Large Cation Engineering in Two-Dimensional Silver–Bismuth Bromide Double Perovskites

Abstract

Double perovskites are promising candidates for less toxic and highly stable metal halide perovskites, but their optoelectronic performances still lag behind those of the lead halide counterpart, due to the indirect nature of the bandgap and the strong electron–phonon coupling. Reducing the dimensionality of Cs2AgBiBr6 down to a 2D layered form is strategic in order to tune the band gap from indirect to direct and provides new insights into the structure–property relationships of double perovskites. Herein, we report on a series of monolayer 2D hybrid double perovskites of formula (RA)4AgBiBr8, where RA represents different primary ammonium large cations with alkyl- and aryl-based functionalities. An in-depth experimental characterization of structure, film morphology, and optical properties of these perovskites is carried out. Interestingly, the variation of the ammonium cation and the interplanar distance between adjacent inorganic monolayers has peculiar effects on the film-forming ability and light emission properties of the perovskites. Experiments have been combined with DFT calculations in order to understand the possible origin of the different emissive features. Our study provides a toolbox for future rational developments of 2D double perovskites, with the aim of narrowing the gap with lead halide perovskite optoelectronic properties.

Silver‐Bismuth Based 2D Double Perovskites (4FPEA)4AgBiX8 (X = Cl, Br, I): Highly Oriented Thin Films with Large Domain Sizes and Ultrafast Charge‐Carrier Localization

“…Although substantial advances have been made recently by Schmitz et al and Pantaler et al elucidating the emissive features and charge-carrier dynamics of Ag-Bi-Br based 2D double perovskites, a comprehensive view is still lacking. [26,49,55,57,58] Here, we observe similarities owing to the related Ag-Bi double perovskite system, but also some differences between the perovskites with halides bromide and iodide. Photoluminescence: Photoluminescence (PL) measurements on thin films show weak intensities and very broad emissions with a FWHM (full width half maximum) of 300 meV for the iodide and 320 meV for the bromide at room temperature (Figure 8a,b).…”

“…This horizontal orientation is typical for single-layered n = 1 2D double perovskite phases and so far, no other orientations have been prepared. [48,49] Furthermore, while for Pb-and Sn-based 2D perovskites higher n phases and vertical orientations have been reported, for Ag-Bi-based double perovskite systems (BA) 2 CsAgBiBr 7 , (iPA) 2 CsAgBiBr 7 and (PEA) 2 CsAgBiBr 7 remain the only known ones exhibiting the n = 2 thickness, though no thin films have been reported thus far. [23,26,27] While other studies have focused on initial thin film syntheses for n = 1 2D double perovskites, [33,34,43,47] here we have optimized the morphology of the thin films on different substrates and were able to observe structural differences depending on the choice of substrate roughness, the different halides, and annealing temperature.…”

“…The characteristic broad emission for these systems is also observed in other 2D silver-bismuth-halide, 2D lead-halide and 3D silver-bismuth-halide systems. [23,45,49,55,59] In Cs 2 AgBiBr 6 , it is generally attributed to substantial electronphonon coupling, leading to self-trapping or localization of the charge carriers or excitons. These self-trapped charge carriers subsequently diffuse to color centers and recombine, leading to the broad and strongly red-shifted PL.…”

2D/3D Hybrid Cs2AgBiBr6 Double Perovskite Solar Cells: Improved Energy Level Alignment for Higher Contact-Selectivity and Large Open Circuit Voltage

“…[67] Additionally, spin-orbit coupling was included to treat the heavy Bi atom. The structure, starting from the single crystal data of Schmitz et al, [49] was optimized until all residual forces on the nuclei were below 1.0 × 10 -3 a.u. Thereby, the following equilibrium unit cell parameters were obtained: 2D Grazing-incidence wide angle X-ray scattering (GIWAXS): GIWAXS measurements were carried out on a Anton-Paar Saxspoint 2.0 with a Primux 100 microfocus source with Cu-K α1 radiation (λ = 1.5406 Å) and a Dectris Eiger R 1M 2D Detector.…”

“…spacer cation by spincoating phenethyl ammonium bromide (PEABr) on top, which is well known to form a 2D silver-bismuth perovskite phase. [47][48][49] We tested the hybrid perovskite formation with different PEABr concentrations in 2-propanol (isopropanol, IPA) ranging from 0.01 mol L -1 (0.01 m) to 0.1 m.…”