Luminescence And Stability Regulation Of Inorganic Perovskite Nanocrystals And Their Application In Solar Cells

Inorganic lead-halide perovskite（CsPbX₃;X=Cl,Br,I）nanocrystals have been developed due to their excellent optoelectronic properties,such as high fluorescence quantum yield,tunable emission color,high color purity,low cost and facile synthesis process,etc.It has attracted extensive attention in optoelectronic applications.As a multifunctional material,they can be used to realize a new generation of solution-processable optoelectronic devices,including light-emitting diodes,lasers,photodetectors,and solar cells.However,almost all lead halide perovskite materials are very sensitive to moisture and easily hydrolyzed,which leads to changes in their emission wavelengths under atmospheric conditions,during hydrolysis and crystal growth.Therefore,the instability of emerging perovskite materials in atmospheric environments is a bottleneck for their potential applications.The emission of inorganic perovskite nanocrystals is related to halogen anions,and the PL（photoluminescence）peaks of CsPbBr₃,CsPbCl₃ and CsPbI₃ nanocrystals are located at 510 nm,410 nm and 700 nm,respectively.Although the luminescence color of nanocrystals can be easily adjusted by adjusting the type and proportion of halogen anions,compared with green CsPbBr₃nanocrystals,blue CsPbCl₃ nanocrystals have more defects,lower fluorescence quantum yield and lower device lifetime.Red light CsPbI₃ has poor stability and low fluorescence efficiency,which are barriers for inorganic perovskite nanocrystals to achieve multicolor luminescence.Although CsPbBr₃ has high fluorescence efficiency,its fluorescence emission wavelength is short,and the light absorption matching of photovoltaic devices is poor,so it is not suitable as an ultraviolet light conversion material for photovoltaic devices.Therefore,it is of great interest to develop other techniques instead of anion exchange to tune the emission color of CsPbX₃,develop reliable red and yellow light perovskite nanocrystals,and simultaneously enhance their stability.In our work,the stability of inorganic perovskite nanocrystals is improved by means of inorganic coating,crystal phase adjustment and organic dispersion,and the feasibility,effectiveness and limitations of different means are analyzed.While enhancing the stability,the emission color of perovskite nanocrystals was regulated and performance optimized by means of quantum confinement,doping,crystal field regulation,etc.,and applied to optoelectronic devices.The details are as follows:1.CsPbBr₃ nanocrystals were prepared,and water-resistant and stable luminescent CsPbBr₃@NaYF₄ and CsPbBr₃@ZnO nanocrystals were synthesized by a simple liquid phase method by using two materials,sodium yttrium tetrafluoro（NaYF₄）and zinc oxide（ZnO）as protective shells.Enhancing the resistance of CsPbBr₃ nanocrystals to water and polar solvents.2.Water-resistant,monodisperse,and stable luminescent CsPbBr₃ perovskite nanocrystals composited with CsPb₂Br₅ were prepared by a modified liquid-phase method.The 2d CsPb₂Br₅ layer was attached to the surface of CsPbBr₃ nanocrystals.Dual-phase CsPbBr₃/CsPb₂Br₅ nanocrystal with stable green emission was obtained,and the emission peak was located at 510 nm.Because CsPb₂Br₅ can enhance the luminescence and structural stability of CsPbBr₃ nanocrystals,the dual-phase CsPbBr₃/CsPb₂Br₅nanocrystals exhibit higher spectral stability than pure CsPbBr₃nanocrystals in atmospheric and polar solvent environments.In addition,the dual-phase CsPbBr₃/CsPb₂Br₅ nanocrystals synthesized at the same temperature are smaller in size than CsPbBr₃ nanocrystals,and the PL spectrum is blue-shifted.Owing to the strong stability,their photoluminescence emission peak can be simply and reliably tuned continuously between 480 nm and 520 nm through the reaction temperature.3.The crystal field of the CsPbCl₃:Mn²⁺nanocrystals was adjusted by co-doping other cations and the concentration of Mn²⁺remained unchanged.And he crystal field strength of different samples was calculated.Compared with the CsPbCl₃:Mn²⁺nanocrystals,the red-orange peak in the fluorescence spectrum of CsPbCl₃:Mn²⁺,Er³⁺nanocrystals was red-shifted from 580 nm to 600 nm and enhanced by 100 times successfully.The same experiment was carried out on CsPbCl₃:Mn²⁺nanoplatelets at the same time to confirm the changed crystal field around Mn²⁺.The effect of co-doping cations on the luminescence properties of Mn²⁺is similar to that in nanocubes.And the mechanism was analyzed in detail.4.In order to enhance the photoelectric conversion efficiencies of crystalline silicon（c-Si）solar cells,CsPbCl₃ nanocrystals（NCs）co-doped with Mn²⁺and Er³⁺(CsPbCl₃:Mn²⁺,Er³⁺NCs)were mixed with ethylene-vinyl acetate（EVA）to form a film which was used as a luminescent down-shifting（LDS）layer.The LDS layer effectively improved the low utilization of near-ultraviolet light of c-Si solar cells.These CsPbCl₃:Mn²⁺,Er³⁺NCs were synthesized via a conventional high-temperature injection method.Mn²⁺is the luminescence center,and the incorporation of Er³⁺greatly enhances the luminescence intensity of Mn²⁺.The absolute photoluminescence quantum yield of the NCs dispersed in toluene reached 79.5%when the NCs were synthesized under the optimum conditions,i.e.,an injection temperature of 180°C and Pb:Mn:Er preparation molar ratios of 6:4:4.The EVA film embedded with NCs at the optimum concentration（0.9 wt%）was used as an LDS layer for c-Si solar module.The short-circuit current(I_SC)and the photoelectric conversion efficiency（η）were increased by 3.42%and 4.02%,respectively,owing to the LDS layer.Moreover,a luminescent solar concentrator（LSC）which was another application of luminescent materials was also demonstrated.For LSC,the relative changes in I_SC andηby using the NCs-dispersed EVA film were+14.9%and+18.0%,respectively.These results indicate a feasible application of luminescent downshifting films in solar modules.