| Metal halide perovskite materials have unique photoelectric properties such as high photoelectric conversion efficiency,continuously adjustable emission spectra,along with their simple and low-cost production process,making them potentially ideal materials for the next generation of high-quality light-emitting diode devices.Allinorganic CsPbBr3 materials show good thermal stability,and their luminescence properties can be adjusted by dopants doping.Therefore,attempts have been made to substitute Pb with smaller divalent cations to obtain high-quality blue emitting with super purity and efficiency.This paper thoroughly analyzes the influence of the Cu dopant on CsPbBr3 materials through a series of theoretical investigations.Firstly,via discussions on the lattice constants,the supercell volumes,the bond lengths and the bond angles,we analyze the geometric changes of the CsPbBr3 system induced by Cu dopant.The optimized geometric structure intuitively reveals the significant lattice distortion introduced by Cu dopants to CsPbBr3,the supercell volume contracts manifestly,and the lattice constants change unevenly in different directions.Comparing the PbBr6 octahedra near the Cu dopant in CsPbBr3 with those octahedra slightly far away from the Cu dopant,we find that due to the strong distortion of CuBr6 octahedron,Pb-Brbond lengths and Br-Pb-Br bond angles near the Cu dopant fluctuate greatly,leading to the relatively obvious distortion of these PbBr6 octahedra.But Pb-Brbond lengths and Br-Pb-Br bond angles further away from the Cu dopant change little,so that the corresponding PbBr6 octahedra still maintain ideal octahedron structures.This phenomenon is consistent both in the low concentration Cu doped system CsPb0.94Cu0.06Br3 and in the high concentration Cu doped system CsPb0.88Cu0.12Br3,indicating that Cu dopants only change the local lattice surrounding them and cannot alter the lattice structure in large scale.Generally,optical emission properties are determined by the electronic structures.We calculate the electronic structures and find that CsPbBr3 is a direct bandgap semiconductor,the bandgap increases with the increase of Cu concentration,which is consistent with the excellent electroluminescence performance and enhanced blue luminescence after doping observed experimentally.The valence band maximum(VBM)of CsPbBr3 is mainly contributed by the Pb-6s orbital and the Br-4p orbital,while the conduction band minimum(CBM)is composed predominantly by the of Pb6p orbital and the Br-4s orbital.Cu orbitals only provide the predominant contribution to the VBM,which leads to the significant alternations of the band structure near VBM.The strong hybridization interactions between Cu-3d orbitals and the orbitals of the host Pb and Br generate new impurity levels,which mix with the VBM and thus increase the bandgap.To further find out the real origin ascribing for the alternation of the electronic structure induced by Cu for CsPbBr3 systems,two artificial CsPbBr3 supercells are constructed in this work,and their electronic properties are analyzed accordingly.Firstly,we construct the#Com system,which is a pure CsPbBr3 supercell but is compressed to a supercell with the lattice constants as that of the Cu-doped CsPbBr3 systems.Via the simulation of electronic structures for system,we find that the decrease of the whole supercell volume leads to a reduction of the CsPbBr3 bandgap,not the experimentally observed increase of bandgap,which is determined by the band edge composition of CsPbBr3 material.Both the CBM and VBM of CsPbBr3 are antibonding states formed by constituent orbital coupling,and their energy levels increase as the lattice and the bond length shrinks.However,the deformation potentials of CBM and VBM of CsPbBr3 behave anisotropically.Due to the smaller energy difference between the constituent orbitals,the energetic shift of VBM is more sensitive to the shrinkage of lattice,leading to a reduction of the bandgap with the lattice contraction.Secondly,we construct the#Dis system,which is still a pure CsPbBr3 supercell,but maintains the local lattice distortion induced by Cu.The analyses about the electronic properties of#Dis system further confirms that the local destruction of the lattice periodicity around Cu dopants plays important role for the increase of the bandgap in Cu doped CsPbBr3 materials.This paper reveals the two critical reasons for the modulation effect of the band structures of CsPbBr3 perovskites after being doped with Cu dopant,i.e.,the contribution of Cu-3d orbitals to the VBM and the localized lattice distortion caused by Cu dopant.This study clarifies the mechanism of how Cu dopants make the bandgaps of CsPbBr3 materials blueshift.Therefore,we provide new views to improve the blue light emission performance of CsPbBr3 materials. |
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