Kenneth Graham: Organic Thermoelectrics and Perovskite Solar Cells

Speaker: Kenneth Graham

Affiliation: U of Kentucky

Abstract: 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, which we attribute in part to the closing of the transport gap as observed with IPES and UPS measurements. 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.

Biography: Ken earned a B.S. in Chemistry from the University of North Carolina at Chapel Hill in 2006, followed by a Ph.D. from the University of Florida in 2011, where he focused on the morphological and photophysical properties of organic semiconductors under the guidance of Prof. John Reynolds. He then worked as a SABIC post-doctoral research fellow in the groups of Prof. Aram Amassian at KAUST and Prof. Mike McGehee at Stanford University, where his research focused on interfacial properties of organic-organic heterojunctions in organic photovoltaics. He started as an assistant professor at the University of Kentucky in 2014. His group specializes in applying photoelectron spectroscopies to understand the energetics and interfacial properties of organic semiconductors and organic metal halide perovskites. In the area of organic thermoelectrics, his group is interested in exploring means to controllably manipulate the energy dependence of charge transport and in determining how dopants influence the electronic structure and charge transport properties in π-conjugated polymers. In the area of halide perovskites, his group is focused on determining how surface chemistry influences interfacial energetics, interfacial charge transfer processes, stability, and photovoltaic device performance. He has received a DOE CAREER award as well as ACS PRF and NSF grants.