ZnO was one of the first metal oxides used in dye-sensitized solar cells (DSCs). It exhibits a unique combination of potentially interesting properties such as high bulk electron mobility and probably the richest variety of nanostructures based on a very wide range of synthesis routes. However, in spite of the huge amount of literature produced in the past few years, the reported efficiencies of ZnO-based solar cells are still far from their TiO 2 counterparts. The origin of this striking difference in performance is analyzed and discussed with the perspective of future applications of ZnO in dye-sensitized solar cells and related devices. In this regard, a change of focus of the current research on ZnO-based DSCs (from morphology to surface control) is suggested.

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The carrier density of ZnO nanowires has been determined by means of electrochemical impedance spectroscopy. A model taking into account the geometry of ZnO nanowires has been developed and the differences with the standard flat model, as curved Mott-Schottky plots, are discussed. The as-grown electrodeposited samples present a high donor density of 6.2×1019cm−3, dramatically reduced by two orders of magnitude after an annealing in air at 450°C during 1h. The results show that the surface of the ZnO nanowires is active; therefore this system appears as a useful structure to support a functionalized nanostructured devices.

Four different types of solar cells prepared in different laboratories have been characterized by impedance spectroscopy (IS): thin-film CdS/CdTe devices, an extremely thin absorber (eta) solar cell made with microporous TiO2/In(OH)xSy/PbS/PEDOT, an eta-solar cell of nanowire ZnO/CdSe/CuSCN, and a solid-state dye-sensitized solar cell (DSSC) with Spiro-OMeTAD as the transparent hole conductor. A negative capacitance behavior has been observed in all of them at high forward bias, independent of material type (organic and inorganic), configuration, and geometry of the cells studied. The experiments suggest a universality of the underlying phenomenon giving rise to this effect in a broad range of solar cell devices. An equivalent circuit model is suggested to explain the impedance and capacitance spectra, with an inductive recombination pathway that is activated at forward bias. The deleterious effect of negative capacitance on the device performance is discussed, by comparison of the results obtained for a conventional monocrystalline Si solar cell showing the positive chemical capacitance expected in the ideal IS model of a solar cell.

The Na–O2 battery offers an interesting alternative to the Li–O2 battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal–O2 batteries, they display significant differences. For instance, Li–O2 batteries form Li2O2 as the discharge product at the cathode, whereas Na–O2 batteries usually form NaO2. A very important question that affects the performance of the Na–O2 cell concerns the key parameters governing the growth mechanism of the large NaO2 cubes formed upon reduction, which are a requirement of viable capacities and high performance. By comparing glyme-ethers of various chain lengths, we show that the choice of solvent has a tremendous effect on the battery performance. In contrast to the Li–O2 system, high solubilities of the NaO2 discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na+ ion solvent shell structure on the NaO2 growth mechanism. Strong solvent–solute interactions in long-chain ethers shift the formation of NaO2 toward a surface process resulting in submicrometric crystallites and very low capacities (ca. 0.2 mAh/cm2 (geom)). In contrast, short chains, which facilitate desolvation and solution-precipitation, promote the formation of large cubic crystals (ca. 10 um), enabling high capacities (ca. 7.5 mAh/cm2 (geom)). This work provides a new way to look at the key role that solvents play in the metal–air system.

A systematic study of the role of KCl on the electrodeposition of ZnO nanowire arrays from the reduction of oxygen in ZnCl 2 solutions was performed. Besides its role as a supporting electrolyte, other major effects were found. An increase of KCl concentration ([KCl]) considerably decreased the rate of O 2 reduction. The consequent decrease in OHproduction rate resulted in an augmentation of the ZnO deposition efficiency, from a value around 3% for [KCl] ) 5 × 10 -2 M to more than 40% for [KCl] ) 3.4 M. The increase of the deposition efficiency mainly resulted in an enhancement of the longitudinal growth rate. However, high [KCl] (>1 M) also favored the lateral growth of the ZnO nanowires, resulting in diameters as big as 300 nm (in comparison to the diameter of 80 nm obtained for [KCl] < 1 M). The observed effects were discussed in terms of Clion adsorption on the cathode surface. The possible preferential adsorption of the anion on the (0001) ZnO surface was emphasized. Transmission electron microscopy revealed that the ZnO nanowires were single crystals, irrespective of [KCl] in the electrolyte. Thus, playing with the chloride content in the solution is an interesting way to obtain ZnO single-crystal nanowire arrays with tailored dimensions under controlled deposition rates. The influence of the nanowire dimensions on the optical properties was also discussed, showing the interest of this study in the frame of nanostructured solar cells.

An innovative route is presented to obtain arrays of single-crystal ZnO nanotubes with tailored dimensions. The three-step process combines electrochemical and chemical approaches. The first step consists in the electrodeposition of ZnO nanowire arrays from the O 2 reduction in an aqueous solution of zinc chloride (ZnCl 2 ) and potassium chloride (KCl). In the second step the core of ZnO nanowires is selectively etched in a KCl solution, resulting in the formation of tubular structures. The influence of KCl concentration, temperature, and immersion time in the ZnO nanotube formation process is investigated, with the finding that the dissolution of the nanowire core occurs for [KCl] g 1 M and the etching rate is enhanced with the temperature. Arrays of ZnO nanotubes with tailored dimensions (200-500 nm external diameter and 1-5 µm length) are obtained by varying the conditions of nanowire array deposition and taking into account the dimensions of the nanowires to adjust the dissolution time. A precise control of the nanotube wall thickness is achieved by performing a further electrodeposition step. The whole process occurs at low temperature (80°C) in aqueous chloride solution at neutral pH, in a couple of hours. The structural properties of obtained ZnO nanotubes are analyzed by transmission electron microscopy, showing their single-crystal character.

CuSCN is proposed as a cost-competitive hole selective contact for the emerging organo-metal halide perovskite-based solar cells. The CuSCN films have been deposited by a solution casting technique, which has proven to be compatible with the perovskite films, obtaining planar-like heterojunction-based glass/FTO/TiO 2 /CH 3 NH 3 PbI 3Àx Cl x /CuSCN/Au solar cells with a power conversion efficiency of 6.4%.Among the photovoltaic parameters, the fill factor (i.e. 62%) suggests good carrier selectivity and, therefore, efficient functionality of the TiO 2 and CuSCN charge carrier selective contacts. However, the open-circuit voltage (V oc ), which remains low in comparison with the state of the art perovskite-based solar cells, appears to be the main limiting parameter. This is attributed to the short diffusion length as determined by impedance spectroscopy. However, the recombination losses are not only affected by the CuSCN, but also by the perovskite film. Indeed, variations of 20 C in the thermal annealing of the perovskite films result in changes larger than 200 mV in the V oc . Furthermore, a detailed analysis of the quantum efficiency spectra contributes significant insights into the influence of the selective contacts on the photocurrent of the planar heterojunction perovskite solar cells.