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 here. 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 the pulsed x rays, they could interrogate each element independently with a resolution of 80 ps. For both materials, the team found that the excited electrons are delocalized in the conduction band, whereas the holes left behind in the valence band are localized at the Br sites. The Br signal consisted of one component that decayed quickly and one that decayed slowly; the photoluminescence kinetics of organic–inorganic perovskite solar cells show a similar biexponential decay. Meanwhile, the Cs⁺ cations showed no evidence of being involved in the charge transport, which suggests that organic cations would likewise be uninvolved. (F. G. Santomauro et al., Struct. Dyn.4, 044002, 2017.)