Time- and Angle-Resolved Photoelectron Spectroscopy of Solar Materials

Natural foundations defining the manner in which a solid material interacts with optical radiation are primarily encoded in the material's momentum-dependent band structure and the microscopic details of the electron-lattice interactions. An access to this fundamental information can facilitate an understanding and help to predict technologically-relevant optoelectronic properties, driving a further optimization of current solar material and device designs, and fueling the search for novel compounds, eliminating unresolved issues. Lead halide perovskites (LHPs) and transition metal oxides (TMOs) are promising solar materials, commonly constituted by earth-abundant components. Particularly, hybrid and fully-inorganic LHP and ZnO semiconductors show a great potential in photonic applications, going beyond light harvesting. Their electronic structure was directly accessed by time- and angle-resolved photoelectron spectroscopy (TR-ARPES). The capabilities of this technique in conjunction with extreme ultraviolet (XUV) Harmonium light source were exploited by performing valence band (VB) mapping of the entire Brillouin zone of three lead tribromide perovskites featuring a different central cation, and of ZnO; and by investigating the ultrafast photocarrier dynamics in CsPbBr3. High quality VB mapping of CsPbBr3 revealed the polaron formation-induced effective mass renormalization, governing carrier transport properties in weak excitation regime. Whereas the temporal and momentum resolutions, surface sensitivity of XUV TR-ARPES, and transport simulations, helped to show the importance of many-body effects and electron-phonon coupling in the ionic lattice in explaining the nature of fluence-dependent quasi-ballistic to diffusive surface-to-bulk transport crossover in strong excitation regime. Future experiments with tunable excitation and surface deposition capabilities can elucidate the influence of excess energy on charge transport in LHPs and TMOs, and contribute to the knowledge of surface properties in presence of adsorbents.