Case of the Bromine Vacancy in Cs₄PbBr₆

Abstract

Typically defect tolerance is equated with a lack of deleterious defects or with abundant defects creating only shallow levels. Here, we address the idea that deep defects, when unavoidable, do not guarantee harmful consequences. Using halogen vacancy as a common defect among halides, we explore its behavior in Cs 4 PbBr 6 . It is a large gap material (band gap of ∼4 eV) known for its green emission at ∼520 nm. We show that its Br-vacancy is indeed a deep defect as obtained from hybrid density functional calculations. An analysis of the configuration coordinate diagram corresponding to the defect's charge transition levels enables us to conclude that the nonradiative recombination cycle will be hampered by an extremely slow hole capture process. Therefore, Br-vacancy will not suppress light emission in Cs 4 PbBr 6 . Although this finding does not signal that all deep defects will behave similarly, it indicates that defect tolerance may be achievable despite their occurrence.

Dimension and Thickness Control Synthesis of Strongly Confined Cesium Lead Iodide Perovskites with Excellent Two-Photon Absorption Properties

“…As we have reported for cesium lead iodide perovskites and Krishnakanth et al reported for cesium lead bromide perovskites, σ 2 for 0D-NCs is more than an order of magnitude higher than that of 3D-NCs. Observed higher TPA properties for 0D can be due to the involvement of defect states for 0D perovskites. − ,− As it is now well reported that the strong fluorescence from Cs 4 PbBr 6 can be attributed to the lattice defects. − ,, Since defect states for different nanocrystals are known to enhance multiphoton properties, − ,, the higher TPA properties observed for 0D-NCs can probably be due to defect centers, which may be arising from halogen vacancies. − ,, …”

“…[20][21][22][23][24][25][26][27][28]55,56 Since defect states for different nanocrystals are known to enhance multiphoton properties, [31][32][33][34][35]57,58 the higher TPA properties observed for 0D-NCs can probably be due to defect centers, which may be arising from halogen vacancies. [20][21][22][23][24][25][26][27][28]55,56 As reported in our reported σ 2 for 2D CsPbI 3 NPL is slightly higher than that reported for 2D CsPbBr 3 NPL by Alvarez et al 35 Interestingly, as reported in Table 3, our reported σ 2 for 1D CsPbI 3 NWs is much higher than that reported for 1D CsPbBr 3 NRs and 1D CsPb(I 0.15 Br 0.85 ) 3 NRs. 41 Due to very high two-photon absorption cross section (σ 2 ) values for 1D CsPbI 3 NWs, we have used two-photon fluorescence microscopic images to image those nanocrystals.…”

“…Materials with high 2PA properties are key for modern quantum photonic technologies and nonlinear photonic devices, where materials with large 2PA cross sections (σ 2 ) are required. − Since σ 2 for commonly used organic materials is very low (σ 2 > 100 GM, where 1 GM = 10 –50 cm 4 s photon –1 ), we and other groups have explored alternatives using perovskite-based quantum dots and other types of nanomaterials such as metal nanoclusters, graphene oxide-based quantum dots, and carbon dots. − In perovskite materials, during the multiphoton absorption process, an electron is excited from the valence band to the conduction band via simultaneously absorbed multiple monochromatic infrared photons without populating any real intermediate state. − After that, the electron rapidly relaxes from the conduction band to the valence band by emitting a photon with energy much higher than the excitation photons. − …”

“…Due to the three-direction space coupling nature, 3D CsPbX 3 are known to exhibit low exciton binding energies, and excitons can dissociate easily to form free charge carriers. − Since low-dimensional CsPbX 3 semiconductor nanocrystals exhibit unique optical and optoelectronic properties, several research groups are working to develop three-dimensional (3D) to zero-dimensional (0D) perovskites with tunable band gaps, which have the capability to be used in different photoelectronic devices such as light-emitting diodes (LEDs), lasers, and photodetectors. − …”