Professor Kenneth R. Graham

Department of Chemistry

University of Kentucky

Host: Brill

Organic thermoelectrics and perovskite solar cells are both promising technologies for generating electricity in a more sustainable manner.  Organic thermoelectrics, which are typically based on doped π-conjugated polymers, provide a means of converting waste heat to electrical energy using low-cost and mechanically flexible devices.  On the other hand, perovskite solar cells rely on low-cost and solution processable organic metal halide perovskites to efficiently convert solar energy to electrical energy.  For both material classes and device types, ultraviolet, inverse, and x-ray photoelectron spectroscopy (UPS, IPES, and XPS, respectively) provide an experimental means to measure the energy of electronic states that help determine material and device performance.  We have developed low-energy UPS and IPES systems that minimize sample damage in sensitive materials, such as organic semiconductors and organic metal halide perovskites. In this talk I will discuss how UPS, IPES, and XPS are applied to better understand the thermoelectric properties of organic semiconductors and interfacial chemistry and energetics within organic metal halide perovskite solar cells.  In the area of thermoelectrics, I will discuss how material blends can be used to manipulate the energy dependence of charge transport and improve the power factor.  Furthermore, I will discuss our recent finding that high levels of p-type doping of π-conjugated polymers can lead to n-type thermoelectric behavior, i.e., negative Seebeck coefficients, and a negative Hall voltage indicative of electrons as the dominant delocalized charge carriers.  IPES and UPS measurements of these doped polymers indicates that the transport gap decreases with increasing doping concentration and the density of states appears semi-metallic at high doping concentrations..  In the area of perovskite solar cells, I will discuss surface ligand binding, the influence of surface ligands on interfacial energetics, and how these energetics impact solar cell performance in both Pb- and Sn-based perovskites.