Electrodeposition of inorganic compound thin films in the presence of certain organic molecules results in self‐assembly of various hybrid thin films with new properties. Examples of new discoveries by the authors are reviewed, taking cathodic formation of a ZnO/dye hybrid as the leading example. Hybridization of eosinY leads to the formation of highly oriented porous crystalline ZnO as the consequence of dye loading. The hybrid formation is a highly complicated process involving complex chemistry of many molecular and ionic constituents. However, electrochemical analyses of the relevant phenomena indicate the possibility of reaching a comprehensive understanding of the mechanism, giving us the chance to further develop them into industrial technologies. The porous crystals are ideal for photoelectrodes in dye‐sensitized solar cells. As the process also permits the use of non‐heat‐resistant substrates, the technology can be applied for the development of colorful and light‐weight plastic solar cells.

A novel room‐temperature method for the preparation of porous TiO2 films with high performance in dye‐sensitized solar cells (DSSCs) has been developed. In this method a small amount of TiIV tetraisopropoxide (TTIP) is added to an ethanolic paste of TiO2 nanoparticles, where it hydrolyzes in situ and connects the TiO2 particles to form a homogenous and mechanically stable film of up to 10 μm thickness without crack formation. Residual organics originating from the TTIP were removed by UV–ozone treatment of the films, leading to a remarkable improvement of the cell efficiency. Intensity‐modulated photocurrent/voltage spectroscopy (IMPS/IMVS) showed that the main effect of the UV–ozone treatment is to suppress the recombination of photogenerated electrons, thereby extending their lifetime. The efficiency was further increased by preheating the TiO2 nanoparticles before the paste preparation to remove contaminants originating from the preparation process of the particles. Solar‐to‐electric energy conversion efficiencies of 4.00 and 3.27 % have been achieved for cells with conductive glass and plastic film substrates, respectively, under illumination with AM 1.5 (100 mW cm–2) simulated sunlight.

Electrical and photoelectrical properties of nondoped and doped zinc oxide films coated on glass plates by the dip-coating method are investigated at room temperature in various ambient atmospheres. The dark conductivity of the nondoped films exponentially decreased with decreasing film thickness while the conductivity under illumination of 350 nm light was almost constant at 100 S·cm-1 irrespective of the film thickness. Consequently thinner films showed larger photoresponse than thicker films. This thickness dependence is explained by the variation of ZnO particle size with the film thickness (fine particle model) and the additional effect of the Schottky barrier generated between the film and gold electrodes.

Electrodeposition of ZnO/eosin Y hybrid thin films from aqueous mixed solution of zinc chloride and eosin Y as promoted by reduction of oxygen has been studied. Highly oriented crystalline hybrid thin films with two distinctive structures have been obtained depending on the redox state of eosin Y. Deposition at potentials more positive than that of eosin Y reduction resulted in a formation of compact ZnO crystals into which eosin Y molecules are entrapped, while that accompanied with the reduction of eosin Y yielded a film consisting of sponge-like ZnO crystals with internal nanoporous structure to which eosin Y molecules are adsorbed. The addition of eosin Y accelerated the film growth both in oxidized and reduced forms due to its catalysis toward the reduction of oxygen. Photoelectrochemical measurement in an I Ϫ /I 3 Ϫ redox electrolyte solution revealed that the hybrid thin film in the latter structure performed as an efficient sensitized photoelectrode because of its porous nature.Chemical and electrochemical depositions are emerging as important synthetic methods to compound thin films not only for their cost effectiveness but also for the high quality of the materials that can be achieve by such methods. One can be convinced about the quality of the materials processed in solutions demonstrated by the possibility of epitaxial growth when appropriate single crystal substrates are chosen. The examples are found in the numerous recent papers for metal chalcogenides such as CdS, 1-3 CdSe, 4 CdTe, 5 and metal oxides such as Bi 2 O 3 , 6 Fe 3 O 4 , 7 Cu 2 O, 8 and ZnO. 9,10 For long-range epitaxial growth to be realized, it is important to achieve the film growth strictly by the crystal growth. 11 Although ideal adlayer structures are often found for ultra-thin films, disordering of the structure can take place as soon as the film gets thicker, 12 because the chemical stability of the surface in contact with solution determines the way that the film continues to grow, rather than the template effect exerted from the substrate. Such a character of solution phase thin film deposition, in return, gives an opportunity to control the film structure by addition of surface acting chemical species to the bath. As one of such examples, we have previously shown a deposition of nanoparticulate CdS thin film with 100 nm thickness by addition of mercaptoethanol which was strongly adsorbed on the surface of CdS to hinder its crystal growth and promote formation of new nuclei during the film growth. 13 The deposition in the absence of mercaptoethanol results in the films consisting of grains ranging over the entire film thickness, because initially deposited seed crystals continue to grow in an atomic layerby-layer fashion. 11 Cathodic electrodeposition of ZnO is a typical electroprecipitation process which realizes layer-by-layer growth of highly crystallized thin films, as proven from the long range epitaxial electrodeposition of a 0.75 m thick film. 9 Base generation by electrochemical reduction of nitrate, 14 dissolved mol...

Cathodic electrodeposition in an aqueous mixed solution of zinc nitrate and water-soluble tetrasulfonated metallophthalocyanines (TSPcMs), in which M ) Zn(II) (TSPcZn), Al(III)-[OH] (TSPcAl), or Si(IV)[OH] 2 (TSPcSi), resulted in a self-assembled growth of zinc oxide (ZnO) thin films whose surface is modified by TSPcMs. It has been found that the adsorption of TSPcM onto the growing surface of ZnO strongly affects the crystal growth and the orientation of the ZnO crystallites. The effect was most prominently seen with TSPcSi, creating a film looking like stacking disks aligned perpendicular to the substrate. Crystallographic studies by X-ray diffraction and TEM observation coupled with the selected area electron beam diffraction have revealed that thin platelike crystals, whose planes and edges correspond to the (002) and (100) crystal faces, respectively, are aligned in the same orientation around the c-axis within the stacks. The evolution of this unique structure is interpreted as arising from the preferential adsorption of TSPcMs at the (002) planes, leading to film growth preferentially in the (100) direction. Optical analysis of the films also revealed high order in the interactions among TSPcMs. Because of the strong intermolecular attraction, TSPcZn forms multilayers of π-stacking aggregates on ZnO as confirmed by the characteristic blueshift of the Q-band absorption. Thus, TSPcZn could be condensed at a very high concentration exceeding 1 M in the deposited film. These surface aggregates were completely removed by dipping the film in a solution of cationic detergent, cetyltrimethylammonium chloride, while only leaving surface-bound monomeric TSPcZn. By contrast, the aggregation was hindered for TSPcAl and TSPcSi because of the presence of axially coordinated OH -, and monomeric adsorption was found in the electrodeposited films.