Bottom-Cell Engineering as the Key to Perovskite/CIGS Tandem Solar Cells

Perovskite/Cu(In,Ga)Se₂ (CIGS) tandem solar cells have emerged as a highly promising thin-film photovoltaic architecture, combining the exceptional efficiency potential of perovskites with the industrial maturity of CIGS technology. Despite rapid advances in perovskite top cells, overall tandem performance remains fundamentally constrained by the CIGS bottom cell, which critically governs bandgap matching, voltage losses, and interfacial recombination. Here, we provide a comprehensive overview of recent progress in CIGS absorber and interface engineering for tandem applications. We highlight key strategies including Ga-gradient bandgap engineering, alkali post-deposition treatment, Ag alloying for low-temperature growth and bifacial operation, as well as buffer-layer and intermediate recombination layer optimization. We further discuss how surface morphology and defect chemistry collectively dictate carrier recombination dynamics and interfacial stability in monolithic device architectures. Finally, we outline a roadmap toward the commercialization of perovskite/CIGS tandem photovoltaics. By linking materials-level engineering with device-level constraints, this review establishes a unified framework to guide the development of high-efficiency, scalable, and industrially viable tandem solar technologies.