Tailorable Indirect to Direct Band-Gap Double Perovskites with Bright White-Light Emission: Decoding Chemical Structure Using Solid-State NMR

Abstract

Efficient white-light-emitting single-material sources are ideal for sustainable lighting applications. Though layered hybrid lead−halide perovskite materials have demonstrated attractive broadband white-light emission properties, they pose a serious long-term environmental and health risk as they contain lead (Pb 2+ ) and are readily soluble in water. Recently, lead-free halide double perovskite (HDP) materials with a generic formula A(I) 2 B′(III)B″(I)X 6 (where A and B are cations and X is a halide ion) have demonstrated whitelight emission with improved photoluminescence quantum yields (PLQYs). Here, we present a series of Bi 3+ /In 3+ mixed-cationic Cs 2 Bi 1−x In x AgCl 6 HDP solid solutions that span the indirect to direct band-gap modification which exhibit tailorable optical properties. Density functional theory (DFT) calculations indicate an indirect− direct band-gap crossover composition when x > 0.50. These HDP materials emit over the entire visible light spectrum, centered at 600 ± 30 nm with full-width at half maxima of ca. 200 nm upon ultraviolet light excitation and a maximum PLQY of 34 ± 4% for Cs 2 Bi 0.085 In 0.915 AgCl 6 . Short-range structural insight for these materials is crucial to unravel the unique atomic-level structural properties which are difficult to distinguish by diffraction-based techniques. Hence, we demonstrate the advantage of using solid-state nuclear magnetic resonance (NMR) spectroscopy to deconvolute the local structural environments of these mixed-cationic HDPs. Using ultrahigh-field (21.14 T) NMR spectroscopy of quadrupolar nuclei ( 115 In, 133 Cs, and 209 Bi), we show that there is a high degree of atomic-level B′(III)/B″(I) site ordering (i.e., no evidence of antisite defects). Furthermore, a combination of XRD, NMR, and DFT calculations was used to unravel the complete atomic-level random Bi 3+ /In 3+ cationic mixing in Cs 2 Bi 1−x In x AgCl 6 HDPs. Briefly, this work provides an advance in understanding the photophysical properties that correlate long-to short-range structural elucidation of these newly developed solid-state white-light emitting HDP materials.

The atomic-level structure of bandgap engineered double perovskite alloys Cs2AgIn1−xFexCl6

“…[24][25][26] and/or (ii) the diamagnetic effect of substitution and lattice contraction. 16 The signals in the diamagnetic region as well as in paramagnetic spectral range around 2950 ppm, indicate multi-component nature of the DPAs, agreeing well with the PXRD results. Based on 115 In and 133 Cs ssNMR results, we conclude that the Fe 3+ ions are incorporated into the matrix replacing In 3+ ions.…”

“…However, the atomiclevel understanding of various dopants/alloys in halide DPAs is at a very early stage; recently, Michaelis and coworkers provided the rst report for the long-to short-range structural elucidation of white-light-emitting DPAs. 16 In this aspect, solid-state NMR (ssNMR), where very small chemical shis within a given nucleus type record precisely the local chemical environment of its chemically inequivalent sites, [17][18][19][20] provide a unique and powerful approach to understand the local atomic-level structures of double perovskite alloys.…”

Uncovering Halide Mixing and Octahedral Dynamics in Vacancy-Ordered Double Perovskites by Multinuclear Magnetic Resonance Spectroscopy

“…The observed broadening and shift of the resonance to higher frequency is associated with the lower site symmetry than cubic as the next-nearest neighbors in the medium-range structure (>5 Å) consists of a mixture of Br and I atoms. 49 The remaining peaks within the sets of 133 Cs resonances in Cs2SnBr1.5I4.5 shift towards higher frequency as I is substituted by Br (Figure 6c and Ta (Table S8) suggests the spectral overlap of multiple positional isomers for unique CsBrmI12−m environments that consists of a mixture of Br and I atoms in 12 positions. The 133 Cs spin-lattice relaxation times decrease linearly from 40 to 14 s with increase in number of Br attached in CsBrmI12−m (Figure 6c).…”

“…Cs2SnCl3Br3 exhibits an inhomogeneously broadened 133 Cs NMR resonance (fwhm = 1.1 kHz) due to the spectral overlap of multiple local CsClmBr12−m (m = 0−12) halide environments according to the random distribution (Table S7 and Figure S16a). 36 The 133 Cs SSB manifold is related to the local CsX12 cuboctahedral symmetry of perovskites, 49 an increase in the 133 Cs SSBs in Cs2SnCl3Br3 indicates a perturbation within the electric field gradient due to the lowering in the local cuboctahedral symmetry upon Cl and Br mixing, and hence an increase in the quadrupole coupling constant ( Figure S17). For the Cs2SnBrxI6−x series, the 133 Cs NMR resonances span across 160 ppm between 115 to −45 ppm.…”

“…However, a low-intensity (<1%) SSB was observed under slow MAS (νr = 5 kHz) for all three Cs2SnX6 parents ( Figure S4), which is associated to intrinsic defects such as halogen vacancies. 49,65 The Sn atoms occupy m3 " m site symmetry in the Cs2SnX6 structure with six nearest-neighbor halogen atoms surrounding Sn to form local SnX6 octahedral units ( Figure 1a). The Cs2SnX6 parents exhibit symmetric 119 Sn NMR resonances with no evidence of SSBs under MAS conditions (Figure 2b), consistent with the highly symmetric local SnX6 octahedral environment.…”