Abstract

The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we review our progress in the understanding of the energy level line-up in finished, polymer-based light-emitting diodes, which exploit a variety of anodic or cathodic buffer-layers to obtain anodes and/or cathodes with low injection barriers. We have used electroabsorption measurements, as they allow the non-invasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/electrode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky-Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g. Bardeen model), except for electrodes which extensively react with the polymer, and introduce deep gap states.

© 2006 Optical Society of America