ABSTRACT
Taking Cs2NaBiCl6, Cs2AgInCl6 and Cs2AgBiCl6 as examples of lead-free double perovskites (DPs), we study the photoluminescence (PL) properties of Mn-doped DPs. The electron localization function (ELF) reveals the more ionic nature of the Na-Cl bond in Cs2NaBiCl6 than that of the Ag-Cl bond in Cs2AgBiCl6. Bader charge calculations confirm the nominal +2 valence state of Mn ions in both DPs. Mn2+ ions introduce two defect levels in the band gap of the Cs2NaBiCl6 host, accounting for the d-d transition (4T1-6A1 transition) of Mn2+ and thus the subsequent orange PL. The changes of the crystal field and their influences on the emission energy of Mn2+ ions in different DPs are evaluated by calculating the Racah parameters (B and C) and the crystal field strength (Dq) obtained from energies of the terms of d5 in the Cs2NaBiCl6:Mn2+ and Cs2AgInCl6:Mn2+ systems. The results show that Dq in Cs2AgInCl6:Mn2+ is stronger than that in Cs2NaBiCl6:Mn2+. The analyses on bonding interactions of the Mn-Cl bond via ELF and the integrated projected pCOHP also confirm the stronger ionic bonding interactions and thus the boost of the crystal field strength in the Cs2AgInCl6:Mn2+ system, which results in the blue-shift of the Mn2+ introduced PL peak from Cs2AgInCl6 to Cs2NaBiCl6. Our results provide a new strategy to modulate the emission wavelengths, i.e., tuning the crystal field.
ABSTRACT
We investigate the internal mechanism of the light-induced phase transition of CsPbBr3 perovskite materials via density functional theory simulations. Although CsPbBr3 tends to appear in the orthorhombic structure, it can be changed easily by external stimulus. We find that the transition of photogenerated carriers plays the decisive role in this process. When the photogenerated carriers transit from the valence band maximum to conduction band minimum in the reciprocal space, they actually transit from Br ions to Pb ions in the real space, which are taken away by the Br atoms with higher electronegativity from Pb atoms during the initial formation of the CsPbBr3 lattice. The reverse transition of valence electrons leads to the weakening of bond strength, which is proved by our calculated Bader charge, electron localization function, and integral value of COHP results. This charge transition releases the distortion of the Pb-Br octahedral framework and expands the CsPbBr3 lattice, providing possibilities to the phase transition from the orthorhombic structure to tetragonal structure. This phase transition is a self-accelerating positive feedback process, increasing the light absorption efficiency of the CsPbBr3 material, which is of great significance for the widespread promotion and application of the photostriction effect. Our results are helpful to understand the performance of CsPbBr3 perovskite under a light irradiation environment.
ABSTRACT
Composition engineering plays an important role in generating novel properties and decreasing the lead (Pb) toxicity for halide perovskite materials. To find out the modulation effect introduced by the composition engineering, namely,B'-site co-metal ions, in (MA)2AgBi1-xSbxBr6systems with various Bi/Sb ratios ofx= 0, 0.25, 0.75, 1.00, series of theoretical simulations and analyses are carried out. For the (MA)2AgBi1-xSbxBr6systems, the Goldschmidt tolerance factortand the octahedral factorµindicate that all samples are in a standard double perovskite structure with alternating AgBr6and Bi/SbBr6octahedra. The calculated electronic structures show that the band gap of (MA)2AgBi1-xSbxBr6decreases with the increase of Sb content, but the indirect band gaps are maintained for all samples. By analyses of the imaginary partÉ2(ω) of dielectric function and the absorption spectra, we find that all (MA)2AgBi1-xSbxBr6systems show absorption in the visible-light region. All these results indicate that the composition engineering adopted in this paper is an effective strategy to modulate the optical properties of (MA)2AgBi1-xSbxBr6systems and may open a new way to put it into applications in the fields of solar cells and other optoelectronic devices.