The First-principle Investigations On The Photoelectric Properties Of Inorganic Double Perovskite Cs₂B′B″X₆（B′=Cu,Ag,Na;B″=Bi,Sb;X=Cl,Br,I）/TiO₂ Heterostructures

The organic-inorganic halide perovskite solar cells have been a hot issue in the photovoltaic field due to its high efficiency,low cost,and easier preparation.However,the instability of perovskites and the toxicity of lead restrict its widespread use in solar cells.Therefore,it is necessary to find the new environmental alternatives for the lead halide perovskites.Recently,the lead-free double perovskites have received the enormous attentions owning to the similar atomic structures with lead halide perovskites and the higher stability in the working conditions.In this paper,we perform the comprehensive study on the optoelectronic properties of inorganic double perovskites and heterostructures based on the density functional theory(DFT),the concrete details and corresponding conclusions are listed as follows:Firstly,we calculate the band structures of bismuth-and antimony-based double perovskites,i.e.Cs2B’BiX6(B’=Ag,Cu;X=Cl,Br),Cs2AgSbCl6,and Cs2NaSbI6.These compounds with band gaps in the visible-light region are suitable for light absorbers of solar cells.The distribution of charge density on the conduction band minimum(CBM)of Cs2B’BiCl6(B’=Ag,Cu)and Cs2AgSbCl6 is more uniform,indicating a higher density of photoinduced electrons.Then,the bismuth-based double perovskites/TiO2 heterstructures are built to investigate the atomic bonding character and charge transfer mechanism across the interfaces,i.e.Cs2B’BiX6(B’=Ag,Cu;X=Cl,Br)/TiO2.It can be seen that a higher covalent character exsits between double perovskites and TiO2 slabs of B’2Bi2X8(B’=Ag,Cu;X=Cl,Br)/TiO2 interfaces by analyzing the electron localization function(ELF).The difference charge density and charge displacement curve(CDC)show that the charges are transported from double perovskites to TiO2 electron transport layer(ETL)in the Cs4X4(X=Cl,Br)/TiO2 interfaces,which enhances the effective separation of photoinduced carriers.Moreover,we find that there is a lower energy barrier at Cs4X4/TiO2(X=Cl,Br)interfaces by comparing the band alignment between double perovskites and TiO2,which is favorable for the charge transfer across the interfaces.Finally,based on the excellent photoelectric properties of the Cs4X4(X=Cl,Br)/TiO2 interfaces in the bismuth-based heterostructures,we further explore the atomic bonding and charge transfer across the interfaces of antimony-based heterostructures(Cs2AgSbCl6/TiO2 and Cs2NaSbl6/TiO2).Similarly,there is a higher covalent character between the double perovskites and TiO2 slab of Ag2Sb2C18/TiO2 and Na2Sb2I8/TiO2 interfaces.Furthermore,a good charge transfer,the charges are transported from Cs2AgSbCl6 to TiO2 slab,displayed in Cs4Cl4/TiO2 interface.It can be seen a smaller energy gradient forms between the CBM of double perovskites and TiO2 in the Cs4I4/TiO2 interface from the locally projected density of states and the carrier will flow easier to TiO2 layers,thus improving the efficiency of solar cells.