Lead-free metal halide (halogenidometallate) semiconductors for optoelectronic applications

The electronic age demands the development of high-performing thin-film semiconductors that are low-cost and scalable. Lead (Pb)-based halide perovskites (LHPs) have proven to be successful in this regard, but their use is limited by environmental and health concerns related to lead toxicity. Lead-free halide compounds offer a promising alternative, with vast compositional space for fine-tuning properties to meet specific application requirements. These materials also offer opportunities for the deliberate introduction of functional properties, providing unparalleled control over their targeted applications. While the call for lead-free halide materials as alternatives to LHPs is echoed several times, the performance of these compounds remains modest as compared to the exponential growth of LHPs. Nevertheless, the compositional space of lead-free halide materials is huge, even bigger than LHPs as they are not restricted by the structural constrains of perovskite structure. This brings their huge potential in future technologies, which are remains untapped as of now. As a meta-analysis, we compare and combine the findings of previously published studies, to assess the optoelectronic properties of ternary and quaternary halide materials and their applications in devices. It details the structures of the various lead-free halide materials including perovskites, perovskite-derivative, and non-perovskites structures and describes the role of dimensionality and composition on their optoelectronic properties. To end, the challenges and perspectives of lead-free materials and devices are given. We hope this review will provide new insights for designing metal halide materials from the viewpoint of the modulation of the basic building blocks metal halide coordination. The future of electronics lies in the hands of lead-free halide materials, and we hope this review will inspire further research in this field.