Research Topic
We use well-defined MAPbBr₃ surfaces to identify where degradation begins and how it proceeds under controlled environments.
Most measurements on perovskite films begin after the surface has already encountered air, solvents, or a transport layer. Those are relevant device surfaces, but they are difficult starting points for identifying a reaction mechanism. A freshly cleaved single crystal provides a simpler reference: large terraces, single-layer steps, and a surface history that is known.
We combine contact-mode AFM, friction measurements, photoelectron spectroscopy, and density-functional calculations. The emphasis is on following the same surface while one experimental variable is introduced, rather than comparing unrelated samples before and after exposure.
01
Two terrace types coexist on cleaved MAPbBr₃. Their height and friction contrast are consistent with an MABr-flat surface and a PbBr₂-vacant surface. Lateral-force microscopy is particularly useful here: it reveals a difference in surface termination that is much less obvious in topography alone.
02
After storage in dark ultrahigh vacuum, PbBr₂-rich clusters appear together with shallow pits in the surrounding crystal. The paired features show that the surface is being redistributed rather than simply covered by an external contaminant. Their slow formation is consistent with a reaction involving trace water or hydroxyl species and the loss of volatile methylammonium products.
03
Ambient-pressure AFM allows us to return to the same area as the water pressure is increased. Near 10⁻⁵ mbar, one-layer-deep patches nucleate and grow. Their boundaries have a distinct friction response. At higher exposure the patches merge, and complementary near-ambient-pressure XPS shows the accompanying loss of surface methylammonium. This sequence provides a direct link between local morphology and surface chemistry.
Current work
We are extending these measurements to controlled gas mixtures, humidity, illumination, electrical bias, and temperature on single crystals and thin films. The practical question is which surface state appears before a measurable loss of photovoltaic performance, and whether that state can be suppressed through composition, passivation, or interface design.
Related publications