Correlation of fluorescence microscopy, electron microscopy, and NanoSIMS stable isotope imaging on a single tissue section.

Correlative light and electron microscopy allows localization of specific molecules at the ultrastructural level in biological tissue but does not provide information about metabolic turnover or the distribution of labile molecules, such as micronutrients. We present a method to directly correlate (immuno)fluorescent microscopy, (immuno)TEM imaging and NanoSIMS isotopic mapping of the same tissue section, with nanometer-scale spatial precision. The process involves chemical fixation of the tissue, cryo sectioning, thawing, and air-drying under a thin film of polyvinyl alcohol. It permits to effectively retain labile compounds and strongly increases NanoSIMS sensitivity for 13C-enrichment. The method is illustrated here with correlated distribution maps of a carbonic anhydrase enzyme isotype, ß-tubulin proteins, and 13C- and 15N-labeled labile micronutrients (and their anabolic derivates) within the tissue of a reef-building symbiotic coral. This broadly applicable workflow expands the wealth of information that can be obtained from multi-modal, sub-cellular observation of biological tissue.

Charge separation pathways in a highly efficient polymer: fullerene solar cell material.

PBDTTPD is one of the best conjugated polymers for solar cell applications (up to 8.5% efficiency). We have investigated the dynamics of charge generation in the blend with fullerene (PCBM) and addressed highly relevant topics such as the role of bulk heterojunction structure, fullerene excitation, and excess energy. We show that there are multiple charge separation pathways. These include electron transfer from photoexcited polymer, hole transfer from photoexcited PCBM, prompt (<100 fs) charge generation in intimately mixed polymer:fullerene regions (which can occur from hot states), as well as slower electron and hole transfer from excitons formed in pure PBDTTPD or PCBM domains (diffusion to an interface is necessary). Very interestingly, all the charge separation pathways are highly efficient. For example, the yield of long-lived carriers is not significantly affected by the excitation wavelength, although this changes the fraction of photons absorbed by PCBM and the amount of excess energy brought to the system. Overall, the favorable properties of the PBDTTPD:PCBM blend in terms of morphology and exciton delocalization allow excellent charge generation in all circumstances and strongly contribute to the high photovoltaic performance of the blend.

Intensity Dependent Femtosecond Dynamics in a PBDTTPD-Based Solar Cell Material.

PBDTTPD is a conjugated polymer with high power conversion efficiency if used in organic solar cells together with fullerene derivatives. We have investigated for the first time the excited state dynamics of pristine PBDTTPD thin film as well as the ultrafast evolution of charge carriers in PBDTTPD:PCBM bulk heterojunction blend using femtosecond transient absorption spectroscopy. In the latter, charges appear within the time resolution of the experiment (<100 fs), but clean spectral signatures allowed to directly follow slower ∼1 ps charge separation. Only the slower quenching component competes with exciton-exciton and exciton-charge annihilation, leading to a reduced yield of charge carriers at high laser fluence. Our excellent measuring sensitivity made it possible to reduce pump power to a point where annihilation is quasi suppressed. In this case >80% of charges survive after 1 ns; the rest recombines (most probably geminately) on the 200 ps time scale.