THE MODIFICATION OF ELECTROCHEMICAL AND PHOTOELECTROCHEMICAL PROPERTIES IN THIN FILMS OF TRI- AND TETRAVALENT METAL PHTHALOCYANINES (GAS SENSORS, PHOTOVOLTAICS, ORGANIC SEMICONDUCTOR(S)).
AuthorKlofta, Thomas James
AdvisorArmstrong, Neal R.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractFour different trivalent and tetravalent metal phthalocyanine systems (chlorogallium, chloroindium, vanadyl, and titanyl phthalocyanines) were used singly to prepare thin films (0.05-2.0 micron thickness) on gold, optically transparent substrates. The photoelectronic properties of these electrodes could be modified either by altering the growth conditions (i.e. rate of sublimation, cleanliness of substrate) or by dosing the thin films with either hydrogen or oxygen at elevated temperatures (150°C). The properties of these thin films were monitored by electron microscopy, UV-visible spectrophotometry, X-ray and Ultra-violet surface spectroscopies, and a variety of electrochemical and photoelectrochemical techniques. All four systems behaved in a manner similar to a p-type semiconductor when prepared at rapid rates (10-20 A/min) on gold substrates. In the dark, for contacting redox couples with Eᵒ’ values negative of +0.6V, the phthalocyanine electrodes showed negligible dark currents. Upon illumination, the photoelectrodes only produced positive photopotentials. Chlorogallium phthalocyanine thin films could be made to produce both positive and negative photopotentials when grown at slow rates (1-5 A/min) on clean, gold substrates. These chlorogallium phthalocyanine electrodes regained the properties of a p-type semiconductor after being dosed with oxygen for 48 hours at 150°C. X-ray Photoelectron Spectroscopy confirmed the presence of a high concentration of oxygen at the surface of all of the p-type phthalocyanine electrodes. The oxygen may accept electron density from the phthalocyanine macrocycle to cause the Fermi level to move down in energy toward its valence band edge. Dosing the film with hydrogen caused the electrode to exhibit its original intrinsic characteristics. This variability in electrical properties as a function of gas dopant may lead to the development of a sensitive gas sensing device. Ultra-violet Photoelectron Spectroscopy, as well as molecular orbital calculations, were applied to the chlorogallium phthalocyanine system to determine the molecular orbital contributions to its valence and conduction bands. Photoelectrochemical cells made from electrodes of chlorogallium and vanadyl phthalocyanines exhibited power conversion efficiencies in excess of 0.1%. The vanadyl and titanyl phthalocyanine electrodes were also effective catalysts for the photoreduction of H⁺ to H₂.