Chemically Localized Resonant Excitons in Silver–Pnictogen Halide Double Perovskites

Abstract

Halide double perovskites with alternating silver and pnictogen cations are an emerging family of photoabsorber materials with robust stability and band gaps in the visible range. However, the nature of optical excitations in these systems is not yet well understood, limiting their utility. Here, we use ab initio many-body perturbation theory within the GW approximation and the Bethe–Salpeter equation approach to calculate the electronic structure and optical excitations of the double perovskite series Cs 2 AgBX 6 , with B = Bi 3+ , Sb 3+ and X = Br – , Cl – . We find that these materials exhibit strongly localized resonant excitons with energies from 170 to 434 meV below the direct band gap. In contrast to lead-based perovskites, the Cs 2 AgBX 6 excitons are computed to be non-hydrogenic with anisotropic effective masses and sensitive to local field effects, a consequence of their chemical heterogeneity. Our calculations demonstrate the limitations of the Wannier–Mott and Elliott models for this class of double perovskites and contribute to a detailed atomistic understanding of their light–matter interactions.

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

“…58 Excitonic effects in lead-based HP instead arise from quantum confinement, as in the case of 2D nanoarchitectures, where breaking the perovskite motif into atomically thin perovskite sheets (and unavoidable dielectric contrast between the organic and the inorganic components 59 ) gives rise to binding energies as large as 450 meV for the thinnest n = 1 layered lead iodide perovskites. 59 In contrast, recent theoretical estimates of the exciton binding energy for the lead-free Cs 2 AgBiBr 6 reference compound amounted to 104, 60 170, 20 and 340 meV, 21 therefore raising the question about the role of quantum confinement in materials already featuring such strong excitonic character in 3D. To address this point, we first extracted qualitative estimates of the exciton binding energy for the investigated 3D and layered double perovskite material, from a Tauc plot and Elliot fit of the corresponding absorption spectrum, similarly to ref ( 68 ) (see the Supporting Information ).…”

“… 21 In contrast, excitonic properties are generally of interest for light emission, as the strong electron–hole interaction results in enhanced radiative recombination mechanisms, supporting their extended application beyond PV, in light-emitting diodes, 22 , 23 photodetectors, 24 , 25 and lasers, 26 , 27 to name a few. In this context, it is important to recall that light emission from the 3D Cs 2 AgBiBr 6 has been observed, 12 , 14 , 15 , 20 , 28 despite its native indirect band gap, usually hindering radiative recombination pathways. Light emission from this material is also characterized by broad lines (full width half-maximum, fwhm, of 400 meV) 29 and Stokes shifts almost reaching 1 eV, 30 which strongly contrast with narrow (fwhm 20–35 meV) 31 and weakly Stokes shifted (40 meV) 32 emissions typical of 3D lead-HP, thus opening the door to the integration of these materials in white color emitter devices.…”

Understanding the Role of Grain Boundaries on Charge‐Carrier and Ion Transport in Cs2AgBiBr6 Thin Films

“…Some previous studies attributed this peak to a resonant exciton associated with the first direct transition of Cs 2 AgBiBr 6 . [15,31,32] There are also calculations showing that this peak could be due to a narrow distribution of states with a direct transition above the indirect band-edges and which have high oscillator strength. [28] Consistent with the GSB peak at 440 nm being due to excitons, the two PIA signals on either side of the main GSB peak (440 nm) are attributed to the broadening of the direct exciton transition introduced by carrier-exciton scattering.…”

Formamidinium dopant effects on double perovskite Cs2AgBiBr6

“…Meanwhile, due to the tiny effects of FA orientation on energy band dispersion, the electron (hole) effective masses differences among these FA‐doped systems are 0.08 (0.04) m e . The calculated exciton binding energy 93 meV of pristine Cs 2 AgBiBr 6 is in the reported experimental range between 70 and 268 meV [45]. The exciton binding energies are 108, 101, 97, 105, and 105 meV for doped systems with FA 1 , FA 2 , FA 3 , FA 4 , and FA 5 configurations, respectively.…”