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Zero-dimensional (0D) halides perovskites, in which anionic metal-halide octahedra (MX 6 ) 4− are separated by organic or inorganic countercations, have recently shown promise as excellent luminescent materials. However, the origin of the photoluminescence (PL) and, in particular, the different photophysical properties in hybrid organic–inorganic and all inorganic halides are still poorly understood. In this work, first-principles calculations were performed to study the excitons and intrinsic defects in 0D hybrid organic–inorganic halides (C 4 N 2 H 14 X) 4 SnX 6 (X = Br, I), which exhibit a high photoluminescence quantum efficiency (PLQE) at room temperature (RT), and also in the 0D inorganic halide Cs 4 PbBr 6 , which suffers from strong thermal quenching when T > 100 K. We show that the excitons in all three 0D halides are strongly bound and cannot be detrapped or dissociated at RT, which leads to immobile excitons in (C 4 N 2 H 14 X) 4 SnX 6 . However, the excitons in Cs 4 PbBr 6 can still migrate by tunneling, enabled by the resonant transfer of excitation energy (Dexter energy transfer). The exciton migration in Cs 4 PbBr 6 leads to a higher probability of trapping and nonradiative recombination at the intrinsic defects. We show that a large Stokes shift and the negligible electronic coupling between luminescent centers are important for suppressing exciton migration; thereby, enhancing the photoluminescence quantum efficiency. Our results also suggest that the frequently observed bright green emission in Cs 4 PbBr 6 is not due to the exciton or defect-induced emission in Cs 4 PbBr 6 but rather the result of exciton emission from CsPbBr 3 inclusions trapped in Cs 4 PbBr 6 . 

All‐inorganic metal halides such as Cs₄PbX₆and CsPbX₃(X = Cl, Br, and I) are attracting global attention owing to their promise in optoelectronic applications. However, the presence of the toxic heavy metal lead (Pb) in these materials is a major concern. Here, a family of nontoxic high‐efficiency blue‐emitting all‐inorganic halides Rb₂CuX₃(X = Br and Cl) is reported; the compounds exhibit 1D crystal structures featuring anionic²⁻ribbons separated by Rb⁺cations. The measured record high photoluminescence quantum yield values range from 64% to 100% for Rb₂CuBr₃and Rb₂CuCl₃, respectively. Furthermore, the measured emission linewidths are quite narrow with full width at half maximum values of 54 and 52 nm for Rb₂CuBr₃and Rb₂CuCl₃, respectively. Single crystals of Rb₂CuCl₃demonstrate an anti‐Stokes photoluminescence signal, shown for the first time for Pb‐free metal halides. The discovery of highly efficient narrow blue emitters based on a nontoxic and inexpensive metal copper paves a way for the consideration of low‐cost and environmentally friendly copper halides for practical applications.