POLYMER LIGHT-EMITTING ELECTROCHEMICAL-CELLS - IN-SITU FORMATION OF ALIGHT-EMITTING P-N-JUNCTION

Solid-state polymer light-emitting electrochemical cells have been fab ricated using thin films of blends of poly(1,4-phenylenevinylene) and poly(ethylene oxide) complexed with lithium trifluoromethanesulfonate. The cells contain three layers: the polymer film (as the emissive lay er) and indium-tin oxide and aluminum films as the two contact electro des. When externally biased, the conjugated polymers are p-doped and n -doped on opposite sides of the polymer layer, and a light-emitting p- n junction is formed in between. The admired polymer electrolyte provi des the counterions and the ionic conductivity necessary for doping. T he p-n junction is dynamic and reversible, with an internal built-in p otential close to the band gap of the redox-active conjugated polymer (2.4 eV for PPV). Green light emitted from the p-n junction was observ ed with a turn-on voltage of about 2.4 V. The devices reached 8 cd/m(2 ) at 3 V and 100 cd/m(2) at 4 V, with an external quantum efficiency o f 0.3-0.4% photons/electron. The response speed of these cells was aro und 1 s, depending on the diffusion of ions. Once the light-emitting j unction had been formed, the subsequent operation had fast response (m icrosecond scale or faster) and was no longer diffusion-controlled.