LIGHT-EMITTING-DIODES BASED ON POLYTHIOPHENE - INFLUENCE OF THE METALWORKFUNCTION ON RECTIFICATION PROPERTIES

Light emission and current-voltage characteristics of conjugated polym er poly(3-octylthiophene) (P3OT) light-emitting diodes (LEDs) are disc ussed for two types of top electrodes. On changing the workfunction of the top contact by 0.9 eV from 4.3 (Al) to 5.2 eV (Au) we find that t he main direction of rectification changes sign: when Al forms the top contact rectification ratios up to 10(4) were obtained, while in the case of Au this ratio is 10(2) in the opposite direction. As a consequ ence, the rectification ratio changes effectively by almost six orders of magnitude. In the case of Au as a top electrode no electroluminesc ence is observed, which suggests that a one-carrier type ('hole-only') device is made. The observation of light emission for indium-tin-oxid e (ITO)/P3OT/A1 devices in both modes of operation is discussed in ter ms of two injection mechanisms (thermionic emission over a Schottky-li ke contact versus tunneling into the transport bands). For these devic es the onset of light emission in forward and reverse modes of operati on occurs at the same current densities (6.25 x 10(-4) A/cm(2)), but a t much higher electric fields in the reverse mode.