Stable perovskite quantum dots synthesis for light-emitting diodes

Abstract

The perovskite quantum dots (PeQDs) are one of the potential candidate materials for next-generation display technologies due to their unique optoelectrical properties. PeQDs have strengths such as high photoluminescence, good defect tolerance, tunable wavelength, narrow emission, and solution processability. However, PeQDs easily lose ligands because of their ionic nature, inducing photoluminescence quenching and merging to bulk. In addition, the uncontrollable hot injection (HT) method intensifies the situation, resulting in broad size distribution and low crystallinity. Consequently, the light-emitting diodes based on PeQDs exhibit a short operation lifetime. The instability impedes the commercial application of PeQDs. This thesis proposes strategies to improve the performance of materials and devices.
Firstly, caesium and halide vacancies are passivated by resurfacing agents. In addition, Ostwald ripening is suppressed. Potent ligands react with hydrobromide acid to form resurfacing agents. The ligand exchange reactions are acid-driven, while extra bromide anions hinder the Ostwald ripening. As a result, weak ligands, such as oleic acid/oleylamine, are substituted by strong ligands via in-situ ligand exchange in the growth process.
Secondly, short chain-length ligands control the nucleation by adjusting the acid/base equilibrium of the solution. The intermediate PbBr2 clusters are superfluous nucleation sources. Homogenous nucleation is achieved by using octanoic acid/octylame, eliminating clusters. Moreover, short chain-length ligands improve the conductivity of materials. The synthesised CsPbBr3 NCs show near-unit photoluminescence efficiency and narrow size distribution. The fabricated device achieves a peak EQE of 24.13% and an operation lifetime T50 of 54 minutes at 10000 cd·m-2.
Thirdly, the same strategies are used to gain stable CsPbI3 QDs successfully. However, it is hard to obtain small-size CsPbI3 QDs due to synthesis difficulties. Different resurface agents are used here. The 2-naphthalene sulfonic acid is introduced as a resurfacing agent to suppress Ostwald ripening, as it is an aromatic ligand and binds strongly to QDs. Moreover, ammonium hexafluorophosphate exchanges long-chain ligands and avoids regrowth during purification. Both ligands ameliorate the charge transport ability of CsPbI3 quantum dots. The pure red CsPbI3 quantum dots achieve 94% photoluminescence quantum efficiency. Devices based on those QDs gain a peak EQE of 26.04%.
The thesis focuses on improving device performance with high-quality PeQDs. Different perovskite quantum dots employ stable transport layers with high carrier mobility and suitable energy levels. The proposed two synthesis strategies ameliorate the crystallinity, size distribution, photoluminescence quantum yield, and conductivity of PeQDs. With the corresponding device structure, high-performance light-emitting diodes are fabricated. The synthesis strategies give new insight into PeQD synthesis.

Date of Award	15 Jul 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Chengbo Wang (Supervisor) & Yong Ren (Supervisor)