Light-assisted tunneling current spectroscopy: a new tool for nanoscale observation of organic semiconductors

摘要

The photovoltaic effect in organic solar cells is a multistep process involving both bulk and interfacial phenomena. In order to probe the local mechanisms of photocurrent production in organic semiconductors, we studied the influence of light on the junction between the tip of a scanning tunnelling microscope (STM) and an organic thin film. We report here on the local through-space I-V and I-ΔZ characteristics (ΔZ-tip-sample distance) of pentacene (5A), a material with potential applications in photovoltaic solar cells. These systems behave as metal-insulator-semiconductor (MIS) tunnel junctions. The influence of light is investigated by coupling the organic nano-junctions to a laser beam. Monitoring the tip-sample distance ΔZ at the angstrom level (i.e. over the 0-20 A range), allows to tune the I-V characteristics of these nanoscale organic tunnel junctions both in the dark and under illumination.