Light-emitting diodes based on polythiophene: influence of the metal workfunction on rectification properties

Light emission and current-voltage characteristics of conjugated polymer poly(3-octylthiophene) (P3OT) light-emitting diodes (LEDs) are discussed 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 the main direction of rectification changes sign: when Al forms the top contact rectification ratios up to 104 were obtained, while in the case of Au this ratio is 102 in the opposite direction. As a consequence, the rectification ratio changes effectively by almost six orders of magnitude. In the case of Au as a top electrode no electroluminescence is observed, which suggests that a one-carrier type (‘hole-only’) device is made. The observation of light emission for indium-tin-oxide (ITO)/P3OT/Al devices in both modes of operation is discussed in terms of two injection mechanisms (thermionic emission over a Schottky-like contact versus tunneling into the transport bands). For these devices the onset of light emission in forward and reverse modes of operation occurs at the same current densities (6.25×10−4 A/cm2), but at much higher electric fields in the reverse mode.