Ultrafast Soft-X Spectroscopy for the Investigation of Electron and Lattice Dynamics in Perovskites

Hybrid Organic-Inorganic Perovskites are soft-lattice ionic semiconductors with a direct bandgap that presents interesting physical properties, such as superconductivity, magnetoresistance, ionic conductivity, ferroelectricity, and piezoelectricity [1]. Furthermore, these semiconductors have been widely used as base materials for the investigation of highly efficient solar cells [2,3].<br/>However, the physics of these materials is still not completely understood. For instance, they show only moderate carrier mobility that is in contrast with their behavior as defect-free semiconductors with long carrier lifetimes, long diffusion length, and slow electron-hole recombination [4].<br/>Ultrafast transient absorption spectroscopy is a powerful tool for the investigation of these material properties. In this approach, an optical excitation by a pump pulse initiates the dynamics. The real-time evolution of the system is directly encoded in the change of the probe spectrum after the interaction with the sample as a function of the delay between the pump and the probe pulses. For instance, ultrafast THz spectroscopy has been used for the understanding of the limited carrier mobility in this class of materials, enlightening the role of large polarons [5].<br/>Recently the development of novel sources of ultrashort pulses based on table-top high harmonic generation has allowed extending transient absorption spectroscopy in the soft-X-ray spectral range with a time resolution reaching the attosecond timescale. Ultrafast soft-X-Ray spectroscopy [6] allows the study of light-matter interaction with an unprecedented temporal and spatial resolution with the further advantages of being element-selective and oxidation- and spin-state specific. Indeed, in this spectral region, the photon absorption occurs locally at the atomic cores. X-ray measurements at atom-specific absorption edges allow the investigation of both the electronic and the structural environment of the probed atom, providing a local probe of the dynamics under study.<br/>In this work, we will report on the development of a beamline for ultrafast soft-X-ray spectroscopy for studying charge transfer in perovskites. The presence of various types of chemical interactions in such complex ionic solids gives them a characteristic “soft” fluctuating structure, prone to a wide set of defects that span from lattice distortions to the presence of mobile ions. Time-resolved X-ray absorption, being site-sensitive will elucidate the mechanisms of charge transfer at the femtosecond time-scale in this material.<br/>References<br/>[1] J. Huang et al.,Nat. Rev. Mater. 2, 17042 (2017).<br/>[2] W. Zhang et al., Nat. Energy 1, 16048 (2016)<br/>[3] Q. A. Akkerman et al., Nat. Energy 2, 16194 (2016)<br/>[4] X.-Y. Zhu and V. Podzorov, J. Phys. Chem. Lett. 6, 4758 (2015)<br/>[5] E. Cinquanta et al., Phys. Rev. Lett. 122, 166601 (2019)<br/>[6] P. Kraus et al., Nat. Rev Chem. 2, 82–94 (2018)