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Special USTAR Seminar: Jianbo Gao

Jianbo Gao
National Renewable Energy Laboratory

Wednesday November 28, 2012
102 JFB

Title: PbS Quantum Dot Solar Cells--An emerging thin film photovoltaic technology


Semiconductor quantum dots (QDs) play an important role in solar energy conversion, due to their unique new physics that arise from quantum confinement effects such as tunable bandgap as a function of QD size and multiple exciton generation (MEG). The power conversion efficiencies (PCEs) of solar cells using thin, densely packed layers of PbS QDs have increased from 2% just a few years ago to approaching 8% today. Furthermore, when multi-junctions cells stack together, more than 68% PCE can be achieved theoretically. Therefore, the perfect marriage of high efficiency and low cost fabrication makes PbS QDs revolutionary materials for next generation multi-junction solar cells. In this talk, I will focus on the strategies to approach 6% PbS QD solar cells through the fundamental study of device architecture, interface, and PbS QD passivation. Also I will present a recent breakthrough in photovoltaic science that demonstrates MEG in a PbSe QD-based solar cell via the observation of external quantum efficiency (EQE) greater than 100%.


Special USTAR Seminar

Vic Liu
General Motors Global R&D

Monday November 19, 2012
1230 WEB

Title: Probing the Local Chemical Physics of Fuel Cell & Battery Materials to Understand Some of the Technical Challenges Facing Fuel Cell- & Battery-Powered Electric Vehicles


To address environmental and resource-limitation constraints, mainstream automobile companies are striving to put more and more electric vehicles (powered by either hydrogen fuel cells or lithium-ion batteries ) on roads in the coming years. Meanwhile, academia and industry research labs are working hard to find alternative high-performing durable materials to make fuel cell and battery technologies costcompetitive with their fossil fuel-based internal combustion analogs. In this presentation, I will briefly outline a part of my research at General Motors related to the hydrogen fuel cells and lithium-ion batteries by reviewing a few representative examples. They are: carbon corrosion, Pt-alloy catalyst development, manganese dissolution, and solid-electrolyte interphase characterization. In these examples, my efforts were to probe the local chemical physics by using state-of-the-art techniques including microscopy, spectroscopy, and three-dimensional tomography. The obtained local information was then connected with the history of the materials in an analyzed system and the system’s overall electrochemical properties. Through these examples, I will show the importance in understanding the local structure and chemistry of materials and their practical relevance to the performance and durability of the fuel cell and battery operating systems. This is because the understanding developed this way will allow us to solve the performance and durability issues at the materials level or develop mitigation strategies at the system and operation levels.


This Week's Colloquium: Maria Loi, Nov. 15, 2012

Maria Loi
Photophysics & Optoelectronics
Zernike Institute for Advanced Materials
University of Groningen, Groningen, The Netherlands

Thursday, Nov. 15, 2012
102 JFB

Refreshments: 3:30 pm in 219 JFB
Lecture 4:00pm (102 JFB)

Title: Organic-Inorganic Hybrids: From Fundamental Properties To Optoelectronic Devices


Colloidal semiconductor nanocrystals (NCs) are solution processable semiconductors that are potentially highly appropriate for optoelectronic device fabrication, owing to their narrow bandwidth, their remarkably broad absorption, their large tunability, their high dielectric constant, and their high stability under ambient conditions. In spite of the interest drawn by these systems their successful application in optoelectronic devices have been limited. Most of the problems reside in the dichotomy between quantum confinement and the necessity of electronic wave function overlap to allow electrical transport. This problem is enhanced by the insulating nature of the organic ligands used to passivate and solubilize the NCs. Recently several authors have reported that the use of small conjugated ligands is a way to overcome these problems. I will report how PbS NCs with benzene dithiols ligands can be use as active layer for efficient solar cells, [1] with power conversion efficiencies approaching 4% and fill factors of 60% under AM1.5 illumination. The effect of different NCs` size on the performance and key parameters of the devices will be discussed together with peculiar features of the device functioning [2]. Finally I will discuss as PbS NCs together with conjugated polymers can give new possibility towards broad absorption of the solar spectrum.[3]

[1] K. Szendrei, W. Gomulya, M. Yarema, W. Heiss and M. A. Loi, Appl. Phys. Lett. 97, 203501 (2010).
[2] K. Szendrei, M. Speirs, W. Gomulya, D. Jarzab, M. Manca, O. V. Mikhnenko, M. Yarema, B. J. Kooi, W. Heiss, and M. A. Loi, Adv. Funct. Mater. 22, 1598 (2012).
[3] C. Piliego, M. Manca, R. Kroon, M. Yarema, K. Szendrei, M. Andersson, W. Heiss and M. A. Loi, J. Mater. Chem., J. Mater. Chem., 22, 24411 (2012).


Astronomers Measure the Deceleration of the Universe before Dark Energy

Young Universe Expanded Slowly During Last 14 Billion Years, Expansion Slowed and then Sped Up

Astrophysicist Kyle Dawson stands in front of the 2.5-meter Sloan Telescope at Apache Point Observatory in New Mexico. Photo Credit: Dan Long, Apache Point Observatory

Nov. 12, 2012 – Like a roller coaster that crawls slowly uphill and then zooms downhill, the universe expanded at a much slower rate 11 billion years ago than it has during the past 5 billion years, says a new study co-authored by Kyle Dawson, a University of Utah astrophysicist.

Light from 60,000 super-bright objects known as quasars served as flashlights to illuminate hydrogen gas between Earth and objects in the distant, early universe.

“We reconstructed a 3-D map of the hydrogen gas, and from the map, we learned about the processes by which the universe expanded and grew in the first 3 billion years,” says Dawson, an assistant professor of physics and astronomy at the University of Utah and a member of the third Sloan Digital Sky Survey, or SDSS-III, which conducted the study.

Scientists believe the universe formed some 13.8 billion years ago in a sudden expansion of matter and energy known as the Big Bang. Previous research showed its expansion has been speeding up for the past 5 billion years. The new study – performed using the 2.5-meter Sloan Telescope at Apache Point Observatory in New Mexico – is the first to make measurements showing expansion of the universe was slowing for the first 3 billion years after the Big Bang. The expansion later crested the top of the “roller coaster” and began to expand more rapidly some 5 billion years ago.

 Full press release.


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