Organometal
halide perovskites show promising features for cost-effective
application in photovoltaics. The material instability remains a major
obstacle to broad application because of the poorly understood degradation
pathways. Here, we apply simultaneous luminescence and electron microscopy
on perovskites for the first time, allowing us to monitor in situ
morphology evolution and optical properties upon perovskite degradation.
Interestingly, morphology, photoluminescence (PL), and cathodoluminescence
of perovskite samples evolve differently upon degradation driven by
electron beam (e-beam) or by light. A transversal electric current
generated by a scanning electron beam leads to dramatic changes in
PL and tunes the energy band gaps continuously alongside film thinning.
In contrast, light-induced degradation results in material decomposition
to scattered particles and shows little PL spectral shifts. The differences
in degradation can be ascribed to different electric currents that
drive ion migration. Moreover, solution-processed perovskite cuboids
show heterogeneity in stability which is likely related to crystallinity
and morphology. Our results reveal the essential role of ion migration
in perovskite degradation and provide potential avenues to rationally
enhance the stability of perovskite materials by reducing ion migration
while improving morphology and crystallinity. It is worth noting that
even moderate e-beam currents (86 pA) and acceleration voltages (10
kV) readily induce significant perovskite degradation and alter their
optical properties. Therefore, attention has to be paid while characterizing
such materials using scanning electron microscopy or transmission
electron microscopy techniques.
Solubilized poly(3-alkylthiophene)s are known to self-assemble into well-ordered supramolecular aggregates upon lowering the solvent quality. This supramolecular organization largely determines the optical and electronic properties of these polymers. However, despite numerous studies the exact mechanism and kinetics of the aggregation process and the role of external stimuli are still poorly understood. Classical characterization techniques such as electronic spectroscopy, dynamic light scattering, and diffraction-based techniques have not been able to provide a full understanding. Here we use second-harmonic scattering (SHS) and third-harmonic scattering (THS) techniques to investigate this supramolecular aggregation mechanism. Our results indicate that the actual supramolecular aggregation is preceded by the formation of structured polymer-solvent clusters consistent with a nonclassical crystallization pathway.
The dissolution profiles of formulations based on mixtures of chitosan/alginate depend on the pH. It is possible to distinguish two processes: (a) a fast kinetic drug release up to 180 min, where the pH value changes from 1.17 to 2.21 and the drug released is controlled by the degree of polymerization and the quantity of chitosan in the formulation; (b) a low kinetic drug release between 210 and 480 min, where the pH value changes from 5.52 to 8.72 and the drug release from the matrix is controlled by the interpolymeric complex. In all formulations the order of release, according to Peppas's model in the range of fast kinetic drug release, was between 0.5 and 1.0. The mechanism of release was non-fickian diffusion, which corresponds to a coupling mechanism of diffusion and relaxation of the polymer.
The
supramolecular organization of star-shaped polythiophenes was
investigated in a solvent/nonsolvent system and upon cooling. Both
systems yield entirely different supramolecular aggregates. Classical
characterization techniques can give some insight; the exact mechanism
is currently not understood. By introducing second- and third-harmonic
light scattering and its combination with other optical techniques,
we were able to fully investigate the evolution of the supramolecular
organization in both regimes. This approach allowed an unprecedented
insight into the different stages of the process. A markedly different
assembly mechanism is proposed for both regimes, which results in
a different supramolecular organization of the polymer.
s are known to self-assemble into well-ordered supramolecular assemblies. The organization influences the optical and electronic properties of these polymers. One way to induce supramolecular self-assembly is by lowering the solvent quality of solubilized poly(3-alkylthiophene) upon the addition of a non-solvent. Recently, the exact mechanism of the supramolecular self-assembly process has been explored for a chloroform/methanol (solvent/non-solvent) system. However, it is not clear how the solvent influences the supramolecular self-assembly mechanism. In this study, poly(3-alkylthiophene) is dissolved in two different solvents and the supramolecular self-assembly is fully explored by combining harmonic light scattering with electronic spectroscopy. Our results indicate that the actual supramolecular self-assembly is very different in both cases and that the choice of solvent has not only a significant influence on the assembly process but also on the structure of the final supramolecular assembly.
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