Irene Gonzalez-Valls

Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review

This work is a brief account of the most recent developments observed in the application of ZnO nanostructured materials in excitonic solar cells (organic, hybrid and dye sensitized solar cells). Special emphasis is made to one-dimensional (1D), vertically-aligned nanostructures (nanowires NW, nanorods NR) of ZnO semiconductor oxide and the extensive research work invested in recent years for its application as an electron acceptor material in solar cells. Our aim is to give the reader a broad overview of this semiconductor oxide and to understand the causes, advantages and disadvantages, for its application in a well-aligned nanostructure form. We briefly describe the most applied methodologies for its synthesis as well as the effect on surface area, electron transport and charge recombination when it is applied as an electron transport material in excitonic solar cells (XSCs). The importance of low-cost and easy-scalable synthesis techniques, as well as stability issues on these solar cells are discussed. Finally, we include a brief analysis of the possible future trends for the application of this interesting semiconductor oxide in XSCs.

Dye sensitized solar cells based on vertically-aligned ZnO nanorods: effect of UV light on power conversion efficiency and lifetime

Electrodes made of vertically-aligned ZnO nanorods (NRs) have been prepared and analyzed in dye sensitized solar cells (DSC). We report a ∼20% power conversion efficiency increase during the first hours of solar cell testing at 1000 W m−2 (AM 1.5). The latter has been attributed to the physisorption/chemisorption of the N-719 dye on the ZnO NRs induced by UV-light irradiation. The ZnO NRs were grown by the hydrothermal method for 6 h obtaining a ZnO layer thickness of about 1.8 μm. The highest solar cell efficiency obtained was 0.69% after UV light irradiation (at 72 °C, 0.63 V, 2.85 mA cm−2, 0.39 FF). The effect of UV light has been monitored by UV-VIS, IV-curves and IPCE analyses with time, and has been related to the solar cell performance.

Aligned TiO2 nanocolumnar layers prepared by PVD-GLAD for transparent dye sensitized solar cells

Transparent thin film electrodes made of vertically aligned nanocolumns of TiO2 with well-controlled oblique angles were grown by physical vapor deposition at glancing incidence (PVD-GLAD). For an electrode thickness of 500 nm, we report a 40% variation on solar cell efficiency (from 0.6% to 1.04%) when the deposition angle was modified between 60° and 85°. Transparent thicker films with higher surface area deposited at the optimal angle of 70° were grown with a zigzag morphology which confers high mechanical strength to the thin films. Using this topology, the application of an electrode thickness of 3 µm in a DSC resulted in a power conversion efficiency of 2.78% maintaining electrode transparency.

Comparison of different vertically aligned ZnO nanostructures in excitonic solar cells: nanorods, nanocore-shells and nanotrees

In this work we present the synthesis and photovoltaic application of four different vertically-aligned ZnO nanostructured electrodes: ZnO nanorods prepared by the a) low-temperature hydrothermal method (LT-HM) and the b) autoclave method (A-HM), c) ZnO nanotrees (NTs) and d) ZnO core-shell NRs with an indium sulfide layer as the shell (CS). The electrodes have been applied in Dye sensitized solar cells (DSCs) and Polymer solar cells (PSCs). The photovoltaic properties of each type of nanostructured electrode were optimized separately. Our results show that the optimal power conversion efficiency depends in great extent on NR dimensions (length and diameter) and the final ZnO nanostructure. In this respect, we have observed an increase in power conversion efficiency when the NR nanostructure is modified as follows: ZnO NRs LT-HM < A-HM < NT< CS for Dye semnsitized solar cells. In the case of PSCs the best power conversion efficiency was obtained for the CS sample.

Vertically Aligned ZnO/InxSy Core–Shell Nanorods for High Efficient Dye-Sensitized Solar Cells

Innovative vertically aligned ZnO/InxSy nanorod (NR) electrodes were prepared by successive ion layer adsorption and reaction (SILAR) technique. The InxSy shell layer was deposited on top of ZnO NR electrodes of two different lengths, ~1.6 μm and ~3.2 μm. Two sulfur contents on the InxSy shell layer with different layer thicknesses were analyzed. These electrodes were fully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD), Energy-dispersive x-ray spectroscopy (EDS), Infrared spectroscopy (FT-IR), x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) and then applied in dye-sensitized solar cells (DSC). Power conversion efficiency of 2.32% was observed when a low-sulfur content InxSy shell layer was applied in comparison to the stoichiometric In2S3 shell layer (0.21%) or the bare ZnO NRs (0.87%). In the case of low sulfur content, a shell layer of In(OH)xSy or/and In(OH)3 is formed as observed by the presence of –OH observed by FTIR analyses. The presence of higher amounts of hydroxide groups modifies the bandgap and work function of the InxSy shell and facilitates dye adsorption, increasing the final solar cell performance.