• Energy Research

AbstractZinc oxide (ZnO) was dissolved in an aqueous solution of ammonia and hydrogen peroxide. According to XRD characterization, this precursor solution yielded crystalline zincoxide after evaporation and thermal treatment at 90 °C for 2 h. The processability of this precursor solution allowed for ZnO deposition on cellulose paper, which is an example of a thermolabile substrate. Cellulose filter paper that was impregnated with this solution and subsequently thermally treated at 90 °C for 2 h had nanostructured crystalline ZnO well adhered to its surface. This composite material was characterized by XRD and scanning electron microscopy (SEM). The described precursor solution also assisted in the preparation of ZnO porous layers for dye‐sensitized solar cells (DSC). For this purpose, it was mixed with ZnO nanopowder dispersed in ethanol. The mixture was then sprayed on transparent conductive oxide substrates. In this way porous films were obtained after heat treatment at 90 °C for 2 h. Under standard testing conditions (AM1.5G, 100 mW cm–2), a device prepared with (Bu4N)2[Ru(dcbpyH)2(NCS)2] (dcbpy = 4,4′‐dicarboxy‐2,2′‐bipyridine) (N‐719) dye and a film thickness of 5 μm exhibited 1.43 % efficiency.

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.

7 páginas, 6 figuras, 2 tablas. 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. To the projects ENE2008-04373 and PIE-200860I134 and the PhD scholarship awarded to I. G-V (BES-2009-028996) from the Spanish Ministry of Science and Innovation, MICINN. To the Consolider NANOSELECT project CSD2007-00041. Peer reviewed

Accurate characterization and reporting of organic photovoltaic (OPV) device performance reniains one of the important challenges in the field. The large spread among the efficiencies of devices with the same structure reported by different groups is significantly caused by different procedures and equipment used during testing. The presented article addresses this issue by offering a new method of device testing using "suitcase sample" approach combined with outdoor testing that limits the diversity of the equipment, and a strict measurement protocol. A round robin outdoor characterization of roll-to-roll coated OPV cells and modules conducted among 46 laboratories worldwide is presented, where the samples and the testing equipment were integrated in a compact suitcase that served both as a sample transportation tool and as a holder and test equipment during testing. In addition, an internet based coordination was used via plasticphotovoltaics.org that allowed fast and efficient communication among participants and provided a controlled reporting format for the results that eased the analysis of the data. The reported deviations among the laboratories were limited to 5% when compared to the Si reference device integrated in the suitcase and were up to 8% when calculated using the local irradiance data. Therefore, this method offers a fast, cheap and efficient tool for sample sharing and testing that allows conducting outdoor measurements of OPV devices in a reproducible manner. (C) 2014 Elsevier B.V. All rights reserved.

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 m in a DSC resulted in a power conversion efficiency of 2.78% maintaining electrode transparency. © 2011 The Royal Society of Chemistry. The authors would like to thank the projects ENE2008-04373, PIE-200860I134 and MAT20010-21218 and the PhD scholarship awarded to I.G.-V. (BES-2009-028996) from the Spanish Ministry of Science and Innovation, MICINN; to the Consolider NANOSELECT project CSD2007-00041 and FUNCOAT CDS2008-0023; to the Xarxa de Referencia en Materials Avancats per a l’Energia, XaRMAE (Reference Center of Advanced Materials for Energy) of the Catalonia Government and Project P09-TEP-5283 from the Junta de Andaluc ía. Peer Reviewed

13 páginas, 11 figuras, 5 tablas. In this work, we study the effect of the transparent conducting oxide (TCO) and the polymer applied (MEH-PPV or P3HT) on the photovoltaic properties of TCO/TiO2/polymer/Ag bi-layer solar cells. The solar cells were analyzed under inert atmosphere conditions resembling an encapsulated or sealed device. We demonstrate that the substrate applied, ITO or FTO, modifies the crystalline structure of the TiO2: on an ITO substrate, TiO2 is present in its anatase phase, on an FTO, the rutile phase predominates. Devices fabricated on an FTO, where the rutile phase is present, show better stability under inert atmospheres than devices fabricated on an ITO, anatase phase. With respect to the polymer, devices based on MEH-PPV show higher Voc (as high as 1 V), while the application of P3HT results in lower Voc, but higher Jsc and longer device stability. These observations have been associated to (a), the crystalline structure of TiO2 and (b) to the form the polymer is bonded to the TiO2 surface. In-situ IPCE analyses of P3HT-based solar cells show a red shift on the peak corresponding to TiO2, which is not present on the MEH-PPV-based solar cells. The latter suggest that P3HT can be linked to the TiO2 though the S-end atom, which results in devices with lower Voc. All these observations are also valid for devices, where the bare TiO2 is replaced by an Nb–TiO2. The application of an Nb–TiO2 with rutile structure in these polymer/oxide solar cells is the reason for their higher stability under inert atmospheres. We conclude that the application of TiO2 in its rutile phase is beneficial for long-term stability devices. Moreover there is an interplay between low Voc and Jsc in devices applying P3HT, since power conversion efficiency can be partially canceled by their lower Voc in comparison with MEH-PPV. These findings are important for polymer/oxide solar cells, but also for organic solar cells, where a layer of semiconductor oxides are in direct contact with a polymer, like in an inverted or tandem organic solar cells. To the Spanish National Research Council (CSIC) for the JAE-Doc contract awarded to Y.Y. and the FPI scholarship awarded to I.G.-V. To the Spanish Ministry of Science and Innovation, MICINN for the Projects ENE2008-04373, PIE-200860I134 and Consolider NANOSELECT (CSD2007-00041). To the Xarxa de Referència en Materials Avançats per a l’Energia, XaRMAE (Reference Center for Advanced Materials for Energy) of the Catalonia Government. Peer reviewed