Watching perovskite photoexcitations, atom by atom

Certain members of the class of crystalline materials known as perovskites have recently shown great promise for optoelectronic applications. Perovskites have the chemical formula ABX₃, where A and B are cations and X is an anion, arranged as shown on page 22. Crystals that combine an organic cation, lead as the second cation, and a halogen anion make for solar cells of remarkably high efficiency despite rather modest charge-carrier mobilities (see Physics Today, May 2014, page 13). Yet the nature and fate of the photoexcited charge carriers remain little understood. A Swiss team led by Majed Chergui of the Swiss Federal Institute of Technology in Lausanne has now peeled back some of that mystery. With time-resolved x-ray absorption spectroscopy, the researchers studied two inorganic perovskites—CsPbBr₃ and CsPb(Cl_xBr_1–x)₃— at the Swiss Light Source. By tuning the energy of...