Lead-free metal halide layered double perovskites as sustainable platforms: composition tuning, synthetic methods and advanced applications

Abstract

The growing demand for environmentally friendly and stable materials has accelerated research into lead-free alternatives for optoelectronic and energy applications. Among these, Layered double perovskites (LDPs) with the general formula A4B(II)B(III)2X12 have gained attention due to their structural versatility, tunable optoelectronic properties and enhanced stability. This review critically examines compositional engineering in mixed-metal halide LDPs, focusing on how targeted modification of cationic and anionic sublattices governs their optoelectronic behaviour. The molecular source of band gap tunability, photoluminescence properties, defect tolerance, and structural stability are highlighted, providing a unified perspective on structure-property correlations in this material class. The impact of synthetic techniques is further investigated, demonstrating how regulated crystallization pathways from bulk and single crystals to nanostructures and thin films dictate morphology, defect landscape, and phase purity, ultimately determining material performance. The integration of composition design with synthesis control has been demonstrated to be necessary for unlocking the full potential of LDPs. Recent advances in the applications of tailored LDP compositions in energy conversion and optoelectronic devices are discussed to illustrate their growing technological relevance. Overall, this review aims to provide a unified understanding of the structure-property-function relationship in lead-free LDPs, while also identifying future directions for high-performance, environmentally benign lead-free perovskite-based systems.