From 0D Cs₃Bi₂I₉ to 2D Cs₃Bi₂I₆Cl₃: Dimensional Expansion Induces a Direct Band Gap but Enhances Electron–Phonon Coupling

Abstract

Alternative all-inorganic halide perovskites are sought to replace the hybrid lead halide perovskites because of their increased stability. Here, the (111)-oriented defect perovskite family A 3 M 2 X 9 based on trivalent M 3+ is expanded through the use of mixed halides, resulting in Cs 3 Bi 2 I 6 Cl 3 . This compound shares the (111)-oriented 2D bilayer structure of α-Cs 3 Sb 2 I 9 (space group P3̅ m1), with Cl occupying the bridging positions of the bilayers and I in the terminal sites, in contrast to the parent compound Cs 3 Bi 2 I 9 , which consists of 0D molecular [Bi 2 I 9 ] 3− dimers. The increased dimensionality induces a direct band gap as calculated by density functional theory but has an absorption edge of 2.07 eV, nearly identical to the indirect band gap compound Cs 3 Bi 2 I 9 . Intriguingly, there is a remarkable lack of Cl orbital contribution to the band edge states of Cs 3 Bi 2 I 6 Cl 3 , despite Bi−Cl bonds binding all octahedra together. This highlights the importance of interlayer interactions in the defect perovskite family, which enhances the effective dimensionality of these 2D and 0D materials and may improve their optoelectronic performance. However, these changes in the excitonic absorption do not reflect free excitons, as Cs 3 Bi 2 I 6 Cl 3 exhibits broad photoluminescence as a result of self-trapped excitons, which appear to be universal in the (111)-oriented defect perovskites.

Synthesis and Localized Photoluminescence Blinking of Lead‐Free 2D Nanostructures of Cs3Bi2I6Cl3 Perovskite

“…The optical properties of bulk crystalline ingot of Cs 3 Bi 2 I 6 Cl 3 were investigated in the solid state initially and subsequently, compared with the solution phase optical properties of Cs 3 Bi 2 I 6 Cl 3 NCs or NSs that dispersed in toluene (Figures a–d). Electronic absorption spectroscopy of the bulk Cs 3 Bi 2 I 6 Cl 3 showed an optical absorption edge at 607 nm (2.04 eV), which is comparable to previously reported band gap value for the Cs 3 Bi 2 I 6 Cl 3 single crystal . Photoluminescence (PL) measurement reveals a broad emission band of bulk Cs 3 Bi 2 I 6 Cl 3 at 615 nm (2.02 eV) in the solid state at the excitation wavelength of 485 nm.…”

“…The PL emission broadening at 77 K comprises of multipeak emissions with slight differences in energies (Figure S6, SI). Interestingly, Kanatzidis and co‐workers observed a single broad PL peak for Cs 3 Bi 2 I 6 Cl 3 single crystals at 13 K, which was completely quenched at around 80 K . The changes of life‐time are fairly mild (Table S1, SI) between 77 K and room temperature, which indicate again the fluctuation of emission events and existence of shallow trap states.…”

“…Till now, all of these 2D perovskites with mixed halide compositions contain Pb and at present, such Pb‐free materials are limited because of the fact that there is slender option of inorganic cations, which can stabilize the desired mixed halide perovskite structure . Recently, Kanatzidis and co‐workers synthesized single crystal of Cs 3 Bi 2 I 6 Cl 3 , the latest member of (111)‐oriented mixed halide 2D perovskite, which is comprising of corner‐shared [BiCl 6/2 I 3 ] 3− octahedra by capping the bilayers with I atoms along with Cl atoms occupied at bridging positions and isostructural to Cs 3 Bi 2 Br 9 and α‐Cs 3 Sb 2 I 9 , as represented in Figure a.…”

Low-dose real-time X-ray imaging with nontoxic double perovskite scintillators

“…Fortunately, previous efforts have discovered many efficient lead-free emitters, e.g., double-perovskite [30][31][32] , copper [33][34][35] and bismuth (Bi) [36][37][38] -based metal halides, which hold potential for X-ray scintillators. Very recently, Rb 2 CuBr 3 33 and Cs 2 NaTbCl 6 39 were shown to be scintillators with high light yield.…”

All‐inorganic Sn‐based Perovskite Solar Cells: Status, Challenges, and Perspectives

“…To seek for efficient Pb-free perovskites, researchers have studied Sn-based, [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Ge-based, [29,30] Bi-based, [31][32][33][34] Sbbased, [31,32,35] and double metal-based perovskites. [31,[36][37][38] However, after several years' development, Pb-free PSCs still encountered with poor photovoltaic performance due to the intrinsic disadvantages of Pb-free perovskites.…”