Polyfluorinated Naphthalene-bis-hydrazimide for Solution-Grown n-Type Semiconducting Films.

Naphthalene tetracarboxylic diimides (NDIs), possessing low-lying and tunable LUMO levels, are of wide interest for their aptitude to provide cost-effective, flexible, and environmentally stable n-type organic semiconductors through simple solution processing. NDI-based aromatic hydrazidimides are herein studied in relation to their chemical and environmental stability and as spin-coated stable thin films. In the case of the pentafluorinated residue, these were found to be crystalline, highly oriented, and molecularly flat (roughness = 0.3 nm), based on optical and atomic force microscopy, X-ray diffraction in specular and grazing incidence geometry, and X-ray reflectivity measurements. A new polymorph, previously undetected during the isolation of bulk powders or in their controlled thermal treatments, is found in the thin film and was metrically and structurally characterized from 2D GIWAXS patterns (monoclinic, P2/c, a = 17.50; b = 4.56; c = 14.24 Å; ß = 84.8°). This new thin-film phase, TF-F5, is formed no matter whether silicon, glass, or polymethylmethacrylate substrates are used, thus opening the way to the preparation of solution-grown flexible semiconducting films. The TF-F5 films exhibit a systematic and rigorous molecular alignment with both orientation and packing favorable to electron mobility (µ = 0.02 cm2 V-1 s-1). Structural and morphological differences are deemed responsible for the absence of measurable conductivity in thin films of polyfluorinated analogues bearing -CF3 residues on the hydrazidimide aromatic rings.

Molecular Design and Crystal Chemistry of Polyfluorinated Naphthalene-bis-phenylhydrazimides with Superior Thermal and Polymorphic Stability and High Solution Processability.

Naphthalene tetracarboxylic diimides (NDIs) are highly promising air-stable n-type molecular semiconductor candidates for flexible and cost-effective organic solar cells and thermoelectrics. Nonetheless, thermal and polymorphic stabilities of environmentally stable NDIs in the low-to-medium temperature regime (<300 °C) remain challenging properties. Structural, thermal, spectroscopic, and computational features of polyfluorinated NDI-based molecular solids (with up to 14 F atoms per NDI molecule) are discussed upon increasing the fluorination level. Slip-stacked arrangement of the NDI cores with suitable π-π stacking and systematically short interplanar distances (<3.2 Å) are found. All these materials exhibit superior thermal stability (up to 260 °C or above) and thermal expansion coefficients indicating a response compatible with flexible polymeric substrates. Optical bandgaps increase from 2.78 to 2.93 eV with fluorination, while LUMO energy levels decrease down to -4.37 eV, as shown per DFT calculations. The compounds exhibit excellent solubility of 30 mg mL-1 in 1,4-dioxane and DMF.

Long-Term Stability of TiS2-Alkylamine Hybrid Materials.

Layered TiS2 intercalated with linear alkylamines has recently attracted significant interest as a model compound for flexible n-type thermoelectric applications, showing remarkably high power factors at room temperature. The thermal and, particularly, environmental stability of such materials is, however, a still an open challenge. In this paper, we show that amine-intercalated TiS2 prepared by a simple mechanochemical process is prone to chemical decomposition through sulfur exsolution, and that the presence of molecular oxygen is likely to mediate the decomposition reaction. Through computational analysis of the possible reaction pathways, we propose that Ti-N adducts are formed as a consequence of amine groups substituting for S vacancies on the internal surfaces of the S-Ti-S layers. These findings provide insights for possible future applications of similar hybrid compounds as devices operating in ambient conditions, and suggest isolating them from atmospheric oxygen.

Cation Engineering for Resonant Energy Level Alignment in Two-Dimensional Lead Halide Perovskites.

Low-dimensional metal halide perovskites are being intensively investigated because of their higher stability and chemical versatility in comparison to their 3D counterparts. Unfortunately, this comes at the expense of the electronic and charge transport properties, limited by the reduced perovskite dimensionality. Cation engineering can be envisaged as a solution to tune and possibly further improve the material's optoelectronic properties. In this work, we screen and design new electronically active A-site cations that can promote charge transport across the inorganic layers. We show that hybridization of the valence band electronic states of the perovskite inorganic sublattice and the highest occupied molecular orbitals of the A-site organic cations can be tuned to exhibit a variety of optoelectronic properties. A significant interplay of A-cation size, electronic structure, and steric constraints is revealed, suggesting intriguing means of further tuning the 2D perovskite electronic structure toward achieving stable and efficient solar cell devices.

Mechanochemical synthesis of three double perovskites: Cs2AgBiBr6, (CH3NH3)2TlBiBr6 and Cs2AgSbBr6.

A wide majority of the organic-inorganic hybrid perovskites employed in photovoltaics contain Pb, which is a negative issue due to its high toxicity and the low stability of the Pb-based three-dimensional (3D) perovskites. The double perovskites or "elpasolites" with the formula A2BB'X6 arise as an alternative to avoid the use of Pb, however, not many of the theoretically predicted structures have been synthesized so far due to several synthetic issues, such as, the formation of stable side products. Herein, we report the synthesis of three double perovskites, Cs2AgBiBr6, MA2TlBiBr6 and Cs2AgSbBr6, through a highly efficient and reproducible mechanochemical approach: the high energy ball milling. This synthetic approach does not require the use of organic solvents, so it is a greener method compared to those reported for other double perovskites. The Cs2AgBiBr6 and MA2TlBiBr6 double perovskites were synthesized with high purity as proved by X-ray diffraction (XRD) and X-ray fluorescence (XRF) measurements. However, the Cs2AgSbBr6 double perovskite was obtained in mixture with Cs3Sb2Br9, a side product of the reaction. Several attempts to prepare the Cs2AgSbBr6 double perovskite by using other synthetic methods have been unsuccessful due to the low formation enthalpy of the Cs3Sb2Br9 side product and only the hydrothermal method afforded Cs2AgSbBr6 in mixture with other compounds. We believe that the low temperature required in the ball milling synthesis is the key factor that allows the formation of the antimony double perovskite. Cs2AgSbBr6 is a brown powder with a bandgap energy of 1.93 eV as shown with diffuse reflectance measurements. The three powders exhibit a very high stability with no changes at all in the crystal structure after several months of storage at room temperature and ambient humidity.

Organization and structure of mixed Langmuir films composed of polydiacetylene and hemicyanine.

Mixed Langmuir monolayers of 10,12-Pentacosadiynoic acid (DA) monomer and an amphiphilic Hemicyanine dye derivative have been formed at the air/water interface. Two derivatives of docosylpyridinium have been used, with either one included in the monolayer in 1:1molar ratio. The DA monomers within the mixed monolayers have been polymerized in situ at the air/water interface. The crystalline structure of the monolayer and the kinetics of polymerization have been probed by grazing incidence X-ray diffraction (GIXD). The polymerization of DA proceeds with no phase segregation, exclusively leading to the red polydiacetylene form. The kinetics of polymerization at the air/water interface has been monitored in situ by GIXD. The experimental results have been combined with Molecular Mechanics computer simulations, revealing that DA molecules are sequentially arranged with molecules of Hemicyanine dye in alternating rows. The hydrophobic chains of the dye molecules act as spacers between the DA monomers. Surprisingly, such molecular arrangement does not hinder the in situ photopolymerization of DA. The mechanism of polymerization of DA within the mixed Langmuir monolayers has been convincingly described in molecular detail. This approach for interfacial polymerization of DA holds great potential for optically active devices and nanostructures comprising self-assembled thin films based in polydiacetylene.

Tailoring a compact and stable Langmuir bi-dimensional PbX-based layered perovskite film at the air-water interface and on solid support.

The present work studies the stability of Langmuir organic-inorganic superlattice materials thin films consisting of layered perovskite-based films with controlled 2D framework as well as to design experimental conditions for increasing the efficiency of the organic-inorganic perovskite motif by mechanical stimulus. Therefore, a whole covering of the air/water interface by a compact and stable lead-based layered perovskite structure is pursued. A 2D layered perovskite-type hybrid structure of the form [(CH3(CH2)19NH3)2(PbX4)], X=Cl, and Br, in which, two-dimensional sheets stabilized by a inner bilayer of organic monoammonium cation matrix, is mechanically tailored by successive compression-expansion cycles. The formation of 2D molecular patterns has been characterized by ΔR, BAM, XRD and XPS.