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)₄AgBiBr₈ (n = 1) and the first reported (PEA)₂CsAgBiBr₇ (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 Cs₂AgBiBr₆ 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.

Figure 1

(a, b) Single crystals and crystal structures for 3D Cs₂AgBiBr₆ (n = ∞) halide perovskites and layered PeABr2 (n = 2) and PeABr1 (n = 1) compounds, respectively, as refined from XRD measurements; (c) XRD patterns for PeABr2 (n = 2) and PeABr1 (n = 1) compounds.

Figure 2

(a) Lateral and (b) top views of a lead-free double perovskite octahedral structure of n = 1 thickness displaying the β and δ tilted angles. (c) Lateral view of an n = 2 perovskite representing the off-centering of the metal from the plane of the equatorial halides.

Figure 3

(a) UV–vis absorption spectra of PeABr1, PeABr2, and 3D Cs₂AgBiBr₆ double perovskites measured on thin films at room temperature. (b) Corresponding band structures as obtained from DFT simulations, based on the standard PBE exchange-correlation functional, including SOC. (c) Weighted contributions in the reciprocal space to the lowest energy, dipole-allowed excited state for the investigated compounds, as obtained from an ab initio solution of the Bethe–Salpeter equation.

Figure 4

(a–c) Photoluminescence spectra of Cs₂AgBiBr₆ (a), PeABr1 (b), and PeABr2 (c), as measured on thin films at 80 K and at 300 K. The dashed lines indicate the UV–vis absorption spectra at the corresponding temperature. (d) PL quantum-yield measurement at room temperature.

Figure 5

Raman spectra measured on thin films of the 3D Cs₂AgBiBr₆ double perovskite and layered PeABr2 (n = 2) and PeABr1 (n = 1) compounds. Measurements were performed at room temperature using a 532 nm laser.

Figure 6

(a) Displacement pattern for the computed vibrational modes with frequencies closer to the most intense Raman bands in Figure 5. (b) Corresponding ground (black)- and excited-state (red) PESs along the normal mode in (a). The Stokes shift was obtained from the difference between the absorption energy (E_abs) and the emission energy (E_em), with the total relaxation energy ΔE given as E_abs – E_em. (c) Contribution from all vibrational modes to the total relaxation energy ΔE.