Selina Olthof1
University of Cologne1
Halide perovskites have revolutionized the field of photovoltaics through their remarkable performance in single as well as tandem solar cell devices. One intriguing property of this material class is the wide tunability of the band gap which enables a fine-tuning of optical and electronic properties. In this talk, I will I revisit some of our earlier work regarding the analysis of the valence and conduction band positions of tin and lead based 3D perovskites. Combining photoelectron spectroscopy with density functional theory we were able to distinguish influences from the atomic level positions, the bond hybridization strength, as well as lattice distortions.<br/>Recently, we also started working on 2D perovskites, which are gaining more and more attention as a strategy to tailor interfaces; they turned out to be a key factor to unlock high efficiencies in perovskite solar cells. Here, the effect of the choice of the bulky cation on the position of the charge transport layers is less clear and published measurements often seem contradicting. I will present a systematic study on alkyl-based organic cations with varying chain length, which are selected to form Ruddlesden Popper as well as Dion-Jacobson structures.<br/>Since such 2D perovskites are most commonly used as a thin interlayer on top of 3D films, I will also present some of our results gathered at such modified surfaces. Notably, using reflection energy loss spectroscopy (REELS) we are able to determine the surface band gap of these samples which helps us to understand the formation of such 2D surface layers.