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Ramón Tena-Zaera

Bio: Ramón Tena-Zaera is an academic researcher from United States Department of Energy. The author has contributed to research in topics: Nanowire & Ionic liquid. The author has an hindex of 39, co-authored 101 publications receiving 5347 citations. Previous affiliations of Ramón Tena-Zaera include University of Valencia & University of Bremen.


Papers
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TL;DR: In this article, a change of focus of the current research on ZnO-based DSCs (from morphology to surface control) is suggested and the origin of this striking difference in performance is analyzed and discussed with the perspective of future applications of ZnOs in dye-sensitized solar cells and related devices.
Abstract: 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 TiO2 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.

517 citations

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TL;DR: In this article, the carrier density of ZnO nanowires has been determined by means of electrochemical impedance spectroscopy, and a model taking into account the geometry of the NN wires has been developed and the differences with the standard flat model, as curved Mott-Schottky plots, are discussed.
Abstract: 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.

287 citations

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TL;DR: The experiments suggest a universality of the underlying phenomenon giving rise to this effect in a broad range of solar cell devices, and an equivalent circuit model is suggested to explain the impedance and capacitance spectra.
Abstract: 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...

221 citations

Journal ArticleDOI
TL;DR: In this paper, a cost-competitive hole selective contact for the emerging organo-metal halide perovskite-based solar cells is proposed, where the CuSCN films have been deposited by a solution casting technique, which has proven to be compatible with the perovsite films, obtaining planar-like heterojunction-based glass/FTO/TiO2/CH3NH3PbI3−xClx/CuSCN/Au solar cells with a power conversion efficiency of 6.4%.
Abstract: 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/TiO2/CH3NH3PbI3−xClx/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 TiO2 and CuSCN charge carrier selective contacts. However, the open-circuit voltage (Voc), 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 Voc. 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.

172 citations


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TL;DR: In this article, the status of zinc oxide as a semiconductor is discussed and the role of impurities and defects in the electrical conductivity of ZnO is discussed, as well as the possible causes of unintentional n-type conductivity.
Abstract: In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

3,291 citations

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TL;DR: In this paper, the authors discussed the steps that have led to this discovery, and the future of this rapidly advancing concept have been considered, and it is likely that the next few years of solar research will advance this technology to the very highest efficiencies while retaining the very lowest cost and embodied energy.
Abstract: Over the last 12 months, we have witnessed an unexpected breakthrough and rapid evolution in the field of emerging photovoltaics, with the realization of highly efficient solid-state hybrid solar cells based on organometal trihalide perovskite absorbers. In this Perspective, the steps that have led to this discovery are discussed, and the future of this rapidly advancing concept have been considered. It is likely that the next few years of solar research will advance this technology to the very highest efficiencies while retaining the very lowest cost and embodied energy. Provided that the stability of the perovskite-based technology can be proven, we will witness the emergence of a contender for ultimately low-cost solar power.

2,506 citations

Journal ArticleDOI
TL;DR: In this paper, three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell, (ii) polymer−smiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell.
Abstract: The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...

2,434 citations