Infoscience

Thesis

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

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.

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