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Perovskite solar cells have drawn much attention recently owing to their world-record-setting photovoltaic performances, whereas their practicality is still limited by the structural instability that often arises from ion migration and defect formation. Despite the general understanding that ion instability is a primary cause for degradation, there is no observation of structural transformation at the atomistic scale. Such observation is crucial to understand how instabilities are induced by external perturbations such as illumination or electrical bias, allowing researchers to devise effective strategies to mitigate them. Here, we designed an in situ transmission electron microscopy setup to enable real-time observation of amorphization in perovskite materials under electrical biasing. To reverse the device performance degradation due to such structural changes, the samples were heated at 50 °C and were found to recrystallize, effectively regaining their performance losses. This work presents vital insights on understanding ion-migration phenomena and addressing instability challenges of perovskite optoelectronics.

Hydrophobic surfaces of multi-walled carbon nanotubes (CNTs) were oxidized by boiling a mixture of CNTs and concentrated acid. A self-organized pellet which can be reversibly dispersed was produced by dehydrating the oxidized CNT solution. Scanning electron microscope pictures showed that the pellet consisted of highly aligned nanotubes. Fourier transformation infrared and X-ray photoelectron spectroscopy indicated that oxygen-containing surface groups were introduced during the acid oxidation. Although an increased number of defects was shown by Raman spectroscopy, X-ray diffraction indicated that graphene layer organization remained after treatment. Thermal stability of treated CNTs was investigated and comparisons between pristine, treated, and heated CNTs were made. This well-packed and soluble pellets can provide ideal samples to study bulk CNT nanostructured materials and CNT composites.

When Mg metal burns in air the resulting rock-salt MgO smoke consists of perfect [100] cubes of about 100 nm. On contrast, we found that intensification of self-burning of Mg micropowder either by injecting it into oxy-hydrogen diffusion flame or under an infrared laser beam switches the growth mechanism producing mostly single-crystalline spheres and terraced nanoparticles. MgO molecule condensation onto primary spherical nanoparticles can account for generation of terraced nanoparticles with regular steps proportional to the nanoparticle size., Comment: 16 pages, 5 figures

Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar cells. While lateral transient photoluminescence microscopies have been popularly used to characterize charge diffusion in perovskites, there exists a discrepancy between low diffusion coefficients measured and near-unity charge collection efficiencies achieved in practical solar cells. Here, we reveal hidden microscopic dynamics in halide perovskites through four-dimensional (directions x, y and z and time t) tracking of charge carriers by characterizing out-of-plane diffusion of charge carriers. By combining this approach with confocal microscopy, we discover a strong local heterogeneity of vertical charge diffusivities in a three-dimensional perovskite film, arising from the difference between intragrain and intergrain diffusion. We visualize that most charge carriers are efficiently transported through the direct intragrain pathways or via indirect detours through nearby areas with fast diffusion. The observed anisotropy and heterogeneity of charge carrier diffusion in perovskites rationalize their high performance as shown in real devices. Our work also foresees that further control of polycrystal growth will enable solar cells with micrometres-thick perovskites to achieve both long optical path length and efficient charge collection simultaneously.