Sensitization of semiconductor electrodes

Much attention has been devoted to the development of optimal photo sensitizers of semiconductor electrodes [36, 43]. Ruthenium(II) polypyridine complexes are especially well suited for this purpose. They are strong light absorbers in the visible spectral region and bpy or tpy ligands can be easily derivatized with anchoring groups. Moreover, localization of the excited electron on the ligand which is attached to the semiconductor surface facilitates the electron injection. 

From the whole of the electrochemical, spectroscopic and photophysical infonnation. achieved from these and previous studies, it has been possible to designed an antenna sensitizer trinuclear complex NC-Ru (bpy)j-CN-Ru°(bpy(COO)j)j-NC-Ru (bpy)2.ChP, which has been shown to perform as an efficient molecular device for the sensitization of semiconductor electrodes in the visible region [16-19]. 

Systems that present spectral sensitization, very important when the light absorption properties of a potentially photoactive (generally, luminescent) species does not permit efficient excitation in the desired wavelength range. This kind of phenomenon is crucial for many applications in different fields, such as, for example, the spectral sensitization of semiconductor electrodes in solar energy conversion. 

Over the years a number of physical methods have been applied to study the mechanism of dye sensitization of semiconductor electrodes. The techniques can be broadly divided into photoelectrochemical and spectroscopic ones. The following are some that fall under the spectroscopic methods flash photolysis (diffuse reflection or internal reflection), photoacoustic spectroscopy. Resonance Raman spectroscopy and microwave absorption. Amongst electrochemical methods, potential modulation and rotating ring disc techniques are the commonly used ones... 

Diaz AF, Logan JA(1980)Electroactive polyanihne films. JElectroanalChem 111 111-114 Noufi R, Nozik AJ, White J, Warren LF (1982) Enhanced stability of photoelectrodes with electrogenerated polyanUine films. J Electrochem Soc 129 2261-2265 Noufi R, Tench D, Warren LE (1981) Protection of semiconductor photoanodes with photoelectrochemicaUy generated polypyrrole films. J Electrochem Soc 128 2596-2599 Jaeger CD, Fan FRF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598 Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

Chlorophyll-Coated Semiconductor Electrodes. Chi has first been employed by Tributsch and Calvin (55,56) in dye sensitization studies of semiconductor electrodes. Solvent-evaporated films of Chi a, Chi b, and bacteriochlorophyll on n-type semiconductor ZnO electrodes (single crystal) gave anodic sensitized photocurrents under potentiostatic conditions in aqueous electrolytes. The photocurrent action spectrum obtained for Chi a showed the red band peak at 673 nm corresponding closely to the amorphous and monomeric state of Chi a. The addition of supersensitizers (reducing agents) increased the anodic photocurrents, and a maximum quantum efficiency of 12.5% was obtained for the photocurrent in the presence of phenylhydrazine. 

The photoelectrochemical activity inherent in thin films of aggregated cyanine dyes permits them to act as the spectral sensitizers of wide bandgap semiconductors [69]. It is seen from Fig. 4.14 that the photoelectrochemical behaviour of semiconductor/dye film heterojunctions fabricated by deposition of 200 nm-thick films of cyanine dyes on the surface of TiC>2 and WO3 electrodes, bears close similarity to that of semiconductor electrodes sensitized by the adsorption of dye aggregates. Thus, both anodic and cathodic photocurrents can be generated under actinic illumination, the efficiency of the photoanodic and photocathodic processes and the potential at which photocurrent changes its direction being dependent on dye and semiconductor substrate [69]. 

Spitler, M. Parkinson, B. A. Efficient infrared dye sensitization of van der Waals surfaces of semiconductor electrodes, Langmuir 1986, 2, 549. 

Dye sensitization of semiconductor surfaces is not considered here, nor are issues related to semiconductor particles, photocatalysis and photoelectrolysis per se. These companion topics may be found elsewhere in Volumes I, IV and V. The discussion is phenomenological and is designed to provide an intuitive grasp of the key issues rather than detailed derivations that would have been prohibitive in terms of space constraints in any case. Indeed, the available theoretical framework is only examined in terms of how and with what confidence the pertinent conclusions can be experimentally verified with semiconductor electrodes. 

Watanabe T., Fnjishima A., Tatsuoki 0. and Honda K. (1976), pH dependence of spectral sensitization at semiconductor electrodes . Bull. Chem. Soc. Jpn. 49, 8-11. 

Using multi-layers of sensitizer does not offer a viable solution to this problem. Only the molecules that are in direct contact with the oxide surface would be photoactive - the remainder filtering merely the light. Apart from poor light harvesting a compact semiconductor film would need to be n-doped to conduct electrons. In this case energy transfer quenching of the excited sensitizer by the electrons in the semiconductor would inevitably reduce the photovoltaic conversion efficiency. For this reason the conversion yields obtained from the sensitization of flat electrodes... 

Jaeger CD, Fan F-RF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598... 

M. Matsumura, S. Matsudaira, H. Tsubomura, M. Takata, and H. Yanagi-da, Dye sensitization and surface structures of semiconductor electrodes, ... 

Other situations may also occur that allow a simple determination of the sensitivity factor. When, for example, a sufficiently negative electrode potential forces all minority carriers to drift into the interior of the semiconductor electrode, where they recombine subject to the bulk lifetime Tfr we will see a limiting PMC signal (given a sufficiently thick electrode). Knowing the photonflux /0 (corrected for reflection), we may expect the following formula to hold ... 

At present, the microwave electrochemical technique is still in its infancy and only exploits a portion of the experimental research possibilities that are provided by microwave technology. Much experience still has to be gained with the improvement of experimental cells for microwave studies and in the adjustment of the parameters that determine the sensitivity and reliability of microwave measurements. Many research possibilities are still unexplored, especially in the field of transient PMC measurements at semiconductor electrodes and in the application of phase-sensitive microwave conductivity measurements, which may be successfully combined with electrochemical impedance measurements for a more detailed exploration of surface states and representative electrical circuits of semiconductor liquid junctions. 

Semiconductor-electrolyte interface, photo generation and loss mechanism, 458 Semiconductor-oxide junctions, 472 Semiconductor-solution interface, and the space charge region, 484 Sensitivity, of electrodes, under photo irradiation, 491 Silicon, n-type... 

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