Dye sensitization, semiconductor electrodes

Chl-coated semiconductor (n-type) electrodes and metal electrodes can act as efficient photoanodes and photocathoes, respectively, for visible light conversion. The former system functions as a dye-sensitized semiconductor electrode, while the latter is presumably driven by the photoconductive properties of a Chi solid layer and/or charge separation involving the Chl-metal contact barrier. 

Study of dye-sensitized semiconductor electrodes has started in the late 1960s as an extension of photographic science where silver hahde grains are photosensitive materials to be spectrally sensitized. Dye molecules adsorbed on the surface of silver halide crystals and photoexcited by absorption of visible light act as electron... 

Photoelectrochemical conversion from visible light to electric and/or chemical energy using dye-sensitized semiconductor or metal electrodes is a promising system for the in vitro simulation of the plant photosynthetic conversion process, which is considered one of the fundamental subjects of modern and future photoelectrochemistry. Use of chlorophylls(Chls) and related compounds such as porphyrins in photoelectric and photoelectrochemical devices also has been of growing interest because of its close relevance to the photoacts of reaction center Chls in photosynthesis. 

A significant drawback of metals for photoelectrochemical applications lies in their ability to efficiently quench excited states via energy transfer processes, as discussed below. Direct detection of photosensitized electron transfer to or from a metal electrode surface has been observed [30]. However, unlike dye-sensitized semiconductor systems, little examination of the kinetics of such systems has yet been undertaken. 

The sensitization of electrodes to visible light by dye molecules is an old area of science with a rich history [2]. A dye-sensitized photoefifect was measured at a semiconductor surface as early as 1887 in Vienna [3]. The accepted mechanisms for the dye sensitization of electrodes emerged from photoelectrochemical studies in the 1960s and 1970s [4-6]. These studies were motivated by a desire to quantify interfacial electron transfer processes and develop cells useful for solar energy conversion. The two most common approaches are shown schematically in Figure 1. 

Dye sensitization of electrodes is an old area of science with a rich history. The field has experienced renewed interest owing to the development of high surface area colloidal semiconductor electrodes. These materials yield impressive solar conversion efficiencies when employed in regenerative solar cells that have already found niche applications and have the real possibility of replacing traditional solid-state photovoltaics. Thus for the first time in history a solar cell designed to operate on a molecular level is useful from a practical point of view. It is also likely that other applications in the growing areas of molecular photonic materials will arise. 

Organic dyes, aside from their role as sensitization agents for wide band gap semiconductors have been employed also for stabilization of narrow band gap semiconductors. The majority of such studies have considered metal or metal-free phthalocyanine films for both sensitization and electrode protection purposes [35]. 

Nazeeruddin, M. K. Graetzel, M. Dyes for Semiconductor Sensitization. In Encyclopedia of Electrochemistry Semiconductor Electrodes and Photoelectrochemistry, Vol. 6 Licht, S., Ed. Wiley-VCH 2002 Darmstadt, pp 407-431. 

There has been a growing interest in inorganic/organic hybrid materials in recent years with expectations for new or improved properties, which are not exhibited or attainable by using the parent materials separately. The usefulness of such materials has been best manifested by recent successful studies of dye-sensitized solar cells, which typically employ thin film electrodes of porous oxide semiconductors such as TiOz, and whose surface is modified with sensitizing... 

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