T. Roland, A. Cheminal, Li Liu, J. Léonard, S. Haacke
The quest for renewable energy sources spurs research for new molecular materials, which enable the conversion of photons into current (hybrid or organic photovoltaic cells) or into chemical energy, as in the case of photocatalytic water splitting. Organic molecules and nanostructures are characterized by high absorption cross sections. But their dielectric constant is low, leading to exciton binding energies far above kbT. Another physical limitation that renders photo-current generation and collection in OPV cells difficult is the notoriously low mobility of excitons and electrons and holes. This has so far limited the power conversion efficiency (PCE) of OPV cells to 10%.
The advantages of these technologies are their ease of production (spin coating, ink printing, roll-to-roll fabrication of large surfaces) and the flexibility and lightweight of these thin film materials. OPV cells thus target application schemes that are alternative to the well-established semiconductor technologies. These may include coverage of otherwise unused curved surfaces, integration in textile or OPV modules for portable devices.
Our research is part of an interdisciplinary approach emerging from collaborations with chemists and material scientists developing innovative molecular systems. This is reflected by the numerous national and international collaborations (see below).
Our studies relying on ultrafast spectroscopy reveal the primary molecular processes of how photons transform into excitons, and how these form charge transfer states or bipolarons in thin films. Our interest in observing and understanding the fundamental molecular processes and material properties that limit the production of free charges is then the basis for an improvement of the molecular design and self-organization abilities. In particular, detrimental charge recombination is at the heart of our investigations.
A detailed quantitative analysis of these effects helps to identify the main bottlenecks of small PCEs: charge generation or collection, or both. However, new experimental approaches still need to be developed in order to bridge the wide range of time (femto-to milliseconds) and length scales (nano-to micrometers) involved in the physics of OPV cells.
Collaborations: S. Méry (IPCMS), N. Leclerc, R. Ziessel (ICPEES), T. Heiser (ICube), I. Burghardt (U Frankfurt), S. Ludwigs (U Stuttgart)
Funding: FP7 InterregIV project Rhin-Solar (www.rhinsolar.eu), ANR project PICASSO, ANR/DFG project MolNanoMat, icFRC (www.icfrc.fr) and LabEX NIE ( http://www.labex-nie.eu/).
Donor-Acceptor conjugated co-oligomers for OPV.
Thomas & Li: DAD
The use of liquid-crystalline molecules combining donor and acceptor properties have proven to be a promising solution to address the common limitations of organic blends (small conductivity and disordered structuration)[1]. Yet, performances achieved are still to be improved, as conversion efficiencies of only a few percent were achieved[2]. Understanding the principles underlying the generation of charges is thus of great interest in order to design better materials.
A family of molecules with such properties have been developed by the team of Stéphane Méry, combining an acceptor based on perylene diimide, and a donor based on fluorene and bithiophene derivatives[3]. Several variations of the donor block have been studied, in order to identify their effect on the photo-dynamic. Using spectro-electro-chemistry (collaboration with Sabine Ludwigs) to obtain the absorption spectra of the charged species and identify it within the transient absorption spectroscopy data, we were able to follow the formation and recombination of the charge transfer state, leading to observations as follow :
-the addition of a terminal amine does increase the lifetime of the charge transfer state, from several hundreds of picoseconds to a few nanoseconds.
-changing the benzene spacer by a benzothiadiazole remove any FRET from the donor to the acceptor, and leads to a slower formation of the charge transfer state (about 100 ps instead of about 10 ps).
– the length of the donor block doesn’t impact significantly the dynamic of the molecule.
Further studies and simulations are under progress in collaboration with the team of Irene Burghartd, in order to understand the physics behind these phenomena.Meijer et Al, Chem. Eur. J., 2002, 8, 19, 4470-4475J. Qu et Al, J. Mater. Chem. A, 2014, 2, 3632-3640P.-O. Schwarz et Al, J. Am. Chem. Soc., 2014, 136, 5981-5992
Small molecules blend BODIPY TB2/PCBM
T. Roland, A. Cheminal, Li Liu, J. Léonard, S. Haacke
The quest for renewable energy sources spurs research for new molecular materials, which enable the conversion of photons into current (hybrid or organic photovoltaic cells) or into chemical energy, as in the case of photocatalytic water splitting. Organic molecules and nanostructures are characterized by high absorption cross sections. But their dielectric constant is low, leading to exciton binding energies far above kbT. Another physical limitation that renders photo-current generation and collection in OPV cells difficult is the notoriously low mobility of excitons and electrons and holes. This has so far limited the power conversion efficiency (PCE) of OPV cells to 10%.
The advantages of these technologies are their ease of production (spin coating, ink printing, roll-to-roll fabrication of large surfaces) and the flexibility and lightweight of these thin film materials. OPV cells thus target application schemes that are alternative to the well-established semiconductor technologies. These may include coverage of otherwise unused curved surfaces, integration in textile or OPV modules for portable devices.
Our research is part of an interdisciplinary approach emerging from collaborations with chemists and material scientists developing innovative molecular systems. This is reflected by the numerous national and international collaborations (see below).
Our studies relying on ultrafast spectroscopy reveal the primary molecular processes of how photons transform into excitons, and how these form charge transfer states or bipolarons in thin films. Our interest in observing and understanding the fundamental molecular processes and material properties that limit the production of free charges is then the basis for an improvement of the molecular design and self-organization abilities. In particular, detrimental charge recombination is at the heart of our investigations.
A detailed quantitative analysis of these effects helps to identify the main bottlenecks of small PCEs: charge generation or collection, or both. However, new experimental approaches still need to be developed in order to bridge the wide range of time (femto-to milliseconds) and length scales (nano-to micrometers) involved in the physics of OPV cells.
Collaborations: S. Méry (IPCMS), N. Leclerc, R. Ziessel (ICPEES), T. Heiser (ICube), I. Burghardt (U Frankfurt), S. Ludwigs (U Stuttgart)
Funding: FP7 InterregIV project Rhin-Solar (www.rhinsolar.eu), ANR project PICASSO, ANR/DFG project MolNanoMat, icFRC (www.icfrc.fr) and LabEX NIE ( http://www.labex-nie.eu/).
