Revealing Weak Dimensional Confinement Effects in Excitonic Silver/Bismuth Double Perovskites

Abstract

Lead-free perovskites are attracting increasing interest as nontoxic materials for advanced optoelectronic applications. Here, we report on a family of silver/bismuth bromide double perovskites with lower dimensionality obtained by incorporating phenethylammonium (PEA) as an organic spacer, leading to the realization of two-dimensional double perovskites in the form of (PEA) 4 AgBiBr 8 ( n = 1) and the first reported (PEA) 2 CsAgBiBr 7 ( n = 2). In contrast to the situation prevailing in lead halide perovskites, we find a rather weak influence of electronic and dielectric confinement on the photophysics of the lead-free double perovskites, with both the 3D Cs 2 AgBiBr 6 and the 2D n = 1 and n = 2 materials being dominated by strong excitonic effects. The large measured Stokes shift is explained by the inherent soft character of the double-perovskite lattices, rather than by the often-invoked band to band indirect recombination. We discuss the implications of these results for the use of double perovskites in light-emitting applications.

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

“…Interestingly, for (PEA) 4 AgBiBr 8 a similar emission shape was observed for spectra recorded at 300 and 80 K, with a redshift or an emerging feature at lower energies 80 K. [26] We further notice a difference between the reported absorption with its maximum at 3.2 eV and our measurement with its absorption maximum at 3 eV. Such prominent Stokes shifts are also commonly reported for Cs 2 AgBiBr 6 , where exact values can depend on the measurement technique, as well as potentially on the morphology of the measured thin films (See Figure S16, Supporting Information, showing the flat and homogeneous surface morphology observed for the samples, compared to the dendritic or needle-shaped morphology of (PEA) 4 AgBiBr 8 thin films reported in [26] ).…”

“…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.…”

“…[ 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.…”

“…In the following, we compare the PL data collected as part of our study of (4FPEA) 4 AgBiBr 8 and (4FPEA) 4 AgBiI 8 with the available data reported in the literature for (PEA) 4 AgBiBr 8 , [ 26 ] (BA) 4 AgBiBr 8 , [ 49 ] and (iBA) 4 AgBiBr 8 . [ 49 ] While the excitation energies and the reported PL ranges differ depending on the employed setup and the publication, they share similar overall features.…”

“…[16] The currently most studied double perovskite is Cs 2 Ag-BiBr 6 , mostly owing to its promising charge-carrier properties and its stability, despite its relatively large band gap of 2.0-2.3 eV. [17][18][19][20][21][22] Following this interest, more recently the effects of the dimensional reduction from 3D to 2D phases of Cs 2 AgBiBr 6 have been reported by several groups by elaborating the theme in the form of (BA) 4 AgBiBr 8 , (BA) 2 CsAgBiBr 7 (BA = butylammonium), [23] (PEA) 4 AgBiBr 8 (PEA = phenethylammonium), [24][25][26] (iPA) 2 CsAgBiBr 7 (iPA = isopentylammonium), [27] (PA) 2 CsAgBiBr 7 (PA = propylammonium), (PA) 4 AgBiBr 8 , (OCA) 4 AgBiBr 8 (OCA = octylammonium), and (BDA) 2 AgBiBr 8 (BDA = 1,4-butyldiammonium). [28] As a way of tuning the band gap of halide perovskites, it is known that changing the halide anion decreases its size in the order of Cl -> Br -> I -.…”