Phthalocyanine electrodes, photocurrent

Film preparation plays a crucial role in determining the photoelectrochemical properties of phthalocyanine electrodes. Since the coupling of individual chromophores strongly depends on their relative orientation, the position of the absorption maximum and its width shows a clear dependence on the structure of thin films. Also the charge transport within phthalocyanine films, a fundamental necessity for the films to work as electrodes, depends upon the overlap of the frontier orbital wave functions. Beyond the microscopic structure of films also the morphology of films plays an important role. In the case of crystalline films, the orientation of crystallites relative to the electrode surface will be relevant because of anisotropies in optical absorption and charge transport. The size of the observed photocurrent directly depends on the real electrode surface area accessible by the electrolyte and this leads to a strong dependence on the porosity of the films. 

To quantitatively determine the rate constants, IMPS measurements were performed at phthalocyanine electrodes when in contact with different redox electrolytes. The Pc working electrode was illuminated by a modulated light-emitting diode as light source and the phase shift (p of the resulting photocurrent and its... 

Photocurrent transient methods [44, 150-155] and intensity-modulated photocurrent spectroscopy (IMPS) [142, 151, 156-161] have been established as good techniques for the investigation of semiconductor surfaces [162]. By these methods the working electrode is illuminated with either a light pulse or with sinusoidal modulated light, respectively, and the resulting photocurrent is monitored. Both techniques have been used at phthalocyanine electrodes to analyze in detail the kinetics of the photoelectrochemical reactions. 

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. 

Figure 17. Effect of front side and back side illumination of semiconductor electrodes coated with phthalocyanine thin films. The direction of the photocurrent could be controlled by illumination direction and the redox couple in the solution.

Chemical substitution at the phthalocyanine ligand with electron-withdrawing substituents led to a dominance of anodic photocurrents as opposed to that of cathodic photocurrents observed at the unsubstituted materials. This was shown for (CN)sPcZn TPyTAPZn and TPzTAPZn" -To restrict the observed differences in the photoelectrochemical characteristics to the influence of the different ligands, the central metal was kept constant in these studies and Aims were prepared by vapor deposition if applicable and also by drop coating for comparison purposes. Figure 10.3 shows the dominance of cathodic photocurrents (high anodic dark currents) at PcZn and the dominance of anodic photocurrents (high cathodic dark currents) at electrodes of... 

Electrodes of two-dimensional sheet polymers of phthalocyanines prepared by in situ reaction of tetracyanobenzene with thin metal films were prepared and characterized in their photoelectrochemical characteristics. Anodic photocurrents were detected, characterizing these films as n-type semiconducting materials. It was also found that such films could be reduced at more positive potentials than unsubstituted phthalocyanines. This change when compared to films of divalent unsubstituted phthalocyanines is caused by unreacted CN groups that were found in the films " . Such films therefore can be looked at as substituted phthalocyanines with electron-withdrawing CN groups which explains their photoelectrochemical characteristics. 

Figure 10.6. Time-resolved photocurrent responses of Fi6PcZn electrodes to a 100 ms flash of white light at different electrode potentials between 100 and 500 mV vs. SCE as indicated. The electrolyte was an aqueous KCl (lmoll ) solution with 10 mol 1 Ce +Z. (Reproduced from ref. [129] with permission from the Society of Porphyrins and Phthalocyanines.)...

The photoelectrochemlcal properties of PcIn(Cl) thin films were also investigated on single-crystalhne layered semiconductor electrodes of SnS2 or MoS2 These surfaces turned out to provide a suitable substrate to deposit highly ordered epitaxial phthalocyanine films by OMBE and to also ahow the study of such films as sensitizers for the n-type semiconductors. Very narrow absorption spectra and also photocurrent action spectra could be obtained speaking... 

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