Submicron Cu(In,Ga)Se ₂ Solar Cells With Over 20% Efficiency Enabled by Novel Construction of U‐Shape Ga‐Gradient

(CIGS) solar cells has long been pursued for its potential in material savings, increased throughput, and reduced production cost. However, progress has remained stagnant, primarily due to the difficulty in constructing a low-defect-density U-shaped Ga gradient within the reduced absorber thickness. Here, we demonstrate a novel approach that exploits the Cu-Se phase to actively manipulate Ga/In interdiffusion, enabling the successful fabrication of a desired U-shaped Ga double gradient in submicron CIGS films while maintaining high film quality. The resulting device exhibits significantly boosted quantum efficiency in the near-infrared region and a substantially reduced open-circuit voltage deficit, yielding a high-performance submicron CIGS solar cell with over 20% efficiency at an absorber thickness of only 935 nm, approximately one-third that of conventional CIGS. The insights gained into the critical role of the Cu-Se phase in forming the Ga double gradient provide a valuable strategy for advancing not only ultrathin CIGS solar cells but also other photovoltaic technologies facing similar bandgap engineering challenges.