Modelling of Ultrafast X-ray Experiments Probing Charge Carrier Dynamics in Solar Cells

Abstract

Improvements of currently achievable power efficiency in solar cells require, as a key prerequisite, a thorough understating of the evolution of photoexcited electronic states in the presence of strong coupling to nuclei. Methods, which enable such understanding empower the application of common strategies for development and discovery of novel solar cell systems, such as introduction of new functional materials and device architectures as well as directed application of synthesis methods. Tracking the dynamics of a molecule initiated in an excited electronic state constitutes a rather challenging task for both theory and experiment. On the one hand the established theoretical methods, which rely on the Born–Oppenheimer approximation are not sufficient to describe such processes. Nonadiabatic molecular dynamics, which takes into account the coupling between the nuclear and electronic sub systems is thus a natural choice to attack the problem theoretically. On the other hand, for experimental investigations, real-time imaging techniques are required. Recent advances in time-resolved x-ray science made new real-time imaging of ultrafast processes with sub-femtosecond temporal resolutions available. In this thesis, I theoretically explore ultrafast charge and nuclear dynamics in a model electron donor-acceptor (D−A) conjugated compound using nonadiabatic molecular dynamics simulations. To this aim, I investigate the use of ultrafast x-ray absorption spectroscopy experiments to directly observe dynamics at the atomic level in which a nonstationary state, initiated by a UV-Vis pump pulse, is tracked by means of a suitable x-ray probe pulse arriving with some time delays. This thesis involves a new expansion to an in-house software XMOLECULE (developed at CFEL-theory division at DESY in Hamburg) for a mixed quantum-classical treatment of molecular dynamics in electronically excited states. As the first project and in order to validate the code, I considered a scenario of investigating ultrafast hole dynamics after photoionization. I demonstrated how changes in the spectral features of the x-ray absorption spectrum at different time delays can be attributed to the ultrafast charge dynamics in the molecule. In the next step, more realistic process of photoexcitation, relevant to photovoltaic applications, is considered. Results showed how the time-resolved x-ray absorption spectroscopy is able to track electronic and structural dynamics during the excited-state relaxation occurring on a timescale of a few tens of femtoseconds.