Submicron Cu(In,Ga)Se₂ Solar Cells With Over 20% Efficiency Enabled by Novel Construction of U-Shape Ga-Gradient

Raising the efficiency of submicron Cu(In,Ga)Se₂ (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.