Younghun Shin

Abstract N‐type doping of polymer semiconductors is necessary to enable printable and efficient organic thermoelectric generators. A recently reported method relies on blending air‐stable benzimidazole derivative dopant molecules with good electron transporting materials, such as the well‐known poly{[ N , N ′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐ alt ‐5,5′‐(2,2′‐bithiophene)}, also known as PNDIT2. One of the main limitations to doping efficiency is miscibility of the dopant with the polymer. In order to overcome such limitation, controlled amounts of the covalently incorporated, meta ‐substituted monomer 1,3‐bis(2‐thienyl)benzene (TPT) (“kinked monomer”) are introduced into the otherwise straight backbone of PNDIT2. Differential scanning calorimetry shows that crystallinity of P(NDI‐ alt ‐[T2‐ co ‐TPT]) first decreases with increasing TPT content up to 5 mol%, but then increases again for higher TPT contents. Miscibility of P(NDI‐ alt ‐[T2‐ co ‐TPT]) with the dopant 4‐(1,3‐dimethyl‐2,3‐dihydro‐1 H ‐benzoimidazol‐2‐yl)‐ N , N ‐diphenylaniline increases with increasing TPT content up to 30 mol%. The electrical conductivity of doped P(NDI‐ alt ‐[T2‐ co ‐TPT]) films is reduced with respect to PNDIT2, owing to a lower charge mobility caused by TPT units which break conjugation. Nevertheless, the doping efficiency at high doping concentration is substantially improved, with an estimated ≈20‐fold increase with respect to PNDIT2, as a result of the improved miscibility of dopant and copolymer.

Organic thermoelectrics are attractive for the fabrication of flexible and cost-effective thermoelectric generators (TEGs) for waste heat recovery, in particular by exploiting large-area printing of polymer conductors. Efficient TEGs require both p- and n-type conductors: so far, the air instability of polymer n-type conductors, which typically lose orders of magnitude in electrical conductivity (σ) even for short exposure time to air, has impeded processing under ambient conditions. Here we tackle this problem in a relevant class of electron transporting, naphthalene-diimide copolymers, by substituting the imide oxygen with sulfur. n-type doping of the thionated copolymer gives rise to a higher σ with respect to the non-thionated one, and most importantly, owing to a reduced energy level of the lowest-unoccupied molecular orbital, σ is substantially stable over 16 h of air exposure. This result highlights the effectiveness of chemical tuning to improve air stability of n-type solution-processable polymer conductors and shows a path toward ambient large-area manufacturing of efficient polymer TEGs.

The ability of dip-pen nanolithography (DPN) to generate nano- or microarrays of soft or hard materials (e.g., small molecules, DNA, proteins, nanoparticles, sols, and polymers) in a direct-write manner has been widely demonstrated. The transporting of large-sized ink materials such as bacteria, however, remains a significant challenge with this technique. The size limitation of the water meniscus formed between the DPN tip and the solid surface becomes a bottleneck in such diffusion-based molecular transport experiments. Herein, we report a straightforward "stamp-on" DPN method that uses a nanostructured poly(2-methyl-2-oxazoline) hydrogel-coated tip and carrier agents to generate patterns of micrometer-sized Escherichia coli JM 109 bacterial cells. We demonstrate that this approach enables the deposition of a single bacterial cell array on a solid surface or arrays of layers of multiple cells by modulating the viscosity of the "ink" solution. Fluorescence microscopy images indicated that the deposited bacterial cells were kept alive on Luria-Bertani-agar layered solid surfaces after DPN patterning.

The use of high stiffness plastics to reduce the weight of mechanical systems has been implemented in various industrial fields. As a result, several studies to increase the adhesion strength of metal/polymer joints to improve the mechanical robustness of a system have been reported. In particular, as an alternative to existing adhesive bonding methods, research on the direct molding method has emerged. The direct molding method, which derives from the insert molding method, is based on the fabrication of microstructures on the metal surface to create mechanical interlocking. In this paper a new direct adhesion method is introduced that does not require additional heat management and instead makes use of an organic solvent. This new adhesion method was used to bond micro/nanostructured aluminum and acrylonitrile butadiene styrene (ABS) using chloroform. The bonding strength of the metal/polymer joints was tested by single-lap shear and T-peel tests. The shear strength of the Al-ABS systems showed a positive correlation with the height of the structures created on the Al roughened surfaces. The peel strength increased dramatically for the micro/nano hierarchical Al structures due to additional vertical shear interactions at the surface. Since this method does not require heat control, it improves the adhesion process efficiency, as well as increases the variety of adhesion designs.

The synthesis of a naphthalene diimide bithiophene copolymer P(EO-NDIT2) with branched, base-stable, and purely ether-based side chains is presented. Stille polycondensation leads to high molecular weights that are limited by methyl transfer and eventually T2 homocouplings. While extensive solution aggregation hampers molecular weight determination by conventional methods, NMR spectroscopy allows identification of both T2- (H and methyl) and NDI-related (methyl) end groups, enabling the determination of absolute number average molecular weights larger than Mn,NMR ∼100 kg/mol. Solvent- and temperature-dependent aggregation in solution is investigated by NMR and UV–vis spectroscopy. These results are used for solution doping of P(EO-NDIT2) with N-benzimidazole-based n-dopants. Spin coating from heated chlorobenzene solutions and using 4-(2,3-dihydro-1,3-dimethyl-1H-benzoimidazol-2-yl)-N,N-diisopropylaniline (N-DiPrBI) as the dopant leads to homogeneous films with highest conductivities up to 10–2 S/cm. Generally, N-DiPrBI concentrations as low as ∼5 wt % are sufficient to increase conductivity by orders of magnitude. Strikingly, maximum power factors up to 0.11 μW/mK2, although limited by conductivity, are achieved for the highest molar mass sample at a low dopant concentration of 2 wt % N-DiPrBI only.