Surface Chemistries and Photoelectrochemistries of Thin Film Molecular Semiconductor Materials

Abstract

Thin films of vacuum deposited trivalent and tetravalent metal phthalocyanines (Pc), and derivatives of perylenes (Pe), have been explored by a combination of ultrahigh vacuum deposition, surface spectroscopic characterization techniques, and measurement of photoconductivity and photocurrent spectral responses involving a) interdigitated array microcircuits (MC), and b) photoelectrochemical cells. Of particular interest has been the way adsorbed molecules such as O 2 and NH 3 affect the photoconductivity of these thin film materials, and the effect on photoactivity of the formation of a sharp interface between a phthalocyanine and a perylene. Multiple chemisorption sites are implicated for molecules such as O 2 and NH 3 on trivalent metal Pc thin film surfaces. These sites may either increase or decrease photoconductivity, depending on the partial pressure of the adsorbant, and the coverage of molecules competing for the same chemisorption sites. Photoelectrochemical modification of the Pc surface introduces submonolayer coverages of metals like Ag, providing a chemisorption site for molecules such as NH 3 , thereby improving the performance of these photoconductivity-based chemical sensors. Photoelectrochemical techniques can be used to characterize Pc/Pe bilayers, since the electrolyte provides a noncorrosive, optically transparent electrical contact. Photocurrent yield spectra, obtained from two different illumination directions, have been used to estimate the width of the interfacial region which is most active for exciton dissociation in the Pc/Pe bilayer. Transient photocurrent yields in Pc/Pe multilayers (up to 16 total layers), show that the photocurrent increases linearly with the number of Pc/Pe interfaces. These increases occur up to the point where the width of the individual layers becomes narrower than the interface resolution that our present vacuum deposition technologies can provide