Stability Of Inorganic Perovskite Materials And Their Application In Light Emitting Diodes

All-inorganic halide perovskite material has attracted much attention due to its simple preparation method,low cost,tunable fluorescent peak with narrow full width at half maximum.Compared with organic halide perovskite materials,all inorganic perovskite materials display good thermal stability.Light-emitting diodes based on all-inorganic perovskite materials have received extensive attentions in recent years because of the high fluorescence and suitable band gap.The current all-inorganic perovskite materials are mainly focused on two types:colloidal perovskite nanocrystals and perovskite solid films.Photoluminescence quantum yield of perovskite nanocrystals is high(up to 100%),solution processing method is simple,but in the synthesis method,people often use oleic acid and oleylamine as ligands.The binding force is weak and the stability is also poor.When the ligand falls off in the solution,the fluorescence is weakened,and the non-radiative combination is severe.At the same time,in the process of application,the nanocrystals need to be purified,and the purification process has great damage to the perovskite nanocrystals with oleic acid and oleylamine as ligands,so it is necessary to consider the polar solvent to the nanocrystals when selecting the anti-solvent.We need to ensure that the excess unreacted precursor is removed,which often brings great difficulty to the application.On the other hand,perovskite film materials are generally prepared from precursors,but the perovskite film fluorescence is very weak because of the large grain size,not only the surface non-radiative recombination is severe,but its exciton binding energy is also smaller than perovskite nanocrystals.In addition?if the all-inorganic perovskite film contains cesium bromide,which has excellent water absorption,resulting in inorganic perovskite films with instability in the humidity environment.Based on the above,this paper mainly includes:1.We used octylphosphonic acid(OPA)to completely replace oleic acid and oleylamine and successfully synthesized all inorganic perovskite nanocrystals with different halide ratios,and regulated different fluorescence wavelengths.The morphology of nanocrystals based on OPA has changed greatly.The original cube has changed into a spheroidal shape due to the short carbon chain and strong interaction between the ligand and the nanocrystals,but the structure phase has not changed.In the crystal phase of perovskite,on the contrary,the crystallinity of the perovskite nanocrystals synthesized by using OPA as a ligand is improved.At the same time,the perovskite nanocrystals synthesized with OPA as a ligand showed great resistance to polar solvents,and the water-oxygen resistance was also greatly improved.2.The highly stable and highly fluorescent OPA-CsPbBr3 nanocrystals solution was used to prepare high-efficiency perovskite nanocrystals light emitting diodes(LED).The strong binding between OPA and CsPbBr3 allows us not only to remove the additional organic residue after purification,but also to maintain its excellent dispersion and high fluorescence quantum yield characteristics.Here,we have prepared a highly efficient green LED with a maximum current efficiency of 18.13 cd A-1,which is equivalent to 6.5%extermal quantum efficiency(EQE).The significant performance improvement of devices based on OPA-CsPbBr3 is attributed to the compactness of the nanocrystals film,uniform morphology and good electrical conductivity.We believe that the OPA-CsPbBr3 nanocrystals solution can be further optimized to improve device performance and stability of perovskite optoelectronic devices.3.Inorganic perovskite CsPbBr3 film was prepared by thermal vacuum depositor method and formed into an ultra-stable core/shell CsPbBr3/CsPb2Br5 structure by air treatment,which has reversible fluorescence adjusted by thermal heating.The formation of the core/shell structure is achieved by introducing additional NaBr or LiBr to accelerate the conversion of a portion of CsPbBr3 to CsPb2Br5.CsPbBr3 embedded in CsPb2Br5 exhibits enhanced fluorescence intensity due to dielectric confinement.At the same time,the fluorescence intensity remains almost 90%after four months of storage in air.The high fluorescence stability is attributed to the water-repellent CsPb2Br5 protective layer while also ensuring thermal stability.In combination with the advantages of the thermal vacuum deposition method,we provide a method for preparing a flexible transient display application by controlling the heating circuit.Flexible displays can switch between various patterns that can be used to display certain dynamic information.The stability and reproducible fluorescence conversion of CsPbBr3/CsPb2Br5 is expected to reach its potential in display applications.At the same time,we have also prepared perovskite light-emitting diodes based on this film,demonstrating the other applications of this film.