A Chemometric Approach for the Sensitization Procedure of ZnO Flowerlike Microstructures for Dye-Sensitized Solar Cells

In this paper, a methodology for the streamlining of the sensitization procedure of flowerlike ZnO nanostructures for dye-sensitized solar cells (DSCs) is reported. The sensitization of ZnO surface with ruthenium-based complexes is a particularly critical process, since one has to minimize the dissolution of surface Zn atoms by the protons released from the dye molecules, leading to the formation of Zn²⁺/dye complexes. The fine-tuning of the experimental parameters, such as the dye loading time, the dye concentration, and the pH of the sensitizing solution, performed through a multivariate optimization by means of a chemometric approach, is here reported. The dye loading procedure was optimized using ZnO microparticles with nanostructured protrusions, synthesized by a simple and low-cost hydrothermal process. Mild reaction conditions were used, and wurtzite-like crystalline structure with a relatively high surface area was obtained once the reaction process was completed. After dispersion of ZnO flowerlike particles in an acetic acid-based solution, a 14 μm-thick ZnO layer acting as DSC photoanode was fabricated. The optimized sensitization procedure allowed minimizing the instability of ZnO surface in contact with acidic dyes, avoiding the formation of molecular agglomerates unable to inject electrons in the ZnO conduction band, achieving good results in the photoconversion efficiency. Moreover, the photoharvesting properties were further enhanced by adding <i>N</i>-methylbenzimidazole into the iodine-based liquid electrolyte. Such an additive was proposed here for the first time in combination with a ZnO photoelectrode, helping to reduce an undesired recombination between the photoinjected electrons and the oxidized redox mediator

First Pseudohalogen Polymer Electrolyte for Dye-Sensitized Solar Cells Promising for <i>In Situ</i> Photopolymerization

The incorporation of selenocianated-based redox couple in a polymer electrolyte for dye-sensitized solar cells is reported for the first time. The pseudohalogen redox mediator was integrated in two kinds of acrylic/methacrylic membranes prepared by photocopolymerization of multifunctional monomers. Before activation, the obtained membranes were transparent, self-standing and flexible, and the physicochemical characterizations of the films showed the formation of highly crosslinked architectures. Membranes were activated by swelling in an optimized solution containing the SeCN^–/(SeCN)₂ redox mediator with 4-<i>tert</i>-butylpyridine in acetonitrile, and the electrochemical behavior of the electrolytes revealed fast charge transfer kinetics. The photovoltaic performances of quasi-solid dye-sensitized solar cells were evaluated and compared with the results of the liquid counterpart, showing promising photoharvesting properties. No diminution in photoconversion efficiencies was evidenced in the comparison between solid and liquid cells, demonstrating an optimal kinetics of the redox species in the polymer cage, associated with a noteworthy increase in device durability, as demonstrated by aging tests. In addition, the <i>in situ</i> photopolymerization in the presence of the redox mediator is presented with outstanding results: this process, hardly feasible for the traditional I^–/I₃^– couple (inhibitor of radical polymerization processes), enables at the same time the creation of an excellent electrode/electrolyte interface and the sealing of the device

Unravelling Resistive Switching Mechanism in ZnO NW Arrays: The Role of the Polycrystalline Base Layer

The physical mechanism involved in resistive switching phenomena occurring in devices based on ZnO nanowire (NW) arrays may vary considerably, also depending on the structure of the switching layer. In particular, it is shown here that the formation of a ZnO base layer between the metallic catalyst substrate and the NW, which is typical of CVD-grown ZnO NW arrays, should not be neglected when explaining the switching physical mechanism. The structural and electrical properties of this layer are investigated after the mechanical removal of NWs. Electrical measurements were performed in the presence of NWs and, after their removal, showed that the base alone exhibits resistive switching properties. The proposed switching mechanism is based on the creation/rupture of an oxygen vacancies conductive path along grain boundaries of the polycrystalline base. The creation of the filament is facilitated by the high concentration of vacancies at the grain boundaries that are oriented perpendicularly to the electrodes, as a direct consequence of the ZnO growth along the <i>c</i>-axis of the wurtzite lattice

Mixed 1T–2H Phase MoS₂/Reduced Graphene Oxide as Active Electrode for Enhanced Supercapacitive Performance

A hybrid aerogel, composed of MoS₂ sheets of 1T (distorted octahedral) and 2H (trigonal prismatic) phases, finely mixed with few layers of reduced graphene oxide (rGO) and obtained by means of a facile environment-friendly hydrothermal cosynthesis, is proposed as electrode material for supercapacitors. By electrochemical characterizations in three- and two-electrode configurations and symmetric planar devices, unique results have been obtained, with specific capacitance values up to 416 F g^–1 and a highly stable capacitance behavior over 50000 charge–discharge cycles. The in-depth morphological and structural characterizations through field emission scanning electron microscopy, Raman, X-ray photoelectron spectroscopy, X-ray diffraction, Brunauer–Emmett–Teller, and transmission electron microscopy analysis provides the proofs of the unique assembly of such 3D structured matrix. The unpacked MoS₂ structure exhibits an excellent distribution of 1T and 2H phase sheets that are highly exposed to interaction with the electrolyte, and so available for surface/near-surface redox reactions, notwithstanding the quite low overall content of MoS₂ embedded in the reduced graphene oxide (rGO) matrix. A comparison with other “more conventional” hybrid rGO-MoX₂ electrochemically active materials, synthesized in the same conditions, is provided to support the outstanding behavior of the cosynthesized rGO-MoS₂