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Bill Pandit Thesis Defense 04/20/12

Thesis Defense

Bill Pandit

Friday, April 20, 2012
3:30pm (334 JFB)

Title: Ultrafast Photophysics of pi-Conjugated Polymers for Organic Photovoltaic Applications"

Abstract:

In this work we used the pump and probe photomodulation (PM) spectroscopy technique to measure the transient excitation dynamics in various pi- conjugated polymers (PCPs) films and blends with appropriate molecular acceptors. Using two different ultrafast laser systems, we extended the PM spectrum to cover a broad spectral range from 0.25 – 2.5 eV in the time domain from 100 fs to 1 ns with 150 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence, electro-absorption, doping-induced absorption, and x-ray diffraction to study the photoexcitations and doping induced states, as well as other optical properties of PCPs and polymer donor-fullerene acceptor blends. In addition to these measurements, we also fabricated organic photovoltaic (OPV) solar cell devices based on poly(3-hexyl-thiophene) [P3HT]/fullerene [PCBM] blend and estimated their power conversion efficiency (PCE) in relation to the optical studies. In (1.2:1) weight ratio of P3HT/PCBM blend that shows maximum donor (D) and acceptor (A) domain separation we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense. Instead there is a built-up of charge-transfer (CT) excitons at the D-A interfaces, which may dissociate into separated ‘free’ polarons in the D and A domains at a later time. Although the CT excitons are photogenerated much faster in D-A blends with a smaller domain size (such as in P3HT with random order), their dissociation is less efficient because of larger binding energy. Our results elucidate the charge photogeneration mechanism in D-A blends, and unravel the important role of the binding energy in generating ‘free’ charge polarons.

We also studied the photophysics of a low band gap polymer, namely poly-thienophene-benzodithiophene (PTB7) film and its blend with acceptor [6,6] phenyl C71 butyric acid methyl ester [PC71BM]. In the CW PM spectrum of PTB7/PC71BM blend, clear signatures of polarons are observed. Whereas PA bands related to triplet excitons and trapped polarons are observed in the PM spectrum of pristine PTB7 film. The transient ultrafast PA of PTB7 is dominated by a singlet exciton band at ~0.95 eV. In the transient ultrafast PA spectrum of PTB7/PC71BM blend we found singlet exciton, charge transfer exciton and a polaron band that are generated simultaneously; this is different from the transient PM spectrum of P3HT/PCBM blend. We also found that the charge transfer exciton in PTB7/PC71BM dissociates faster than in P3HT/PCBM blends. This may be one of the reasons for getting higher PCE of ~7.4 % in the PTB7/PC71BM based solar cells compared to PCE ~4 % in P3HT/PCBM based solar cells.

[1] “Two-step charge photogeneration dynamics in polymer/fullerene blends for photovoltaic applications”
Bill Pandit*, Sanjeev Singh*, Tek. P. Basel, & Z. V. Vardeny, under review in Phys. Rev. B.* equal contributions.

[2] “Photoexcitation dynamics in Poly [[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) and its fullerene blend”
Bill Pandit, Tek. P. Basel, Ella Olejnik, Uyen Hyunh & Z. V. Vardeny, in preparation for Phys. Rev. B.

[3] “Ultrafast optical studies of ordered poly (3-thienylene-vinylene) films”
E. Olejnik, B. Pandit, T. Basel, E. Lafalce, C.-X Sheng, C. Zhang, X. Jiang, & Z. V. Vardeny, under review in Phys. Rev. B.

[4] “Photoexcitation dynamics in isotope exchanged DOO-PPV”
Ella Olejnik, Bill Pandit, Tek Basel & Z. V. Vardeny, submitted to Appl. Phys. Lett.

[5] “Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications”
C.-X. Sheng, T. Basel, B. Pandit & Z. V. Vardeny, Organic Electronics 13, 1031 (2012).

[6] “Evidence for excimer photoexcitations in an ordered pi – conjugated polymer film”
K. Aryanpour, C-X. Sheng, E. Olejnik, B. Pandit, D. Psiachos, S. Mazumdar, & Z. V. Vardeny, Phys. Rev. B 83,155124 (2011).

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RECON: Self-Gated Cardiac Perfusion MRI: Edward DiBella

Self-Gated Cardiac Perfusion MRI by Edward DiBella

Thursday Apr. 19, 2:00-2:45pm,
INSCC Large Conference Room

RECON (UCAIR REsearch CONnections) provides a scientific forum for Ph.D.–level exchange of ideas, concepts, and research of broad interest in medical imaging.  Meeting 2-3 times monthly, 30 min. presentations are given by UCAIR faculty, postdocs, collaborators, and late-stage graduate students.  Topics will range broadly across medical imaging and the related sciences, and should include general-interest content that is not modality-specific.  The emphasis is on physics and engineering research, with some translational clinical components; more clinically-oriented presentations can be found at the monthly UCAIR Seminar held at SOM Classroom A/B.  Special-interest topics that delve deeper into a specific modality, problem, or theory will also be occasionally presented. RECON attendees are asked to request special-interest topics that would be appealing to a wide audience.  Everyone with an interest in medical imaging, small animal imaging, image processing, or just science in general is invited to attend (attendance by UCAIR students and trainees is mandatory).  Each RECON will begin with a few minutes of general UCAIR announcements and accolades, etc.; and coffee-&-bagel snacks will be provided.

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Dustin Winslow Thesis Defense 03/12/12

Thesis Defense

Dustin Winslow

Thursday, March 12, 2012
3:00pm (334 JFB)

Title: Atomic Scale Study of Dielectric Trap States Using Single Electron Tunneling Force Spectroscopy

Abstract:

The rapid advancement of technology has led to increasingly faster and smaller solid state devices. One reason for this rapid development is the dedicated effort to characterize the defects in the dielectric materials used in solid state electronics. However there are no techniques which allow for characterization of localized electron and hole trap states, in completely nonconducting films, with atomic scale spatial resolution. This talk will focus on the force detected tunneling techniques developed in the Williams lab over the last decade, with an emphasis on the recently developed single electron tunneling force spectroscopy (SETFS) technique. The density of localized trap states in SiO2, Si3N4 and HfO2 measured using SETFS will be compared to experimental results and theoretically predicted values found in the literature. The SETFS results from measurements made on each of these films is in good agreement with standard techniques. However, several states have been characterized that have only been theoretically predicted or are not identified in the literature. Finally, evidence of mobile charge in HfO2 will be presented and a possible mechanism proposed to explain the irreversible nature of the surface charging.

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The Nature of Things Keynote Lecture Dr. Brian Greene

From the Natural History Museum of Utah's website.

"Dr. Brian Greene
Professor of Physics and Mathematics at Columbia University

Location: Kingsbury Hall at the University of Utah
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Tickets $10.00 per person + $2.00 handling and service fee
Buy Tickets at KingTix Now

Join Dr. Brian Greene for a whirlwind exploration of one science story -- the quest to unravel some of the biggest mysteries of space and time -- and come away with a visceral understanding of why science matters.

Greene distills impossibly complex theoretical physics, like the concept of the multiverse, into a form we can all understand, and is dedicated to sharing science stories with broad audiences to combat perceptions of science as cold, distant, or irrelevant. Greene argues that understanding the transformative power of science – just like great music, art, or literature -- should be considered an indispensable part of what makes life worth living.

String theorist and author of The Elegant Universe, Brian Greene is one of the world’s leading theoretical physicists and a brilliant, entertaining communicator of cutting-edge scientific concepts. The Washington Post described him as “the single best explainer of abstruse concepts in the world today.”

In addition to writing best-selling books, Greene shares science stories through a range of interdisciplinary media.  He co-founded the World Science Festival, an annual celebration of science from cutting-edge research to works in theatre, film, and the arts inspired by scientific ideas. A graduate of Harvard and a Rhodes Scholar at Oxford, Greene is a professor in both Physics and Mathematics at Columbia University.

Buy Tickets at KingTix Now

Special Discount:  University of Utah Students, Faculty and Staff get a 10% ticket price discount when purchasing tickets at the Kingsbury Hall Box Office.  Not available on Internet or Phone orders. View Google Map

Nature of Things 2012 is underwritten by the R. Harold Burton Foundation. 

Complete Nature of Things 2012 Series Information"

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