Degradation of the Formamidinium Cation and the Quantification of the Formamidinium–Methylammonium Ratio in Lead Iodide Hybrid Perovskites by Nuclear Magnetic Resonance Spectroscopy

The highest efficiency in perovskite solar cells is currently achieved with mixed-cation hybrid perovskites. The ratio in which the cations are present in the perovskite structure has an important effect on the optical properties and the stability of these materials. At present, the formamidinium cation is an integral part of many of the highest efficiency perovskite systems. In this work, we introduce a nuclear magnetic resonance (NMR) spectroscopy protocol for the identification and differentiation of mixed perovskite phases and of a secondary phase due to formamidinium degradation. The influence of the cooling rate used in a precipitation method on the FA/MA ratio in formamidinium–methylammonium lead iodide perovskites (FA_<i>x</i>MA_1–<i>x</i>PbI₃) was investigated and compared to the FA/MA ratio in thin films. In order to obtain the FA/MA ratio from fast and accessible liquid-state ¹H NMR spectra, the influence of the acidity of the solution on the line width of the resonances was elucidated. The ratio of the organic cations incorporated into the perovskite structure could be reliably quantified in the presence of the secondary phase through a combination of liquid-state ¹H NMR and solid-state ¹³C NMR spectroscopic analysis

Anisotropic Atomic Layer Deposition Profiles of TiO₂ in Hierarchical Silica Material with Multiple Porosity

Anisotropic deposition profiles of TiO₂ in Zeotile-4 ordered mesoporous silica material are obtained using Atomic Layer Deposition (ALD) involving alternating pulses of tetrakis­(dimethylamino) titanium (TDMAT) and water. TiO₂ concentration profiles visualized by transmission electron microscopy (TEM) on particle cross sections reveal the systematic deeper penetration of the deposition front along the main channels and the more limited penetration in the perpendicular direction through the narrower slit-like mesopores. In ordered mesoporous material with one-dimensional pore system ALD leads to pore plugging. Diffusion limited ALD is shown to be useful for TiO₂ deposition in anisotropic mesoporous support materials

Enhanced Organic Solar Cell Stability by Polymer (PCPDTBT) Side Chain Functionalization

Organic photovoltaics represent a promising thin-film solar cell technology with appealing mechanical, aesthetical, and cost features. In recent years, a strong growth in power conversion efficiency (to over 10%) has been realized for organic solar cells through extensive material and device research. To be competitive in the renewable energy market, further improvements are mandatory though, both with respect to efficiency and lifetime. High intrinsic stability of the photoactive layer is obviously a crucial requirement for long lifetimes, but the generally applied bulk heterojunction blends and their components are prone to light-induced and thermal degradation processes. In the present contribution, the high-<i>T</i>_g polymer strategy is combined with specific side chain functionalization to address the thermal stability of polymer solar cells. These two design concepts are applied to a prototype low bandgap copolymer, PCPDTBT. Accelerated aging tests (at 85 °C) indicate an improved thermal durability of the PCPDTBT:PC₇₁BM blends and the resulting devices by the insertion of ester or alcohol moieties on the polymer side chains. The different stages in the efficiency decay profiles are addressed by dedicated experiments to elucidate the (simultaneously occurring) degradation mechanisms

Biocompatible Label-Free Detection of Carbon Black Particles by Femtosecond Pulsed Laser Microscopy

Although adverse health effects of carbon black (CB) exposure are generally accepted, a direct, label-free approach for detecting CB particles in fluids and at the cellular level is still lacking. Here, we report nonincandescence related white-light (WL) generation by dry and suspended carbon black particles under illumination with femtosecond (fs) pulsed near-infrared light as a powerful tool for the detection of these carbonaceous materials. This observation is done for four different CB species with diameters ranging from 13 to 500 nm, suggesting this WL emission under fs near-infrared illumination is a general property of CB particles. As the emitted radiation spreads over the whole visible spectrum, detection is straightforward and flexible. The unique property of the described WL emission allows optical detection and unequivocal localization of CB particles in fluids and in cellular environments while simultaneously colocalizing different cellular components using various specific fluorophores as shown here using human lung fibroblasts. The experiments are performed on a typical multiphoton laser-scanning microscopy platform, widely available in research laboratories

Simultaneous Enhancement of Solar Cell Efficiency and Stability by Reducing the Side Chain Density on Fluorinated PCPDTQx Copolymers

The performance of polymer solar cells is strongly dependent on the morphology of the photoactive layer, which can be optimized by tuning the polymer side chain pattern. Whereas most studies focus on length and bulkiness, the side chain density receives much less attention. In this work, the effect of the number of side chains on PCPDTQx­(2F) low bandgap copolymers on material properties and solar cell characteristics is investigated. The active layer morphology is strongly affected, affording more favorable finely intermixed blends when decreasing the side chain density. As a result, the efficiency increases to a maximum of 5.63% for the device based on the copolymer with intermediate side chain density. Moreover, removal of the side chains also has a positive effect on device stability under prolonged thermal stress. A single structural parameteralkyl side chain densityis hence used for simultaneous enhancement of both solar cell efficiency and lifetime

Improved Photovoltaic Performance of a Semicrystalline Narrow Bandgap Copolymer Based on 4<i>H</i>-Cyclopenta[2,1-<i>b</i>:3,4-<i>b</i>′]dithiophene Donor and Thiazolo[5,4-<i>d</i>]thiazole Acceptor Units

A solution processable narrow bandgap polymer composed of alternating 2,5-dithienylthiazolo­[5,4-<i>d</i>]­thiazole and asymmetrically alkyl-substituted 4<i>H</i>-cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene units (<b>PCPDT-DTTzTz</b>) was synthesized by Suzuki polycondensation and the donor–acceptor copolymer was thoroughly characterized. Thermal analysis and X-ray diffraction studies disclosed the semicrystalline nature of the material. When blended with PC₇₁BM and integrated in bulk heterojunction organic solar cells, a moderate power conversion efficiency of 2.43% under AM 1.5 G (100 mW/cm²) conditions was obtained. However, upon purification of the semiconducting copolymer by preparative size exclusion chromatography, a noticeable rise in power conversion efficiency to 4.03% was achieved. The purified polymer exhibited a relatively high field-effect carrier mobility of 1.0 × 10^–3 cm²/(V s). The active layer morphology was explored by atomic force microscopy and transmission electron microscopy studies, showing phase segregation on the nanometer scale

Factors Influencing the Conductivity of Aqueous Sol(ution)–Gel-Processed Al-Doped ZnO Films

Solution processing of Al-doped ZnO (AZO) is interesting from an economical point of view for reducing synthesis and deposition costs in comparison to that for vacuum methods. Several (aqueous) chemical solution deposition routes have been explored for AZO, but the question that has never been answered is how state-of-the-art conductivity is achieved. Here, we fine tune an aqueous solution precursor for AZO, resulting in resistivities within the 10^–3 Ohm cm range after a reductive treatment. Profound insights are gained through the study of the density of the film, the crystal phase, the optimum Al doping, and the effect of Al positioning in the ZnO lattice, as determined by ²⁷Al magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy in combination with ¹H NMR, in order to understand the conductivity mechanism. As the conductivity of the AZO films drops as a function of time, the position of Al is studied with respect to the observance of charge carriers using Fourier transform infrared spectroscopy. The influences of all of these different factors on conductivity are summarized in a general overview