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Dipartimento di Fisica - Politecnico di Milano

Ultrafast X-ray transient absorption spectroscopy for the investigation of carrier dynamics in perovskites

This thesis project concerns the investigation of photoinduced ultrafast dynamics in perovskite semiconductors by few femtosecond near edge X-ray absorption fine structure.
With these measurements, we aim at gaining insight in the photoinduced ultrafast dynamics, with particular emphasis on the dependence of photophysical properties and charge transport properties on the chemical composition and the presence/type of defects in perovskite semiconductors, such as MAPbI3 and MAPbBr3

Recently the development of novel sources of ultrashort pulses at large-scale facilities, such as synchrotrons and Free Electron Lasers (FELs), and based on table-top high harmonic generation (HHG) process, has allowed extending transient absorption spectroscopy in the X-ray spectral range with a time resolution reaching the attosecond timescale. Ultrafast X-Ray spectroscopy allows the study of light-matter interaction with 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. The investigation of the properties of core electrons at ultrafast time scales promises to enlighten the dynamics occurring in complex materials, such as topological matter, quantum matter, and light-harvesting complexes.
Several impressive results have been recently obtained with tabletop sources based on HHG. Time-resolved X-ray absorption spectroscopy experiments performed at the C K-edge with a high harmonic source have already provided new insights into the transient electronic structure during ultrafast bond-breaking and ring opening of gas-phase molecules with femtosecond time resolution.
Time-resolved Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy probes the photo-excited system by measuring the absorption spectrum of a broadband X-ray pulse that covers all the absorption edges of interest. Electronic transitions from the core to unoccupied valence orbitals are probed, and reflect changes of the electronic structure and ultrafast charge carrier dynamics occurring in materials. In the related technique of Extended X-ray Absorption Fine Structure (EXAFS), the X-ray photon excites the core electron into the ionization continuum. Scattering of this electron with electrons from neighboring atoms on its way out leads to oscillations in the X-ray absorption spectrum, that reflect the chemical coordination environment of the absorbing element.

Solar cells prototypes based on perovskite (PSCs) have reached the remarkable conversion efficiency of 20%, making this material the most intriguing and reliable option for future photovoltaics. Two main hurdles hamper the market exploitation of PSCs, namely the presence of moving defects affecting the dynamics of polaronic carriers and the presence of Lead - a toxic constituent. Our ability to understand fundamental processes involving interactions between electrons, lattice (phonons) and photons that are at play in perovskites is an essential prerequisite for further development and is currently limited.
Time-resolved X-ray absorption/reflectivity, being site-sensitive will elucidate the mechanisms of charge transfer at the femtosecond time-scale in this material, which is still not completely understood.

This thesis work will be performed in the ERC-granted UDynI laboratory:



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