Photocurrent, efficiency

The mixed SAM under discussion here has been designed to overcome this limitation and to increase the photocurrent efficiency in the visible part of the spectrum. The strong overlap of the emission spectrum of BoDy with the absorption of Fc-P-C6o in the 500 nm region is anticipated to facilitate efficient energy transfer between the two species. The photochemical action spectra of the mixed monolayer show contributions from both BoDy and Fc-P-C6o- The overall behavior of the assembly is therefore in agreement with the scheme shown in Figure 5.34. At 510 nm, a much increased quantum yield for photocurrent production of 50 % is obtained. This study shows that photocurrents can be created at metal electrodes. Relatively high photocurrent efficiencies are obtained however, the fact that flat surfaces are used rather than the nanocrystalline approach taken for semiconductor substrates (see Chapter 6) means that the incident photocurrent efficiency (IPCE) values are... 

Figure 5.58 (a) Schematic illustration of the binding in a Au-dithiol-CdS-dithiol-CdS multilayer, where the CdS nanoparticles are formed from reversed micelles onto a modified gold surface, (b) Action spectra of composite films and absorption spectra of the CdS nanoparticle dispersion before being immobilized continuous and dashed lines show the action spectra on different scales and represent the relative photocurrent efficiencies of Au-dithiol-CdS and Au-dithiol-CdS-dithiol-CdS, respectively. Reprinted with permission from O. Nakanishi, B. Ohtani and K. Uoskai, /. Phys. Chem., B, 102, 1571 (1998). Copyright (1998) American Chemical Society... 

Fig. 7 Photocurrent efficiency (charge transfer/photon flux) of rhenium (ig)and iron (A)cyanide complexed Ti02 photoelectrodes with that of untreated Ti02 ( )for comparison. The extension of the spectral response into the visible is evident.

Fig. 8.18. The upper plot shows the experimental IMPS response measured at very low light intensities for the photodissolution of n-Si in 1.0 NH4F at pH 4.S. The lower plot is the calculated best fit. Note that the photocurrent efficiency varies from 4 at low frequencies to 1 at high frequencies as expected for the scheme shown in Fig. 8.16.

Sensitized n-type semiconductors have been known to yield higher photocurrent efficiencies when electron donors such as iodide, hydroquinone or triethanolamine were added to an aqueous electrolyte. These donors are often referred to as supersensitizers because their presence so dramatically increases the photocurrents. The mechanism of supersensitization was often attributed to efficient reduction of the oxidized sensitizer allowing a greater fraction of the injected electrons to be converted to a photocurrent. This was a perfectly reasonable suggestion as most of these cells were designed to oxidize water at the photoanode, which is an inherently sluggish process. 

O Regan B. and Schwartz D. T. (1998), Large enhancement in photocurrent efficiency caused by UV illumination of the dye-sensitized heterojunction TiOa/ RuLL NCS/CuSCN initiation and potential mechanisms , Chem. of Materials 10, 1501-1509. 

Photoelectrochemlcal conversion from visible light to electric and/or chemical energy has been investigated with chlorophyll thin membranes deposted on semiconductor or metal electrodes (71). Chlorophyll-coated metal (platinum) electrodes derived cathodic photocurrent in acidic electrolyte solutions, although the photocurrent efficiencies tend to be low compared to those of chlorophyll-semiconductor electrodes. The cathodic photoresponse may result from the p-type photoconductive nature of a solid chlorophyll layer and/or the formation of a contact barrier at the metal-chlorophyll interface, which contributes to light-induced carrier separation and leads to photocurrent generation. 

The terminology used for the transfer function requires some care. If the input function is an intensive quantity such as voltage (also called an across function), and the output function is an extensive quantity such as current (also called a through function), the ratio of output to input can be referred to as an admittance. If the type of input and output functions are reversed, then the ratio becomes an impedance. If the input and output functions are both of the same type, the ratio of output to input can be referred to as a gain function. However, these conventions, which are usually employed in network analysis, have not always been followed consistently in the development of new photoelectrochemical techniques. For example, the frequency dependent photocurrent efficiency, O, shown in Table 1 is often referred to as the opto-electrical admittance and its inverse as the opto-electrical impedance [62, 64-66], in spite of the fact that it is the ratio of two through functions. It would be preferable to use the term opto-electrical transfer function. The inverse of has also been called the photoelectrochemical impedance [53, 70]. To avoid confusion, the use... 

The ratio g/I(0) defines the photocurrent efficiency . In the absence of surface recombination, qg corresponds to the photocurrent density Jphow measured in the external circuit. The Gartner equation has been used successfully to explain the photocurrent-potential characteristics of many semiconductor electrodes under conditions where surface recombination is absent. Plots of ln — O) against dsc (which according to the Mott-Schottky relationship is proportional to (1/ — have... 

When losses due to surface recombination are taken into account, the photocurrent efficiency, O, is given by... 

As discussed in section 2, photogenerated holes can readily reach the surface of the nanostructured network and it is assumed that they react with Red generating Ox. If the holes are consumed rapidly, Ep,p will remain close to Ep,redox- In many systems, by contrast, it appears that electrons react much more slowly at the interface than holes. The reason for this kinetic selection of carriers is not clear at present, but it is clearly a prerequisite for the observation of substantial photocurrent efficiencies. If electrons do not react rapidly at the interface, their concentration increases substantially under illumination, so that becomes higher than Ep, edox- Since elec-... 

Fig. 43. Complex plane IMPS plot for 14 micron thick nanoporous GaP layer on n-GaP under depletion conditions (potential 2.5V vs. SCE) in acid electrolyte (pH = 1.0). Illumination from the electrolyte side 350 nm). The steady state photocurrent efficiency is unity. The transit time r d) derived from a> i is 5 10-3 s ...

Reaction of redox systems Photocurrent efficiencies reached Possible application CO VO N>... 

Solarska R, Augustynski J, Sayama K (2006) Viewing nanocrystalline Ti02 photoelectrodes as three-dimensional electrodes effect of the electrolyte upon the photocurrent efficiency. Electrochim Acta 52 694-703... 

Solar-to-hydrogen efficiency. Photocurrent efficiency obtained under zero-bias conditions and using AM 1.5 G light spectrum. 

Photocurrent efficiencies of about 0.1% and consistent photoaction spectra were measured, when the modified Au plates were immersed in a 5 mM solution... 

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