Structure-property relationships in 2D/quasi-2D lead-free halide double perovskites for optoelectronic devices

Two-dimensional (2D) and quasi-2D lead-free halide double perovskites (LFHDPs) have gained significant attention as promising alternatives to conventional lead-based perovskites due to their superior environmental stability, reduced toxicity and versatile structural chemistry. This review provides a comprehensive overview of recent advances in the structural design, scalable synthesis strategies, and optoelectronic functionalities, with particular emphasis on bandgap tunability, strong excitonic effects, and photoluminescence quantum yields of 2D LFHDPs. We examine emerging trends in dimensional engineering, particularly the development of Ruddlesden–Popper and Dion–Jacobson phases incorporating monovalent and trivalent cations, which enable enhanced moisture resistance and tailored quantum confinement. Key challenges remain, including indirect bandgaps, limited carrier mobilities, defect-related trap states, and difficulties in large-area processing. To address these issues, innovative approaches such as compositional alloying, halide mixing, defect passivation, and hybrid device architectures are discussed. Furthermore, we discuss challenges such as limited light absorption, poor carrier mobility, and defect tolerance, and explore innovative strategies to overcome these restrictions. Finally, we outline future research directions that couple computational screening with experimental synthesis, integrate LFHDPs into flexible and neuromorphic devices, and pursue sustainable, low-cost compositions, emphasizing their potential in sustainable optoelectronic technologies.