Presenter: Prof. Norbert Koch (Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, Berlin, Germany; Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany)
Topic: Fundamental structural and electronic properties of perovskites and transition metal dichalcogenide monolayers
Time: 10:00 AM, Sep. 17th (Monday)
Location: 909-B
Abstract:
Photovoltaic cells based on halide perovskites and possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential, but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, used in practical solar cells, we employ single crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3) we reveal the band dispersion in two high symmetry directions, and identify the global valence band maxima (VBM), which have not yet been reported. With these benchmark data, we construct “standard” photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature of determining the valence band onset with high reliability. With appropriate consideration of all contributing factors, sample- and instrument-specific, the valence band onset of polycrystalline thin film perovskites can thus be determined with confidence better than 50 meV.
Optical and electrical properties of two-dimensional transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. Here we demonstrate how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy (LFM) and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS2) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable us to acquire the fourth-order elastic constants of monolayer WS2 experimentally. Our results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs, insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such two-dimensional semiconductors.
Contact: Prof. Steffen Duhm
Editor: Juan Yang