Halide Perovskite Single Crystals and Nanocrystal Films as Electron Donor‐Acceptor Heterojunctions

Halide perovskites are materials for future optical displays and solar cells. Electron donor‐acceptor perovskite heterostructures with distinguishing halide compositions are promising for transporting and harvesting photogenerated charge carriers. Combined e‐beam lithography and anion exchange are promising to develop such heterostructures but challenging to prepare multiple heterojunctions at desired locations in single crystals. We demonstrate swift laser trapping‐assisted band gap engineering at the desired locations in MAPbBr3 microrods, microplates, or nanocrystal thin films. The built‐in donor‐acceptor double and multi‐heterojunction structures let us transport and trap photogenerated charge carriers from wide‐band gap bromide to narrow‐band gap iodide domains. We discuss the charge carrier transport and trapping mechanisms from the viewpoints of engineered bands and band continuity. This work offers a convenient method for designing single‐, double‐ and multi‐heterojunction donor‐acceptor halide perovskites for photovoltaic, photonic, and electronic applications. Using focused near‐infrared laser trapping, bromide‐iodide perovskite heterojunctions are prepared by local anion exchange reactions in methylammonium lead bromide perovskite microcrystals or nanocrystal thin films. The resulting donor‐acceptor double and multi‐heterojunctions efficiently transport and trap photogenerated carriers. This method offers a route to heterojunction fabrication in single crystals for solar cells and LEDs.