An explanation for the high stability of polycarboxythiophenes in photovoltaic devices—A solid-state NMR dipolar recoupling study

Continuous operation of a polymer photovoltaic device under accelerated conditions for more than 1 year has been demonstrated (8760 h at 72 °C, 1000 W m⁻², AM1.5, under vacuum). Formation of hydrogen-bonded networks is proposed to be responsible for the long lifetime and high stability observed in photovoltaic devices employing polythiophene substituted with carboxylic-acid moieties under oxygen free conditions. ¹H and ¹³C solid-state NMR, IR, and ESR spectroscopy of unmodified and isotopically labeled polythiophenes were studied. Distances between the isotopically labeled carboxylic acid carbon atoms were measured by ¹³C solid-state magic-angle-spinning (MAS) NMR using symmetry-based double-quantum (2Q) dipolar recoupling. This revealed the presence of ¹³C–¹³C distances of 3.85 Å, which correspond to the C–C distance in hydrogen-bonded carboxylic acid dimers. In spite of the presence of carboxylic groups in the polymer as demonstrated by ¹³C CP/MAS NMR and IR spectroscopy, the absence of carboxylic protons in solid state ¹H NMR spectra indicate that they are mobile. We link the extraordinary stability of this system to the rigid nature, cross-linking through a hydrogen-bonded network and a partially oxidized state.     

Mechanistic and kinetic evaluation of organic disinfection by-product and assimilable organic carbon (AOC) formation during the ozonation of drinking water

Ozonation of drinking water results in the formation of low molecular weight (LMW) organic by-products. These compounds are easily utilisable by microorganisms and can result in biological instability of the water. In this study, we have combined a novel bioassay for assessment of assimilable organic carbon (AOC) with the detection of selected organic acids, aldehydes and ketones to study organic by-product formation during ozonation. We have investigated the kinetic evolution of LMW compounds as a function of ozone exposure. A substantial fraction of the organic compounds formed immediately upon exposure to ozone and organic acids comprised 60-80% of the newly formed AOC. Based on experiments performed with and without hydroxyl radical scavengers, we concluded that direct ozone reactions were mainly responsible for the formation of small organic compounds. It was also demonstrated that the laboratory-scale experiments are adequate models to describe the formation of LMW organic compounds during ozonation in full-scale treatment of surface water. Thus, the kinetic and mechanistic information gained during the laboratory-scale experiments can be utilised for upscaling to full-scale water treatment plants.     

Low band gap polymers for organic photovoltaics

Low band gap polymer materials and their application in organic photovoltaics (OPV) are reviewed. We detail the synthetic approaches to low band gap polymer materials starting from the early methodologies employing quinoid homopolymer structures to the current state of the art that relies on alternating copolymers of donor and acceptor groups where strategies for band gap design are possible. Current challenges for OPV such as chemical stability and energy level alignment are discussed. We finally provide a compilation of the most studied classes of low band gap materials and the results obtained in photovoltaic applications and give a tabular overview of rarely applied materials.