First-principles Studies On The Optoelectric Properties Of Double Perovskites Modulated By Mn Dopant

Metal-halide perovskites are widely used in solar cells,light-emitting diodes,photoelectric detectors,X-ray detectors and other fields because of their intrinsic excellent properties,such as extremely high light absorption,low electron and hole effective mass,long diffusion length and high carrier mobility,etc.However,lead-based perovskite represented by MAPbI3(MA=CH3NH3+)is not stable,and its composition element Pb is harmful to the environment.While Halide double perovskite(HDPs)materials such as Cs2AgInCl6 and Cs2NaBiCl6 can overcome these problems of stability and biological toxicity of the traditional halide perovskite materials effectively.Therefore,the research related to HDPs has attracted much attention recently.When HDPs are used in the field of light-emitting devices,researchers used to control its lightemitting performance via doping impurities.In the field of luminescence devices,Mn is one of the common elements to modulate luminescence properties.It will introduce a new PL spectrum into the double perovskite materials,however,the PL peaks of Mn2+in Cs2AgInCl6,Cs2AgBiCl6 and Cs2NaBiCl6 are different,they locate at 620 nm,600 nm and 577 nm,respectively.To explain this phenomenon,we take Cs2AgInCl6,Cs2NaBiCl6 and Cs2AgBiCl6 as t examples and use the first-principles method to simulate their geometric structures and electronic structures,the bonding properties of Mn-Cl in Cs2AgInCl6 and Cs2NaBiCl6 in addition to the effect of local environment on the PL peak of Mn2+were studied in detail.Taking Cs2NaBiCl6 and Cs2AgBiCl6 as examples,we analyzed the distribution of Mn in Bi-based double perovskite materials.Firstly,we study the similarities and differences of Cs2NaBiCl6 and Cs2AgBiCl6 in crystal structure using the electron localization function(ELF).Our ELF results show that the electrons near Na-Cl are more localized,so Na-Cl bond shows a more ionic nature than that of Ag-Cl bond and has a longer bond length.When Mn is doped into Cs2NaBiCl6 and Cs2AgBiCl6,we find that Mn tends to form MnBi defects in Bi-based double perovskite materials.This defect introduces the weakest distortion in the lattice,so its formation energy is also the smallest one.We calculate the ELF in these systems when Mn is incorporated into Cs2NaBiCl6 and Cs2AgIn Cl6 to form MnCl6 octahedron.The results show that the ionic character of the Mn-Cl bond is strong in these systems,and its ionic property is the strongest in Cs2AgInCl6.We calculate the electronic structures such as band structures,density of states(DOS)and charge distributions of Cs2NaBiCl6 before and after doping Mn.The doped Cs2NaBiCl6 possesses an indirect band structure,which is consistent with the experimental results.The calculated DOS indicates that the conduction band minimum(CBM)of Cs2NaBiCl6 is mainly contributed by the Bi 6p orbitals,while the valence band maximum(VBM)is attributed to the hybridization between the Bi 6s orbitals and the Cl 3p orbitals.We take Cs2NaBiCl6 with MnBi type defects as representation of the Mn-doped Bi-based double perovskite.After Mn is doped into Cs2NaBiCl6 and MnBi defects forms,the band structure of this system is still indirect,the CBM of the Mn-doped Cs2NaBiCl6 is mainly contributed by the Bi 6p orbitals and Mn 3d orbitals,while the VBM is attributed to the hybridization between the Cl 3p orbitals and the Mn 3d orbitals.The most remarkable change of band structure is that the Mn introduces two defect levels in the forbidden gap of Cs2NaBiCl6,one is close to the CBM and the other appears near the VBM.These two levels are contributed by the 3d orbit of Mn and their energy difference is 2.63 eV.Therefore,the d-d transition of Mn can produce photoluninescence(PL)of 2.63 eV,which is consistent with the experimental phenomenon.At the same time,we also analyze the Bader charges of the double perovskite material containing Mn impurities.We find that Mn is+2 valence after occupying the Bi site,which indicates that the outmost electron configuration of Mn ions in the double perovskite material is 3d5,so we can conclude that the d-d transition spectrum comes from the 4T1→6Ai transition of Mn2+.To explain the phenomenon that the PL peak wavelengths of Mn2+of Cs2NaBiCl6 and Cs2AgInCl6 are different,we compare the crystal field strength,i.e.,the Dq around Mn2+in both perovskites.We calculate Dq and the integrated projected COHP(IpCOHP)around MnCl6 octahedron by crystal field theory and the theoretical energy term,respectively.Before calculating IpCOHP,we use ELF to judge the bonding type of Mn-Cl.The results show that Mn-Cl bonds in both systems are ionic bonds,but the Mn-Cl bond in Cs2AgInCl6 is more ionic.Using the experimentally measured emission peak of the photoluminescence excitation spectrum(PLE),we calculate the Racah parameters B and C in the crystal field and obtain the Dq of two double perovskites.The Dq around the Mn2+in the Cs2AgInCl6 and Cs2NaBiCl6 are 1895 cm-1 and 1195 cm-1,respectively.Employing the crystal orbital Hamilton populations(COHP),we visualize the bonding nature of Mn-Cl and calculate the IpCOHP.These two results of Dq and IpCOHP show that the crystal field strength of MnCl6 octahedron in Cs2AgInCl6 is stronger than that in Cs2NaBiCl6.It also shows that the bond strength of Mn-Cl is positively correlated with the crystal field strength of octahedron around Mn2+.The local environment of Mn2+differs in different double perovskite,and its crystal field is also different.The Dq of MnCl6 octahedron decreases from Cs2AgInCl6 to Cs2NaBiCl6,so the PL peak produced by the d-d transition of Mn2+blue shifts.