The impressive rise in efficiencies of solar cells employing the three-dimensional (3D) lead-iodide perovskite absorbers APbI (A = monovalent cation) has generated intense excitement. Although these perovskites have remarkable properties as solar-cell absorbers, their potential commercialization now requires a greater focus on the materials' inherent shortcomings and environmental impact. This creates a challenge and an opportunity for synthetic chemists to address these issues through the design of new materials. Synthetic chemistry offers powerful tools for manipulating the magnificent flexibility of the perovskite lattice to expand the number of functional analogues to APbI. To highlight improvements that should be targeted in new materials, here we discuss the intrinsic instability and toxicity of 3D lead-halide perovskites. We consider possible sources of these instabilities and propose methods to overcome them through synthetic design. We also discuss new materials developed for realizing the exceptional photophysical properties of lead-halide perovskites in more environmentally benign materials. In this Forum Article, we provide a brief overview of the field with a focus on our group's contributions to identifying and addressing problems inherent to 3D lead-halide perovskites.

When the bonds of a quantum magnet are modulated with a periodic pattern, exotic quantum ground states may emerge. Newly synthesized crystalline barlowite (Cu 4 (OH) 6 FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin-1 2 kagome lattice. Our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a quantum spin liquid (QSL). The presence of interlayer spins eventually leads to novel pinwheel q=0 magnetic order. Partially Zn-substituted barlowite (Cu 3.44 Zn 0.56 (OH) 6 FBr) has an ideal kagome lattice and shows QSL behavior, demonstrating the robustness of the QSL against local impurities. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.Identifying the ground state for interacting quantum spins on the kagome lattice is an important unresolved question in condensed matter physics owing to the great difficulty in selecting amongst competing states that are very close in energy. Antiferromagnetic (AF) spins on this lattice are highly frustrated, and for spin-1 2 the ground state does not achieve magnetic order and is believed to be a quantum spin liquid (QSL). (1-9) The QSL is an unusual magnetic ground state, characterized by long-range quantum entanglement of the spins with the absence of long-range magnetic order. (10-13) The recent identification of herbertsmithite (Cu 3 Zn(OH) 6 Cl 2 ) (14-17) as a leading candidate QSL material has ignited intense interest in further understanding similar kagome materials.Often, real kagome materials have interactions that relieve the frustration and drive the moments to magnetically order. (18,19) In contrast, for the ideal S = 1 2 kagome Heisen-

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