The dependence of the surface photovoltage of methine bridge substitute porphyrins on the method of film fabrication

A series of methine bridge substituted porphyrin dye films have been fabricated both by vacuum sublimation and by the evaporation of a solution of the dye in methanol. The saturation surface photovoltage of these films at 4 × 10^?7 Torr has been characterized by measuring the optical modulation of the transmission of incident electrons, and all of the films exhibit surface depletion layers with characteristic photovoltaic relaxation times on the order of 10 ms. There are appreciable differences, however, in the observed photovoltage resulting from the different methods of fabrication and the corresponding differences in crystallinity. In fact, this dependence on the structure appears to outweigh the much smaller dependence on the chemical nature of the methine bridge substituent. The sublimed films give evidence of additional electron levels in the bandgap which can be experimentally removed by annealing the films. The spectral dependence of the photovoltage does not indicate the presence of such levels in the spectral region examined.