Study On Material Synthesis And Photoelectric Properties Of Efficient And Stable Perovskite Based LEDs

Metal halide perovskite with charming photoelectric properties including tunable bandgap,superb defects toleration,facile processing and high emissive-efficiency are seen as the ideal candidate for next generation of electroluminescence.Owing to the successful pace achieved in perovskite based solar cell,the accumulated experiences on materials’ design and device optimization rapidly boost the progresses of corresponding LEDs.However,despite being the rising star that even could be comparable to OLEDs and QLEDs,the efficiencies and especially the operational lifetimes of perovskite based LEDs(PeLEDs)still have large scope to be enhanced.With aim at developing next generation of full color-gamut display,specific lighting applications and solid laser device,it is significant to achieve highly efficient and stable PeLEDs.In this dissertation,we conduct researches in perspective of materials design and modulation for perovskite active layers,meanwhile,the investigations on device physics and materials’recombination kinitics are also carried out,by which we sucessfully fabricate highly efficient PeLED in green and blue regions.Furthermore,we also devote to unravel the underlying mechanisms that determine PeLEDs stability.By designing experiments and summarizing the rules,insights and corresponding strategies for realizing stable PeLEDs are systematically elucidated and disscused.(1)In Chapters 1 and 2,we introduce the research history,operational principles and basic parameters of LEDs.The photophysical process and factors that influence luminous efficiency during electroluminescence are fully analyzed.In terms of new generation of lighting source,we provide systematical introduction on OLEDs,QLEDs and the PeLEDs.In addition,a series of background about perovskite materials including materials structure,film deposition,composition engineering,defects physics and ion nature are comprehensively described in detail.(2)In Chapter 3,based on the understanding of perovskite emitters,we novelly introduce amino acid ligand into perovskite active layer,and realize in-situ core-shell structured perovskite quantum dots film.The results demonstrate that the film formation of perovskite emitter can be significantly modulated by amino acid ligand,leading to large exciton bonding energy and suppressed defects density in product active layer.Owing to the ideal recombination kinitics and reduced energy loss,the fluorescence quantum yield(PLQY)of green perovskite film is dramatically increased to over 80%.Finally,we fabricate PeLEDs with external quantum efficiency(EQE)over 15%.(3)In Chapter 4,we try to boost the device operational stability from the aspect of device physics.The PeLED with unique low driven voltage is systematically investigated.We verify that the interfacial Auger effect modulated by interfacial minority carrier determines devices’ turn-on voltage.Furthermore,the boundary conditions and physical model of interfacial Auger effect are setup in followed studies.Finally,the lifetime of PeLEDs is enhanced by several folds in compare with that of control device.(4)In Chapters 5 and 6,we pursue highly efficient blue PeLEDs by employing pure Br and mixed halide(Br/Cl)perovskite emissive layers,respectively.Firstly,we develop a new long chain spacer with double functional amido terminals to achieve stable deep blue quasi-2D perovskite film,where the blueshifted emission is stem from quantum confinement in low dimensional perovskite structure.Furthermore,a mixed spacer system that controllably modulate the domain distribution and excited state kinitics in quasi-2D perovskite film is established.The optimized perovskite films delivered ideal PLQY about 80%.The fabricated PeLEDs exhibited a deep blue emission with an EQE of 2.6%.Secondly,we demonstrate that incorporation of the cationic π-conjugated polymer(PFNBr)can improve both the morphological and optoelectronic properties of mixed Cl/Br quasi-2D perovskite films,resulting in efficient and stable blue LEDs.This improvement in the device performance with respect to previous reports is based on a substantially increased PLQY of～82%,which originates from a drastic reduction in the trap density in the PFNBr-perovskite films.We speculate the charged quaternary ammonium and bromide in PFNBr have the capability to passivate charged ionic defects at grain boundary of perovskite.In addition,the quaternary ammonium ionic group and extra Br-ions could mitigate the migration of halide ions through a weak electrostatic interaction and healing halide defects at grain boundary of perovskite film respectively.The sky-blue perovskite LEDs present peak EQE of 11%.(5)In Chapter 7,we focus on the study of blue PeLEDs with new branch of perovskite emitter-quantum dots.The theoretical calculations suggest that Cl" vancencies are the primary defects species determine the recombination behavior and spectral stability of blue mixed halide perovskite quantum dots.In this regard,we propose the strategy by using organic pseudohalides for targeted passivation of intrinsic Cl-defects.Finally,we successfully improve the EQE of true blue PeLEDs to 6%.(6)In Chapter 8,we try to understand the failure mechanism of mixed halide blue perovskite quantum dots based LEDs.Our findings explicitly point out that the stacked multi-layer QDs emitter as ion acceptor could receive Cl-from adjoined quantum dots under electric field,which create Cl-deficiency region resulting in redshifted EL and pretty short operational lifetime.Such issue was successfully overcome by fabricating mono-layer QDs emitter which is confirmed to restrict the ion drifting pathway.Furthermore,we unravel that electrochemical-oxidation of Cl-in form of Cl2 is the other underlying reason for device degradation,and such process could be catalyzed by joule heat during device operation.The further strategy on modulation of hole current and operational temperature efficiently boost the device lifetime over hours,which is two orders of magnitude longer than that of initial devices.