Dynamic recombination of triplet exciton with trapped counterion in conjugated polymers
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THE EUROPEAN PHYSICAL JOURNAL B
Regular Article
Dynamic recombination of triplet exciton with trapped counterion in conjugated polymers Yadong Wang 1,a , Jianjun Liu 1 , Yanxia Liu 1 , Xiru Wang 2 , and Yan Meng 3,b 1 2 3
College of Information Science and Engineering, Hebei North University, Zhangjiakou 075000, P.R. China Department of Electronics, Polytech Nice Sophia, Nice 06200, France Department of Physics, Xingtai University, Xingtai 054001, P.R. China Received 20 June 2020 / Received in final form 21 July 2020 / Accepted 28 July 2020 Published online 7 September 2020 c EDP Sciences / Societ`
a Italiana di Fisica / Springer-Verlag GmbH Germany, part of Springer Nature, 2020 Abstract. The dynamic recombination of an on-chain triplet exciton with an off-chain counterion trapped in an impurity molecule has been investigated using a molecular dynamics method. It has been found that luminous composite states including the trion and the singlet exciton are the main products in the triplet exciton-counterion reaction and their yields depend sensitively on the impurity potential and the coupling between the polymer chain and the impurity molecule. We demonstrate that the triplet exciton-counterion reaction can enhance the electroluminescence efficiency considerably, and that the triplet exciton plays an important role in the photoelectric process of polymer. Our results provide a new clue to understand the electroluminescence mechanism in the polymer light-emitting devices.
1 Introduction Polymer light emitting diodes (PLEDs) are currently of broad interest due to their scientific and commercial value [1,2]. These devices generally consist of a luminescent polymer layer introduced between two metal electrodes. When the PLED is in operation, due to the strong electron–lattice interactions and instability, holes and electrons are injected from the electrodes into the luminescent layer and lead to the formation of self-localized excitations that include both charge and lattice distortion, such as polaron [3]. When oppositely charged polarons meet they can recombine to form excitons, including emissive singlet excitons and nonemissive triplet excitons. In theory, the branching ratio for formation of singlet excitons and triplet excitons should be 1:3 so the electroluminescence (EL) efficiency of PLEDs is limited to 25%. However, many studies have demonstrated that the EL efficiency in PLEDs ranges between 22% and 83%, which greatly breaks through the theoretical limitation derived from spin statistics [4–6]. From the high EL efficiency mentioned above, it can be expected that there might be additional mechanisms involved in the EL in PLEDs beyond the oppositely charged polarons recombination. For example, several collision and recombination processes, such as the polaron–polaron interaction-induced bipolaron [7,8], and the polaron–bipolaron scattering [9,10], can also arise significant contributions to the enhancement of the EL a b
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efficiency in PLEDs by form
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