Synthesis Of Lead-Free Halide Perovskite For Light-Emitting Diodes

Metal halide perovskite materials have many excellent optoelectronic properties,such as strong light absorption,high photofluorescence quantum yields(PL QYs),adjustable fluorescence emission with narrow full width at half maximum(FWHM),and facile synthetic protocols.Moreover,the intrinsic ionic nature of halide perovskite allows it to be prepared at lower temperatures,and the high defect tolerance of halide perovskite can lead to production of bright emitters without any passivated surface.Due to these optoelectronic properties,halide perovskite has been a promising material for solar cells,light-emitting diodes,lasers,and photodetectors and so on.Despite these fascinating characteristics,nearly all high-performance perovskites contain heavy metal lead,which poses a great threat to the ecological environment and human health.Therefore,it is urgently desired to explore low toxic materials with comparable optoelectronic properties of lead-hal ide perovskite.In this thesis,two kinds of high-quality lead-free perovskite materials were successfully prepared and applied into photoluminescent LED devices as solid phosphors.The details are as follows:1.We have demonstrated a facile aqueous acid-based synthetic protocol to prepare two-dimensional(2D)(OCTAm)2SnBr4 tin halide perovskites.The whole synthesis process was carried out in humid and in ambient environment with high reproducibility and chemical yield.The obtained tin halide perovskites exhibited strong PL emission centered at 600 nm with broad FWHM(136 nm),large Stokes shifts(250 nm),high absolute PL QYs near unity in solid-state,zerooverlap between their absorption and emission spectra,and an ultra-long lifetime(3.3 ?s).The 2D tin halide perovskites perovskite showed no changes within 240 days storage at room temperature in ambient air and humid conditions.2.Lead-free all-inorganic halide materials(Cs3MnBr5 and CsMnBr3)with different Mn2+ ion coordination were synthsized by using a convenient and facile method under ambient air.Cs3MnBr5 exhibits a bright green emission(522 nm),unique single-exponential(single-rate)lifetime(?avg = 236 ?s),and PL QYs up to 82±5%.While,CsMnBr3 emits a red emission,corresponding to two-exponential fluorescence decay curve with lifetime 1.418 ?s(93%)and 18.328 ?s(7%)respectively.In addition,both of them show clearly paramagnetic behavior and excellent thermal stability.3.Photo pumped LED devices were successfully fabricated using two newly developed phosphors.Among them,the 2D(OCTAm)2SnBr4 perovskite(yellow phosphor)also cover the red light region because of its large half width(136?nm),which is more cost effective than the addition of one more red phosphor to broaden the emission band.A white LED device was fabricated by mixing it with a blue phosphor and a green phosphor on a UV chip in a weight ratio of 4:1:0.15.On the other hand,the all-inorganic CsnMnBrn+2 phosphor with a feed ratio of 1.4:1 was used as a one-component color conversion layer in the LED,and a warm white LED device was obtained without superposing other phosphor materials.