Elemental de-mixing-induced epitaxial kesterite/CdS interface enabling 13%-efficiency kesterite solar cells

The conversion efficiency of kesterite solar cells has been stagnated at 12.6% since 2013. In contrast to chalcopyrite solar cells, the performance of kesterite solar cells is seriously limited by heterojunction interface recombination. Here we demonstrate kesterite/CdS heterojunction is constructed on a Zn-poor surface due to the dissolution of Zn2+ during chemical bath deposition. The occupation of Cd2+ on the Zn site and re-deposition of Zn2+ into CdS creates a defective and lattice-mismatched interface. Low-temperature annealing of the kesterite/CdS junction drives migration of Cd2+ from absorber back to CdS and Zn2+ from absorber bulk to surface, achieving a gradient composition and reconstructing an epitaxial interface. This greatly reduces interface recombination and improves device open-circuit voltage and fill factor. We achieve certified 12.96% efficiency small-area (0.11 cm2) and certified 11.7% efficiency large-area (1.1 cm2) kesterite devices. The findings are expected to advance the development of kesterite solar cells.