Challenges and Opportunities for Layered Double Perovskite Solar Cells

Layered lead‐free halide double perovskites (LDPs) have emerged as promising candidates for environmentally benign and intrinsically stable photovoltaic absorbers. Despite their chemical robustness and compositional flexibility, their power conversion efficiencies remain well below those of Pb‐based perovskites. This discrepancy arises from a combination of intrinsic electronic structure constraints, defect‐driven nonradiative recombination, processing‐induced inhomogeneity, and device‐architecture mismatches. In this review, we systematically examine performance limitations in LDPs and emerging solutions that can overcome these fundamental obstacles. We dissect four interdependent bottlenecks, namely, (1) electronic and excitonic limitations arising from reduced dimensionality and heavy carrier effective mass mismatches, (2) defect physics and self‐trapping phenomena, (3) synthesis and thin‐film processing challenges, and (4) interface and device‐level losses. For each category, we discuss emerging mitigation strategies, including band engineering, defect passivation, crystallization control, and interface optimization. By consolidating insights into a unified diagnostic perspective, we aim to provide a roadmap toward overcoming performance limitations in LDP photovoltaics.