FERMI-LEVEL SHIFT IN PHOTOCONDUCTIVE ORGANIC PIGMENT FILMS MEASURED BY KELVIN VIBRATING CAPACITOR METHOD

Fermi level shift in n-type perylene pigment film was measured by the Kelvin vibrating capacitor method. Purification of pigment by reactive sublimation under methylamine atmosphere caused negative shift of the Fermi level due to the effective removal of unknown but specific impurity acting as an acceptor. On the other hand, bromine doping of pigment film caused marked positive shift of Fermi level to near the valence band, resulting in reversal of the photocurrent flow arising from the Schottky junction with Au. This is a clear demonstration of the control of conduction type from n-type to p-type, which was observed for the first time in pigment-based organic semiconductors. This result enabled construction of an organic homojunction composed of p- and n-type perylene pigment films.