Optoelectronic insights of lead-free layered halide perovskites

Abstract

Two-dimensional organic-inorganic halide perovskites have emerged as promising candidates for a multitude of optoelectronic technologies, owing to their versatile structure and electronic properties. The optical and electronic properties are harmoniously integrated with both the inorganic metal halide octahedral slab, and the organic spacer layer. The inorganic octahedral layers can also assemble into periodically stacked nanoplatelets, which are interconnected by the organic ammonium cation, resulting in the formation of a superlattice or superstructure. In this perspective, we explore the structural, electronic, and optical properties of lead-free hybrid halides, and the layered halide perovskite single crystals and nanostructures, expanding our understanding of the diverse applications enabled by these versatile structures. The optical properties of the layered halide perovskite single crystals and superlattices are a function of the organic spacer layer thickness, the metal center with either divalent or a combination of monovalent and trivalent cations, and the halide composition. The distinct absorption and emission features are guided by the structural deformation, electron-phonon coupling, and the polaronic effect. Among the diverse optoelectronic possibilities, we have focused on the photodetection capability of layered halide perovskite single crystals, and elucidated the descriptors such as excitonic band gap, effective mass, carrier mobility, Rashba splitting, and the spin texture that decides the direct component of the optical transitions.

Fig. 1. Schematic representation of the transformation from the 3D to 2D layered halide perovskites along (100), and their existence as single crystals, and NPLs (top panel). Basic structural classification of the layered halide perovskite into major categories: RP, DJ, ACI, HLDP RP, and HLDP DJ (bottom panel).

Fig. 2. Schematic representations of the optoelectronic descriptors of 2D halide perovskites. (a) Variation of band gap with change in A, B, X sites; structural rearrangement via ligand or metal engineering; octahedral tilting; lattice contraction and variation of n value. (b) Variation of dielectric constant from 3D to 2D layered structure, and changes in spacer layer thickness. (c) Carrier mobility through a polycrystalline film in the photodetector device (top), and dark current–voltage curve by SCLC measurement, where the calculated trap-state voltage (V_TFL) is directly related to the hole mobility (μ). (d) Rashba splitting, dictated by the strength of the Rashba SOC (RSOC) in the system: α_R, and the energy splitting of the sub-bands: Δk_R = 2α_Rm*/ℏ². (e) Variation of spin texture with the spacer layer thickness in DJ (xNx)SnBr₄ (x = 8N8, 6N6, 4N4). The z-component of the spin (colour bar on the right) is associated with the highest occupied band, and the lowest unoccupied band along the X–Γ–Y symmetry direction (top). Dependence of the direct or indirect transition probability on the spin texture including the spacer layer thickness and nature of ammonium cation (RP vs. DJ) (bottom). Reproduced with permission from ref. , copyright (2023) American Chemical Society of Chemistry.

Fig. 3. (a) UV-vis absorption, and (b) PL spectra of (8N8)SnX₄ (X = Br⁻ and/or I). (c) Bar plots showing the ambient-stability of PLQY between the fresh and 30 days stored DJ (8N8)SnBr₄ and RP (8N)₂SnBr₄ samples under relative humidity of 55% and 25 °C, at different λ_ex. (d) Digital images of the corresponding DJ (8N8)SnBr₄ and RP (8N)₂SnBr₄ crystals showing better ambient stability of the DJ perovskite. (e) UV-vis absorption and (f) PL spectra at λ_ex = 360 nm for xNx-Br/I (x = 8N8, 6N6, 4N4) Sn-mixed halide DJ perovskites. Reproduced with permission from ref. and , copyright (2022 & 2023) American Chemical Society of Chemistry.

Fig. 4. (a) PL spectra of (8N8)SnBr₄ in the range 5 to 150 K, at λ_ex = 380 nm. Schematics of (b) the energy level diagram demonstrating the BE and STE emission, and (c) the exciton self-trapping in the perovskite lattice. (d) FWHM (in eV) as a function of temperature at λ_ex of 360 and 380 nm for (8N8)SnBr₄. (e) Schematics showing the broad emission through the sub-band gap states, which are present in a system with stereochemically active ns² lone pairs, along with a parallel phenomenon of J–T distortion, together promoting the STE. Reproduced with permission from ref. , copyright (2022) American Chemical Society of Chemistry.

Fig. 5. Schematic representation of (a) the octahedra with M(I) and M(III) centers in the inorganic layer of double perovskite, and HLDP, and (b) the corresponding octahedral tilting. (c) Variations of bond length (in Å) and bond angle (in degrees) in (4N4)₂AgBiBr₈, (4N4)₂AgSbBr₈, (6N6)₂AgBiBr₈, and (6N6)SbBr₅. Optical absorption, and PL spectra of (d) (4N4)₂AgBiBr₈, and (4N4)₂AgSbBr₈, and (e) (6N6)₂AgBiBr₈, and (6N6)SbBr₅. (f) Schematic for the formation of LDPs from single layered 〈111〉 oriented perovskites. Reproduced with permission from ref. , copyright (2023) Royal Society of Chemistry.

Fig. 6. (a) Schematic showing the transformation of 3D FAPbI₃ NCs to 2D RP superlattice of L₂FA_n−1[Sn_xPb_1−x]_nI_3n+1 (x ≥ 0.011), and the TEM analysis of S2.2-T NPL superlattice. Sequentially magnified TEM images show the superlattice structure, and its correlation with Λ = d_IB + d_OS = 4.35 ± 0.06 nm. IB and OS represent the inorganic block with n = 2, and the organic spacer, respectively. (b) PXRD patterns with increasing Sn/Pb ratio, illustrating the gradual phase change from cubic NCs to 2D RP NPLs. (c) TEM image, and AFM image (inset) for the S2.2-T superlattice. (d) TEM images and schematics showing the NPL stacking with (i), and without (ii) edge offset. FFT mask filter analysis of the NPLs with (e) edge offset stacking, and (f) perfect stacking. The lower left insets show the model crystal lattice of a single NPL, and upper left insets demonstrate the stacking of four lattice planes. (g) Optical absorbance spectra, and (h) PL spectra (λ_ex = 510 nm) of S0-T, S0.3-T, S1.1-T, S2.2-T and S4.0-T. (i) XRD patterns of the RP NPLs (x = 0.022, n = 2) with hexylamine and octylamine as the spacer. Reproduced with permission from ref. , copyright (2023) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 7. (a) Schematic demonstration of the out-of-plane and in-plane photodetector devices. Statistical bar diagrams of (b) R and EQE (%) of the HLDP structures, and (c) R and D of 1D 4N4-BiI, and 4N4-SbI. (d) Wavelength dependant variation of R, and statistical bar diagram of R and D under (e) potential-biased, and (f) self-powered conditions for the DJ (xNx)Sn(Br/I)₄ (x = 8N8, 6N6, 4N4). (g) The spatially resolved charge density corresponding to the CBM of 4N4-Br, 6N6-Br, and 8N8-Br. The bond lengths of the inorganic network formed by the Sn atoms are also shown. Reproduced with permission from ref. , copyright (2023) Royal Society of Chemistry, and ref. , copyright (2023) American Chemical Society of Chemistry.

Vishwadeepa Hazra

Arnab Mandal

Sayan Bhattacharyya