Electronic and Morphological Studies on Materials for Organic Solar Cells

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Sylvain Massip

Abstract

This thesis discusses electronic and morphological effects in new materials for the active layers, substrates and electrodes of organic solar cells.
The performance and loss mechanisms in polymer:fullerene blends based on a new family of donor-acceptor polymers differing only by the presence and position of their side-chains are discussed. By comparing transient absorption spectroscopy and device measurements, a kinetic model of the excitation in these organic solar cells is built. We show that subnanosecond geminate recombination limits device efficiencies and explains the difference between the three blends.
The morphology of polymer solar cells is studied by diverse electron mi-roscopy techniques. Thin cross-sections of solar cells are prepared by focused ion-beam milling and are imaged by transmission electron microscopy. Thus, the morphology of polymer:nanoparticle blends could be resolved. A higher concentration of CdSe at the bottom of the film in OC 1 C 10 PPV:CdSe blends is observed and a favourable change of morphology upon the random addition of 10% of phenanthroline-containing groups to a quinoxaline-fluorene alternating copolymer is demonstrated. In addition, by combining scanning transmission electron microscopy with energy dispersive X-ray spectroscopy, the concentration profile of P3HT in P3HT:PCBM blends is measured and discussed.
Finally, the fabrication of organic solar cells on alternative substrates is studied. It is shown that PET substrates release water that damages the electrodes. This problem is addressed by heating the substrate in a nitrogen atmosphere. We also demonstrate that, because of its non-uniform conduction on the micrometre level, a conductive coating based on a nanowire dispersion requires to be coated with high-conductivity PEDOT:PSS in order to be used as a transparent electrode for organic photovoltaics.

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