—CANCELLED— Please note that our speaker forTOMORROW’scolloquium has had to CANCELhis visit.

No colloquium tomorrow. Our apologies.

Western University
DEPARTMENT OF PHYSICS AND ASTRONOMY

PHYSICS & ASTRONOMY COLLOQUIUM

Date:           Thursday, 25^th October 2012
Time:          1:30 p.m.
Location:    Physics & Astronomy Seminar Room 100

Dr. Carlos Silva
Département de Physique & Regroupement québécois sur les matériaux de pointe
Université de Montréal

“The dependence of charge recombination dynamics on the solid-state microstructure in neat, semicrystalline polymeric semiconductors”

ABSTRACT

We investigate the dynamics of geminate-polaron-pair recombination in the neat polymeric semiconductor regioregular poly(3-hexylthiophene) (P3HT) by means of time-resolved photoluminescence (PL) spectroscopy as a function of the solid-state microstructure, imposed by the molecular weight of the polymer and the thin-film processing route. At 10K, we observe delayed PL that originates from recombination of long-lived geminate-polaron pairs. The delayed PL decays over microsecond timescales, which is orders of magnitude longer than the exciton lifetime. We find that >36% of the total PL in solution-processed films of high-molecular-weight (331 kg/mol) material, which features a two-phase semicrystalline microstructure of alternating chain-entangled amorphous phases and crystalline lamella. In this microstructure, both the yield and the decay dynamics depend upon molecular weight. However, in films with dominant one-phase chain-extended microstructures, delayed PL is strongly suppressed. We thus find that a two-phase morphology leads to a higher yield of delayed PL with a higher distribution of decay rates than one-phase, chain-extended morphologies. By invoking a kinetic model including interconversion of excitons and geminate polarons, we conclude that geminate electron-hole pairs are on average further apart in two-phase morphologies, and that these microstructures result in a higher yield of geminate pairs that do not recombine within the timeframe of our experiment.

Refreshments will be available.

Department of Physics and Astronomy

MSc Public Lecture

Tahani Aldahri

Supervisor: Dr. John de Bruyn

“The behaviour of a falling particle in a funnel”

DATE:                  WEDNESDAY, 31^st October 2012

TIME:                   11:00 a.m.

PLACE:               Physics & Astronomy Room 100

All are welcome to attend.

Professor Emeritus Patrick Whippey spoke to the North London Rotary Club on October 17, 2012.

Patrick spoke of the London and District Science Fair, with which he has been associated for many years.

The Science Fair is an international program that students from grades 4-5 through
to grade 12 can participate in. Science projects on various topics can be submitted by those students who choose to participate.

Winners can work their way through the ranks of city, regional, national and international competitions. Over the years there have been a number of London students who have done extremely well and have continued in the field of science and done very well not only for themselves
but for mankind. The local committee is quite sure that there is something in our water that helps
to produce these great scientific gene pools. This is a totally volunteer program and helpers are needed in many areas. Financing is also done on a fund raising / donation basis.

After his talk, Patrick was presented with a ‘Polio Plus’ award for his contribution.

Poster: http://www.physics.uwo.ca/colloquia/Colloquia%20Notices/2012_2013/JOHN%20Colloquium.pdf

Western University
DEPARTMENT OF PHYSICS AND ASTRONOMY

PHYSICS & ASTRONOMY COLLOQUIUM

Date:               Thursday, 1^st November 2012
Time:              1:30 p.m.
Location:        Physics & Astronomy Seminar Room 100

Dr. Sajeev John
Department of Physics
University of Toronto

“Light Trapping and Solar Energy Harvesting in Thin Film Photonic Crystals”

ABSTRACT

Photonic crystals are widely known for their light-trapping capabilities. This is often associated with the occurrence of a photonic band gap or other suppression in the electromagnetic density of states. This enables guiding of light on an optical micro-chip and unprecedented forms of strong-coupling between light and matter. In the past, practical applications of these effects have focused on information technology. More recently, an important opportunity has emerged in the area of energy technology. This arises from light-trapping in the higher bands of a photonic crystal, where the electromagnetic density of states is enhanced rather than suppressed. This enables unprecedented strong absorption of sunlight in a material with weak intrinsic absorption.

We describe designs of 3D photonic crystal silicon-based solar cells that enhance the overall absorption of sunlight using architectures consisting of just 1 micron (equivalent bulk thickness) of silicon. These crystals trap light through a parallel-to-interface negative refraction (PIR) effect and other optical resonances that occur over a broad angular and frequency range. These 3D photonic crystals exhibit an enhanced electromagnetic density of states, consisting of slow group velocity modes, in which the flow of energy is transverse to the depth of a thin film of material. In the case of a modulated nanowire photonic crystal solar cell, it is possible to absorb roughly 75% of all available sunlight in the wavelength range of 400–1100 nm, using one micron of silicon. In the case of conical nano-pore silicon photonic crystal, roughly 85% of all available sunlight is absorbed. The power conversion efficiencies of these one-micron photonic crystals rival those of present-day solar cells using up to 300 microns of silicon. These photonic crystals offer additional opportunities for solar spectral reshaping to rival and possibly surpass the famous Shockley-Queisser power conversion efficiency limit of 33%.

Coffee & snacks will be available.