Dye-modified ZnO thin films were prepared by electrochemically induced crystallization from aqueous mixtures of zinc nitrate and water-soluble dyes. A direct crystallization of semiconductor/ dye composites without heat treatment is seen as a significant advantage of this method. Moreover, characterization of these materials has revealed ordered growth of ZnO crystallites as well as formation of ordered dye assemblies, thus characterizing this method as electrochemical self-assembly. The photoelectrochemical properties of these unique ZnO-dye thin film electrodes were investigated in photocurrent transient measurements in the ms-regime and by steady- state voltammetric measurements. Two sets of electrodes are discussed, employing either metal complexes of tetrasulfophthalocyanines (TSPcMt; Mt = Zn, Al, Si) or the xanthene dye Eosin Y. For aggregates of TSPcMt on ZnO, efficient charge-transfer to the electrolyte is found, leading to low surface charging and low surface recombination of photogenerated holes with electrons from the ZnO, at however, rather low injection efficiencies of electrons into the conduction band of ZnO. This efficiency was higher for adsorbed monomers of TSPcMt leading to a considerably higher quantum efficiency of the photocurrent in spite of increased surface charging and recombination of holes. Higher photocurrents were observed for ZnO sensitized with monomers of Eosin Y caused by both, efficient electron transfer from the dye to ZnO as well as hole transfer from the dye to the electrolyte. Not only dye molecules which were directly accessible from the electrolyte, but also those which were enclosed within matrix cavities proved to be photoelectrochemically active.
Dye-modified ZnO thin films were prepared by electrochemically induced crystallization from aqueous mixtures of zinc nitrate and water-soluble dyes. A direct crystallization of semiconductor/ dye composites without heat treatment is seen as a significant advantage of this method. Moreover, characterization of these materials has revealed ordered growth of ZnO crystallites as well as formation of ordered dye assemblies, thus characterizing this method as electrochemical self-assembly. The photoelectrochemical properties of these unique ZnO-dye thin film electrodes were investigated in photocurrent transient measurements in the ms-regime and by steady- state voltammetric measurements. Two sets of electrodes are discussed, employing either metal complexes of tetrasulfophthalocyanines (TSPcMt; Mt = Zn, Al, Si) or the xanthene dye Eosin Y. For aggregates of TSPcMt on ZnO, efficient charge-transfer to the electrolyte is found, leading to low surface charging and low surface recombination of photogenerated holes with electrons from the ZnO, at however, rather low injection efficiencies of electrons into the conduction band of ZnO. This efficiency was higher for adsorbed monomers of TSPcMt leading to a considerably higher quantum efficiency of the photocurrent in spite of increased surface charging and recombination of holes. Higher photocurrents were observed for ZnO sensitized with monomers of Eosin Y caused by both, efficient electron transfer from the dye to ZnO as well as hole transfer from the dye to the electrolyte. Not only dye molecules which were directly accessible from the electrolyte, but also those which were enclosed within matrix cavities proved to be photoelectrochemically active.
Perylene- and phthalocyanine- pigment molecules were systematically modified and consequences were studied for their solid state properties. Thin films (1 - 150 nm) were prepared by physical vapor deposition. Intermolecular interactions were probed by optical measurements in absorption and emission. Atomic force microscopy served to analyze the morphology of films. Different interactions among the molecules and with the substrate surfaces allowed to prepare either crystalline or amorphous films. Crystalline films of perylene pigments were typically characterized by strong chromophore coupling leading to a characteristic splitting, well- defined shifts of the optical absorption bands and emission mainly from excimer species whereas the chromophore coupling in amorphous films was suppressed sufficiently to provide a significantly increased optical emission yield from uncoupled monomer states. Temperature-dependent optical emission experiments are presented which allow a detailed discussion of monomer vs. excimer emission. Decoupling of the chromophores could be obtained by appropriate chemical substitutions at the aromatic core system of phthalocyanines and perylene pigments that led to strong deviations from planarity. This was achieved by the introduction of bulky substituents in the bay position of the aromatic perylene core and by changes in the coordination number of the central group in phthalocyanines. The strategy led to a strongly enhanced optical emission for both classes of materials. This could be obtained, however, either in an amorphous arrangement of the molecules or under conservation of crystallinity, both offering alternative advantages.
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