The surface of halide perovskites from nano to bulk

The surface of a semiconductor often has a key role in determining its properties. For metal halide perovskites, understanding the surface features and their impact on the materials and devices is becoming increasingly important. At length scales down to the nanoscale regime, surface features become dominant in regulating the properties of perovskite materials, owing to the high surface-to-volume ratio. For perovskite bulk films in the micrometre range, defects and structural disorder readily form at the surface and affect device performance. Through concerted efforts to optimize processing techniques, high-quality perovskite thin films can now be fabricated with monolayer-like polycrystalline grains or even single crystals. Surface defects therefore remain the major obstacle to progress, pushing surface studies to the forefront of perovskite research. In this Review, we summarize and assess recent advances in the understanding of perovskite surfaces and surface strategies towards improving perovskite materials and the efficiency and stability of perovskite devices. The surface features of metal halide perovskites have a crucial role in determining their properties. This Review introduces the recent advances in the understanding of perovskite surfaces and surveys the surface strategies towards improving the properties of perovskite materials and the performance of perovskite optoelectronic devices.