Fall 2020
CONDENSED MATTER PHYSICS Seminar
Joseph Ngai
University of Texas-Arlington
Title: Electrically coupling multifunctional oxides to semiconductors through charge transfer
Semiconducting heterojunctions (e.g., pn-junction, isotype junctions, etc.) are the building blocks for virtually all electronic device technologies, ranging from solar cells to transistors. Such heterojunctions exhibit built-in electric fields that arise from a net transfer of itinerant charge between two electrically dissimilar semiconductors. Recent advancements have enabled charge transfer and built-in fields to be studied in heterojunctions between semiconductors and crystalline multifunctional oxides. The mixed covalent and ionic character of such hybrid heterojunctions could enable functionality that cannot be achieved using either material alone. After a brief introduction about multifunctional oxides, we will discuss our recent efforts in understanding charge transfer and the formation of built-in electric fields in SrNbxTi1-xO3-δ / Si heterojunctions. Magneto-transport measurements reveal charge transfer and the formation of a hole-gas in Si as Nb content in the oxide is varied. Hard x-ray photoelectron spectroscopy measurements allow us to map out built-in electric fields across heterojunctions by analyzing asymmetries in the core-level line shapes. Owing to the properties of oxides, hybrid heterojunctions exhibit behavior not found in conventional semiconducting heterojunctions, including the ability to tune band-alignments with doping. Such hybrid heterojunctions could address emerging challenges in energy harvesting and information technology.
November 20, 2020, 4:00PM
ONLINE (via Zoom: contact Dr. Mario Borunda, mario.borunda@okstate.edu if you would like to attend)
CONDENSED MATTER PHYSICS Seminar
Makhsud Saidaminov
University of Victoria, Department of Chemistry and Department of Electrical & Computer Engineering
Title: Perovskite solar cells: why they perform well and degrade fast?
Perovskite solar cells (PSCs) have recently reached a certified power conversion efficiency (PCE) of 25.2%, the fastest advance among all photovoltaic technologies. This has been enabled in significant part by combinatorial optimization of composition that now contains six or more components instead of three in the conventional perovskite. The key question is why perovskites need to be so much “contaminated” in order to perform well?
In this talk, I will discuss how mixing improves the efficiency and stability of perovskite solar cells. We found that the grains form with a gradient composition in mixed perovskite thin films, exhibiting a natural passivation layer on the surface of grains; this shields photogenerated carriers in solar cells [1]. I then show that alloyed perovskites have a remarkably reduced density of atomic vacancies, a major source of decomposition due to their high affinity for water and oxygen molecules [2]. Thus, alloying overcomes some of the problems that single cation/anion perovskites are prone to, such as formation of point defects in the crystal lattice and defective surfaces.
[1] M. I Saidaminov, K. Williams, M. Wei, A. Johnston, R. Quintero-Bermudez, M. Vafaie, J. M. Pina, A. H. Proppe, Y. Hou, G. Walters, S. O. Kelley, W. A. Tisdale, E. H. Sargent. Multi-cation perovskites prevent carrier reflection from grain surfaces. Nature Materials, 19, 412 (2020).
[2] M. I. Saidaminov, J. Kim, A. Jain, R. Quintero-Bermudez, H. Tan, G. Long, F. Tan, A. Johnston, Y. Zhao, O. Voznyy, and E. H. Sargent. Suppression of atomic vacancies via incorporation of isovalent small ions to increase the stability of halide perovskite solar cells in ambient air. Nature Energy, 3, 648 (2018).
November 6, 2020, 4:00PM
ONLINE (via Zoom: contact Dr. Mario Borunda, mario.borunda@okstate.edu if you would like to attend)
CONDENSED MATTER PHYSICS Seminar
Brandon K. Durant
University of Oklahoma, Homer L. Dodge Department of Physics and Astronomy
Title: Effects of Temperature and Proton Irradiation on Mixed Tin-Lead Halide Perovskites, Implications for Space Applications
Mixed organic-inorganic halide perovskite solar cells (PSC’s) have garnered attention in recent years for their impressive solar to electrical power efficiency gains and potentially lower material and processing costs for optoelectronic applications. Here we investigate the properties of mixed formamidinium tin and methylammonium lead iodide (FASn)0.6(MAPb)0.4I3 perovskites which lower the lead content as well as the bandgap, making them more attractive for the absorber material in PSCs. In addition to terrestrial applications, PSCs are of interest to the space power markets for their low cost, low weight, adaptability to flexible architecture, and tolerance to high energy particle irradiation (mainly protons). Through current density-voltage (JV) characterization at lower temperatures, a barrier to photogenerated carrier extraction is evident and attributed to the changing bandgap of the absorber layer relative to the energy selective contacts in the device. Although the architecture used here hinders the performance at temperatures below 225 Kelvin, the tolerance to high energy protons is respectable compared to the current industry technologies.
October 16, 2020, 3:00PM
ONLINE (via Zoom: contact Dr. Mario Borunda, mario.borunda@okstate.edu if you would like to attend)
CONDENSED MATTER PHYSICS Seminar
TeYu Chien
University of Wyoming, Department of Physics and Astronomy
Title: Electronic properties of novel materials - photovoltaic, 2D magnetic, and topological materials
September 17, 2020, 4:00PM
ONLINE (via Zoom: contact Dr. Mario Borunda, mario.borunda@okstate.edu if you would like to attend)