Rb₄Ag₂BiBr₉: A Lead-Free Visible Light Absorbing Halide Semiconductor with Improved Stability

Abstract

Replacement of the toxic heavy element lead in metal halide perovskites has been attracting a great interest because the high toxicity and poor air stability are two of the major barriers for their widespread utilization. Recently, mixed-cation double perovskite halides, also known as elpasolites, were proposed as an alternative lead-free candidate for the design of nontoxic perovskite solar cells. Herein, we report a new nontoxic and air stable lead-free all-inorganic semiconductor Rb 4 Ag 2 BiBr 9 prepared using the mixedcation approach; however, Rb 4 Ag 2 BiBr 9 adopts a new structure type (Pearson's code oP32) featuring BiBr 6 octahedra and AgBr 5 square pyramids that share common edges and corners to form a unique 2D layered non-perovskite structure. Rb 4 Ag 2 BiBr 9 is also demonstrated to be thermally stable with the measured onset decomposition temperature of T o = 520 °C. Optical absorption measurements and density functional theory calculations suggest a nearly direct band gap for Rb 4 Ag 2 BiBr 9 . Room temperature photoluminescence (PL) measurements show a broadband weak emission. Further, temperature-dependent and power-dependent PL measurements show a strong competition between multiple emission centers and suggest the coexistence of defect-bound excitons and self-trapped excitons in Rb 4 Ag 2 BiBr 9 .

Rb2CuX3 (X = Cl, Br): 1D All‐Inorganic Copper Halides with Ultrabright Blue Emission and Up‐Conversion Photoluminescence

“…The presence of monovalent Cu + cations in these materials and their ionic nature makes them susceptible to degradation in ambient air (Figures S4–S6, see Supporting Information for further details) However, larger single crystals of Rb 2 CuCl 3 do not exhibit any noticeable degradation when left in ambient air (Figure S6, Supporting Information). On the other hand, our thermogravimetric analysis (TGA) measurements suggest that unlike most hybrid organic–inorganic lead halide perovskites, Rb 2 CuX 3 show no significant weight loss up to 475 °C (Figure S7, Supporting Information), which is in agreement with the reports of improved thermal stability of all‐inorganic metal halides such as Cs 3 Cu 2 Br 5− x I x ,7a Rb 4 Ag 2 BiBr 9 , and Cs 2 SnI 6 . Differential scanning calorimetry (DSC) data show that Rb 2 CuCl 3 and Rb 2 CuBr 3 have peritectic decompositions at 274 and 271 °C, respectively, consistent with the reported phase diagrams …”

“…Results show that Rb 2 CuX 3 compounds have excitation‐independent emission shapes and a linear dependence of the PL intensity as a function of excitation power. This fact suggests the intrinsic nature of Rb 2 CuX 3 blue emission, and the absence of saturation at high excitation power excludes the presence of permanent defects emissions . Therefore, we attribute this intense blue‐emission to STEs often observed in metal halide all‐inorganic systems 7a,14,19,21.…”

“…This fact suggests the intrinsic nature of Rb 2 CuX 3 blue emission, and the absence of saturation at high excitation power excludes the presence of permanent defects emissions . Therefore, we attribute this intense blue‐emission to STEs often observed in metal halide all‐inorganic systems 7a,14,19,21. Based on our single exponential fitting of the time‐resolved PL data for Rb 2 CuCl 3 single crystals (Figure S13, Supporting Information), a decay lifetime of 12.21 µs was extracted.…”

Ultrafast Excited-State Localization in Cs2AgBiBr6 Double Perovskite

“…The phenomenon of self-trapping occurs when the local lattice distortion caused by a photoexcited charge carrier is sufficiently strong that the charge carrier rapidly relaxes into the energetic state associated with this local deformation, 27 such that its localization length may approach the length of a single unit cell of the lattice. 28 Self-trapping of charge carriers has been reported in related materials, whether for electrons in CsPbI 3 29 or for holes in CsPbBr 3 , 30 in other bismuth-based materials such as Rb 4 Ag 2 BiBr 9 31 and Cs 3 Bi 2 Br 9 , 32 and layered metal halide perovskites. 33 For Cs 2 AgBiBr 6 , the proposed self-trapping 34 has also been synonymously 35 described as the formation of small polarons 24 or color centers, 25 with broad photoluminescence (PL) emission 25 , 34 and low charge-carrier mobility 24 attributed to its occurrence.…”

(CH3NH3)AuX4⋅H2O (X=Cl, Br) and (CH3NH3)AuCl4: Low‐Band Gap Lead‐Free Layered Gold Halide Perovskite Materials

“…As the structural dimensionality is lowered, the size constraints as dictated by the tolerance factor are gradually lifted leadingt oawiders election of A, organic cations,a nd M, inorganic cations including divalent metal ions such as Pb 2 + ,S n 2 + , Cd 2 + ,Z n 2 + ,P d 2 + and Co 2 + or trivalent cationss uch as Sb 3 + [20] and Bi 3 + . [21] However, while dimensionalr eduction enables richer chemistry,going from the 3D parent perovskite structure to 2D layered, 1D chain and 0D cluster structures result in progressivelyw ider band gaps, [1b] which is undesirable for solar cell applications.I ndeed, while there is still as trong interesti n 2D layered halide perovskites as potential solar cell absorbers, [22] challenges of 2D layered compounds such as notably higherb and gaps and poorer transport properties must be addressed.…”