Resolving the backbone tilt of crystalline poly(3-hexylthiophene) with resonant tender X-ray diffraction

The way in which conjugated polymers pack in the solid state strongly affects the performance of polymer-based optoelectronic devices. However, even for the most crystalline conjugated polymers the precise packing of chains within the unit cell is not well established. Here we show that by performing resonant X-ray diffraction experiments at the sulfur K-edge we are able to resolve the tilting of the planar backbones of crystalline poly(3-hexylthiophene) (P3HT) within the unit cell. This approach exploits the anisotropic nature of the X-ray optical properties of conjugated polymers, enabling us to discern between different proposed crystal structures. By comparing our data with simulations based on different orientations, a tilting of the planar conjugated backbone with respect to the side chain stacking direction of 30 ± 5° is determined

Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues

A series of polythiophenes with varying side chain density was synthesized, and their electrical and thermoelectric properties were investigated. Aligned and non-aligned thin films of the polymers were characterized in the neutral and chemically doped states. Optical and diffraction measurements revealed an overall lower order in the thin films with lower side chain density, also confirmed using polarized optical experiments on aligned thin films. However, upon doping the non-aligned films, a sixfold increase in electrical conductivity was observed for the polythiophene with the lowest side chain density compared to poly(3-hexylthiophene) (P3HT). We found that the improvement in conductivity was not due to a larger charge carrier density but an increase in charge carrier mobility after doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On the other hand, doped aligned films did not show the same trend; lower side chain density instead led to a lower conductivity and Seebeck coefficient compared to those for P3HT. This was attributed to the poorer alignment of the polymer thin films with lower side chain density. The study demonstrates that optimizing side chain density is a synthetically simple and effective way to improve electrical conductivity in polythiophene films relevant to thermoelectric applications

Drastic Enhancement of X-ray Scattering Contrast between Amorphous and Crystalline Phases of Poly(3-hexylthiophene) at the Sulfur K-Edge

Semicrystalline semiconducting polymers such as poly(3-hexylthiophene) exhibit hierarchical molecular ordering that influences their optoelectronic properties. As well as possessing crystalline order on the molecular scale, P3HT also exhibits regular ordering of crystalline lamellae on the nanoscale. This layering of crystalline and amorphous regions is characterized by the so-called "long period"which can be measured with small-angle X-ray scattering (SAXS). The weak scattering contrast between amorphous and crystalline phases generally requires SAXS measurement of bulk powder samples. Here, we show that by utilizing polarized tender X-rays tuned to the sulfur K-edge, strong contrast between amorphous and crystalline phases can be generated allowing for the long period of thin film P3HT samples to be easily observed. Furthermore, we show that the contrast generated results from differences in orientational order in the amorphous and crystalline regions. The use of resonant tender X-ray scattering is thus a promising technique for studying nanoscale ordering not only in semiconducting polymers but also for other soft matter systems

Diketopyrrolopyrrole based molecular semiconductors with intrinsic conductivity

Intrinsically conducting diketopyrrolopyrrole (DPP) compounds functionalised with pendant quaternary ammonium groups have been synthesised. The hydroxide forms were found to be the product of base hydrolysis of the DPP amide during synthesis, likely made possible by restricted resonance. This gave mixed chemical compositions of solutions and conductive films formed by drop-casting. Chemical decomposition of the hydrolysed compounds was also observed at temperatures above 85 ◦C. Conductivities of approximately 3 × 10− 4 S m− 1 were observed in processed films of DPP ammonium hydroxides. It was also found that trifluoroacetic acid (TFA) DPP precursors gave clear electron paramagnetic resonance (EPR) signals indicative of doping and exhibited conductivities one order of magnitude larger than the hydroxides. This opens the possibility of using ammonium TFAs as moieties for doping organic semiconductors

Critical analysis of self-doping and water-soluble n-type organic semiconductors: structures and mechanisms

Self-doping organic semiconductors provide a promising route to avoid instabilities and morphological issues associated with molecular n-type dopants. Structural characterization of a naphthalenetetracarboxylic diimide (NDI) semiconductor covalently bound to an ammonium hydroxide group is presented. The dopant precursor was found to be the product of an unexpected base catalyzed hydrolysis, which was reversible. The reversible hydrolysis had profound consequences on the chemical composition, morphology, and electronic performance of the doped films. In addition, we investigated the degradation mechanism of the quaternary ammonium group and the subsequent doping of NDI. These findings reveal that the products of more than one chemical reaction during processing of films must be considered when utilizing this promising class of water-soluble semiconductors