Roberto Argazzi

Testing of dye sensitized TiO2 solar cells. I: Experimental photocurrent output and conversion efficiencies

Abstract Recently, a paper was published by the Lausanne Group headed by Dr. M. Graetzel which reported a simple low cost 7% efficient photo electrochemical solar cell made from a trinuclear Ru dye complex adsorbed on the very rough surface of a colloidal TiO 2 film. In the current paper, a verification of this result is presented using procedures described in the literature. Measurements are reported in stimulated and natural sunlight which confirm that the efficiency is indeed in the range previously reported. Predicted Air Mass 1.5 photo currents are compared to those obtained from fabricated dye sensitized cells. Although current densities of 12 mA/cm 2 and voltages of over 0.6 V are measured,it is found that corresponding fill factors, less than 0.6, limit the performance of the cell under solar illumination. The basic economics of such a device are outlined and it is proposed that cell costs of

Molecular and supramolecular sensitization of nanocrystalline wide band-gap semiconductors with mononuclear and polynuclear metal complexes

0.6 per peak watt could be possible if the longevity of the cell is at least 15 years.

Ruthenium polyoxometalate water splitting catalyst: very fast hole scavenging from photogenerated oxidants

Some strategies for the design of mononuclear sensitizers and of artificial supramolecular systems useful in the sensitization of wide band-gap semiconductors, and featuring functions such as photoinduced charge separation and the antenna effect for their use in sensitization of semiconductors are reviewed. Such functions depend on the choice of specific molecular components which may control the kinetics of the interfacial and intercomponent electron transfer processes. Examples of molecular devices, which may prevent interfacial charge recombination in sensitized semiconductor cells, and examples of polynuclear complexes, supporting efficient intramolecular energy transfer to sensitizer units, are discussed.

Efficient dye-sensitized solar cells using red turnip and purple wild sicilian prickly pear fruits.

The tetraruthenium polyoxometalate water oxidation catalyst 1 performs very fast hole scavenging from photogenerated Ru(iii) polypyridine complexes, both in homogeneous solution and at a sensitized nanocrystalline TiO(2) surface.

Efficient Photoelectrochemical Water Splitting by Anodically Grown WO3 Electrodes

Dye-sensitized solar cells (DSSCs) were assembled by using the bougainvillea flowers, red turnip and the purple wild Sicilian prickly pear fruit juice extracts as natural sensitizers of TiO2 films. The yellow orange indicaxanthin and the red purple betacyanins are the main components in the cocktail of natural dyes obtained from these natural products. The best overall solar energy conversion efficiency of 1.7% was obtained, under AM 1.5 irradiation, with the red turnip extract, that showed a remarkable current density (Jsc = 9.5 mA/cm2) and a high IPCE value (65% at λ = 470 nm). Also the purple extract of the wild Sicilian prickly pear fruit showed interesting performances, with a Jsc of 9.4 mA/cm2, corresponding to a solar to electrical power conversion of 1.26%.

Natural dye senstizers for photoelectrochemical cells

The potentiostatic anodization of metallic tungsten has been investigated in different solvent/electrolyte compositions with the aim of improving the water oxidation ability of the tungsten oxide layer. In the NMF/H(2)O/NH(4)F solvent mixture, the anodization leads to highly efficient WO(3) photoanodes, which, combining spectral sensitivity, an electrochemically active surface, and improved charge-transfer kinetics, outperform, under simulated solar illumination, most of the reported nanocrystalline substrates produced by anodization in aqueous electrolytes and by sol-gel methods. The use of such electrodes results in high water electrolysis yields of between 70 and 90% in 1 M H(2)SO(4) under a potential bias of 1 V versus SCE and close to 100% in the presence of methanol.

Electronic coupling between remote metal centers in cyanobridged polynuclear complexes

In nature, fruit, vegetable, leaves, flowers and algae contain several dyes which can be easily extracted and employed in dye sensitized photoelectrochemical cells. In this contribution, the most significant advances made in the search for efficient and convenient natural sensitizers are reported through meaningful examples and case studies. To date, selected chlorophyll derivatives, raw anthocyanine and betalain extracts are the most successful natural sensitizers, resulting in the generation of monochromatic photon to current conversion yields exceeding 60%. Maximum overall conversion efficiencies above 2% under simulated sunlight have been achieved, which is comparable to that of natural photosynthesis. Finding appropriate additives for improving VOC without causing dye degradation might result in a further enhancement of cell performance, making the practical application of such systems more suitable to economically viable solar energy devices for our society.

Combination of Cobalt and Iron Polypyridine Complexes for Improving the Charge Separation and Collection in Ru(terpyridine)2‐Sensitised Solar Cells

Abstract Several cyanide-bridged polynuclear complexes have been synthesized in the context of intramolecular energy and electron transfer studies. A valencelocalized description is generally appropriate for such complexes. Within such a localized picture, however, the cyanide bridge is found to provide a remarkable degree of metal-metal electronic coupling. This conclusion can be drawn from a variety of experimental results. Specific attention is devoted here to the spectroscopic observation of appreciable second-order interactions between remote (i.e., non directly bridged) sites in a polynuclear complex.

Photocatalytic Hydrogen Evolution with a Self‐Assembling Reductant–Sensitizer–Catalyst System

Mixtures of polypyridine Fe(II) and Co(II) complexes are used as electron mediators in Ru-thienyltpy-sensitised solar cells (tpy=terpyridine). The use of the metalorganic redox couples allows for improved charge-collection efficiency with respect to the classical iodide/iodine couple which, when associated to Ru-tpy(2) dyes, usually produces poor performance. The improved charge collection is explained by a combination of effective dye regeneration and decreased recombination with the oxidised electrolyte on the basis of data obtained by transient spectroscopy and photoelectrochemical measurements. The efficiency of the regeneration cascade is also critically dependent upon the ability of the Co(II) complex to intercept Fe(III) centres, as clearly indicated by chronocoulometry experiments.

New Components for Dye-Sensitized Solar Cells

A noble-metal-free system for photochemical hydrogen production is described, based on ascorbic acid as sacrificial donor, aluminium pyridyl porphyrin as photosensitizer, and cobaloxime as catalyst. Although the aluminium porphyrin platform has docking sites for both the sacrificial donor and the catalyst, the resulting associated species are essentially inactive because of fast unimolecular reversible electron-transfer quenching. Rather, the photochemically active species is the fraction of sensitizer present, in the aqueous/organic solvent used for hydrogen evolution, as free species. As shown by nanosecond laser flash photolysis experiments, its long-lived triplet state reacts bimolecularly with the ascorbate donor, and the reduced sensitizer thus formed, subsequently reacts with the cobaloxime catalyst, thereby triggering the hydrogen evolution process. The performance is good, particularly in terms of turnover frequencies (TOF=10.8 or 3.6 min(-1), relative to the sensitizer or the catalyst, respectively) and the quantum yield (Φ=4.6%, that is, 9.2% of maximum possible value). At high sacrificial donor concentration, the maximum turnover number (TON=352 or 117, relative to the sensitizer or the catalyst, respectively) is eventually limited by hydrogenation of both sensitizer (chlorin formation) and catalyst.