Studies On Quasi-two-Dimensional Perovskite Light-Emitting Diodes Based On Thin Film Microstructure And Quantum-well Width Distribution

Quasi-two-dimensional(quasi-2D)perovskite light-emitting diodes(PeLEDs)are semiconductor devices that use quasi-2D perovskite materials as the light-emitting layer to emit light driven under an electric field or current.Quasi-2D PeLEDs have the advantages of solution processability,simple preparation process,high color purity,high photoluminescence quantum yield(PLQY)and adjustable exciton binding energy,etc.,and are considered to be an ideal choice for next-generation lighting and flat-panel display technologies.Benefitting from the material synthesis and device fabrication experience accumulated in the field of perovskite solar cells and organic light-emitting diodes(OLEDs),PeLEDs have progressed significantly since 2016,and the external quantum efficiencies(EQE)over 20%have been achieved in the past a few years,highlighting their great potential for commercial application.Quasi-2D perovskites were often formed by in-situ self-assembly of precursor solutions during the film formation,which brings about scientific problems different from traditional optoelectronic devices.The material synthesis and device preparation of traditional light-emitting diodes such as OLEDs proceed separately.In the case of luminescent material synthesis,the adverse effects of by-products and impurities can be reduced by purification.In the process of device preparation,the intrinsic photoelectric properties of the luminescent materials will not change.However,the quasi-2D perovskites are often synthesized in-situ during the device fabrication,the resultant defects,thin film microstructures and quantum-well width distribution will strongly impact the device performance.At present,quasi-2D PeLEDs cannot still meet the requirements of commercialization in terms of light-emitting efficiency and stability,and it is a necessity to further explore the crystal structure-material property-device performance relationship for the quasi-2D perovskite materials and the resultant PeLEDs.The thin film microstructure and quantum-well width distribution of quasi-2D perovskite have a great influence on the optoelectronic properties of quasi-2D perovskite and the device performance of PeLEDs.Therefore,this thesis is dedicated to exploring the influence of the microstructure and quantum-well width distribution of quasi-2D perovskite films on the optoelectronic properties of quasi-2D perovskites and the resultant PeLEDs.Three parts have been accomplished around the research topic toward improving the qualities of the quasi-2D perovskites and the coordinated regulation of the quantumwell width distribution and defect density of the quasi-2D perovskites.High quality thin-film quasi-2D perovskites with good film microstructure and narrow quantum-well width distribution were prepared,with which the high-efficiency PeLEDs were fabricated.The achievements are promising to promote the development of quasi-2D perovskite light-emitting materials and high-performance PeLEDs.In part 1,the research was carried out on improving the microstructure of the quasi-2D PEA2Csn-1Pbn(BrxCl1-x)3n+1 perovskite film in the horizontal direction.To mitigate the issue of excessive organic ligands impeding grain growth and increasing grain boundary-related defects in the film’s horizontal direction,the additive TPPO was dissolved in various anti-solvents to regulate the crystallization process of quasi-2D PEA2Csn-1Pbn(BrxCl1-x)3n+1 perovskites.Blue quasi-2D perovskites with enlarged grain size were prepared with this method.A specific combination of antisolvent and additive has been shown to promote the formation of regular-shaped quasi-2D perovskite nanoplates.The addition of TPPO enlarges grain size and reduces defect density in quasi-2D PEA2Csn-1Pbn(BrxCl1-x)3n+1 perovskites.The average grain size is increased from 38 nm with EA antisolvent treatment to 110 nm with EA-TPPO treatment.Moreover,the PLQY increased from 18%with EA treatment to 49%with EA-TPPO treatment.This paves the way for in situ fabrication of quasi-2D perovskite nanoplates.Low miscibility between DMSO and its antisolvent EA-TPPO promotes the formation of oriented perovskite nanoplates with enhanced PLQY.Furthermore,the quasi-2D perovskite film may have a vertical double-layer structure:an upper discontinuous perovskite nanoplate layer and a below continuous and dense quasi-2D perovskite film layer.The PeLEDs utilizing quasi-2D perovskite films with oriented nanoplates as light-emitting layers exhibit sky-blue electroluminescence with a peak wavelength of 488 nm and CIE chromaticity coordinates of(0.07,0.24).These PeLEDs achieve a maximum EQE of 9.5%,a high luminance of 4256 cd m-2,and exhibit stable electroluminescence spectra.This study demonstrates an effective approach for fabricating oriented perovskite nanoplates and highlights the potential for enhancing the light-emitting efficiency of blue PeLEDs through the use of oriented nanostructures.In part 2,the effects of the buried interfaces between the hole transport layer and the perovskite light-emitting layer in PEA2Csn-1Pbn(BrxCl1-x)3n+1 quasi-2D perovskite films were studied.The buried interfaces were passivated using GBA-modified PEDOT:PSS.The results show that GBA modification effectively passivated the buried interfaces of the quasi-2D perovskite film.The hole defect density is reduced from 1.26×1017 cm-3 of the quasi-2D perovskites based on the pristine PEDOT:PSS to 8.56×1016 cm-3 of the quasi-2D perovskites based on the GBA-modified PEDOT:PSS,and the PLQY of the corresponding quasi-2D perovskite films is significantly increased from 3.5%to 60.8%.It is found that the GBA primarily impacts the photoluminescence(PL)spectra of quasi-2D perovskite films during the spin-coating process.The PL spectra of unannealed quasi-2D perovskites based on the pristine PEDOT:PSS and GBA-modified PEDOT:PSS were 478 nm and 470 nm,respectively,with a shift of 8 nm.Similarly,the PL spectra of annealed quasi-2D perovskites based on the pristine PEDOT:PSS and GBA-modified PEDOT:PSS were 496 nm and 485 nm,respectively,with a shift of 11 nm.The ionized ammonium group(-NH3+)of GBA may contribute to the passivation of halogen vacancy defects.Additionally,the lone pair electrons of the amine or imine group(-NH2 or-NH)can coordinate with incompletely coordinated Pb2+ defects.Such a coordination reduces the charge trap density of the film and passivates the buried interfaces.The strong interaction between the amine groups and Pb2+ results in heterogeneous nucleation.This affects the formation of lead-halogen octahedra at the nucleation sites,leading to the formation of a densely packed and highcoverage quasi-2D perovskites film.The electroluminescence spectra of the quasi-2D PeLEDs prepared on the GBA-modified PEDOT:PSS hole transport layer is sky blue with an emission peak of 488 nm.The maximum EQE of these devices is 9.41%,representing an improvement over the traditional blue PeLEDs.This study presents a straightforward and effective approach for passivating the buried interfaces of quasi2D perovskite films.The results demonstrate that the characteristics of the below holetransport layers significantly influence the optoelectronic properties of the upper quasi2D perovskite light-emitting layer and the electroluminescent performance of corresponding PeLEDs.In part 3,the study is focused on the coordinated regulation of quantum-well width distribution and defect density in PEA2CSn-1PbnBr3n+1 quasi-2D perovskite films.The objective is to produce quasi-2D perovskite films with an optimal distribution of quantum-well widths and a minimum density of defects.A quasi-2D perovskite film with high PLQY was prepared using two additives,i.e.,3-(Diaminomethylidene)-1,1dimethylguanidine(Metformin)and 1,4,7,10,13,16-hexaoxacyclooctadecane(Crown).The preferential interactions of the additives with different quasi-2D perovskite precursors is proposed to synergistically passivate the defects and regulate the quantumwell width distribution of the quasi-2D perovskites.Metformin additive interacts strongly with lead bromide and mainly passivates the defects.Crown additive has preferential interactions with organic spacer phenethylammonium bromide,which dominates the quantum-well width distribution and induces quasi-2D perovskites with uniform dimensions.The dual additives of Metformin and Crown interact preferentially with distinct precursors,allowing to regulate the quantum-well width distribution and defect density of quasi-2D perovskites synergistically.A PLQY as high as 91.5%is achieved for the quasi-2D perovskites prepared with the dual-additive strategy.Moreover,the high-efficiency quasi-2D PeLEDs were successfully fabricated with a maximum external quantum efficiency of 21.3%and a maximum luminance over 30000 cd m-2.This work provides a preferential interaction-guided dual-additive strategy to fabricate high-efficiency quasi-2D PeLEDs without anti-solvent treatment.