Speaker: Alexander Weber-Bargioni, Molecular Foundry, Lawrence Berkeley National Laboratory
Time: October 15, 2015, 2:00 - 3:00
Understanding and eventually controlling opto electronic processes at the native length scale, e.g. deliberately transporting excitons to predetermined sites where they perform work, will provide the access to a new parameter space for the development of next generation light harvesting materials.
Two fascinating material systems of interest to study local optoelectronic processes are perovskites and 2-D Transition Metal Dichalcogenites: Lead Halide perovskites based solar cell have recently reached 20% power conversion efficiency, albeit the lack of understanding properly the mechanism. Using Photo Conductive AFM in pin-point mode and near field optical nano PL measurements we map the local mobility, short circuit current, open circuit voltage and PL and find substantial anisotropy within and amongst individual grains with specific grains showing 30% higher local PCE. Our analysis suggest that this heterogeneity originates from crystal facets with higher trap states and hence a spatially varying electric field introducing a locally varying Rashba splitting of Valence and conduction band, effecting strongly the local charge carrier lifetime.
MoSe2 is a fascinating new 2-D materials with direct band gap, high exciton binding energy and an equivalent effective mass of hole and electron, opening an enormous application space from valley electronics to ultra thin photo voltaic elements. We used near field optics to map edge states and on the search of an excitonic signature in photo excited Scanning Tunneling Spectroscopy, we found by accident 1-D mirror grain boundaries that create 1-D metallic structures exhibiting high temperature charge density waves.
Using STS mapping we show the various periodicities of the in gap states and CO-functionalize AFM tips provide an exact atomic picture of the 1-D defect structure.