Ligand Engineering Of Alkyl Ammonium Bromide On Quantum Dot Surface And Its Effect On QLED Performance

Quantum dots（QDs）can be used as quantum dot light emitting diodes（QLED）in the display domain due to their wide absorption peaks,narrow emission peaks and the fact that the position of the light emitting peaks can be continuously tuned by changing the growth particle size of the material.QLED has become the most promising next-generation lighting and display technology.The bottom-out light structure of QLED device is consist of anode,hole injection layer（HIL）,hole transport layer（HTL）,QDs light-emitting layer（EML）,electron transport layer（ETL）and cathode.Among them,modulating the balance of electron and hole injection to form excitons for luminescence,the injection of carriers at the interface between the EML and the carrier transport layer（CTL）and the transport of carriers in the QDs layer are three main factors to improve the efficiency and stability of QLEDs.Meanwhile,ligand exchange methods can be used to modified the surface of QDs in order to improve the interface quality between EML and HTL（ETL）by improving interfacial carrier injection and transport.In the current study,although the impact of spatial site blocking induced by the chain length of ligand exchange and the F?rster energy resonance transfer caused by short chain length on the ligand exchange and properties of QDs are known,the mechanism in the chain length of the alkylammonium bromide ligand leads to changes in the performance of QLED devices is not well understood.In addition,when the dynamic balance of carrier injection and transport and the type of current in the device are analysed,the current paths formed by carrier injection and transport in the device are also not detailed.In order to explore the influence of Ligand exchange on the luminescence properties of QDs,the exchange of alkylammonium bromide ligands for QDs and the effect of ligand chain length,concentration and reaction time on the performance of QLEDs are investigated in this thesis.The high electrical conductivity and high internal quantum efficiency QDs were obtained.In addition,the sources of current generation during injection transport in devices are explored and classified in detail.Finally,high performance green QLED devices were obtained by designing device structures with more rational interfacial contacts using ligand exchange modified QDs with better carrier injection balancing and efficient transport in the device.The detailed results are as follows:（1）Ligand exchange and performance effects of alkylammonium bromide on green QDs surfaceIn order to obtain a higher internal quantum efficiency within the device,the defect sites on the surface of QDs are further passivated to though the ligand exchange to enhance their luminescence efficiency.To improve the device performance by enhancing carrier injection and transport in device,the conductive ligand alkylammonium bromide is used to realize the enhanced conductivity of the QDs surface.The chain length of the alkylammonium bromide ligand was optimised to prevent non-radiative complexation induced by F?rster energy resonance transfer and exciton bursts of surface defect states during the exchange process,and the best performance of the QDs with the dilauryl dimethyl ammonium bromide（DDAB）ligand was obtained.Then,the reaction concentration and reaction time of the DDAB ligand were optimised and the mechanism of the QDs and surface ligands was analyzed by second-order conduction spectroscopy.Effective ligand exchange was ensured by changes in the vibrational peaks of the carboxylic acid groups by infrared absorption spectroscopy（FT-IR）and changes in the elemental content of Br by X-ray photoelectron spectroscopy（XPS）testing.In order to improve the level of carrier-injected QDs and find the reasons,The energy level changes before and after ligand exchange were tested under optimal conditions for UV photoelectron spectroscopy（UPS）and absorption.The fluorescence spectra and spectral lifetimes were tested to obtain QDs that effectively passivated defects and suppress non-radiative complexes by modulating the distance between QDs,achieving improved luminescence performance.The Transmission electron microscopy（TEM）and atomic force microscopy（AFM）images of the films were tested to demonstrate that QDs particle size and film-forming morphology were unchanged.The conductive atomic force microscopy（C-AFM）of the films was tested and found that the QDs with good conductivity improve from 77.3 p A to 1.59 n A,further validating the success of ligand exchange.（2）Dynamic equilibrium of carrier injection and transport for green QLEDs by surface ligands In order to understand the transport path and its variation during the carrier injection and transport dynamic equilibrium of green QLEDs,the thickness of the charge transport layer was firstly optimised for green QLEDs.And then the influence of the film-forming state with different luminescent layer concentrations and rotational speeds on the carrier injection and transport was investigated,by constructing QDs devices with luminescent layers under different concentrations and rotational speed conditions.The J-V characteristic curves,as well as the influence of AFM film-formation analysis of films with different concentrations of QDs solution film-formation wettability and equivalent device performance effects under different conditions ruled out that the variation in current with voltage was due to differences in film-formation and wettability.Taking the equivalent circuit diagram of the impedance,the energy level energy band schematic of the device structure and four partitions of the J-V characteristic curve into considerations:the ohmic region,the trap space charge limited region（T-SCLCs）,the trap filled region（TFL）and the space charge limited region（SCLCs）,the variation of current with voltage in the device is divided into four types of currents:1.leakage currents at low voltages through the defective resistance of the QDs surface ligands and the defective resistance of the charge transport layer,and leakage currents through the internal defective capacitance of the QDs to the charge transport layer.2.complex currents in the T-SCLCs region due to complex luminescence through the valence and conduction bands of the QDs.3.non-radiative complex currents in the TFL region due to the injection of excess carriers.4.Breakdown currents in SCLCs where electrons（or holes）can be freely injected into the HTL（or ETL）causing irreversible damage to the functional layer.This classification provides guidance for a good modification of the device from the source,based on which the J-V characteristic curves,impedance and stability analysis of green QLED devices with different ligand states on the surface are combined to further validate the four types of currents.And based on reasonable carrier injection and transport,a dual QDs light-emitting layer（6H/QD）was designed to effectively balance charge carrier injection and transport,resulting in an increase in device efficiency from 14.62%to 17.01%,an increase in device luminance from 185498.09 cd/m²to236756.15 cd/m²,and the device life T₅₀under 1000 cd/m²luminance is increased from 3592.5 h to 7387.5h.