No abstract

Hybrid thin films of crystalline ZnO modified by 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrinato zinc (TSTPPZn) and 2,9,16,23-tetrasulfophthalocyaninatozinc(II) (TSPcZn) were prepared by electrochemical deposition from aqueous zinc salt solutions. A "one-step" process with the sensitisers adsorbed during ZnO deposition represented the most simple approach. ZnO was also grown independently in the presence of Eosin Y as a structure-directing agent, which was then removed and the sensitisers were chemisorbed from solutions ("readsorption" method). The photoelectrochemical characteristics of the electrodes were studied by photocurrent spectra and by time-resolved photocurrent measurements in an acetonitrile-based solution containing I3(-)/I(-) as the redox electrolyte. In films with both sensitisers present, both sensitisers worked in parallel providing panchromatic sensitisation. Recombination of electrons injected into the conduction band of ZnO with remaining holes in the HOMO of the sensitisers was indicated for the one-step films, but was considerably suppressed for the films prepared by the readsorption method. Films prepared by the readsorption method showed a significantly increased efficiency by an increased surface area and suppressed recombination.

Sensitized mesoporous ZnO films were prepared by a three-step procedure comprising the electrodeposition of ZnO in the presence of Eosin Y serving as a structure directing agent, its desorption and the adsorption of the sensitizer cisbis͑isothiocyanato͒bis͑2,2Ј-bipyridyl-4,4Ј-dicarboxylato͒-ruthenium͑II͒ often referred to as N3 or N535. The obtained optimized electrodes exhibit high external quantum yields ͑IPCE͒ of up to 55% in the absorption maximum of N3, being clearly superior to electrodes obtained earlier by a one-step procedure involving the addition of the N3 dye to the deposition bath. Compared to other reported N3/ZnO electrodes, the newly developed ones are to be preferred because of a high IPCE for substantially thinner films prepared in a low-temperature process. The changes in the film surface area following individual preparation steps were monitored by krypton sorption measurements. The N3 adsorption conditions were optimized in order to maximize the dye load but to maintain an open pore structure and to avoid dye agglomeration. Time-resolved photocurrent measurements in the millisecond regime were used to analyze the electrode kinetics and detect recombination reactions.The development of dye-sensitized solar cells ͑DSSCs͒ is a very promising approach to provide an economically feasible way of solar energy conversion. 1 In these cells chemically stable widebandgap oxide semiconductors are combined with sensitizer molecules strongly absorbing in the visible range. Studies on dye sensitization initially made important progress in the 1960s by using single-crystal ZnO electrodes in aqueous solutions containing the photosensitizer. 2 The performance of the electrodes was significantly improved by adsorption of the sensitizers on sintered ZnO, whose surface area was substantially larger providing an increased amount of sensitizers bound to the semiconductor surface. 3 In 1991 nanoparticulate TiO 2 with a large surface area was prepared by a sol-gel technique on a conductive transparent substrate, sintered at 400-500°C in order to achieve a sufficient contact between the particles, and subsequently loaded with cis-bis͑isothiocyanato͒bis͑2, 2Ј-bipyridyl-4,4Ј-dicarboxylato͒-ruthenium͑II͒ called N3. 4,5 The conversion efficiency of such electrodes reached 10%, a significant step towards the application of such electrodes in DSSCs.Recently, zinc oxide has seen a renaissance as an important alternative to TiO 2 in the use as a dye-sensitized electrode material. A method was reported for the cathodic electrodeposition of crystalline ZnO from aqueous solutions of Zn 2+ salts on conductive substrates. 6-8 Depending on the deposition conditions, crystals of different shape and size were obtained. This method allows preparation of films with good contact among the ZnO crystals in the growth direction perpendicular to the substrate, so that any subsequent annealing steps can be avoided. A variety of substrates can be used as back contacts without the requirement of thermal stability beyond 150°C. These include a ...

Hybrid thin films of crystalline zinc oxide and the zinc complex of 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin (ZnTPPS) have been prepared by cathodic electrodeposition from different aqueous zinc salt solutions. Films from a solution containing zinc nitrate and the porphyrin derivate show a rough surface and porosity, whereas films prepared from an oxygen-saturated zinc chloride solution were very smooth. The presence of the porphyrin derivative in the deposition solution has a clear influence on the morphology of the hybrid films compared with pure zinc oxide films. In aqueous zinc nitrate solution the addition of the porphyrin derivative hinders the growth of zinc oxide. In contrast, the addition of the dye to the oxygen-saturated zinc chloride solution leads to an increase of the growth rate of ZnO/porphyrin hybrid films. An increased spectral absorbance was reached when two dyes (zinc complex of tetrasulfonated phthalocyanine and porphyrin) were added to the zinc nitrate solution since the electrodeposited hybrid thin films contained both dyes. UV-vis spectra revealed the presence of both macrocyclic metal complexes in the ZnO films.

Dye-sensitized zinc oxide thin films were prepared, characterized and optimized for applications as photoelectrochemically active electrodes. Conditions were established under which crystalline thin films of ZnO with a porous texture were formed by electrochemically induced crystallization controlled by structure-directing agents (SDA). Dye molecules were adsorbed either directly as SDA during this preparation step or, preferably, following desorption of a SDA. The external quantum efficiency (IPCE) could thereby be increased significantly. Particular emphasis was laid on dye molecules that absorb in the red part of the visible spectrum. Model experiments under ultrahigh vacuum (UHV) conditions with dye molecules adsorbed on defined crystal planes of single crystals aimed at a deeper understanding of the coupling of the chromophore electronic π-system within molecular aggregates and to the semiconductor surface. Detailed photoelectrochemical kinetic measurements were used to characterize and optimize the electrochemically prepared dye-sensitized ZnO films. Parallel electrical characterization in vacuum served to distinguish between contributions of charge transport within the ZnO semiconductor matrix and the ions of the electrolyte in the pore system of the electrode.