Early Studies of Dye Sensitization

Very early studies of dye sensitization have been on metal electrodes and aromatic hydrocarbon crystals [6,7]. On metal electrodes, the excited states of dye molecules are rapidly deactivated (by energy transfer to the broad continuum states of the metal) and there are no evidences of electron transfer quenching. Organic crystals are insulators with a wide bandgap ( 3eV) and narrow conduction and valence bands. Hole injection in the fully occupied valence bands of organic crystals such as anthracene, perylene or phenanthrene through excited dye molecules (e.g, Rhodamine B) was discovered in 1963 and the process has been examined extensively. The efficiency of sensitized hole generation in these cases is directly related to the 

An important practical application of dye-sensitization phenomenon is in the silver halide photography [11]. Various cyanine and related dyes are adsorbed onto silver halide microcrystals to extend the spectral response in the visible light region. The optimum efficiency of spectral sensitization of silver halide grains is known to be very high. 

Some of the early studies of spectral sensitization of semiconductors have been on ZnO and CdS. The first report was published by Putzeiko and Terenin in 1949 when they reported sensitization of pressed ZnO powder by adsorbed Rhodamine B, Eosin, Erythrosin and by Cyanine dyes [12]. Hauffe and Gerischer coworkers made some pioneering studies in the seventies and these have been extended by many others [13-18]. Most of these early studies focussed on organic dyes of interest to photographic industry (e.g. Xanthenes such as Rhodamine B or Eosin or Cyanines). 

Bard et al. [19] have studied the sensitization properties of thin films (100-250A) of several metallophthlocyanines, MPcs (M =Mg, Zn, Al(Cl), TiO, Fe, H2) deposited on n-type Ti02 and WO3 single crystal electrodes. Anodic photocurrents corresponding to IPCE of lO -lO were found for visible light irradiation in the presence of electron donors such as hydroquinone. Phthalocyanines behave as p-type semiconductors and for this reason, cathodic photocurrents can also be observed at negative potentials and in the presence of suitable electron acceptor molecules. 

Yields are significant only in cases where there is mono or submonolayer of the dye on the semiconductor surface. Light absorption by a single monolayer of the dye is extremely low ( 1%) and hence overall white light conversion efficiency is negligibly small. Even the monochromatic conversion efficiency decreases rapidly with thicker layers. The origin of such low quantum efficiencies are not fully understood, though several 

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