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The high defect tolerance of metal halide perovskites, in terms of their exceptional optoelectronic properties, is assumed to be due to the very fact that most native point defects are shallow, which does not contribute to the non‐radiative recombination of free carriers. Here, a systematic study is presented, which concerns the evolution of shallow‐defect signatures observed at low temperatures in the photoluminescence (PL) spectra of mixed organic‐cation lead iodide perovskite single crystals (FAxMA1−xPbI3, where MA stands for methylammonium and FA for formamidinium). Below ≈100 K, a number of peak‐like features become clearly apparent in the PL spectra at energies lower than the strong free‐exciton emission, which are related to the radiative recombination of bound exciton complexes associated with native shallow defects (donors and/or acceptors). Based on state‐of‐the‐art ab initio calculations, a tentative assignment is provided for all PL features to different shallow‐defects (Pb, I, and MA vacancies as well as I interstitials) typically present in hybrid perovskites. The defect‐related signatures exhibit a clear trend regarding the mixed‐crystal composition, indicating that the material becomes less prone to defect formation with increasing FA content. [ABSTRACT FROM AUTHOR]

The microstructure of blend films of conjugated polymer and fullerene, especially the degree of mixing and crystallization, impacts the performance of organic photovoltaic devices considerably. Mixing and crystallization affect device performance in different ways. These phenomena are not easy to screen using traditional methods such as imaging. In this paper, the amorphous regiorandom poly(3-hexylthiophene) is blended with the potentially crystalline fullerene [6,6]-phenyl-C61-butyric acid methyl ester PCBM and the amorphous bis-adduct. First, the degree of mixing of polymer: fullerene blends is evaluated using UV-Vis absorption, steady-state and ultra-fast photoluminescence spectroscopy. The blue-shift of the polymer emission and absorption onset are used in combination with the saturation of the polymer emission decay time upon fullerene addition in order to infer the onset of aggregation of the blends. Second, the crystallinity of the fullerene is probed using variable angle spectroscopic ellipsometry (VASE), electroluminescence and photoluminescence spectroscopy. It is shown that the red-shift of charge transfer emission in the case of PCBM based blends cannot be explained solely by a variation of optical dielectric constant as probed by VASE. A combination of optical spectroscopy techniques, therefore, allows to probe the degree of mixing and can also distinguish between aggregation and crystallization of fullerenes. [ABSTRACT FROM AUTHOR]

Abstract: The main challenges toward massive fabrication of silicon nanowires are the way to control the crystal orientation and size. In the present work, we report advances in the pattern transfer process to obtain arrays of holes on a Si substrate, aiming at the fabrication of ordered arrays of quantum wires. The wires obtained from this procedure can be uniform in terms of doping profile, crystal orientation and size, while enormously simplifying metallic contacting for applications where parallel biasing is needed. The samples have been fabricated using a combination of electron-beam lithography and reactive-ion etching. Electron-beam lithography is performed at 10keV on a 100nm thick 950K PMMA. A specific recipe for deep reactive-ion etching was developed in order to minimize any widening or under-etching of the holes, as well as any type of wall roughness. Holes with diameters from 30nm up to 900nm, and pitch from 90nm up to 1000nm were fabricated, achieving no observable scalloping. The sample has been oxidized for further reduction of the size of interconnects (the site between two holes) and interstitials (the site between three holes). As a result, thin nanowires have been fabricated. [Copyright &y& Elsevier]