Photoinduced electron dynamics at the chromophore-semiconductor interface: A time-domain ab initio perspective

The chromophore-semiconductor interface offers a classic example of an interaction between an organic molecular species and an inorganic bulk material. The interface provides the foundation for a new, promising type of solar cell and presents a fundamentally important case study for several fields, including photo-, electro- and analytical chemistries, molecular electronics, and photography. Scientists employ different concepts and terminologies to describe molecular and solid states of matter, and these differences make it difficult to describe the interface with a single model. At the basic atomistic level of description, however, this challenge can be largely overcome. Recent advances in non-adiabatic molecular dynamics and time-domain density functional theory have created a unique opportunity for simulating the ultrafast, photoinduced processes on a computer very similar to the way that they occur in nature. The progress report is a review of these state-of-the-art theoretical tools. It offers a comprehensive picture of a variety of electron transfer processes that occur at the interface. The topics of discussion include electron injection from the chromophore to the semiconductor, electron relaxation and delocalization inside the semiconductor, back-transfer of the electron to the chromophore and to the electrolyte, and regeneration of the neutral chromophore by the electrolyte. The ab initio time-domain modeling is particularly valuable for understanding these dynamic features of the ultrafast electron transfer processes, which cannot be represented by a simple rate description. For example, the simulations show that what appears as a single step, such as electron injection, is in fact an average over many distinct elementary processes, and that very different vibrational modes drive electron transfer, depending on the process, the system, and the experimental conditions. The report focuses in particular on the electronic donor-acceptor interaction, atomic motions, electron-vibrational coupling, surface termination, thermal effects, electron transfer mechanisms and fluctuations from the average behavior.