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: Charge-transfer absorption and emission in polymer:
fullerene solar cells
Presenter: Koen Vandewal
Organization: Linkoping University
Date: April 13, 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/vandewal-041310.html
Abstract
For an efficient conversion of photons to electrons by organic materials,
the presence of a material interface between an electron donating and
electron accepting material is crucial. Most successful active layers for
organic solar cells today comprise a blend of conjugated polymers as
electron donors and fullerenes as electron acceptors, reaching power
conversion efficiencies (h) up to 8%. In order to find pathways to
increase h further, fundamental properties of the electronic states at the
donor/acceptor (D/A) interface and their role in determining and limiting
h, are investigated in this work.
In order to probe these interfacial properties directly, highly sensitive
measurements of the photocurrent and electroluminescent spectra of organic
D/A photovoltaic devices are performed. For all material combinations
exhibiting a decent charge carrier generation, weak absorption and
emission involving a CT state with energy ECT lower than the lowest
optical gap of both donor and acceptor can be observed. Efficient
population and subsequent dissociation of the CT state determines the
short-circuit current (Jsc).
Also the open-circuit voltage (Voc) can be related to CT properties: The
principles of reciprocity and detailed balance provide a relation between
the CT absorption and emission properties, and Voc. This relation is
shown to be valid for a range polymer:fullerene photovoltaic devices, at
different temperatures and illumination intensities. When measured under
solar illumination, we find an energetic difference between ECT and qVoc
of ~0.6 eV for this type of photovoltaic devices. The origin of this
difference is twofold. About 0.25 eV of this energetic loss is due to the
radiative recombination through the CT state. The remaining ~0.35 eV is
due to additional, non-radiative recombination mechanisms.
To conclude, upper bounds for Jsc, Voc and h are derived, under the
assumption of perfect conditions for charge generation and recombination.
Ideal values for CT state properties and optical gap of the main absorber
are discussed.
Bio
Koen Vandewal received his master’s degree in electro-optical engineering
from Gent University (Belgium) in 2004 and his Ph.D. from Hasselt
University (Belgium) in 2009. Currently, he is a postdoctoral researcher
at Linköping University (Sweden). Research interests include charge
generation and recombination processes in organic opto-electronic devices.