Investigation Of Perovskite Light-Emitting Diodes Based On Zinc Oxide Electron Transport Layer

Metal halide perovskites are regarded as one of the promising luminescent materials owing to their outstanding optoelectronic properties,high carrier mobility,tunable bandgap,narrow emission width,cost-effective,and high defects tolerance.Despite great achievements in improving the efficiency of perovskite light-emitting diodes(PeLEDs),the peak external quantum efficiency(EQE)of current PeLEDs was usually obtained at low current density and low brightness,while a serious EQE roll-off issue exists at high brightness,which severely limits their practical application.Zinc oxide(ZnO)semiconductors have been ubiquitously utilized as electron transport layer(ETL)in the well-developed quantum dot LEDs(QLEDs),due to their excellent charge mobility,high transmittance,and cost-effective.However,the implementation of such promising ETL in PeLEDs has not received enough investigation,with focus only on the near-infrared devices.This is mainly ascribed to the unwanted photoluminescence(PL)quenching phenomenon of perovskite on the ZnO surface.It is of great importance to understand the PL quenching mechanism,and then rationalize suitable ZnO for efficient and stable PeLEDs.Therefore,this thesis thoroughly investigates the PL quenching mechanism of 2-Phenylethylamine Hydrobromide(PEABr)-based quasi-two-dimensional(quasi-2D)perovskite films on the ZnO surface.We found that the PL quenching should be mainly attributed to the decomposition of quasi-2D perovskite into 3D perovskite,which increases the defects and retard the exciton recombination rate.Furthermore,we investigated that only a special crystal face of ZnO is capable to trigger a deprotonation process,hence decomposing the ammonium cation of PEA+.The key factor determining the rate of the deprotonation reaction is the basic strength of the crystal face.Based on our above finding,we proposed a strategy to passivate the active facets of ZnO with an atomic layer deposition(ALD)deposited Al2O3,which shows great potential to address the PL quenching issue.Finally,an inverted structure green PeLED device with high EQE and low roll-off was successfully demonstrated based on the Al2O3 passivated ZnO ETL.The main contents of this paper as followed:1.We preformed absorption,fluorescence spectra,and temperature-dependent PL to get insight into the PL quenching mechanism of quasi-2D perovskite on the ZnO surface.Due to the basic nature of the ZnO surface,the long-chain organic ammonium salt undergoes a deprotonation reaction,converting the quasi-2D phase into the 3D phase.This is firmly based on the FTIR measurement,where the streatching and scissoring vibration of NH3+disappear and arise new absorption signal belong to NH2.The undesirable phase decomposition not only slows down the exciton recombination rate but also may bring about numerous defects within the perovskite films.In addition,the small energy offset between 3D perovskite could accelerate the exciton dissociation at the perovskite/ZnO surface,which also devoted to the PL quenching.These above detrimental factors,eventually,lead to the photoluminescence quantum yield(PLQY)of the perovskite film on the ZnO surface lower than 2%.Through using a ZnO single crystal substrate,we further revealed that the deprotonation process is highly dependent on the crystal orientation of ZnO.In specific,the reaction rate is extremely low on the(0001)zinc polar plane of ZnO and shows a negligible effect on the PL quenching,as evidenced by the PL image of perovskite films on different crystal planes.We proceed to use X-ray photoelectron spectroscopy(XPS)to understand how the different crystal planes influence the deprotonation reaction rate by characterizing the surface chemistry.The obtained results indicated that the main factor determining the deprotonation rate is alkalinity of the crystal surface rather than the concentration of hydroxyl group on ZnO surface.2.By depositing an insulating Al2O3 layer on the ZnO surface,the deprotonation reaction can be remarkably suppressed,thereby preventing the decomposition of organic ammonium salt ligands and suppressing the PL quenching phenomenon.Our study has shown that even a 0.1-nm-thick Al2O3 shielding layer can enhance the PLQY of perovskite from 2%to 44%,while 0.5 nm Al2O3 can completely suppress the PL quenching.This is because of the existence of an interaction between the unoccupied 3p orbital of the aluminum atoms and the lone pair electrons of the active oxygen atoms on the ZnO surface during the ALD reaction.This suppressed PL quenching of the ZnO/Al2O3 substrate remains even baked at a high temperature.In addition,the Al2O3 treated ZnO surface allowed us to target smooth perovskite films upon,and the rootmean-square(RMS)roughness value of the perovskite film was reduced from 5.03 nm(W/O Al2O3)to 3.56 nm.3.An inverted green PeLED with high EQE,low turn-on voltage,and less noticeable efficiency roll-off was fabricated using Al2O3 passivated ZnO ETL.The highest EQE of the device was 17.7%,the maximum brightness was 38000 cd m-2,and the operation lifetime was 16.6 h with the emission peak at 514 nm,with all the parameters significantly improved compared to the devices without Al2O3.Moreover,in contrast to the conventional device,the efficiency roll-off of the inverted PeLEDs at high current density was significantly suppressed.In specific,the EQE value of our device was still reserved more than half of the peak value at 108 mA cm-2 current density and 28000 cd m-2 brightness.In addition,the device lifetime was improved by 15 times compared to the PeLEDs with the commonly used conventional structure.