Preparation And Optical Properties Of Lead-free Halide Double Perovskites Nanomaterials

Recently,lead halide perovskites have become a new favorite in the photovoltaic fields due to its high absorption coefficient,long carrier diffusion length,and simple manufacturing processes.Organic halide perovskite solar cells are currently the fastest growing photovoltaic material,with excellent power conversion efficiency.After the first groundbreaking work by Miyasaka et al.,The power conversion efficiency of organic lead halide perovskite solar cells has rapidly increased to 23.3%of NREL certification within a few years.However,despite the excellent photovoltaic performance,the toxicity of heavy metals lead in these materials to biological and environmental hampers their further commercial development in the future.It is therefore important to find less toxic metals to replace lead.According to previous reports,Sn and Ge have been used to develop lead-free perovskites as elements similar to Pb in the IVA group,but the problem is that they have poor stability in air;the stability of bismuth-based halide perovskite Cs₃Bi₂I₉ is high,which is now assembled in solar cells and achieves a power conversion efficiency of more than 1%.Similar to bismuth,antimony-based perovskite Cs₃Sb₂I₉ has also been developed and reported.Nevertheless,the fact is that only a few divalent or trivalent cations can replace Pb²⁺to form non-toxic and stable perovskites.Therefore,another feasible approach is to explore the formation of a double perovskite by replacing two lead ions with a monovalent cation and a trivalent cation.Its crystal structure is composed of B^IX₆ and B^IIIX₆ octahedrons with sharing alternating angles.The surrounding large space is occupied by A-site Cs⁺cations,forming an ordered rock salt structure.This double perovskite shows extremely high stability when exposed to light,moisture and ambient air,and has good light and energy absorption,showing extraordinary potential in the application of photovoltaic devices in the future.In this paper,three new types of lead-free double perovskite quantum dots were successfully prepared by the ligand-assisted reprecipitation method(LARP)at room temperature.They are organic-inorganic hybrid MA₂AgBiI₆(MA=CH₃NH₃),all inorganic Cs₂AgSbX₆(X=Cl,Br,I),all inorganic Cs₂AgInCl₆ and Na-doped Cs₂Na_xAg_1-xIn Cl₆(0?x?1).Through a series of characterization methods,its structure and optical properties were investigated in depth.(1)MA₂AgBiI₆ perovskite quantum dots was prepared at room temperature using iodomethylamine,silver iodide,and bismuth iodide as raw materials.They have a quasi-spherical shape with a uniform size of 4.4 nm,showing no obvious lattice defects.It has a sharp exciton absorption peak at 370 nm,which indicates a distinct quantum confinement effect compared to its bulk material;the fluorescence emission peak is 580nm with a large Stokes shift.The fluorescence quantum yield(PLQY)of MA₂AgBiI₆perovskite quantum dots is 15.69%,and the fluorescence emission is mainly driven by the recombination of exciton radiation.It was found that the organic solvent chloroform can control the pure phase of perovskite.In addition,organic ligands can affect the fluorescence emission peak:after adding 10%oleic acid(OA),compared with the one without any ligand,the intensity can be increased by 75%,and the emission peak is slightly red-shifted.The combined addition of trioctylphosphine(TOP)and OA can further increase the fluorescence emission intensity.(2)Cs₂AgSbX₆(X=Cl,Br,I)perovskite quantum dots were prepared at room temperature using cesium halide,silver halide and antimony halide as raw materials.The blue-emitting Cs₂AgSb Cl₆ quantum dots occupied the cubic Fm3m space group with quasi-spherical uniform morphology and size of about 4.7 nm;they have high crystallinity and no obvious lattice defects.Particularly,it has high stability:the dry Cs₂AgSb Cl₆ perovskite powder can be stored in dark air for 6 months without the formation of impurities;Besides,the fluorescence intensity of Cs₂AgSb Cl₆ perovskite powder re-dispersed in the solvent is only 10%less than the initial,Confirming its excellent air stability.The ligand oleic acid in this system not only plays a role in inhibiting the accumulation of quantum dots and controlling the synthesis of pure phase perovskite,but also can improve the fluorescence emission performance by passivating the surface defects of the quantum dots.Proper amounts of OA can control its PLQY of as high as 31.33%.By changing the proportion of halogen in the precursors,the fluorescence emission peak of Cs₂AgSbX₆(X=Cl,Br,I)perovskite quantum dots can be tuned in the spectral range of 409 nm to 557 nm.(3)All inorganic Cs₂AgInCl₆ and Na-doped Cs₂Na_xAg_1-xIn Cl₆ perovskite quantum dots were prepared at room temperature using cesium chloride,silver chloride,sodium chloride,and indium chloride as raw materials.They are all quasi-spherical with uniform morphology and size of about 1.45 nm and 2.05 nm with high crystallinity and no obvious lattice defects.Cs₂AgInCl₆ quantum dots emit warm orange fluorescence at489 nm.In order to further optimize its optical performance,Na ion was doped therein.With the increasing proportion of Na,its X-ray diffraction peak shifts to a low angle,and its fluorescence emission peak also red-shifts,resulted from the lattice shrinkage caused by the reduced exciton Bohr radius.When 75%Na⁺was added,the PLQY of Cs₂Na_0.75Ag_0.25In Cl₆ perovskite quantum dots can be up to 39.76%,which is nearly four times higher than that of undoped Cs₂AgInCl₆.The photoluminescence lifetime also increases after the doping of Na⁺.In short,the PLQY and stability of Na-doped perovskite quantum dots have been greatly improved compared to the undoped Cs₂AgInCl₆ perovskite quantum dots.