The role of organic cations as additives in photovoltaic perovskites.

The design of additives for perovskite-based solar cells seeks to improve the balance between stability and power conversion efficiency. Organic molecules such as theophylline, theobromine and caffeine (xanthines) have proved to be a good engineering solution. As an alternative, we present a first-principles study of the use of organic cations as additives. These cations are obtained when the free nitrogen of the imidazole unit of the aforementioned molecules is quaternized. We have found that the interaction between the organic cations and the MAPbI3 perovskite surface is stronger compared to the organic molecules. The Pb-O and I-H bonds of the interface dominated these interactions. In addition, organic cations showed higher charge transfer through the interface and shallow states that are harmless and could improve the charge carrier mobility. These characteristics show that quaternized xanthines should be a promising additive for perovskite materials in photovoltaic applications.

Enhancing the stability and efficiency of MAPbI3 perovskite solar cells by theophylline-BF4 - alkaloid derivatives, a theoretical-experimental approach.

Perovskite solar cells (PSCs) are an evolving photovoltaic field with the potential to disrupt the established silicon solar cell market. However, the presence of many transport barriers and defect trap states at the interfaces and grain boundaries has negative effects on PSCs; it decreases their efficiency and stability. The purpose of this work was to investigate the effects on efficiency and stability achieved by quaternary theophylline additives in MAPbI3 PSCs with the structure FTO/TiO2/perovskite/spiro-OMeTAD/Ag. The X-ray photoelectron spectroscopy (XPS) and theoretical calculation strategies were applied to study the additive's interaction in the layer. The tetrafluoroborinated additive results in an increase in device current density (J SC) (23.99 mA cm-1), fill factor (FF) (65.7%), and open-circuit voltage (V OC) (0.95 V), leading to significant improvement of the power conversion efficiency (PCE) to 15.04% compared to control devices (13.6%). Notably, films exposed to controlled humidity of 30% using the tetrafluoroborinated additive maintained their stability for more than 600 hours (h), while the control films were stable for less than 240 hours (h).

On the stability of Cu x Te polytypes: phase transitions, vibrational and electronic properties.

Based on the experimental structures reported for the Cu x Te (1 ⩽ x ⩽ 2) system, a theoretical study on stability and phase transitions has been performed. Three theoretical structures derived from rickardite (Cu1.5Te) were considered to represent different Cu/Te ratios (1, 1.5 and 2). The structural, electronic, and vibrational properties were calculated by density functional theory and compared to the experimental data available to date. This analysis showed that the proposed CuTe and Cu1.5Te structures are energetically and dynamically stable (unlike Cu2Te), and that their vibrational modes may play an important role in the reported Raman spectra for Cu x Te films. As well, it was found that being vulcanite the most stable phase for x = 1, the addition of Cu atoms to this structure induces a gradual flattening of the Cu planes, producing significant changes in the electronic band structure. A thorough review of the experimental reports on the electrical properties of the system was carried out. The experimental data showed that, in agreement with the calculations, the electrical conductivity is higher for phases with x ⩽ 1.5, decreasing as x gets closer to 2. Hence, the low-copper concentration phases are the best choice for solar cell applications due to their electrical properties and stability.

Magnetic Force Microscopy Study of Multiscale Ion-Implanted Platinum in Silica Glass, Recorded by an Ultrafast Two-Wave Mixing Configuration.

This study explores magnetization exhibited by nanoscale platinum-based structures embedded in pure silica plates. A superposition of laser pulses in the samples produced periodic linear arrangements of micro-sized structures. The samples were integrated by PtO2 microstructures (PtOΣs) with dispersed Pt oxide nanoparticles in their surroundings. The characterization of the materials was performed by high transmission electron microscopy studies. Furthermore, topographical and magnetic effects on the sample surfaces were analyzed by atomic force microscopy and magnetic force microscopy, respectively. The magnetic measurements indicated an enhancement in the gradient phase shift and in the gradient force related to the magnetic PtOΣs. The possibility of tuning the magnetic characteristics of the samples through contact with a Nd2Fe14B magnet was demonstrated. This process corresponds to an innovative method for obtaining magnetic PtOΣs induced by laser pulses. Moreover, an increase in the compactness of the silica with platinum-based structures was confirmed by an evaluation of the effective elastic modulus with reference to pure silica. The multimodal magnetic structures studied in this work seem to be candidates for developing high-density magnetic storage media.

Segmentation and additive approach: A reliable technique to study noncovalent interactions of large molecules at the surface of single-wall carbon nanotubes.

This investigation explores a new protocol, named Segmentation and Additive approach (SAA), to study exohedral noncovalent functionalization of single-walled carbon nanotubes with large molecules, such as polymers and biomolecules, by segmenting the entire system into smaller units to reduce computational cost. A key criterion of the segmentation process is the preservation of the molecular structure responsible for stabilization of the entire system in smaller segments. Noncovalent interaction of linoleic acid (LA, C18 H32 O2 ), a fatty acid, at the surface of a (10,0) zigzag nanotube is considered for test purposes. Three smaller segmented models have been created from the full (10,0)-LA system and interaction energies were calculated for these models and compared with the full system at different levels of theory, namely ωB97XD, LDA. The success of this SAA is confirmed as the sum of the interaction energies is in very good agreement with the total interaction energy. Besides reducing computational cost, another merit of SAA is an estimation of the contributions from different sections of the large system to the total interaction energy which can be studied in-depth using a higher level of theory to estimate several properties of each segment. On the negative side, bulk properties, such as HOMO-LUMO (highest occupied molecular orbital - lowest occupied molecular orbital) gap, of the entire system cannot be estimated by adding results from segment models. © 2016 Wiley Periodicals, Inc.