Professor Chad Risko

Department of Chemistry

University of Kentucky

Host:  Brill

Abstract:

Organic semiconductors are derived from π-conjugated molecules or polymers, whose chemical composition and structure are only limited by the imagination of the synthetic chemist – or, perhaps in the future, are determined through machine-learned paradigms. The structure of organic semiconductors are also determined by their processing environment. Currently, how molecular design and processing are interwoven to result in semiconducting materials with optimal performance remains a mixture of art and science. Here we will discuss the development of atomistic-scale models that bring together molecular topology and processing conditions to provide first-principles insight into the physicochemical connections that are required to be controlled.

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.

The energy crisis and critical need of advanced materials for engineering, pharmaceutical, and medical applications are calling for the development of new materials to response quickly and efficiently to such demand. For such imperative request, the fundamental understanding of materials, the origin of the properties in correlation with structure could be the main pathway toward the successful design and discovery. The principle of homologous series has been successful in creating new materials with control of specific module in the structure, to improve the electronic, magnetic and optical properties [1, 2]. For example, we can create and manipulate cooperatively in within the same crystal lattice of complex transition metal chalcogenide, ferromagnetism and semiconductivity, two properties difficult to combine in a conventional inorganic compound. However, ferromagnetic semiconductors are very attractive and might result in new physical phenomena and novel applications such as spintronic that has the particularities to use both the charge and the spin of electron to process and store information [3, 4]. In this talk, we will discuss my recent work using that principle of homologous series to develop a model of high Curie temperature ferromagnetic semiconductors using the complex metal chalcogenides. We will mainly focus on my recent discovery of both FeSb2Se4 [5] a p-type and FeBi2Se4 [6] n-type ferromagnetic semiconducting materials with a Curie temperature of 450K [5, 6] that can be tuned through doping with Sn or In.

_{Reference: [1] R. J. Cava, J.Am.Ceram.Soc. 83 [1] 5-28(2000) [2] A. Mrotzek and M. G. Kanatzidis, Accoun. Chem. Re. 36(2) 111-119 (2003) [3] H.Ohno et al. Appl. Phys. Lett. 69, 363-365 (1996) [4] M. N. Leuenberger et al. Nature, 410, 789-793 (2001) [5] H. Djieutedjeu et al. Angew. Chem. Int. Ed. 49, 9977-9981 (2010) [6] K. G. S. Ranmohotti et al. J. Am. Chem. Soc. 137, 691-698 [7] H. Djieutedjeu et al. Manuscript in progress.}