Photovoltaic Rudorffites: Lead-Free Silver Bismuth Halides Alternative to Hybrid Lead Halide Perovskites

Hybrid CPbX3 (C: Cs, CH3NH3; X: Br, I) perovskites possess excellent photovoltaic properties but are highly toxic, which hinders their practical application. Unfortunately, all Pb-free alternatives based on Sn and Ge are extremely unstable. Although stable and non-toxic C(2)ABX(6) double perovskites based on alternating corner-shared AX(6) and BX6 octahedra (A = Ag, Cu; B = Bi, Sb) are possible, they have indirect and wide band gaps of over 2 eV. However, is it necessary to keep the corner-shared perovskite structure to retain good photovoltaic properties? Here, we demonstrate another family of photovoltaic halides based on edge-shared AX(6) and BX6 octahedra with the general formula A(a)B(b)X(x) (x = a + 3b) such as Ag3BiI6, Ag2BiI5, AgBiI4, AgBi2I7. As perovskites were named after their prototype oxide CaTiO3 discovered by Lev Perovski, we propose to name these new ABX halides as rudorffites after Walter Rudorff, who discovered their prototype oxide NaVO2. We studied structural and optoelectronic properties of several highly stable and promising Ag-Bi-I photovoltaic rudorffites that feature direct band gaps in the range of 1.79-1.83 eV and demonstrated a proof-of-concept FTO/c-m-TiO2/Ag3BiI6/PTAA/Au (FTO: fluorine-doped tin oxide, PTAA: poly[bis(4-phenyl)(2,4,6-trimethyl-phenyl)amine], c: compact, m: mesoporous) solar cell with photoconversion efficiency of 4.3%.