PHOTOELECTROCHEMISTRY OF THIN FILM CHLORO-GALLIUM PHTHALOCYANINE ELECTRODES FOR SOLAR ENERGY CONVERSION.

Abstract

An organic Schottky barrier cell, consisting of a thin layer of the organic semiconductor, GaPc-Cl, in contact with gold on one side and an electrolyte containing a redox couple on the other, was developed as a solar energy conversion device. Schottky barriers were formed at both interfaces. Film morphology, as determined by the sublimation rate, was the major determinant of the photoelectrochemical behavior. An optimum film consisted of a single layer of crystallites about 1.0 micron in thickness, tightly packed together to give a non-porous film. Thinner films did not develop the full theoretical photopotential, and pores acted as recombination sites, decreasing the efficiency. Both negative and positive photopotentials could be developed, depending on the redox couple used. The photopotential, was found to be proportional to the differences between the Fermi level of the Au and the formal potential of the redox couple. Hydrogen evolution was possible with up to 0.1% solar efficiency on a platinized version of the optimum electrode. Results from photocurrent action spectra and pulsed laser photocoulostatics, showed the potential drop across the film was not linear, but formed a potential well about 0.1 eV deep, which captured charge carriers and decreased the efficiency. From scanning electron microscope studies, phthalocyanines, such as AlPc-Cl, GaPc-Cl, and InPc-Cl, with bulky anions were found to form block-like crystal structures favorable for use in Schottky barrier cells. Phthalocyanines with transition metals in the +2 oxidation state, such as FePc and MgPc, were found to form long needles, which were not favorable for use in Schottky barrier cells.