Perovskite photovoltaics (PV) is becoming a competitive technology to CIGS, CdTe and crystalline Si solar cells, especially after addressing its key issues, such as hysteresis [1], stability [2], and scalability [3]. However, the issue of Pb toxicity still remains an obstacle that can hinder potential of perovskite PV. According to the WHO, the blood Pb level for children should not exceed 5 mg/L, which is the amount of Pb contained in only 5x5 mm² of a perovskite absorber layer. Although, Pb-based perovskite PV does not lose its advantages for solar power stations located in non-residential areas, the neurotoxicity of Pb should be considered extremely seriously in the case of building integrated perovskite PV systems. For that reason, the development of non-toxic Pb-free halide semiconductors for photovoltaic applications is highly desirable.

In my presentation I am going to review recent progress in Bi-based photovoltaic rudorffites. In contrast to double-perovskites C₂ABX₆ (C=Cs, MA; A=Ag, Cu; B=Bi, Sb; X=Br, I) consisting of corner shared AX₆ and BX₆ octahedra and hybrid C₃B₂X₉ halides, such as MA₃Bi₂I₉, consisting of isolated face-shared B₂X₉ bioctahedra, the family of rudorffite materials with a general formula of A_aB_bX_x (A=Ag, Cu; B=Bi, Sb; X=Br, I, and x=a+3b) has a structural motif based on edge-shared AX₆ and BX₆. Several members of this family, such as Ag₃BiI₆, Ag₂BiI₅, AgBiI₄, AgBi₂I₇, have been extensively studied for solar cell applications during the last three years [4,5]. This family of materials was named rudorffites, after Walter von Rudorff, who discovered their prototype oxide NaVO₂. Rudorffites feature direct bandgaps in the range of 1.76-1.83 eV corresponding to possible PCE of up to 18% by assuming Voc of 1.2V, typical optical losses and FF for optimized solar cells. It turns out, however, that rudorffite solar cells do not follow the rapid PCE improvement trend that we observed for Pb-based hybrid perovskites. In contrast to electronically clean hybrid perovskites, where native point defects usually do not create mid-gap levels acting as recombination centers, the control of electronically active point defects in rudorffite absorbers is highly important issue as it is in the case of GIGS and CZTS. I am also going to highlight my recent achievements in fabrication of morphologically perfect rudorffite layers through iodination of Ag-Bi bimetallic films and discuss further technological improvements that can help to reveal full potential of rudorffite solar cells.