Cation Exchange Synthesis of Two-dimensional Vertical Cu2S/CdS Heterojunctions for Photovoltaic Device Applications
Two-dimensional (2D) vertical heterojunctions possess peculiar electronic and optoelectronic properties according to their complementary composition, ultrathin thickness, and unique geometry. However, it is still a challenge to synthesize the 2D vertical heterojunctions based on non-layered materials due to strong covalent bonds and abundant surface dangling bonds. Here we developed a partial cation exchange method for the synthesis of 2D vertical Cu2S/CdS heterojunctions. The large-size and single-crystalline CdS nanosheets prepared by a van der Waals epitaxy method served as host lattices for cation exchange reactions. The evolution of the nanosheets in terms of their morphology, composition and crystal structure were elaborately characterized. The as-prepared 2D vertical heterojunctions have two-layer ultra-thin structures and form large-area junctions at their interfaces. Under the light irradiation, the photogenerated electron-hole pairs can be quickly separated under the electric field of the junction region, minimizing the possibility of charge recombination. As a result, the device based on 2D vertical Cu2S/CdS heterojunction exhibits a higher photoelectric conversion capability compared with the device based on single-component 2D CdS or Cu2S Schottky junction. Our work opens up opportunities to explore the 2D vertical heterojunctions based on non-layered materials for high-performance optoelectronic devices.