’, or a non-emitting ‘triplets’. Previous theories have suggested that the ratio between these two types should be around 1:3, which would produce a light emission efficiency of 25%. However, subsequent experiments showed that the real value can be as high as 83%. We found that this higher-than-expected efficiency can be reached through interactions between triplet excitons, and impurities embedded in the polymer.

Y D Wang, J J Liu, Y X Liu, X R Wang, Y Meng, Dynamic Recombination of Triplet Exciton with Trapped Counterion in Conjugated Polymers, Eur. Phys. J. B 93, 173 (2020)
[Abstract]