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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]

• 3D X-ray diffraction yields per-grain strain, texture, twin relations and structure. • Applicable to a broad scope of functional polycrystalline materials. • Demonstrated by application to a kesterite (CZTS) thin film solar cell. • CZTS is a technologically interesting case with complex phase composition. • 3DXRD may detect and characterize secondary phases not otherwise distinguishable. [Display omitted] The non-destructive structural characterization of individual grains in thin-films photovoltaics based on polycrystalline materials is a powerful tool for revealing important details of the microstructure of solar cell absorbers. Here, we use three-dimensional X-ray diffraction (3DXRD) to obtain statistics on the phase, size, orientation and strain tensors of the grains, as well as their twin relations in Cu 2 ZnSnS 4 (CZTS) absorbers. Moreover, this powerful approach allows the non-ambiguous determination of phases with distinct optical and electrical properties but similar lattice parameters, such as CZTS and ZnS. Our analysis over cumulative statistics of nearly 600 grains in polycrystalline CZTS reveals that a fraction of 2.5% corresponds to the ZnS secondary phase. Statistics of the strain distribution in the polycrystalline CZTS layer indicate an average tensile stress in the plane of the film of ~70 MPa and a compressive stress along the normal to the film of ~145 MPa. We found that 41% of the total number of grains in CZTS absorbers are Σ3 twins. We calculate the frequency of the six types of Σ3 boundaries, revealing that the 180° rotation along the axis<221>is the most frequent. Accessing the microstructure opens the possibility to study its influence on the properties of the film, such as bandgap variations due to strain or the role of twin boundaries in the charge transport mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Intrinsic polymer parameters such as regio-regularity, molecular weight, and crystallinity play an important role when studying polymer stability. 18 different batches of poly-3-hexyl-thiophene (P3HT) were degraded in a solar simulator (AM1.5G, 1000 W/m2) and the degradation kinetics were monitored. The results suggest that the radical reaction responsible for the photodegradation takes place at terminal thiophene rings exposed at points were the conjugation is broken. This proposed mechanism is supported by the fact that stability scales with regio-regularity following the ratio of head-to-tail connected thiophene units. Annealing was found to relax the P3HT films and increase conjugation length and, in turn, increase stability observed as a delayed spectral blueshift caused by photochemical degradation. Crystallinity was found to play a minor role in terms of stability. Oxygen diffusion and light shielding effects were shown to have a negligible effect on the photochemical degradation rate. The results obtained in this work advance the understanding of polymer stability and will help improve the design of materials used for polymer solar cells resulting in longer lifetimes, which will push the technology closer to large-scale applications. [Copyright &y& Elsevier]