Javier Navas

ZnO-based dye solar cell with pure ionic-liquid electrolyte and organic sensitizer: the relevance of the dye–oxide interaction in an ionic-liquid medium

The use of non-volatile electrolytes and fully organic dyes are key issues in the development of stable dye-sensitized solar cells (DSCs). In this work we explore the performance of ZnO-based DSCs sensitized with an indoline derivative coded D149 in the presence of a pure ionic-liquid electrolyte. Commercial nanostructured zinc oxide and an electrolyte composed of iodine plus (1) pure 1-propyl-3-methyl imidazolium iodide (PMII) and (2) a blend of PMII with low-viscosity ionic liquids were employed to construct the devices. Without further additives, the fabricated devices exhibit remarkable short-circuit photocurrents and efficiencies under AM1.5 simulated sunlight (up to 10.6 mA cm−2, 2.9% efficiency, 1 sun, active area = 0.64 cm2) due to the high surface area of the ZnO film and the high absorptivity of the D149 dye. Impedance spectroscopy is used to characterize the devices. It is found that the addition of the low-viscosity ionic-liquid improves the transport features (leading to a better photocurrent) but it does not alter the recombination rate. The robustness of the dye–oxide interaction is tested by applying continuous illumination with a Xenon-lamp. It is observed that the photocurrent is reduced at a slow rate due to desorption of the D149 sensitizer in the presence of the ionic liquid. Exploration of alternative ionic-liquid compositions or modification of the ZnO surface is therefore required to make stable devices based on ZnO and fully organic dyes.

Introducing “UCA-FUKUI” software: reactivity-index calculations

A new software (UCA-FUKUI) has been developed to facilitate the theoretical study of chemical reactivity. This program can calculate global parameters like hardness, softness, philicities, and Fukui condensed functions, and also local parameters from the condensed functions. To facilitate access to the program we have developed a very easy-to-use interface. We have tested the performance of the software by calculating the global and local reactivity indexes of a group of representative molecules. Finite difference and frontier molecular orbital methods were compared and their correlation tested. Finally, we have extended the analysis to a set of ligands of importance in coordination chemistry, and the results are compared with the exact calculation. As a general trend, our study shows the existence of a high correlation between global parameters, but a weaker correlation between local parameters.

Electronic and structural properties of highly aluminum ion doped TiO(2) nanoparticles: a combined experimental and theoretical study.

This study presents the experimental and theoretical study of highly internally Al-doped TiO2 nanoparticles. Two synthesis methods were used and detailed characterization was performed. There were differences in the doping and the crystallinity, but the nanoparticles synthesized with the different methods share common features. Anatase to rutile transformation occurred at higher temperatures with Al doping. X-ray photoelectron spectroscopy showed the generation of oxygen vacancies, which is an interesting feature in photocatalysis. In turn, the band-gap energy and the valence band did not change appreciably. Periodic density functional calculations were performed to model the experimentally doped structures, the formation of the oxygen vacancies, and the band gap. Calculation of the density of states confirmed the experimental band-gap energies. The theoretical results confirmed the presence of Ti(4+) and Al(3+) . The charge density study and electron localization function analysis indicated that the inclusion of Al in the anatase structure resulted in a strengthening of the TiO bonds around the vacancy.

Tm-doped TiO2 and Tm2Ti2O7 pyrochlore nanoparticles: enhancing the photocatalytic activity of rutile with a pyrochlore phase

Summary Tm-doped TiO2 nanoparticles were synthesized using a water-controlled hydrolysis reaction. Analysis was performed in order to determine the influence of the dopant concentration and annealing temperature on the phase, crystallinity, and electronic and optical properties of the resulting material. Various characterization techniques were utilized such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and UV–vis spectroscopy. For the samples annealed at 773 and 973 K, anatase phase TiO2 was obtained, predominantly internally doped with Tm3+. ICP–AES showed that a doping concentration of up to 5.8 atom % was obtained without reducing the crystallinity of the samples. The presence of Tm3+ was confirmed by X-ray photoelectron spectroscopy and UV–vis spectroscopy: the incorporation of Tm3+ was confirmed by the generation of new absorption bands that could be assigned to Tm3+ transitions. Furthermore, when the samples were annealed at 1173 K, a pyrochlore phase (Tm2Ti2O7) mixed with TiO2 was obtained with a predominant rutile phase. The photodegradation of methylene blue showed that this pyrochlore phase enhanced the photocatalytic activity of the rutile phase.

Preparation of Au nanoparticles in a non-polar medium: obtaining high-efficiency nanofluids for concentrating solar power. An experimental and theoretical perspective

This paper presents the preparation of Au nanoparticles in a non-polar medium, which is a fluid composed of the eutectic mixture of biphenyl and diphenyl oxide commonly used in Concentrating Solar Power (CSP) plants. The nanofluids prepared showed enhanced thermal properties, presenting thermal conductivity values 70% higher than those of base fluids, and isobaric specific heat values up to 10% higher. In turn, an increase of up to 36% was observed in their heat transfer coefficient, which is their efficiency as a heat transfer fluid (HTF). Also, the stability of the nanofluids was analysed using UV-vis spectroscopy, and particle size and ζ potential. The nanofluids with lower concentrations agglomerate slowly, which is considered stable for this application. Thus, these nanofluids are a promising, interesting alternative to the HTF often used in CSP plants. Also, molecular dynamics calculations were performed to better understand how the Au-nanofluid behaves in the presence of a surfactant within a temperature range between 50 and 600 K. The isobaric specific heat and thermal conductivity values followed the same experimental tendency. The analysis of the radial distribution functions (RDFs) and spatial distribution functions (SDFs) showed that, as the temperature rose, an exchange took place between the surfactant and diphenyl oxide molecules in the first layer of molecules around the metal. This movement incorporated a directionality that may play a part in the enhanced thermal properties. The surfactant participates as an active component within the Au-nanofluid, contributing to efficient heat transfer processes.

TiO2 and pyrochlore Tm2Ti2O7 based semiconductor as a photoelectrode for dye-sensitized solar cells

This study presents the use of TiO2 nanoparticles with Tm as photoelectrodes in DSSCs. The nanoparticles were annealed at 1173 K and the predominant TiO2 phase was rutile. XRD and Raman spectroscopy revealed the presence of a crystalline pyrochlore phase of the mixed oxide Tm2Ti2O7. In turn, XPS confirmed the presence of Ti4+ and Tm3+, so the inclusion of Tm did not affect the oxidation state of the Ti. UV–Vis spectra showed that the presence of the pyrochlore phase led to new electronic states in the band gap. The use of the pyrochlore phase in the photoelectrode had a positive effect, improving the efficiency of the pure TiO2 cells. The efficiency increased by between 2.32% and 3.16% when pure TiO2 was replaced with a mixture of rutile TiO2 and pyrochlore Tm2Ti2O7, so the controlled use of a pyrochlore phase can produce good results in dye-sensitized solar cells. Another important effect of the pyrochlore phase was to increase the open-circuit voltage values by around 7% and can be explained by the flat band voltage values. The samples with Tm showed two flat band voltage values, which generated two possible electronic injection mechanisms in the cells.

Convergent study of Ru–ligand interactions through QTAIM, ELF, NBO molecular descriptors and TDDFT analysis of organometallic dyes

We report a theoretical study of a series of Ru complexes of interest in dye-sensitised solar cells, in organic light-emitting diodes, and in the war against cancer. Other metal centres, such as Cr, Co, Ni, Rh, Pd, and Pt, have been included for comparison purposes. The metal–ligand trends in organometallic chemistry for those compounds are shown synergistically by using three molecular descriptors: quantum theory of atoms in molecules (QTAIM), electron localisation function (ELF) and second-order perturbation theory analysis of the natural bond orbital (NBO). The metal–ligand bond order is addressed through both delocalisation index (DI) of QTAIM and fluctuation index (λ) of ELF. Correlation between DI and λ for Ru–N bond in those complexes is introduced for the first time. Electron transfer and stability was also assessed by the second-order perturbation theory analysis of the NBO. Electron transfer from the lone pair NBO of the ligands toward the antibonding lone pair NBO of the metal plays a relevant role in stabilising the complexes, providing useful insights into understanding the effect of the ‘expanded ligand’ principle in supramolecular chemistry. Finally, absorption wavelengths associated to the metal-to-ligand charge transfer transitions and the highest occupied molecular orbital (HOMO)--lowest unoccupied molecular orbital (LUMO) characteristics were studied by time-dependent density functional theory.

On-line thermal dependence study of the main solar cell electrical photoconversion parameters using low thermal emission lamps

This paper presents a non-conventional methodology and an instrumental system to measure the effect of temperature on the photovoltaic properties of solar cells. The system enables the direct measurement of the evolution of open-circuit voltage and short-circuit current intensity in relation to a continuously decreasing temperature. The system uses a high-intensity white light-emitting diode light source with low emissions of radiation in the infrared region of the electromagnetic spectrum, resulting in a reduced heating of the photovoltaic devices by the irradiation source itself. To check the goodness of the system and the methodology designed, several measurements were performed with monocrystalline silicon solar cells, dye-sensitized solar cells, and thin-film amorphous silicon solar cells, showing similar tendencies to those reported in the literature.

Experimental analysis and computer simulation of a methodology for laser focusing in the solar cell characterization by laser beam induced current

This paper presents a quick methodology for focusing a laser beam on a photoactive surface based on performing a single line scan while simultaneously modifying the lasers position over the surface and the distance between the focusing lens and the active surface. The methodology was tested using the computer simulation technique. Several configurations were computer simulated by programming different experimental situations to discover the situations in which this focusing methodology would provide optimum results. The conclusions obtained from computer simulation methodology were checked by means of experimental tests using several solar cells, such as a thin-film amorphous silicon solar cell, a monocrystalline silicon solar cell, and a polycrystalline silicon solar cell. From the tests performed, we concluded that optimum focusing is achieved in systems in which the laser beam induced current signal generated by the photoactive surface has no large heterogeneities (e.g., fingers or grain boundaries), artefacts, or defects. Thus, the best results are achieved in systems where the surface of the photovoltaic device has a certain degree of homogeneity.

Hybrid Perovskite, CH3NH3PbI3, for Solar Applications

The effect of the incorporation of NH4+ into the CH3NH3+ sites of the tetragonal perovskite CH3NH3PbI3 is analysed. Also, how it affects the introduction of Cd2+ cations into Pb2+ sites for a perovskite with 25źat.% of NH4+ is addressed. The incorporation of NH4+ into perovskite leads to a dramatic loss of crystallinity and to the presence of other phases. Moreover, the NH4PbI3 was not found. The less formation of perovskite when NH4+ is incorporated is due to geometrical factors and not changes in the chemical state bonding of the ions. Also, the samples where perovskite is formed show similar band gap values. A slight increase is observed for samples with x=0.5 and 0.75. For the sample with x=1, a drastic increase of the band gap is obtained. Periodic-DFT calculations agree with the experimental structural tendency when NH4+ is incorporated and the density of states analysis confirmed the experimental band gap. The perovskite with 25źat.% of NH4+ was selected for studying the effect of the concentration of Cd on the structural and electronic properties. The theoretical band gap values decreased with the Cd concentration where the narrowing of Cd s-states in the conduction band plays an important role.