Bromide Decorated Eco‐Friendly ZnSeTe/ZnSe/ZnS Quantum Dots for Efficient Blue Light‐Emitting Diodes

Abstract

Multi‐shelled ZnSeTe/ZnSe/ZnS quantum dots (QDs) have served as a promising eco‐friendly emitter for blue quantum dot light‐emitting diodes (QLEDs). While extensive studies have concentrated on the optimization of the shell species and thickness of the multi‐shelled QDs to raise the QLED electroluminescent performance, very few reports focus on the QD surface states involving ligand and defect modulations which are essential for high‐performance QLEDs. Herein, the strategy of bromide decoration is theoretically and practically demonstrated to simultaneously diminish the QD surface defects by passivating the unsaturated Zn for strengthened carrier radiative recombination and removing the superabundant oleic acid through ligand exchange for efficient carrier transport. As a result, the merits of bromide decoration benefit a large increase in photoluminescence quantum yield (PLQY) from 39.7% to 86.2% at the wavelength of 443 nm, as well as a great enhancement of the device performance with over sevenfold improvement in external quantum efficiency (EQE) from 0.74% to 5.46% and a distinct decrease in turn‐on voltage from 6.7 to 5.9 V. Consequently, this work contributes an effective approach of the multi‐shelled QD surface decoration toward enhanced QLED performance.

Bright and Stable ZnSeTe Core/Shell Quantum Dots Enabled by Surface Passivation with Organozinc Halide Ligands

“…According to the previous study, the calculated binding energy value of 4MBZC fell within a reasonable range, indicating that the ligand allowed for stable passivation of the CQD surfaces, enabling the development of CQDs with an excellent PLQY. When metal halide materials are considered X-type ligands, the binding energies obtained by DFT simulations are higher than those obtained in this work. , For example, Kim et al described that the bonds between Cl – of ZnCl 2 and the zinc blende surface of the ZnSeTe/ZnS/ZnS CQDs exhibited binding energies ranging from −1.5 to −2.5 eV depending on the ligand density . This is explained by setting differences, including the ligand molecular structure, which was supposed to be a halide ion form, and a higher unsaturated coordination number of the surface atoms.…”

“…To examine the impact of the organozinc halide ligand on the optical properties and stabilities of CQDs, we selected blue-emissive ZnSeTe-based CQDs as a model system. This system is chosen among various semiconductor nanocrystal materials for the following reasons: (1) ZnSeTe is a state-of-the-art eco-friendly CQD platform that can facilitate the development of commercial-grade blue QLEDs upon further optimization of the CQDs; and (2) there are few reports on surface ligands capable of improving PLQYs in ZnSeTe-based CQD systems, ,− and to the best of our knowledge, there are even no reports on the utilization of organozinc halide compounds, which can provide dual-ion passivation and ensure the colloidal stability of CQDs. ZnSeTe/ZnSe/ZnSeS/ZnS CQDs were synthesized by hot injection methods with a multishelling protocol at 300 °C .…”

“…The mole fractions of the ligands in the CQDs can be estimated from the weight percentages of the Zn­(St) 2 -QDs and modified CQDs, the molecular weights of the inorganic and organic components of the ligands, and the following additional assumptions. We assumed that (1) Carboxylate ligands were substituted with ZnCl 2 or 4MBZC, but Zn­(St) 2 was not exchanged due to its high binding energy (binding energies of the ligands are discussed in the following part); (2) ZnCl 2 or 4MBZC also formed new bonds with extra unpassivated sites; , (3) the change in total mass of CQDs before and after ligand exchange was negligible; and (4) the molecular weight of the organic ligand system in Zn­(St) 2 -QDs can be considered as the average molecular weight of the stearic acid and oleic acid chain, 283.47 g mol –1 . According to the estimation (Table S1), the mole fractions of total carboxylates (oleate, stearate, and stearate of Zn­(St) 2 ) and ZnCl 2 on the ZnCl 2 -QD surfaces were 0.53 and 0.47.…”

Heavy-metal-free blue-emitting ZnSe(Te) quantum dots: synthesis and light-emitting applications

“…9c). 91 Meanwhile, a strong binding affinity between Br and Zn can further drive the ligand exchange to remove the superabundant oleic acid for efficient carrier transport. The density of states (DOS) calculation results of ZnS shell on the QD surface in Fig.…”

Blue-Emissive ZnSeTe Quantum Dots and Their Electroluminescent Devices

“…Recently, Chen et al reported an improvement in PL QY from 39.7 to 86.2% by employing bromine for ZnSeTe QD surface passivation (Figure 3d). 49 DFT calculations demonstrated that Br ions effectively passivate QD surfaces by eliminating trap states (Figure 3e). Moreover, the stronger binding energy of Br to Zn compared with that of OA on the QD surface explains the observed ligand exchange from OA to Br (Figure 3f).…”