Enhanced Visible Light Absorption in Layered Cs₃Bi₂Br₉ Halide Perovskites: Heterovalent Pb²⁺ Substitution-Induced Defect Band Formation

Abstract

We have successfully substituted trivalent Bi3+ with divalent Pb2+ in Cs3Bi2Br9-layered perovskites. Controlled heterovalent Pb substitution in these Cs3Bi2Br9-layered perovskites reduces the band gap because of the emergence of defect states in between the bands. These heterovalent Pb-substituted Cs3Bi2Br9 bulk perovskite compounds are successfully synthesized for the first time by chemical reprecipitation method. X-ray photoelectron spectroscopy analysis indicate that lead substitution in the structure is in Pb2+ form, which creates a charge imbalance in the compound as it replaces Bi3+ from the layered perovskite structure. Such charge imbalance is compensated either by bromine vacancies (VBr) or interstitial cesium (Csi) additions. VBr or Csi in Cs3Bi2Br9 along with PbBi creates defect states in between the bands, which results in redshift in the layered perovskite band. Band structure calculations indeed confirm the onset of such defect states, responsible for the redshift. A more detailed defect physics simulation indicates that the defect complex PbBi + VBr is more probable to form if Pb is rich in the environment, which consequently introduces a few deep level defects responsible for the reduction of the band gap. Understanding of the electronic structure and defect physics of such heterovalent Pb-substituted Cs3Bi2Br9 will strengthen the future photovoltaic and optoelectronic applications.

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping

“…2 The cubic A 2 BX 6 structure, commonly called the vacancy-ordered double perovskite structure, 3 consists of B(IV) cations such as Sn, Te, Ti, Pt etc, while the composition A 3 B 2 X 9 contains B(III) cations such as Sb and Bi, and forms several different crystal structures with varying degrees of connectivity in the B-X sublattice. [4][5][6][7][8][9][10] The large majority of reported A 3 B 2 X 9 compounds adopt a hexagonal structure in the space group P6 3 /mmc. 11,12 This structure can be thought of as hexagonal close packed A 3 X 9 with B cations ordered in octahedral interstitial sites, resulting in isolated [B 2 X 9 ] 3dimers, an arrangement often described as zero dimensional (0D).…”

“…Two other main structure types are known, each adopted by only a handful of reported compositions: infinite onedimensional (1D) double chains based on corner-sharing [BX 6 ] 3octahedra, 13,14 and twodimensional (2D) corrugated layers featuring corner-connected [BX 6 ] 3octahedra. 8,[15][16][17][18][19] Of these, the 2D layered structure is most closely related to the cubic perovskites, and can be described as a vacancy-ordered triple perovskite, formed by removing every third B site cation from cubic ABX 3 . The resulting local B cation coordination environment is irregular and exhibits a trigonal distortion that produces three short and three long B-X bonds, with the shorter linkages shared between neighbouring octahedra in each layer.…”

“…The wide range of ions that can be incorporated on both the cation and anion sites of an choice of composition. 2 Several strategies have been employed, from alloying on the halide site, 8,[23][24][25] which usually produces a band gap response similar to that seen in the perovskites, to doping on the B-metal site, which may lead to introduction of filled states within the band gap, 26,27 or band bowing. [26][27][28] Recently, Ghosh et al studied a mixed valence perovskite (CH 3 NH 3 )AuBr 3 , containing Au(I) and Au(III), which resulted in significant visible light absorption.…”

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(II) / Sn(IV) doping

“…Two other main structure types are known, each adopted by only a handful of reported compositions: infinite one-dimensional (1D) double chains based on corner-sharing [BX6] 3octahedra, 13,14 and two-dimensional (2D) corrugated layers featuring corner-connected [BX6] 3-octahedra. 8,[15][16][17][18][19] Of these, the 2D layered structure is most closely related to the cubic perovskites, and can be described as a vacancyordered triple perovskite, formed by removing every third B site cation from cubic ABX3. The resulting local B cation coordination environment is irregular and exhibits a trigonal distortion that produces three short and three long B-X bonds, with the shorter linkages shared between neighbouring octahedra in each layer.…”

“…2 The cubic A2BX6 structure, commonly called the vacancy-ordered double perovskite structure, 3 consists of B(IV) cations such as Sn, Te, Ti, Pt etc, while the composition A3B2X9 contains B(III) cations such as Sb and Bi, and forms several different crystal structures with varying degrees of connectivity in the B-X sublattice. [4][5][6][7][8][9][10] The large majority of reported A3B2X9 compounds adopt a hexagonal structure in the space group P63/mmc. 11,12 This structure can be thought of as hexagonal close packed A3X9 with B cations ordered in octahedral interstitial sites, resulting in isolated [B2X9] 3dimers, an arrangement often described as zero dimensional (0D).…”