Abstract

Functional interfaces have acquired a primary role in many areas of science and technology, ranging from photovoltaics and photocatalysis to electronics and biosensing. A fundamental property of functional interfaces is the alignment of the quasiparticle energy levels between the two materials. Such alignment underpins a variety of complex phenomena such as charge-transfer doping, carrier injection, and exciton dynamics. In the context of solar energy technology the level alignment determines the ability of the interface to transfer energy from a donor to an acceptor by exchanging photoexcited charges. While the physics of the energy-level alignment at conventional semiconductor heterojunctions is currently well established, little is known about functional interfaces involving metal oxides and soft materials such as polymers and light-harvesting complexes. Here I will review our recent activity in the computational modelling of TiO2 interfaces for dye-sensitized solar cells, and of experimental probes such as X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. In this area the first challenge that we have to face is to determine the structure of the interface at the atomic scale. Standard optimization techniques are inadequate for this task due to the large number of possible interfacial morphologies. I will argue that a possible way forward is to build interface models by reverse-engineering experimental data using first-principles computational spectroscopy. This notion will be illustrated by discussing core-level photoemission spectra at the TiO2/N3 interface. The second challenge is the development of robust computational methods for studying the energy-level alignment at the interface. Standard density-functional techniques are rarely in quantitative agreement with photoemission data, and in some cases they fail to correctly describe the interfacial charge transfer. In order to perform a quantitative comparison with experiment it is important to develop a comprehensive theory of electron removal and addition processes in molecules and solids. Such theory needs to capture the complex interplay of image charges, thermal broadening, and configurational disorder. Here I will discuss a recent proposal of our group for addressing this challenge.