RESUMEN
Polydiacetylenes are well-established one-dimensional organic semiconductors that have been generated by photochemical and thermal polymerizations of diacetylenes in single crystals, gel phases, thin films, and membranes. Their formation in mesophases, such as liquid crystals, has been surprisingly little studied although higher-ordered mesophases should support the topochemical polymerization of diacetylenes (1,3-butadiyne groups) and may give access to large domains of uniformly aligned materials. The polymerization of diacetylenes in a mesophase may also increase the stability of the self-assembled supramolecular structure. Here, the dye and discotic mesogen tetraazaporphyrin was decorated with eight diacetylene-containing alkyl chains to probe its mesomorphism and conversion into multifunctional polydiacetylene materials. While the incorporation of diacetylene groups supports columnar mesomorphism, successful photopolymerization required the presence of directing amide groups that suppressed columnar in favor of nematic mesomorphism. Still, the polymerization of the nematic mesophase generated a soluble nematic polydiacetylene of significantly higher molecular weight (Mn = 77 kDa or 25 monomer units by gel permeation chromatography) than what has been obtained in gel phases of related compounds. The formation of polydiacetylene was confirmed by Raman spectroscopy, and its nematic structure was verified by UV-vis spectroscopy, polarized optical microscopy, and X-ray diffraction. Both its nematic structure and the incorporation of eight side chains per discotic unit provide the polydiacetylene with sufficient solubility for casting thin films on substrates. Atomic force microscopy studies of films on silicon wafers revealed a grid-like structure of connected nanofibers. This study demonstrates the requirements for the formation of multifunctional mesomorphic polydiacetylene materials from mesomorphic precursors and their advantages. Optimization of the presented molecular design should give access to other mesophases and, consequently, functional polydiacetylene materials with tunable structures and optoelectronic properties.
RESUMEN
Organic bulk heterojunction solar cells are promising candidates as future photovoltaic technologies for large-scale and low-cost energy production. It is, therefore, not surprising that research on the design and preparation of these types of organic photovoltaics has attracted a lot of attention since the last two decades, leading to constantly growing values of energy conversion and efficiency. Combined with the possibility of a large-scale production via roll-to-roll printing techniques, bulk heterojunction solar cells enable the fabrication of conformable, light-weight and flexible light-harvesting devices for point-of-use applications. This perspective review will highlight the recent advances toward mechanically robust and intrinsically stretchable bulk heterojunction solar cells. Mechanically robust fullerene-based and all-polymer devices will be presented, as well as a comprehensive overview of the recent challenges and characterization techniques recently developed to overcome some of the challenges of this research area, which is still in its infancy.
RESUMEN
A new strategy for influencing the solid-state morphology of conjugated polymers was developed through physical blending with a low-molecular-weight branched polyethylene. This nontoxic and low-boiling-point additive was blended with a high-charge-mobility diketopyrrolopyrrole-based conjugated polymer, and a detailed investigation of the new blended materials was performed by various characterization tools, including X-ray diffraction, UV-vis spectroscopy, and atomic force microscopy. Interestingly, the branched additive was shown to reduce the crystallinity of the conjugated polymer while promoting aggregation and phase separation in the solid state. Upon thermal removal of the olefinic additive, the thin films maintained a lower crystallinity and aggregated morphology in comparison to a nonblended polymer. The semiconducting performance of the new branched polyethylene/conjugated polymer blends was also investigated in organic field-effect transistors, which showed a stable charge mobility of around 0.3 cm2 V-1 s-1 without thermal annealing, independent of the blending ratio. Furthermore, using the new polyethylene-based additive, the concentration of a conjugated polymer required for the fabrication of organic field-effect transistor devices was reduced down to 0.05 wt %, without affecting charge transport, which represents a significant improvement compared to usual concentrations used for solution deposition. Our results demonstrate that the physical blending of a conjugated polymer with nontoxic, low-molecular-weight branched polyethylene is a promising strategy for the modification and fine-tuning of the solid-state morphology of conjugated polymers without sacrificing their charge-transport properties, thus creating new opportunities for the large-scale processing of organic semiconductors.