Cathodic photocurrents

Figure 21. (a) PMC potential and (b) cathodic photocurrent-potential curves for a p-Si (111) electrode (resistivity, 10 ft cm). Electrolyte, 1 M NH4F light intensity 1 mW cm-2. Sweep toward negative potentials. 

In principle, the same photoexcited reactant, if it is liable both to oxidation and reduction, can inject both electrons (into an -type semiconductor) and holes (into a p-type semiconductor). Such a material is, for example, the crystal-violet dye. Figure 25 shows the spectra of cathodic photocurrent iph at p-type gallium phosphide and anodic photocurrent at n-type zinc oxide both in a solution, which does not absorb light (dashed lines), and in the presence of crystal violet the absorption spectrum of the latter is also shown for comparison. 

Fig. 25. Absorption spectrum of the crystal-violet dye (a) and the spectra of anodic photocurrent at an electrode of n-type ZnO (b) and of cathodic photocurrent at an electrode of p-type GaP (c) in the presence of crystal violet (in pA/cm2). The dashed line is the photocurrent in the absence of the dye. [From Gerischer (1977b).]...

Figure 28.9 Spectral dependence of the photocurrents at a p-type GaP electrode with sensitization by N, N -diethylpseudoisocyanine. [Cathodic photocurrent, electrolyte 1 M KC1 + N,N -diethylpseudoisocyanine chloride (1 mL of 10 2 M ethanol to 10 mL of water).] 1, N2 flushing 2, 02 flushing 3, addition of piperidine (10 2 M) 4, trace of the background photocurrent at p-type GaP without dye. [From Ref. 42.]...

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. 

We have investigated the photocurrent behavior of multilayers of a Chi a-DPL (molar ratio 1/1) mixture on platinum in an aqueous electrolyte without added redox agents (80). Cathodic photocurrents with quantum efficiencies in the order of 10- were obtained with films consisting of a sufficient number of monolayers. The photocurrent was increased in acidic solutions. However, no appreciable photocurrent was observed with a single monolayer coated on platinum. The latter fact most probably results from minimal rectifying property of the metal surface and/or an efficient energy quenching of dye excited states by free electrons in... 

Aizawa and Suzuki (83,84,85,86) utilized, as an ordered system, liquid crystals in which Chi was immobilized. Electrodes were prepared by solvent-evaporating a solution consisting of Chi and a typical nematic liquid crystal, such as n-(p-methoxybenzyl-idene)-p -butylaniline, onto a platinum surface. Chl-liquid crystal electrodes in acidic buffer solutions gave cathodic photocurrents accompanied by the evolution of hydrogen gas (83). This was the first demonstration of photoelectrochemical splitting of water using in vitro Chi. Of particular interest in these studies is the effect of substituting the central metal in the Chi molecule. 

Photoelectrochemical behavior of metal phthalocyanine solid films (p-type photoconductors) have been studied at both metal (93,94,95,96) and semiconductor (97,98) electrodes. Copper phthalocyanine vacuum-deposited on a Sn02 OTE (97) displayed photocurrents with signs depending on the thickness of film as well as the electrode potential. Besides anodic photocurrents due to normal dye sensitization phenomenon on an n-type semiconductor, enhanced cathodic photocurrents were observed with thicker films due to a bulk effect (p-type photoconductivity) of the dye layer. Meier et al. (9j>) studied the cathodic photocurrent behavior of various metal phthalocyanines on platinum electrodes where the dye layer acted as a typical p-type organic semiconductor. 

We have demonstrated photoactivity at solid electrolyte/Cond. glass interfaces. The positive photopotentials could, however, be a Dember-type photovoltage. Cathodic photocurrents passed when the cell was discharged. Illumination may yield iodine on the illuminated side with some reduced species (probably silver) in the bulk of the electrolyte. 

Interfacial electron transfer between a metal and an excited sensitizer, A -L- B where B represents a metal electrode, may be reductive, whereby the electron transfers from the conduction band of the metal to the singly occupied HOMO state of the excited adsorbed molecules, thus resulting in A -L-B and a cathodic photocurrent at the electrode. Alternatively, it may be an oxidative process, wherein the electron is transferred from the adsorbate to the metal, so resulting in A+-L-B and an anodic photocurrent at the electrode. 

Dye IV to operate in a similar manner generating anodic current under positive bias and cathodic photocurrent under negative bias. 

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]. 

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