Polymer LEDs (PLEDs) are devices that contain single layers of luminescent polymers, closely packed between a pair of metal electrodes.
PLEDs emit light as the metal layers inject electrons and holes into the polymer, forming distortions which can integrate to develop two diverse types of electron-hole pair—non-emitting “triplets” or ”light-emitting “singlets.”
Earlier theories have proposed that the ratio between these two types of electron-hole pair should be about 1:3, which would yield a light emission efficiency of 25%. But subsequent experiments revealed that the actual value can be as much as 83%.
In a new study published in The European Physical Journal, Chinese physicists, headed by Yadong Wang from Hebei North University, discovered that this higher-than-anticipated efficiency can be realized via interactions between triplet excitons, and impurities integrated in the polymer.
Due to their commercial and scientific advantage, PLEDs are becoming a more and more popular area of research. Going forward, the discoveries of Wang’s research team may lead to more extensive applications of the devices.
Within the layers of PLED polymer, excitons are said to be formed by the recombination of “polarons”—that is, distortions in electrical charges which appear and disappear as electrons travel via solid materials.
But other mechanisms must also be present to explain the reason behind their relatively higher luminescence efficiencies than what was predicted in previous theories. One proposal recommends that their optical and electrical properties of PLEDs are greatly influenced by unpaired electrons caught inside molecular impurities.
Light-Emitting Singlet Excitons
Wang and collaborators investigated this concept via simulations of molecular dynamics, which enabled them to reproduce collisions between an unpaired impurity electron and a non-emitting triplet exciton in a polymer chain.
The team’s calculations showed that light-emitting singlet excitons are among the key products of this reaction; with their general proportion differing with its degree of coupling with the polymer chain and the size of the impurity.
The result provides decisive proof for the first time that impurities can considerably improve the efficiency of PLEDs, and provides new clues regarding the molecular mechanisms involved.
Wang, Y, D., et al. (2020) Dynamic Recombination of Triplet Exciton with Trapped Counterion in Conjugated Polymers. The European Physical Journal. doi:10.1140/epjb/e2020-10310-4.