Charge mobility in perovskites

In recent years, lead-halide perovskites have become the most studied material for photovoltaic applications. The latter rely, of course, on the generation of charges (electrons and hole) upon absorption of light. These charges then migrate through the material to generate an electrical current. One of the crucial physical properties in this respect is the so-called charge mobility. Despite their remarkable performances in light-to-electricity conversion, a limitation of perovskites is their charge mobilities, which are orders of magnitude smaller than those of conventional semi-conductors used in photovoltaics.

This is rationalized by invoking polarons, which represent a deformation of the crystal lattice around a charge. As the latter moves across the lattice it induces this polaronic deformation, which limits its mobility. The polaron hypothesis is widely used in the literature on perovskites, yet the polaronic distortion has never been directly observed, let alone been quantified.

In a recent study, published in the Journal of the American Chemical Society, an international team of scientists led by Giulia Mancini (also at the University of Pavia) and M. Chergui at EPFL have used X-ray absorption spectroscopy to address this question. The team includes the Advanced Photon Sources at Argonne National Labs (USA) and the Paul-Scherrer-Institut (PSI, Villigen) where the experiments were conducted, as well as the group of Maksym Kovalenko at ETH-Zürich, which provided the samples.

The researchers monitored with element-selectivity the photoinduced deformation around the lead (Pb) and bromine (Br) atoms in the inorganic CsPbBr₃ perovskite. Using cutting-edge computational simulations, carried out by Nicola Colonna at PSI and EPFL, they could identify the structural deformations caused by the polarons formed around Br atoms. They also showed that the much invoked photoinduced thermal effects are negligible in determining the structural changes, which are predominantly due to the polarons. These results clarify the origin of the low charge mobilities and provide a basis for strategies to improve it.

“The insight gained on inorganic lead-halide perovskites are of general validity and should be applicable to the even more popular inorganic-organic perovskites,” says Giulia Mancini. “Our work also shows the importance of combining different approaches (X-ray and optical spectroscopy, X-ray diffraction) in order to nail down the details of photoinduced processes in these materials.”