Daniel Ramirez

Growth and characterization of ZnO nanowire arrays electrodeposited into anodic alumina templates in DMSO solution

ZnO nanowire arrays were grown by potentiostatic cathodic electrodeposition on aluminum anodic oxide template (AAO) from dimethyl sulfoxide (DMSO) solutions containing zinc chloride and molecular oxygen as precursors. The nanowires presented high aspect ratio and exhibited a very high crystallinity with a wurtzite crystal structure with preferential orientation along the (0001) crystallographic axis. Chronoamperometric experiments were performed on gold bulk electrodes in order to model this preferential mode growth of ZnO nanowires, which has not been previously reported for similar precursors in DMSO solution. The analysis of the corresponding chronoamperograms revealed that chloride ions influence the oxide nucleation and growth mechanism. It was found that in the absence of KCl as a supporting electrolyte, the data fitted an instantaneous three-dimensional diffusion-controlled (IN-3D)diff nucleation and growth mechanism (NGM). The presence of KCl, instead favored a progressive three-dimensional (PN-3D)diff NGM. With these results, a model for the more complex nanowire’s growth inside the pores of the AAO template is proposed.

Self-Functionalization Behind a Solution-Processed NiOx Film Used As Hole Transporting Layer for Efficient Perovskite Solar Cells

Fabrication of solution-processed perovskite solar cells (PSCs) requires the deposition of high quality films from precursor inks. Frequently, buffer layers of PSCs are formed from dispersions of metal oxide nanoparticles (NPs). Therefore, the development of trustable methods for the preparation of stable colloidal NPs dispersions is crucial. In this work, a novel approach to form very compact semiconducting buffer layers with suitable optoelectronic properties is presented through a self-functionalization process of the nanocrystalline particles by their own amorphous phase and without adding any other inorganic or organic functionalization component or surfactant. Such interconnecting amorphous phase composed by residual nitrate, hydroxide, and sodium ions, proved to be fundamental to reach stable colloidal dispersions and contribute to assemble the separate crystalline nickel oxide NPs in the final film, resulting in a very homogeneous and compact layer. A proposed mechanism behind the great stabilization of the nanoparticles is exposed. At the end, the self-functionalized nickel oxide layer exhibited high optoelectronic properties enabling perovskite p-i-n solar cells as efficient as 16.6% demonstrating the pertinence of the presented strategy to obtain high quality buffer layers processed in solution at room temperature.

Photophysics behind highly luminescent two-dimensional hybrid perovskite (CH3(CH2)2NH3)2(CH3NH3)2Pb3Br10 thin films

Two-dimensional (2D) Ruddlesden–Popper perovskites have emerged as a new class of hybrid materials with high photoluminescence and improved stability compared to their three-dimensional (3D) counterparts. Studies of the photophysics of these new 2D perovskites are essential for the fast development of optoelectronic devices. Here, we study the power and temperature dependences of the photoluminescence properties of the (PA)2(MA)2Pb3Br10 hybrid perovskite. High electron–phonon coupling near room temperature was found to be dominated by longitudinal optical (LO) phonons via the Frohlich interaction. However, we show that the presence of free carriers is also possible, with lower trapping states and higher and more stable emission compared to the 3D MAPbBr3. These characteristics make the studied 2D material very attractive for optoelectronic applications, including solar cells and light emitting diodes (LEDs). Our investigation provides new fundamental insights into the emission characteristics of 2D lead halide perovskites.

Structural and Electrochemical Evaluation of Three- and Two-Dimensional Organohalide Perovskites and Their Influence on the Reversibility of Lithium Intercalation

Organic-inorganic hybrid perovskite materials have recently been investigated in a variety of applications, including solar cells, light emitting devices (LEDs), and lasers because of their impressive semiconductor properties. Nevertheless, the perovskite structure has the ability to host extrinsic elements, making its application in the battery field possible. During the present study, we fabricated and investigated the electrochemical properties of three-dimensional (3D) methylammonium lead mixed-halide CH3NH3PbI3- xBr x and two-dimensional (2D) propylammonium-methlylammonium lead bromide (CH3NH3)2(CH3(CH2)2NH3)2Pb3Br10 hybrid perovskite thin films as electrode materials for Li-ion batteries. These electrodes were obtained by solution processing at 100 °C. CH3NH3PbBr3 achieved high discharge/charge capacities of ∼500 mA h g-1 /160 mA h g-1 that could account also for other processes taking place during the Li intercalation. It was also found that bromine plays an important role for lithium intercalation, while the new 2D (CH3NH3)2(CH3(CH2)2NH3)2Pb3Br10 with a layered structure allowed reversibility of the lithium insertion-extraction of 100% with capacities of ∼375 mA h g-1 in the form of a thin film. Results suggest that tuning the composition of these materials can be used to improve intercalation capacities, while modification from 3D to 2D layered structures contributes to improving lithium extraction. The mechanism of the lithium insertion-extraction may consist of an intercalation mechanism in the hybrid material accompanying the alloying-dealloying process of the Li xPb intermetallic compounds. This work contributes to revealing the relevance of both composition and structure of potential hybrid perovskite materials as future thin film electrode materials with high capacity and compositional versatility.

Effect of cooling induced crystallization upon the properties of segmented thermoplastic polyurethanes

Abstract An investigation on the cooling-induced crystallization in three thermoplastic polyurethanes based on MDI, PTMG, and 1.4-BD as chain extender with different hard segment content is reported. Thermal transitions were determined using differential scanning calorimetry (DSC) measurements at different cooling rates, and thermal stability was studied by thermogravimetric analysis. Changes in Raman spectra were useful to correlate the thermal transitions with changes in the morphology of the polymers. The dissimilarity in the composition gave different rheological behavior in the molten state, indicated by the temperature dependence of the viscosity. The mechanical properties and the crystallinity was influenced not only by the cooling rate but also by the hard segment content. Thermoplastic polyurethanes with more hard segment content formed more crystalline hard domains as evidenced by the DSC and atomic force microscopy results.

Study of the Crystallization of Metal Halide Perovskites Containing Additives via Differential Scanning Calorimetry

Control of the crystallization and morphology has been one of the major challenges in the design and fabrication of perovskite thin films. Recently, additives of different nature have been used to improve the crystallization and the quality of perovskite films. In this work, we study in detail the role of water (H2O), 1,8-diiodooctane (DIO), and dimethylsulfoxide (DMSO) as additives in the precursor solution of the CH3NH3PbI3−xClx perovskite. Differential scanning calorimetry analysis revealed the thermal transitions for the crystallization of CH3NH3PbI3−xClx perovskite depending on the chemical nature and physical properties of the additive. Information from thermograms enabled us to find optimal annealing ramps for each additive. We found that DIO produced changes mainly due to the interaction of this additive with the precursor salts in solution, while H2O due to its different boiling point compared to the DMF solvent. DMSO dramatically modified the thermal transitions to higher temperatures; however, high quality films were obtained only when the solvent engineering method is used.

New nickel-based hybrid organic/inorganic metal halide for photovoltaic applications

In this work, we have synthesized and fabricated solar cells with the hybrid metal halide compounds with the general formula ABX3, where the A cation is methylammonium, the B cation is nickel, and the X anion is chlorine or a mixture of chlorine and iodine. We obtained experimental evidence that this material is a semiconductor with an orthorhombic crystalline structure which pertains to the space group Cmcm. The bandgap can be modulated from 1.4 eV to 1.0 eV by changing the chlorine anion to iodine. Therefore, we were able to obtain solar cells with efficiencies up to 0.16% with the CH3NH3NiCl2I composition. We have also studied by means of first-principles calculations, taking into account van der Waals dispersive forces, the ground state properties of these materials such as their crystal structure and formation and decomposition energies. We have found that these energies are lowered by the lighter mass anion, and the calculated decomposition energies show that only CH3NH3NiCl3 is stable with respect to the most probable decomposition pathway. The electronic band structure and band edge alignments have been calculated using quasiparticle effects through the GW0 approximation; these materials show an indirect bandgap with the valence band maxima at -6.93 and -5.49 eV with respect to vacuum and the conduction band minima at -5.62 and -4.60 eV with respect to vacuum for CH3NH3NiCl3 and CH3NH3NiI3, respectively. This work provides a pathway to explore new hybrid A+B2+X3--type semiconductor materials.