Yaroslav I. Odarchenko

Tailoring the microstructure and charge transport in conjugated polymers by alkyl side-chain engineering

Charge transport in conjugated polymers is critical to most optoelectronic devices and depends strongly on the polymer structure and conformation in the solid state. Understanding the correlations between charge carrier mobility, energy disorder and molecular assembly is therefore essential to improve device performances. Alkyl side-chains contribute to intermolecular interactions and are key to controlling the polymer microstructure and electronic properties. Investigating a set of polymers with common conjugated units but different side-chain functionalization provides new insights into the complex structure–transport relationship. Here, field-effect transistors and space-charge-limited current devices are used together with in situ grazing-incidence wide-angle X-ray scattering to study charge transport and morphology in a series of donor–acceptor copolymers. Probing hole mobility as a function of carrier density and orientation permits us to assess energy disorder and hopping rate anisotropy, while X-ray diffraction allows us to link transport properties to the polymer microstructure. We show that branched side-chains enhance structural and energy disorder and lead to isotropic transport, whereas linear chains induce either a common lamellar structure or a more exceptional pseudo-hexagonal columnar phase with a helicoidal polymer conformation. The latter enhances out-of-plane mobility but increases energy disorder possibly due to larger interring torsion angles.

Tailoring of mechanical properties of derivatized natural polyamino acids through esterification and tensile deformation

Tensile deformation was applied to the naturally produced poly-γ-glutamic acid, which can be enzymatically degraded and is, therefore, of interest for biomedical use. However, natural polyamino acids have a similar chemical structure to synthetic polyamides (“nylons”), which are known to feature strong inter-molecular hydrogen bonding that prevents large-scale molecular motion in their solid state. Through esterification, this hydrogen bonding was partially shielded, allowing orientation of the polyamino acid macromolecules through tensile deformation. An increase in Youngs modulus and tensile strength was achieved of solution-cast films of the chemically modified poly-γ-glutamic acids, consistent with enhanced uniaxial polymer chain orientation. The latter was confirmed by both wide-angle X-ray scattering and polarized Raman spectroscopy. The films thus produced were found to be non-cytotoxic. These mechanically tailorable, biocompatible polymers may be excellent candidates for use in musculoskeletal tissue engineering applications that have different loading requirements within the body.

Non‐Radial Growth of Helical Homopolymer Crystals: Breaking the Paradigm of the Polymer Spherulite Microstructure

Radial symmetry is essential for the conventional view of the polymer spherulite microstructure. Typically it is assumed that, in the course of the spherulite morphogenesis, the lamellar crystals grow radially. Using submicron X-ray diffraction, it is shown that in banded spherulites of poly(propylene adipate) the crystals have the shape of a helix with flat-on crystals winding around a virtual cylinder of about 6 µm in diameter. The helix angle of 30° implies that the crystal growth direction is tilted away from the spherulite radius by this angle. The implications of the helical crystal shape contradict the paradigm of the spherulitic microstructure. The radial growth rate of such spherulites does not correspond to the crystal growth rate, but to the propagation rate of the virtual cylinder the ribbons wind around.

Morphological and structural interface characterization in multilayer inverted polymer solar cells

The interfaces in multilayer inverted polymer solar cells based P3HT / PCBM bulk heterojunction were prepared and studied on a flexible polyethylene naphthalate (PEN) substrate. The annealing effect on each interface of the PEN/ITO/ZnO multilayer and also ZnO/P3HT:PCBM on the structural, photophysical and photovoltaic properties was investigated and scrutinized directly on OPV device using the tomographic atom probe (TAP), scanning electron microscope (SEM) and microfocus X-rays techniques. We carried out a three-dimensional reconstruction of each interfaces of the multilayer containing PEN / ITO, ZnO / ITO and P3HT: PCBM / ZnO to analyze interface property and its influence on the morphology of the photoactive film. These analyzes provide strong evidence that the morphology of the interfaces induced by the modification of the structure of each layer of the BHJ devices leading to an anisotropic morphology and good orientation of P3HT crystals with PCBM Aggregates and ZnO and thus to lead to a significant change of Characteristics of photovoltaic devices.