Effects of electrode modifications on the performance of polymer light-emitting electrochemical cells

Abstract

Effects of electrode modifications on the performance of polymer light-emitting electrochemical cells (LECs) were studied by measuring and comparing the current–luminance–voltage characteristics and the electroluminescence (EL) external quantum efficiency of the corresponding devices. A block copolymer of 2,5-dimethoxy-1,4-di(α-(2-naphthalene vinylene)benzene) and tri(ethylene oxide) (MDNVB–TEO) was used as the luminescent polymer in the LECs. Insertion of an ultrathin layer of barium at the polymer/cathode interface yields higher injection current and light intensity, lower threshold for light output, but lower device EL efficiency due to the doping of barium ions into the copolymer. With a conducting polymer layer of poly(ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS) on the indium-tin-oxide (ITO) anode, the operation stability and the lifetime of the LEC devices are significantly improved. However, the larger sheet resistance of the PEDOT-PSS layer results in lower injection current and light intensity, and higher threshold for light output. Oxygen-plasma treatment on the ITO anode increases the injection current and light intensity of the LECs. The threshold for the current injection consistently keeps unchanged for the LEC devices with and without the electrode modifications, which confirms the electrochemical doping mechanism of the LECs.