Fullerène | EPFL Graph Search

Un fullerène est une molécule composée de carbone pouvant prendre une forme géométrique rappelant celle d'une sphère, d'un ellipsoïde, d'un tube (appelé nanotube) ou d'un anneau. Les fullerènes sont similaires au graphite, composé de feuilles d'anneaux hexagonaux liés, mais contenant des anneaux pentagonaux et parfois heptagonaux, ce qui empêche la feuille d'être plate. Les fullerènes sont la troisième forme connue du carbone. Les fullerènes ont été découverts en 1985 par Harold Kroto, Robert Curl et Richard Smalley, ce qui leur valut le prix Nobel de chimie en 1996. Le premier fullerène découvert est le C60, il se compose de 12 pentagones et de 20 hexagones. Chaque sommet correspond à un atome de carbone et chaque côté à une liaison covalente. Il a une structure identique au dôme géodésique ou à un ballon de football. Pour cette raison, il est appelé « buckminsterfullerène » (en hommage à l'architecte Buckminster Fuller qui a conçu le dôme géodésique) ou « footballène ». Hi

Photochemical Transformations in Fullerene and Molybdenum Oxide Affect the Stability of Bilayer Organic Solar Cells

Okan Deniz, Roland Hany, Jakob Heier, Hui Zhang

Thin films of fullerene C-60 and molybdenum oxide (MoO3) are ubiquitously used as the electron acceptor material and hole extraction interfacial layer for the fabrication of organic photovoltaic (OPV) cells. It is well known that light exposure induces color changes in MoO3 (photochromism) and the formation of intermolecular bonds between C-60 molecules (photopolymerization). The influence of these photoinduced reactions on the long-term stability of OPV cells, however, has not previously been studied in detail. Here, a study and discussion of the early (

Wiley-V C H Verlag Gmbh2015

Ultrafast dynamics of photoinduced charge separation in cyanine- and polymer-based organic photovoltaic systems

Jelissa Risse

Organic photovoltaics (OPV) have the potential to provide low-cost solar-to-electricity converting devices. Improving such devices requires a deeper understanding of the ultrafast photoinduced processes occurring after light absorption. We have performed femtosecond (fs) transient absorption spectroscopy, as well as time-resolved electroabsorption (Stark effect) on model OPV systems to follow electron and hole transfer dynamics, as well as charge carrier motion in the active layer to the electrodes. Solid-state cyanine borate (Cy3-B) films undergo intra-ion pair reductive quenching on the picosecond (ps) timescale. We have found that when intermixing Cy3-B with a fullerene acceptor (PCBM), photoexcitation leads to the appearance of oxidized Cy3 in less than 70 fs. We correlated the rate and yield of Cy3 oxidation to the PCBM loading. We then investigated a cyanine Cy3-P in a bilayer geometry with fullerene C60. In this case, ultrafast electron transfer in less than 100 fs is followed by slower dissociation of interfacial charge transfer states in the presence of an electric field. A photoinduced Stark effect observed in neat C60 enabled to identify an initially delocalized excited state, which localizes in 360 fs. Lastly, we analyzed how microstructural changes in polymer pBTTT:PCBM blends impact free charge carrier formation and motion. We identified that the intermixed phase efficiently produces charge carriers within 100 fs, and that pure fullerene and pBTTT domains attract the charge carriers separating them further apart. This is ascribed to an energy cascade toward the neat domains. This thesis reveals ultrafast charge transfer at donor-acceptor interfaces in such OPV systems by means of fs transient absorption spectroscopy. Results emphasize that a second step is needed for charge separation in order to successfully compete with charge pair recombination in these systems. An electric field applied can act as a driving force to dissociate interfacial charge transfer states. Pure donor or acceptor domains attract holes or electrons, respectively, and reduce geminate charge recombination in blends. These second steps in charge separation were derived from photoinduced electroabsorption dynamics in transient absorption measurements, as well as time-resolved electroabsorption based on the Stark effect.

EPFL2015

Charge carrier dynamics at organic interfaces for polymer photovoltaics

Mariateresa Scarongella

The increased energy demand and the importance to find renewable energy resources have been addressed by a scientific effort to improve the power conversion efficiency of organic solar cells. In particular, polymer-based solar cells have experienced a great increase in power conversion efficiency, which recently reached a value over 10%. The possibility to further improve the performance of polymer-based solar cells has encouraged the research to have a close look at the key parameters, which play an important role in the mechanism of charge generation. Specifically, the photo-physical processes and the time scales involved in converting excitons to charges are still poorly understood. Also, the microscopic origin and the nature of the driving force allowing interfacial electron-hole pairs to become free charges and the role played by the sample microstructure in a bulk heterojunction have to be clarified. We have used transient absorption spectroscopy, which provides information about the excited states dynamics with

EPFL2015

Ultrafast dynamics of photoinduced charge separation in cyanine- and polymer-based organic photovoltaic systems

Jelissa Risse

EPFL2015