Thomas Charles Henry Rossi
In modern ultrafast optoelectronic technologies based on wide band gap insulators, the non-equilibrium dynamics of photogenerated charges plays a major role. Unravelling the mechanisms of interaction between these charge carriers and their environment is crucial to support the design of more efficient devices. Especially, the absorption of photons with more energy than the optical band gap generates electrons-hole pairs (EHP) whose transport and dissociation as excitons or free charge carriers is of pivotal importance for the charge separa- tion at photovoltaic interfaces. The advent of ultrafast pump-probe optical spectroscopy in the deep-UV provides access to the dynamics of EHP through a variety of transient modifications of the optical spectrum. In this Thesis, the cooling of electrons in ZnO and methylammonium lead bromide perovskite (MAPbBr3), two broadly used direct band gap semiconductors, is tracked as they end up and accumulate at the bottom of the conduction band. ZnO has a fast cooling time of the order of 100 fs which efficiently converts photon excess energy into lattice heat while MAPbBr3 has a rather slow electron cooling time of the order of 400 fs, favorable for charge separation at interfaces as it drives the injection of hot carriers. Additionally, the high energy carried by the UV probe photons provides access to the photodynamics at different high symmetry points in the Brillouin zone. In a second step, these two semiconductors have been associated to other materials to form prototypical photovoltaic assemblies such as dye-sensitized (ZnO/N719) and solid-state lead perovskite sensitized (TiO2/MAPbBr3) interfaces. Upon electron injection into the transition metal oxide (ZnO or TiO2), dramatic changes are observed in the absorption close to the optical band gap from which the timescale of electron injection is obtained. It highlights how slight screening length and chemical potential changes can generate large modifications in the optical properties through many-body effects. At both interfaces, a delayed appearance of the electron at the bottom of the conduction band is observed which is due the formation of a bound state. This Thesis extends beyond the band insulators to the photodynamics of NiO, a strongly correlated charge-transfer insulator. Excitation above the optical band gap generates a large photoinduced absorption below the optical band gap which is characteristic of the interplay between electronic and low energy bosonic excitations. The dressing of electronic excitations with bosonic modes generates low energy excitation modes on ultrafast timescales which generate a competition between itinerant and localized resonances with a high degree of tunability. The progress made in resonant X-ray based time-resolved spectroscopies allows the investigation of the degree of charge localization around specific atoms in a system. Photogenerated electron localization in NiO is studied by time-resolved X-ray absorption spectroscopy, causing bond elongations and the formation of an electron-polaron in less than 100 ps. In a last part, the linear dichroism of the steady-state X-ray absorption spectrum of anatase TiO2 at the K-edge is studied which provides an unambiguous assignment of the pre-edge transitions as their orbital composition is strongly anisotropic and sensitive to the crystal orientation.
EPFL2018
