Abstract
Electron overcharge causes rapid luminescence quenching in the quantum dot (QD) emission layer in QD light–emitting diodes (QD–LEDs), resulting in low device performance. In this paper we describe the application of different aromatic thiol ligands and their influence on device performance as well as their behavior in combination with an electron blocking material (EBM). The three different ligands, 1–octanethiol (OcSH), thiophenol (TP), and phenylbutan–1–thiol (PBSH), were introduced on to InP/ZnSe/ZnS QDs referred to as QD–OcSH, QD–TP, and QD–PBSH. PBSH is in particular applied as a ligand to improve QD solubility and to enhance the charge transport properties synergistically with EBM probably via π–π interaction. We synthesized poly-[N,N-bis[4-(carbazolyl)phenyl]-4-vinylaniline] (PBCTA) and utilized it as an EBM to alleviate excess electrons in the active layer in QD–LEDs. The comparison of the three QD systems in an inverted device structure without the application of PBCTA as an EBM shows the highest efficiency for QD–PBSH. Moreover, when PBCTA is introduced as an EBM in the active layer in combination with QD–PBSH in a conventional device structure, the current efficiency shows a twofold increase compared to the reference device without EBM. These results strongly confirm the role of PBCTA as an EBM that effectively alleviates excess electrons in the active layer, leading to higher device efficiency.
Highlights
Functional Polymer Systems, Fraunhofer Institute for Applied Polymer Research (IAP), Geiselbergstrasse 69, GFZ German Research Center for Geosciences, Helmholtz Centre Potsdam, Telegrafenberg, 14473 Potsdam, Abstract: Electron overcharge causes rapid luminescence quenching in the quantum dot (QD) emission layer in QD light–emitting diodes (QD–LEDs), resulting in low device performance
The defect-related emission of the device with QD–TP in the range of 700–750 nm was significantly more pronounced compared to QD–OcSH and QD–PBSH (Figure S5a). This defect emission is in accordance with the results described in Section 3.1, where we concluded that the QD surface was not well passivated with TP ligands compared to QD–OcSH and QD–PBSH, which resulted in relatively low efficiency in the EL device for QD–TP
We have successfully demonstrated the application of concepts developed for Cdbased QDs and QD–LEDs in InP-based systems
Summary
The Restrictions of Hazardous Substances (RoHS) directive limits the concentration of hazardous materials in electrical equipment and affects the usage of Cd–. Cd-based QD–LED structure incorporating an active QD layer sandwiched by ultrathin double charge-blocking–layers using a poly(9–vinlycarbazole) (PVK) layer to prevent accumulation of excess electrons, and a polyethylenimine ethoxylated (PEIE) layer to reduce the hole-injection barrier. This structure improved the charge imbalance in the active layer and showed highly enhanced performance with a current efficiency of 89.8 cd/A and a maximum brightness of 72,814 cd/m2 [18]. Bae et al showed that a well–dispersed QD/HTL hybrid active layer composed of a grafted polymer on the QD surface contributes to the suppression of efficiency roll-off at high current density, resulting in an enhanced charge carrier balance in the QD–LEDs [19]. The results showed evidence for a reduced phase separation in the hybrid active layer with QD–PBSH as compared to that with QD–OcSH
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