No abstract

Organic-inorganic halide perovskite solar cells have rapidly evolved over the last 3 years. There are still a number of issues and open questions related to the perovskite material, such as the phenomenon of anomalous hysteresis in current-voltage characteristics and long-term stability of the devices. In this work, we focus on the electron selective contact in the perovskite solar cells and physical processes occurring at that heterojunction. We developed efficient devices by replacing the commonly employed TiO2 compact layer with fullerene C60 in a regular n-i-p architecture. Detailed spectroscopic characterization allows us to present further insight into the nature of photocurrent hysteresis and charge extraction limitations arising at the n-type contact in a standard device. Furthermore, we show preliminary stability data of perovskite solar cells under working conditions, suggesting that an n-type organic charge collection layer can increase the long-term performance.

Using the X-ray standing wave method, scanning tunneling microscopy, low energy electron diffraction, and density functional theory, we precisely determine the lateral and vertical structure of hexagonal boron nitride on Ir(111). The moiré superstructure leads to a periodic arrangement of strongly chemisorbed valleys in an otherwise rather flat, weakly physisorbed plane. The best commensurate approximation of the moiré unit cell is (12 × 12) boron nitride cells resting on (11 × 11) substrate cells, which is at variance with several earlier studies. We uncover the existence of two fundamentally different mechanisms of layer formation for hexagonal boron nitride, namely, nucleation and growth as opposed to network formation without nucleation. The different pathways are linked to different distributions of rotational domains, and the latter enables selection of a single orientation only.

We investigated the structural and electronic properties of vacuum sublimed perfluoro-pentacene ͑PFP͒ thin films on Ag͑111͒ substrates using x-ray standing waves ͑XSW͒, x-ray diffraction ͑XRD͒ and ultraviolet photoelectron spectroscopy ͑UPS͒. XSW results reveal a flat adsorption geometry of the monolayer PFP/Ag͑111͒ with a relatively large bonding distance of 3.16 Å for both, the carbon and fluorine atoms. Multilayers PFP/Ag͑111͒ adopt a herringbone structure with the molecular long axis parallel to the substrate and a vertical lattice spacing of 3.06 Å as evidenced by XRD. The strong intramolecular polar bond character of the fluorine-carbon bonds in PFP leads to an orientation dependent ionization energy ͑IE͒ that is experimentally observed by UPS for the monolayer-multilayer transition: The inclined molecular plane orientation in the multilayer herringbone arrangement leads to an increase of the PFP IE by Ͼ0.4 eV compared to the flat lying monolayer.

The emergence of oxidized oxygen RIXS features at high voltages for Ni-rich layered oxide cathodes.