Thick epitaxial multilayer silicene films with a √3 × √3R(30°) surface structure show only mild surface oxidation after 24 h in air, as measured by Auger electron spectroscopy. X-ray diffraction and Raman spectroscopy measurements performed in air without any protective capping, as well as, for comparison, with a thin Al2O3 cap, showed the (002) reflection and the G, D and 2D Raman structures, which are unique fingerprints of thick multilayer silicene.

Solution‐processed organic–inorganic lead halide perovskite solar cells (PSCs) are considered as one of the most promising photovoltaic technologies thanks to both high performance and low manufacturing cost. However, a key challenge of this technology is the lack of ambient stability over prolonged solar irradiation under continuous operating conditions. In fact, only a few studies (carried out in inert atmosphere) already approach the industrial standards. Here, it is shown how the introduction of MoS2 flakes as a hole transport interlayer in inverted planar PSCs results in a power conversion efficiency (PCE) of ≈17%, overcoming the one of the standard reference devices. Furthermore, this approach allows the realization of ultrastable PSCs, stressed in ambient conditions and working at continuous maximum power point. In particular, the photovoltaic performances of the proposed PSCs represent the current state‐of‐the‐art in terms of lifetime, retaining 80% of their initial performance after 568 h of continuous stress test, thus approaching the industrial stability standards. Moreover, it is further demonstrated the feasibility of this approach by fabricating large‐area PSCs (0.5 cm2 active area) with MoS2 as the interlayer. These large‐area PSCs show improved performance (i.e., PCE = 13.17%) when compared with the standard devices (PCE = 10.64%).

One year after the publication of the seminal paper on monolayer 33 reconstructed silicene grown on a silver (111) substrate, evidence of the synthesis of epitaxial 33 reconstructed multilayer silicene hosting Dirac fermions was presented. Although a general consensus was immediately reached in the former case, in the latter one, the mere existence of multilayer silicene was questioned and strongly debated. Here, we demonstrate by means of a comprehensive x-ray crystallographic study, that multilayer silicene is effectively realized upon growth at rather low growth temperatures (~200°C), while, instead, three-dimensional growth of silicon crystallites takes place at higher temperatures, (~300°C). This transition to bulk like silicon perfectly explains the various data presented and discussed in the literature and solves their conflicting interpretations.

Among the hybrid metal-organic perovskites for photovoltaic applications FAPbI 3 (FAPI) has the best performance regarding efficiency and the worst regarding stability, even though the reports on its stability are highly contradictory. In particular, since at room temperature the cubic α phase, black and with high photovoltaic efficiency, is metastable against the yellow hexagonal δ phase, it is believed that α−FAPI spontaneously transform into δ−FAPI within a relatively short time. We performed X-ray diffraction and thermogravimetric measurements on loose powder of FAPI, and present the first complete dielectric and anelastic spectra of compacted FAPI samples under various conditions. We found that α−FAPI is perfectly stable for at least 100 days, the duration of the experiments, unless extrinsic factors induce its degradation. In our tests, degradation was detected after exposure to humidity, strongly accelerated by grain boundaries and the presence of δ phase, but it was not noticeable on the loose powder kept in air under normal laboratory illumination. These findings have strong implications on the strategies for improving the stability of FAPI without diminishing its photovoltaic efficiency through modifications of its composition. Graphical TOC Entry1 arXiv:1905.02992v1 [cond-mat.mtrl-sci] 8 May 2019Although MAPbI 3 (MAPI, MA = methylammonium CH 3 NH 3 ) is the most studied hybrid metal-organic perovskite for photovoltaic applications, 1 better performance in terms of photovoltaic efficiency are found in FAPbI 3 (FAPI, FA = formamidinium CH(NH 2 ) 2 ). This is due both to a smaller bandgap of FAPI and to the fact that the FA + ion, in spite of a smaller electric dipole with respect to MA + , has a much larger quadrupole and faster reorientation dynamics that better screen the photoexcited carriers, enhancing their lifetime. 2 FAPI also has a better stability than MAPI at high temperature but its major flaw is that the black cubic α phase, which has the high photovoltaic efficiency, is metastable at room temperature, where instead the stable phase, and the one obtained by standard chemical methods, is the yellow hexagonal δ phase. For these reasons, major efforts are directed now at trying to stabilize the cubic α phase of FAPI through partial substitutions of FA with MA, Cs, etc. or I with Br, although this approach increases the bandgap. 3,4 It has been discussed, based on neutron diffraction measurements and simulations, that the α → δ transformation is complex and occurs through various intermediate stages, requiring to overcome a free energy barrier estimated in the order of hundreds of milli-electron volt. 5 This explains why the α phase of FAPI is kinetically trapped, resulting in a large thermal hysteresis between the δ → α transition at T h δα = 350 K and the α → δ at T c δα = 290 K. 5 Actually, the barrier for the α → δ transition has not been measured, and there is complete uncertainty on the kinetics of this transition. Indeed, also the reported temperatures for the δ → α transition during heati...

Despite the progress on organic photovoltaic (OPV) performance, the photoactive layer degradation during prolonged solar illumination is still a major obstacle. In this work, an approach to mitigate the degradation pathway related to structural/morphological changes of the photoactive layer occurring upon continuous illumination in air is presented. It is shown, for the first time, that the incorporation of Ag nanoparticles in poly(3‐hexylthiophene) (P3HT) and [6‐6]‐phenyl‐C61‐butyric acid methyl ester bulk heterojunction (BHJ) leads to improved structural and morphological properties of the composite BHJ solar cells and to better photovoltaic (PV) stability after long periods of continuous illumination. This is evidenced by an original approach based on joint in‐situ time‐resolved X‐ray and atomic force microscopy monitoring. Besides the structural stability improvement and reduced photodegradation rate, it is shown that the composite blends exhibit superior PV performance compared to the pristine BHJs. It can be postulated that the incorporation of metallic nanoparticles in the BHJ leads to a dual enhancement, a plasmon absorption mediated effect, causing improved initial cell efficiency, and a structural stability effect giving rise to reduced degradation rate upon prolonged illumination. The results are in favor of the exploitation of polymer–nanoparticle composites as a promising approach to mitigate the aging effects in OPVs.