Photocurrent multiplication at organic/metal interface and surface morphology of organic films

Photocurrent multiplication occurring at an organic pigment/metal interface was revealed to be strongly dependent on the surface morphology of the organic film. The polycrystalline pigment film having a microscopically rough surface showed large photocurrent multiplication reaching the gain of more than ${10}^4$ -fold, while amorphous pigment film having a smooth surface and Au film showed little multiplication. Because of imperfect contact with an Au electrode deposited on a bumpy film surface, there exists a large number of blind alleys (structural trap) which can hold off photogenerated holes. The accumulation of hole carriers builds up high electric field at organic/metal interface and triggers the tunneling injection of electrons from the Au electrode. Emergence of the photocurrent multiplication phenomenon was successfully demonstrated to be controllable through changing the organic film morphology. The present results also pointed out the existence of a large number of blind alleys acting as structural traps at organic/metal interface.