Abstract
Capacitances ( C) of the molecular solids of zinc tetraphenylporphyrin (ZnTPP) are measured as functions of an applied voltage ( V), the frequency, and the thickness of the ZnTPP film. The analysis of the 1 C 2 vs V plots for the frequencies far below 0.1 Hz reveals that the ZnTPP solids behave well as a semiconductor without any complication due to surface states, etc. and thus form an almost ideal Schottky contact with Al. A space charge concentration is evaluated to be 6 × 10 16cm −3. At higher frequencies, Al/ZnTPP/Au cells exhibit voltage-independent capacitances, which are inversely proportional to the thickness of the ZnTPP layer in accord with the parallel-plate capacitor model. This finding in the high frequency region is explained in terms of an extremely slow kinetics of space charges. In addition, a change in photocurrent with the film thickness of ZnTPP is explained by the formation of a depletion layer with 90 nm-width.
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