Monica Lira-Cantu

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.

Influence of Doped Anions on Poly(3,4-ethylenedioxythiophene) as Hole Conductors for Iodine-Free Solid-State Dye-Sensitized Solar Cells

Poly(3,4-ethylenedioxythiophene) (PEDOT) is an excellent hole-conducting polymer able to replace the liquid I(-)/I3(-) redox electrolyte in dye-sensitized solar cells (DSCs). In this work we applied the in situ photoelectropolymerization technique to synthesize PEDOT and carried out a careful analysis of the effect of different doping anions on overall solar cell performance. The anions analyzed in this work are ClO4(-), CF3SO3(-), BF4(-), and TFSI(-). The best solar cell performance was observed when the TFSI(-) anion was used. Photoelectrochemical and impedance studies reveal that the doped anions in the PEDOT hole conductor system have great influences on I-V curves, conductivity, and impedance. The optimization of these parameters allowed us to obtain an iodine-free solid-state DSC with a maximum J(sc) of 5.3 mA/cm2, V(oc) of 750 mV, and a conversion efficiency of 2.85% which is the highest efficiency obtained so far for an iodine-free solid-state DSC using PEDOT as hole-transport material.

Hybrid organic-inorganic nanocomposite materials for application in solid state electrochemical supercapacitors

Integration into a conducting polymer matrix to form a hybrid material is an effective way to harness the electrochemical activity of nanosized oxide clusters. By anchoring them into polyaniline, the reversible redox chemistry of the otherwise soluble polyoxometalate clusters can be combined with that of the conducting polymer and be put to work in energy storage applications. We present here preliminary results that show how the resulting hybrid polymer displays the combined activity of its organic and inorganic components to store and release charge in a solid state electrochemical capacitor device.

The Organic-Inorganic Polyaniline/V2O5 System. Application as a High-Capacity Hybrid Cathode for Rechargeable Lithium Batteries

This work was partially supported by CICYT (Spain, MAT98-0807-C02-02). We also thank the Ministerio de Educacion y Ciencia (Spain, MEC-AECI) and CONACYT (Mexico) for a predoctoral fellowship to M.L.C.

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

Low-temperature, solution-processed, layered V2O5 hydrate as the hole-transport layer for stable organic solar cells

Layered V2O5 hydrate has been applied as the hole transport layer (HTL) in organic solar cells (OSCs). V2O5 is obtained from a sodium metavanadate solution in water under ambient conditions, resulting in a final thin film of formula V2O5·0.5H2O. The 0.5 water molecules are not removed from the V2O5 layered structure unless the sample is heated above 250 °C, which makes the thin film highly stable under real working conditions. The HTL was used in OSCs in the normal and the inverted configurations, applying metallic Ag as the back-metal electrode in both cases. Fabrication of both OSC configurations completely by solution-processing printing methods in air is possible, since the Al electrode needed for the normal-configuration OSC is not required. The work function (WF) and band gap energy (BG) of the V2O5 thin films were assessed by XPS, UPS and optical analyses. Different WF values were observed for V2O5 prepared from a fresh V2O5–isopropanol (IPA) solution (5.15 eV) and that prepared from a 24 h-old solution (5.5 eV). This difference is due to the gradual reduction of vanadium (from V5+ to V4+) in IPA. The OSCs made with the V2O5 thin film obtained from the 24 h-old V2O5–IPA solution required photo-activation, whereas those made with the freshly obtained V2O5 did not. Outdoor stability analyses of sealed OSCs containing a V2O5 HTL in either configuration revealed high stability for both devices: the photovoltaic response at T80 was retained for more than 1000 h.

Detrimental Effect of Inert Atmospheres on Hybrid Solar Cells Based on Semiconductor Oxides

We report the improvement observed in J sc , V oc , and current-voltage (1-V) curves when hybrid solar cells (HSCs) are transferred from inert conditions to ambient atmosphere. The effect is observed regardless of the semiconductor oxide applied and has been attributed to the reversible incorporation of oxygen from the atmosphere into the semiconductor oxide surface during illumination. The HSCs were prepared as bilayers of thin-film semiconductor oxides (TiO 2 , Nb 2 O 5 , and ZnO) made by the sol-gel technique and the polymer poly[(2-methoxy-5-ethylhexyloxy)-l,4-phenylenevinylene] (MEH-PPV), applying a final device configuration of the type indium tin oxide/oxide thin film /MEH-PPV/Ag. The photovoltaic response was studied in terms of inert atmosphere by recording the initial values of open-circuit voltage (V oc ) and current density (J sc ). Solar decay curves, I-V curves, the effect of filter and resting time, as well as photophysical analyses were also carried out for each type of device.

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.

Chemical polymerization of polyaniline and polypyrrole by phosphomolybdic acid : In situ formation of hybrid organic-inorganic materials

The oxidative polymerization of aniline or pyrrole by phosphomolybdic acid (H 3 PMo 12 O 40 ) is described. These reactions take place in the absence of other anions and result in the incorporation of the photoactive and electroactive PMo 12 O 40 3- anion into the lattice of the polymer. The resulting hybrid materials present the good conductivity and polymeric nature of the matrix plus the added electroactivity of the inorganic cluster, which is anchored in the polymer and not exchanged upon reduction. The hybrids can work as electrodes harnessing the electrochemical properties of the polyoxometalic clusters.