MOLECULAR-SOLID OF ZINC TETRAPHENYLPORPHYRIN AS A MODEL ORGANIC SEMICONDUCTOR WITH A WELL-DEFINED DEPLETION LAYER

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 x 10(16) cm(-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 him thickness of ZnTPP is explained by the formation of a depletion layer with 90 nm-width.