(CH₃ NH₃ )AuX₄ ⋅H₂ O (X=Cl, Br) and (CH₃ NH₃ )AuCl₄ : Low-Band Gap Lead-Free Layered Gold Halide Perovskite Materials

Perovskite solar cells have recently enabled power conversion efficiency comparable to established technologies such as silicon and cadmium telluride. Ongoing efforts to improve the stability of halide perovskites in ambient air has yielded promising results. However, the presence of toxic heavy element lead (Pb) remains a major concern requiring further attention. Herein, we report three new Pb-free hybrid organic-inorganic perovskite-type halides based on gold (Au), (CH₃ NH₃ )AuBr₄ ⋅H₂ O (1), (CH₃ NH₃ )AuCl₄ ⋅H₂ O (2), and (CH₃ NH₃ )AuCl₄ (3). Hydrated compounds 1 and 2 crystallize in a brand-new structure type featuring perovskite-derived 2D layers and 1D chains based on pseudo-octahedral AuX₆ building blocks. In contrast, the novel crystal structure of the solvent-free compound 3 shows an exotic non-perovskite quasi-2D layered structure containing edge- and corner-shared AuCl₆ octahedra. The use of Au metal instead of Pb results in unprecedented low band gaps below 2.5 eV for single-layered metal chlorides and bromides. Moreover, at room temperature the three compounds show a weak blue emission due to the electronic transition between Au-6s and Au-5d, in agreement with the density function theory (DFT) calculation results. These findings are discussed in the context of viability of Au-based halides as alternatives for Pb-based halides for optoelectronic applications.