Ultra-thin γ-In₂Se₃ buffer layer for efficient and all-dry Cd-free Cu(In,Ga)Se₂ solar cells

The search for a high-efficiency, low-cost, and sustainable cadmium (Cd)-free buffer has long challenged the commercialization of Cu(In,Ga)Se₂ (CIGSe) photovoltaics. Here, we report an all‑dry, scalable vacuum deposition strategy of an ultrathin γ-In₂Se₃ buffer layer using only absorber‑native elements, enabling high-performance Cd-free CIGSe solar cells. Fully compatible with vacuum manufacturing and requires no additional equipment. Owing to its strong thickness-dependent electronic and optical properties, an unconventionally thin ( ~ 10 nm) γ-In₂Se₃ layer exhibits a widened bandgap ( ~ 3.10 eV), avoiding the parasitic absorption and poor conductivity associated with thicker films. By optimizing growth conditions, thickness, and post-treatments, certified conversion efficiency exceeding 20% and good thermal stability (98.7% retention after 1032 h at 90 °C) are achieved. Techno-economic analysis suggests that compatibility with existing vacuum manufacturing processes could reduce the levelized cost of electricity by ~10%.