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

Abstract

Hybrid CPbX₃ (C: Cs, CH₃ NH₃ ; 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 C₂ ABX₆ double perovskites based on alternating corner-shared AX₆ 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 AX₆ and BX₆ octahedra with the general formula A_a B_b X_x (x=a+3 b) such as Ag₃ BiI₆ , Ag₂ BiI₅ , AgBiI₄ , AgBi₂ I₇ . 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 Rüdorff, 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 /Ag₃ BiI₆ /PTAA/Au (FTO: fluorine-doped tin oxide, PTAA: poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], c: compact, m: mesoporous) solar cell with photoconversion efficiency of 4.3 %.

Keywords: halides; iodobismuthate; lead-free; photovoltaics; solar cells.