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Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs AgBiBr ) solar cells using the planar structure are demonstrated. The prepared Cs AgBiBr films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community hold different views towards the toxic lead in these high-performance perovskites, it is certainly preferred to replace lead with non-toxic, or at least less-toxic, elements while maintaining the superior properties. Here, we present the design rules for lead-free perovskite materials with structural dimensions from three dimension (3D) to zero dimension (0D). We review recent progress on lead-free halide perovskites, and summarize the relationships between the structures and fundamental properties, in particular optical, electric, magnetic-related properties.3D perovskites, especially A2B + B 3+ X6-type double perovskites, demonstrate very promising optoelectronic prospects; while low-dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic and multifunctional applications.Furthermore, based on these structure-property relationships, strategies on multifunctional perovskite design is proposed. We also highlight the challenges and future directions on leadfree perovskite applications, with emphasis on materials development and device fabrications.The research on lead-free halide perovskites at Linköping University benefits from inspirational discussions with Prof. Olle Inganäs. We would also like to take this opportunity to congratulate his great achievements on a range of different areas throughout his scientific career. coordination with Xanions, so that Sn and X can form BX6 octahedra easily, resulting in highly symmetric 3D structures. [19] On the contrary, in Cu + -based double perovskite Cs2Cu + Bi 3+ Br6, Cu + cations energetically favor 4-fold coordination and form the CuX4 tetrahedra, instead of 6fold coordination as required for CuX6 octahedra. [20] The coordination tendency explains why Cu + -based double perovskite materials (such as Cs2Cu + Bi 3+ Br6, Cs2Cu + In 3+ Cl6 etc.) are unstable.According to the above three key factors, we present the design rules for lead-free perovskites. We start with the classical 3D AB 2+ X3 perovskites and derive a wide range of leadfree perovskites, using the B-site, A-site, and A+B-site substitution strategies. Note that we do not include the X-site substitution in this paper. Although X-site substitution can affect the properties of perovskites (e.g. bandgap, stability, etc.), it does not result in much difference of the structure. We now discuss the different substitution strategies in detail. i) B-site substitution.All the three factors, ionic radius, ionic valence and coordination type, can be possibly changed after B-site substitution, providing wide structural flexibility. As a result, eight different types2X12, 3D A2B 4+ X6, 3D A4B 3+ B 5+ X12, have been derived. The structure dimensions, which strongly relate to their fundamental properties, range from 3D to 0D. ii) Asite substitution. Both of the ionic radius and the ionic...