Günter Reiter

Anisotropic Charge Transport in Spherulitic Poly(3-hexylthiophene) Films

Poly(3-hexylthiophene) (P3HT) is one of the most important semiconducting polymers for organic photovoltaics and optoelectronics.[1] Yet, because of the structural complexity of semicrystalline polymers, comparatively little is known conclusively about the relationship between charge transport and microstructure in P3HT-containing semiconductors.[2–9] This lack of information arises mainly from an inability to manufacture samples with spontaneous long-range crystalline order approaching macroscopic dimensions, that allow characterisation in a similar fashion to inorganic single crystals. We have overcome this limitation and grown well-ordered P3HT spherulites, sufficiently large to measure charge transport within single oriented crystal domains, enabling measurements of the charge mobility both parallel and perpendicular to the characteristic π-stacked nanocrystalline lamellae and across spherulite boundaries. The importance of local order and molecular orientation in determining the electronic properties of polymer semiconductor films is now well established.[3] In the case of the poly(alkylthiophenes) (PAT)s an edge-on orientation of the molecules, such that the alkyl sidechains (a-axis) stand vertically and both the chain backbone (c-axis) and π-stacking direction (b-axis) lie in the plane of the film, is critical in achieving high in-plane

Controllable Processes for Generating Large Single Crystals of Poly(3‐hexylthiophene)

Sowing the seeds: A simple strategy based on self-seeding allows large single crystals of long regioregular poly(3-hexylthiophene) chains to be grown from solution. When appropriately crystallized, materials differing in their degrees of regioregularity and molecular weights formed monoclinic form II crystals with interdigitated hexyl side groups (see picture).

Light absorption of poly(3-hexylthiophene) single crystals

We report the local UV-Vis absorption behaviour of single crystals of conjugated poly-3-hexylthiophene (P3HT), obtained by crystallization in dilute solutions at elevated temperatures based on a self-seeding approach and characterized by high internal structural order.

Probing Properties of Polymers in Thin Films Via Dewetting

In our quest to make functional devices smaller, the thickness of polymer films has reached values even smaller than the diameter of the unperturbed mole- cule. Many experimental studies have been devoted to the determination of the behavior of such thin films as a function of film thickness and temperature. However, despite enormous efforts over the last few decades, our understanding of the origin of some puzzling properties of such thin films is still not satisfactory and several peculiar observations remain rather mysterious. In this context, we explore the consequences of film preparation, i.e., the transition from a dilute polymer solution to the glassy state, with respect to the properties of polymers in thin films. This transition is likely to result in residual stresses arising from out- of-equilibrium chain conformations due to rapid solvent loss. Consequently, depending on thermal history and ageing time, such films exhibit significant changes even in the glassy state, which we can quantify by performing detailed studies of visco-elastic dewetting of thin polystyrene films on solid substrates. We explore relaxation times, residual stresses, and temporal changes to the stability of non-equilibrated thin films as they progress toward stable equilibrium behaviors. We present some tentative ideas on the relation between the observed atypical mechanical and relaxational behavior and the metastable states introduced by sample preparation.

Covalent functionalization by cycloaddition reactions of pristine, defect-free graphene

Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken, and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

Generating long supramolecular pathways with a continuous density of states by physically linking conjugated molecules via their end groups.

Self-assembly of conjugated 2,5-dialkoxy-phenylene-thienylene-based oligomers on epitaxial monolayer graphene was studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). The formation of long one-dimensional (1D) supramolecular chain-like structures has been observed, associated to a physical linking of their ends which involved the rotation of the end thiophene rings in order to allow π-π stacking of these end-groups. dI/dV maps taken at an energy corresponding to the excited states showed a continuous electronic density of states, which tentatively suggests that within such molecular chains conjugation of electrons is preserved even across physically linked molecules. Thus, in a self-organization process conjugation may be extended by appropriately adapting conformations of neighboring molecules. Our STM results on such self-organized end-linked molecules potentially represent a direct visualization of J-aggregates.

Polymer crystallization under nano-confinement of droplets studied by molecular simulations

Fabrication of polymer nano-crystals proceeds usually through hierarchical ordering of the different-scale structures. Nano-scale patterns are produced first, which serve as a spatial template for subsequent polymer crystallization under nano-confinement. We begin with a survey of the effects of nano-confinement on polymer crystallization, mainly on the basis of the knowledge obtained from molecular simulations. After that, we report dynamic Monte Carlo simulations of polymer crystallization confined in nano-droplets. We observed that the shape of droplets on a solid substrate appears as a pancake, and both initiation and development of crystallization are depressed with the decrease of droplet size. Surface-induced crystal nucleation guides the dominant edge-on crystal orientation at high temperatures; however, its contribution to nucleation rates is not much greater than crystal nucleation in the volume of the droplet. At low temperatures, edge-on crystals are frequent at both substrate/polymer and polymer/air interfaces. In conclusion, molecular simulations can shed light on the microscopic mechanisms of polymer crystallization under nano-confinement.

Enhancing nucleation and controlling crystal orientation by rubbing/scratching the surface of a thin polymer film

We used the tip of an atomic force microscope (AFM) in the contact mode to scratch/rub the surface of a glassy polymer thin film, i.e., isotactic polystyrene (i-PS) at room temperature. After subsequent isothermal crystallization, an extremely high nucleation density of edge-on crystals within the rubbed region or at the edge of the scratched area was observed. Furthermore, a transition from edge-on to flat-on lamellae occurred beyond a certain distance from the edge of the scratched region. Our results demonstrate that both, soft rubbing or hard scratching, allow to lower the nucleation barrier for polymer crystallization and to control the orientation of the resulting crystalline lamellae. The role of scratching/rubbing on chain deformation and its relation to nucleation and crystal orientation in polymer thin films is discussed.

Surface-induced breakout crystallization in cylinder-forming P(I-b-EO) diblock copolymer thin films

Abstract.We have found very slow crystallization in thin films of cylinder-forming poly(isoprene-b -ethyleneoxide) (P(I-b -EO)) diblock copolymers with PEO being the minority block. The film was crystallized at room temperature after melting at 62 °C . Imaging methods were combined with X-ray reflectometry and grazing-incidence small-angle X-ray scattering and diffraction. Initially, hexagonally packed, amorphous PEO cylinders lie in the film plane. After 148 days, crystallized, finger-like terraces were observed over the entire film surface. The terrace height is 20% higher than the repeat distance in the as-prepared film. Thus, at the film surface, the cylinders have been destroyed by crystalline lamellae lying in the film plane. The PEO chain stems are perpendicular to the substrate surface and are once-folded and fully interdigitated. The substrate-near layers still consist of the hexagonally packed, amorphous PEO cylinders within the PI matrix.

Transient Cooperative Processes in Dewetting Polymer Melts.

We compare the high velocity dewetting behavior, at elevated temperatures, of atactic polystyrene (aPS) and isotactic polystyrene (iPS) films, with the zero shear bulk viscosity (η_{bulk}) of aPS being approximately ten times larger than iPS. As expected, for aPS the apparent viscosity of the films (η_{f}) derived from high-shear dewetting is less than η_{bulk}, displaying a shear thinning behavior. Surprisingly, for iPS films, η_{f} is always larger than η_{bulk}, even at about 50 °C above the melting point, with η_{f}/η_{bulk} following an Arrhenius behavior. The corresponding activation energy of ∼160±10  kJ/mol for iPS films suggests a cooperative motion of segments which are aligned and agglomerated by fast dewetting.