Liquid‐Phase Transfer of Organic–Inorganic Halide Perovskite Films for TEM Investigation and Planar Heterojunction Fabrication

Abstract Organic–inorganic halide perovskites (OIHPs) show high promise in optical and electronic applications such as solar cells, light‐emitting diodes, and nonlinear optics. However, the fundamental knowledge of the atomic‐scale microstructures in OIHP thin films is limited due to the challenge in characterizing them using transmission electron microscopy (TEM). Here a solution‐phase “release‐and‐transfer” method is demonstrated, which entails the lifting of OIHP films from their original substrates while maintaining the film integrity, followed by a sequential transfer onto a TEM grid. The freestanding nature of the OIHP films with a nanoscale thickness, prepared as such, allows a direct TEM observation in the plan view, complementing those typical cross‐sectional views enabled by focus‐ion‐beam specimen fabrication. Using low‐dose scanning TEM, the atomic‐scale microstructure of transferred OIHP films is confirmed to be generally maintained, while the microstrain existing in original films is largely relaxed. This “release‐and‐transfer” method is generic to both standard 3D and low‐dimensional OIHPs. Based on a simple layer‐by‐layer transfer, the fabrication of a 2D–3D planar heterojunction with a good interfacial contact and optoelectronic properties is achieved. This unique methodology offers new opportunities to accelerate the fundamental and practical developments of OIHP materials and devices.