Photocurrent quantum efficiencies

Another type of Chi interfacial layer employed on a metal electrode was a film consisting of ordered molecules. Villar (79) studied short circuit cathodic photocurrents at multilayers of Chi a and b built up on semi-transparent platinum electrodes in an electrolyte consisting of 96% glycerol and 4% KCl-saturated aqueous solution. Photocurrent quantum efficiencies of multilayers and of amorphous films prepared by solvent evaporation were compared. The highest efficiency (about 10 electrons/ absorbed photon, calculated from the paper) was obtained with Chi a multilayers, and the amorphous films of Chi a proved to be less efficient than Chi b multilayers. 

Tetraphenylporphine (TPP) and other metal porphyrine derivatives coated on platinum (87,88,89) or gold (89,90) electrodes have been investigated in photoelectrochemical modes. Photocurrents reported are cathodic or anodic, depending on the pH as well as the composition of the electrolyte employed. Photocurrent quantum efficiencies of 2% (89) to 7% (87) were reported in systems using water itself or methylviologen as the redox species in aqueous electrolyte. Photocurrent generation at Zn-TPP-coated metal cathodes (89) was interpreted in terms of a rectifying effect of the Schottky barrier formed at a metal-p-type... 

Fig. 8 Photocurrent quantum efficiency of ruthenium (4 )and osmium (S) cyanide complexed Ti02 electrodes compared with that of untreated TiOz ( )...

The photoanodic dissolution of n-silicon in acidic fluoride media provides an example of the complexity of multistep photoelectrochemical reactions [33, 34]. The reaction requires the transfer of four electrons, but it is clear that not all of the steps involve photogenerated holes because the photocurrent quantum efficiency is between 2 and 4. The explanation of the high quantum efficiencies is that the initial hole capture step can be followed by a series of steps in which intermediates with low electron affinity inject electrons into the conduction band. These intermediates can be assigned nominal oxidation states as shown in the following scheme. 

Reaction (8.39d) is responsible for photocurrent doubling since it results in the two electron oxidation of formic acid to C02 for the absorption of only one photon. The electron injection step competes with the hole capture reaction, (8.39c), and as a result the photocurrent quantum efficiency depends on illumination intensity. At high intensities, the supply of photogenerated holes to the surface favours reaction (8.39c), and the quantum efficiency is I. At low light intensities, electron injection becomes predominant, and the quantum efficiency tends towards 2. 

The photocurrent quantum efficiency, Y, of n-Si electrode in fluoride solutions depends on current as shown in Fig. 5.12. It is high at low current densities and decreases with increasing current densities to a plateau value of about 2 Fig. 5.12 shows... 

Substrate Chi. layer Electrolyte Photocurrent Quantum efficiency Ref. 

Undoubtedly, the most convincing evidence in support of the hypothesis of the electron-injection process is the observation of the photocurrent multiphcation observed during anodization of n-type in HF solutions. [8, 53-56]. More than one charge carrier per absorbed photon contributes to the photocurrent measured in the external circuit. Photocurrent quantum efficiency, Q, (i.e. number of charge carriers measured in the external current by absorbed photon) is dependent on the incident photon flux. At low light... 

The wavelength dependence of the photocurrent quantum efficiency of PDS(Th)m is shown in Fig. 2. Comparing the photocurrent quantum efficiency curves with their absorption spectra, the position of each maximum peak for the photocurrent quantum efficiencies appears at 30 - 40 nm longer wavelength than that of the respective absorption maximum. These peaks correspond to the absorption edge of the polymers. This means that almost all incident light would be absorbed in the vicinity of the illuminated surface of the films in the region of the absorption bands of the polymers. 

The generated excitons must be quenched because the density of the excitons in this area is probably very high. Consequently, the peaks of the photocurrent quantum efficiency appear at the edge of the absorptions, which establish bulk excitation in the whole film. 

The photocurrent quantum efficiencies of undoped PDS(Th)4 and PDS(Th)4 doped with 1.5 wt % Cao versus wavelength are shown in Fig. 5. The photocurrent quantum efficiency of the doped sample is greatly enhanced, compared with that of the undoped one. In fact, the photocurrent quantum efficiency of undoped PDS(Th)4 was only 0.5 % E= 3x10 V cm ) upon irradiation at 480 nm. The photocurrent quantum efficiency of the Ceo-doped PDS(Th)4, however, increased to 11.5 % E = 1.5x10 V cm" ) upon irradiation at 470 nm. Furthermore, Fig. 6 shows the field... 

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