Tuning charge transport dynamics via clustering of doping in organic semiconductor thin films

A significant challenge in the rational design of organic thermoelectric materials is to realize simultaneously high electrical conductivity and high induced-voltage in response to a thermal gradient, which is represented by the Seebeck coefficient. Conventional wisdom posits that the polymer alone dictates thermoelectric efficiency. Herein, we show that doping — in particular, clustering of dopants within conjugated polymer films — has a profound and predictable influence on their thermoelectric properties. We correlate Seebeck coefficient and electrical conductivity of iodine-doped poly(3-hexylthiophene) and poly[2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl)-alt-(2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′-diyl)] films with Kelvin probe force microscopy to highlight the role of the spatial distribution of dopants in determining overall charge transport. We fit the experimental data to a phonon-assisted hopping model and found that the distribution of dopants alters the distribution of the density of states and the Kang–Snyder transport parameter. These results highlight the importance of controlling dopant distribution within conjugated polymer films for thermoelectric and other electronic applications

THE PROPERTIES OF GAMMA IRRADIATED ELASTOMERIC NANOCOMPOSITES BASED ON CHLOROSULFONATED POLYETHYLENE

In the case of irradiation of polymeric materials, the progress in oxidative degradation depends on absorbed dose, dose rate, exposure environment, energy of irradiation, chemistry of material, and previous state of ageing. It is known that the main effect of the interactions between gamma rays and rubber macromolecules is the formation of free radicals, whose further evolution can cause crosslinking with increase in the crosslinking density or chain scission. Chlorosulfonated polyethylene is specifically recommended for sheeting of cables in nuclear energetic plants. Elastomers based on this network precursor are resistant to wear and repeated deformation and have excellent irradiation resistance. The goal of the current work was to study the effects of γ-ray radiation for elastomers based on chlorosulfonated polyethylene in combination with nitrile rubber. The reinforcing filler was nano-silica. The irradiation was performed in the Co60 radiation sterilization unit. The level of ageing was evaluated using the hardness and swelling measurement. The use of silica nano-particles improved the swelling resistance in toluene after irradiation ageing

Thermoelectric Enhancement by Compositing Carbon Nanotubes into Iodine-Doped Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

Free-standing iodine-doped composite samples of poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with carbon nanotubes (NTs) showed thermoelectric (TE) power factors (PFs) up to 33 μW·m^–1·K^–2 after optimizing multiple factors, including: (1) sample fabrication solvent, (2) doping time, (3) average MEH-PPV molecular weight, (4) NT fraction in the composite, and (5) use of single-wall versus multi-wall nanotubes (SWNT and MWNT, respectively). Composite fabrication from halogenated solvents gave the best TE performance after iodine doping times of 2–4 h; performance drops substantially in ∼20 h doped samples. TE performance dropped after at least 24 h of removal from iodine vapor but was fully restored upon re-exposure to the dopant. Longer-chain MEH-PPV gave not only mechanically stronger films but also higher PFs in doped SWNT composites. MWNT composites gave low PFs, attributed to poor NT dispersion. Scanning electron microscopy showed increasingly extensive network formation as NT fraction increased in the composites; this phase separation provides charge transport pathways that improve thermoelectric PFs. The results support a strategy of producing phase-separated materials having both electrical conduction enhanced regions and Seebeck thermopower retaining regions to maximize organic TE response