E Erwan Sourty

Three-Dimensional Nanoscale Organization of Bulk Heterojunction Polymer Solar Cells

In this study, the three-dimensional (3D) nanoscale organization in the photoactive layers of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) is revealed by electron tomography. Morphologies suggested by previous experimental evidence were, for the first time, observed directly with a nanometer resolution and studied in detail. After annealing treatment, either at elevated temperature or during slow solvent evaporation, genuine 3D nanoscale networks are formed with high crystalline order and favorable concentration gradients of both P3HT and PCBM through the thickness of the photoactive layer. These favorable morphological changes account for a considerable increase of the power conversion efficiency in corresponding solar cell devices.

Electron Tomography on Micrometer-Thick Specimens with Nanometer Resolution

Transmission electron microscopy (TEM) is a well-established technique to explore matter down to the atomic scale. TEM tomography methods have been developed to obtain volume information at the mesoscopic dimensions of devices or complex mixtures of multiphase objects with nanometer resolution, but these methods are in general only applicable to relatively thin specimens with a few hundred nanometer thickness at most. Here we introduce an approach based on scanning TEM (STEM) tomography that pushes the resolution in three dimensions down to a few nanometers for several micrometer ultrathick specimens using a conventional TEM with 300 kV accelerating voltage, and we demonstrate its versatility for materials research and nanotechnology.

The microstructure of petroleum vacuum residue films for bituminous concrete: a microscopy approach

Selected carbon‐rich refinery residues (‘binders’) mixed with mineral particles can form composite materials (‘bituminous concrete’) with bulk mechanical properties comparable to those of cement concrete. The microstructural mechanism underlying the remarkable composite properties has been related to the appearance of a rigid percolating network consisting of asphaltenes and mineral particles [ Wilbrink M. et al. (2005) Rigidity percolation in dispersions with a structured visco‐elastic matrix. Phys. Rev. E71, 031402]. In this paper, we explore the microstructure of thin binder films of varying thickness with a number of microscopic characterization techniques, and attempt to relate the observed microstructure to the distinctive mechanical behaviour. Two binders, only one of which has been proven to be suitable for bituminous concrete were investigated, and their microstructure compared. Both binders show the formation of asphaltene aggregates. The binder suitable for bituminous concrete is distinguished by the fact that the asphaltenes show a stronger tendency towards such aggregation, due to a higher concentration and less stabilization in the maltene phase. They also show a clear affinity to other species (such as waxes) and may act as nucleation sites for crystals and aggregates of those species.

High-Angle Annular Dark Field Scanning Transmission Electron Microscopy on Carbon-Based Functional Polymer Systems

Two purely carbon-based functional polymer systems were investigated by bright-field conventional transmission electron microscopy (CTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For a carbon black (CB) filled polymer system, HAADF-STEM provides high contrast between the CB agglomerates and the polymer matrix so that details of the interface organization easily can be revealed and assignment of the CB phase is straightforward. For a second system, the functional polymer blend representing the photoactive layer of a polymer solar cell, details of its nanoscale organization could be observed that were not accessible with CTEM. By varying the camera length in HAADF-STEM imaging, the contrast can be enhanced between crystalline and amorphous compounds due to diffraction contrast so that nanoscale interconnections between domains are identified. In general, due to its incoherent imaging characteristics HAADF-STEM allows for reliable interpretation of the data obtained.

CONTROLLING THE 3D NANOSCALE ORGANIZATION OF BULK HETEROJUNCTION POLYMER SOLAR CELLS

In this study, the three dimensional nanoscale organization in the photoactive layers of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) is revealed by transmission electron tomography. After annealing treatment, either at elevated temperature or during slow solvent evaporation, nanoscale interpenetrating networks are formed with high crystalline order and favorable concentration gradients of both components through the thickness of the photoactive layer. Such a tailored morphology accounts for the considerable increase of the power conversion efficiency in corresponding solar cell devices.

Annular dark-field scanning transmission electron microscopy (ADF-STEM) tomography of polymer systems

We have utilized bright-field conventional transmission electron microscopy tomography and annular dark-field scanning transmission electron microscopy (ADF-STEM) tomography to characterize a well-defined carbon black (CB)-filled polymer nanocomposite with known CB volume concentration. For both imaging methods, contrast can be generated between the CB and the surrounding polymer matrix. The involved contrast mechanisms, in particular for ADF-STEM, will be discussed in detail. The obtained volume reconstructions were analysed and the CB volume concentrations were carefully determined from the reconstructed data. For both imaging modes, the measured CB volume concentrations are substantially different and only quantification based on the ADF-STEM data revealed about the same value as the known CB loading. Moreover, when applying low-convergence angles for imaging ADF-STEM tomography, data can be obtained of micrometre-thick samples.