Center for Excitonics
Seminar Series Announcement
The Center for Excitonics is an Energy Frontier Research Center funded by
the
U.S. Department of Energy, Office of Science and Office of Basic Energy
Sciences
The Center for Excitonics invites you to join us at the next seminar of
the
Spring 2010 series. Please forward this information on to others who
might be
interested in attending this and other center seminars.
Title: Polymer Solar Cells: New Materials, 3D Morphology, and
Tandem Devices
Presenter: René Janssen
Organization: Molecular Materials and Nanosystems, Eindhoven University
of Technology
Date: March 2, 2010
Time: 3:00 - 4:00pm
Place: Haus Room 36-428
Center URL:
www.rle.mit.edu/excitonics
Seminar URL:
www.rle.mit.edu/excitonics/janssen-030210.html
Abstract
Polymer solar cells offer an opportunity for low-cost, large area
renewable energy production. These devices use a phase separated blend of
two organic semiconductors with energy levels that lead to intermolecular
charge transfer after photoexcitation. The power conversion efficiency of
polymer solar cells depends on the quantum and energy efficiency by which
photons from the sun are absorbed and can be converted into current in an
external circuit. To be efficient, absorption of light as well as charge
generation, transport and collection all have to occur with high quantum
efficiency and with minimal losses in energy. Presently, the best polymer
solar cells reach power conversion efficiencies over 7% in solar light.
The lecture will address three recent advances in this field.
Design strategies and synthesis of new conjugated polymers for efficient
(>5%) solar cells will be shown. The new materials are characterized by an
extended optical absorption and feature high, balanced mobilities for
charges. The materials have an appreciable tendency to aggregate and can
be processed together with C60 and C70 derivatives into efficient single
layer solar cells, depending on molecular weight and processing
conditions. Prospects for further enhancement will be discussed.
An important aspect of polymers solar cells is the nanoscale morphology of
the active layer. High resolution three-dimensional electron tomography
provides unprecedented insights into the actual heterojunctions present in
hybrid polymer:metal oxide solar cells. The nanoscale morphology can be
related to the device performance via exciton diffusion and exciton
quenching, and the presence of suitable continuous percolation pathways
for photogenerated charge carriers to reach the respective electrodes in
both phases. The data provide a unique new insight in the operation of
bulk heterojunction devices and provides directions to further
improvements.
By conserving a larger fraction of the incident photon energy, tandem
solar cells form a promising strategy to further increase the power
conversion efficiency of organic photovoltaics beyond the limits of single
junctions. The performance of tandem cells can be accurately predicted by
combining the characteristics of representative single junction cells and
allows establishing the optimal device layout. Using this strategy a
solution processed polymer tandem cell with an efficiency of 4.9% under
AM1.5G conditions has been obtained, higher than the efficiency that can
be reached with the individual materials for optimized solar cells.
Bio
René Janssen is full professor in chemistry and physics at the Eindhoven
University of Technology (TU/e) since 2000. He received his Ph.D. in 1987
from the TU/e for a thesis on electron spin resonance and quantum chemical
calculations of organic radicals in single crystals. He was lecturer at
the TU/e since 1984, and a senior lecturer in physical organic chemistry
since 1991. In 1993 and 1994 he joined the group of Professor Alan J.
Heeger (Nobel laureate in 2000) at the University of California Santa
Barbara as associate researcher to work on the photophysical properties of
conjugated polymers. René Janssen has been visiting professor at the
University of Angers in France from 2000-2006 and was Tarrant Lecturer at
the University of Florida in 2009.
The research of his group focuses on functional-conjugated molecules,
macromolecules, nanostructures, and materials that may find application in
advanced technological applications. Synthetic organic and polymer
chemistry are combined with advanced time-resolved optical spectroscopy,
electrochemistry, morphological characterization and the preparation of
prototype devices to accomplish these goals. In recent years many of the
activities have concentrated on organic and polymer solar cells. He has
co-authored more than 330 scientific papers in this field and has
supervised 30 PhD students.
Teaching activities at the TU/e involve various courses in Bachelor,
Master, and graduate levels on subjects like chemical bonding, molecular
spectroscopy and photophysics, organic electronics, and solar cells.
In 1999 René Janssen received the ‘Pionieer’ award from the Chemistry
Science Branch of the Netherlands Organization for Scientific Research and
in 2000 he was co-recipient of the René Descartes Prize from the European
Commission for outstanding collaborative research.
René Janssen currently serves as editor of “Organic Electronics” and is
member of the editorial boards of Advanced Functional Materials, Journal
of Materials Chemistry, and Advanced Energy Materials.