Ordered Vacancy Compounds and Nanotube Formation in CuInSe₂−CdS Core−Shell Nanowires

CuInSe 2 related materials-based heterojunction diodes have received much attention, owing to their highest power conversion efficiency (19.5%) among all the thin-film solar cell technologies. Important issues on the microstructure and formation mechanism of CuInSe 2 −CdS p-n heterojunction persist due to the complexity of polycrystalline films and invasive sample preparation for characterization. Here, we investigated the microstructure, chemical composition, and formation mechanism of the junction with CuInSe 2 −CdS core−shell nanowires, where nanowire geometry affords single-crystalline nanograins for direct characterization. A coherent CdS shell can be epitaxially deposited onto the CuInSe 2 nanowire with chemical bath deposition even at 60 °C. For the first time, ordered vacancy compound nanodomains induced by fast outward diffusion of Cu ions were directly observed near the interface of epitaxial CIS−CdS heterostructure. The core−shell nanowires can be transformed into nanotubes with chemical bath deposition progression through a nanoscale Kirkendall effect. Our results provide important understanding of CuInSe 2 −CdS heterojunctions for developing better CuInSe 2 solar cells.