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Spin-coated poly(9,9-di-n-octylfluorene-alt-benzothiacliazole) (F8BT) films of different molecular weights (Mn = 9—255 kg/mol), both in the pristine and annealed state, were studied in an effort to elucidate changes in the polymer packing structure and the effects this structure has on the optoelectronic and charge transport properties of these films. A model based on quantum chemical calculations, wide-angle X-ray scattering, atomic force microscopy, Raman spectroscopy, photoluminescence, and electron mobility measurements was developed to describe the restructuring of the polymer film as a function of polymer chain length and annealing. In pristine high molecular weight films, the polymer chains exhibit a significant torsion angle between the F8 and BT units, and the BI units in neighboring chains are close to one another. Annealing films to sufficiently high transition temperatures allows the polymers to adopt a lower energy configuration in which the BT units in one polymer chain are adjacent to F8 units in a neighboring chain ("alternating structure"), and the torsion angle between F8 and BT units is reduced. This restructuring, dictated by the strong dipole on the BT unit, subsequently affects the efficiencies of interchain electron transfer and exciton migration. Films exhibiting the alternating structure show significantly lower electron mobilities than those of the pristine high molecular weight films, due to a decrease in the efficiency of interchain electron transport in this structure. In addition, interchain exciton migration to low energy weakly emissive states is also reduced for these alternating structure films, as observed in their photoluminescence spectra and efficiencies. [ABSTRACT FROM AUTHOR]

Poly(ethylene oxide) (PEO) thin films of different thicknesses (220, 450, and 1500 nm) and molecular masses (4000, 8000, and 20 000 g/mol) have been fabricated by spin-coating of methanol solutions onto glass substrates. All these samples have been recrystallized from the melt using a directional thermal gradient technique. Millimeter-size domains with crystallites uniformly oriented in the direction of the thermal gradient are observed. Futhermore, the crystallites size and orientation distribution are enhanced (e.g., increases and decreases, respectively) when film thickness is decreased, ultimately leading to a single-crystal-like behavior for 220 nm thick PEO films of mass 8000 g/mol. Interestingly, this fine microstructure is partially retained after melting and subsequent cooling back to ambient temperature for the highest molecular weight polymer allowing, in this particular case, to significantly decrease the distribution of crystal orientation obtained after crystallization using the thermal gradient technique. [ABSTRACT FROM AUTHOR]

Conjugated polymers and small molecules based on alternating electron-donating (D) and electron-accepting (A) building blocks have led to state-of-the-art organic solar cell materials governing efficiencies beyond 10%. Unfortunately, the connection of D and A building blocks via cross-coupling reactions does not always proceed as planned, which can result in the generation of side products containing D-D or A-A homocoupling motifs. Previous studies have reported a reduced performance in polymer and small molecule solar cells when such defect structures are present. A general consensus on the impact of homocouplings on device performance is, however, still lacking as is a profound understanding of the underlying causes of the device deterioration. For differentiating the combined effect of molecular weight and homocouplings in polymer solar cells, a systematic study on a small molecule system (DTS(FBBTh2)2) is presented. The impact of homocouplings on nanomorphology, thermal, and electro-optical properties is investigated. It is demonstrated that small quantities of homocouplings (<10%) already lead to suboptimal device performance, as this strongly impacts the molecular packing and electronic properties of the photoactive layer. These results highlight the importance of material purity and pinpoint homocoupling defects as one of the most probable reasons for batch-to-batch variations. [ABSTRACT FROM AUTHOR]

Studying noninvasively the internal nanoporous structureof a singleTussah silk fiber under different humidity conditions, we demonstratefor the first time the feasibility of in-situ ptychographic tomography.The resulting 3D images of the silk fiber interior, obtained at bothdry and humid conditions, yield quantitative information about thespatial density variations in the form of detailed maps of the size,shape, and orientation distributions of the nanopores inside the silkfiber, revealing that the fiber swells anisotropically in humid conditions,with the expansion taking place solely normal to the fiber axis. Exploitingquantitative information on the fiber’s electron density, hydrationwas found to proceed through interaction with the silk protein ratherthan filling of pores. [ABSTRACT FROM AUTHOR]