Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(II) / Sn(IV) doping

Abstract

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs3Bi2Br9, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr2 to the solution-phase synthesis of Cs3Bi2Br9 leads to substitution of up to 7% of the Bi(III) ions by equal quantities of Sn(II) and Sn(IV). The nature of the substitutional defects was studied by X-ray diffraction, 133 Cs and 119 Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(II) and Sn(IV) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(II) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(II) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structureproperty relationships of lead-free vacancy-ordered perovskite structures.

Frenkel Excitons in Vacancy-ordered Titanium Halide Perovskites (Cs₂TiX₆)

“…Strategies to replace the divalent B-site cation in halide perovskites, while retaining the BX 6 octahedral motif, have led to the exploration of A 2 BB'X 6 double perovskites with a pair of monovalent and trivalent cations at the B and B' sites, 1,5,6 as well as the A 3 B 2 X 9 'vacancy-ordered perovskites', where a trivalent B cation is combined with a 1/3 vacancy of the B-site to satisfy electroneutrality. [7][8][9] Issues of indirect and/or large band gaps in these materials has led to the emergence of A 2 BX 6 vacancy-ordered double perovskites (VODPs), where now the combination of a tetravalent cation and a 50 % vacancy of the B site is employed, giving a checkerboard arrangement (Fig. 1).…”

Frenkel Excitons in Vacancy-ordered Titanium Halide Perovskites (Cs₂TiX₆)

“…Strategies to replace the divalent B-site cation in halide perovskites, while retaining the BX 6 octahedral motif, have led to the exploration of A 2 BB'X 6 double perovskites with a pair of monovalent and trivalent cations at the B and B' sites, 1,4,5 as well as the A 3 B 2 X 9 'vacancyordered perovskites', where a trivalent B cation is combined with a 1/3 vacancy of the B-site to satisfy electroneutrality. [6][7][8] Issues of indirect and/or large band gaps in these materials has led to the emergence of A 2 BX 6 vacancy-ordered double perovskites (VODPs), where now the combination of a tetravalent cation and a 50 % vacancy of the B site is employed, giving a checkerboard arrangement (Fig. 1).…”

Identifying the ground state structures of point defects in solids

“…The standard modelling approach, based on local optimisation of a defect containing crystal, is prone to miss the true ground state atomic arrangement, however. The chosen initial configuration, which is often initiated as a vacancy/ substitution/interstitial on a known crystal site (Wyckoff position) and all other atoms retaining their typical lattice positions, may lie within a local minimum or on a saddle point of the potential energy surface (PES), trapping a gradient-based optimisation algorithm in an unstable or metastable arrangement [1][2][3][4][5][6][7][8][9] as illustrated in Fig. 1.…”