Polymer Light-Emitting Electrochemical Cells: In Situ Formation of a Light-Emitting p−n Junction

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Abstract
Solid-state polymer light-emitting electrochemical cells have been fabricated 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 layer) and indium−tin oxide and aluminum films as the two contact electrodes. 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 admixed polymer electrolyte provides the counterions and the ionic conductivity necessary for doping. The p−n junction is dynamic and reversible, with an internal built-in potential 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 observed with a turn-on voltage of about 2.4 V. The devices reached 8 cd/m2 at 3 V and 100 cd/m2 at 4 V, with an external quantum efficiency of 0.3−0.4% photons/electron. The response speed of these cells was around 1 s, depending on the diffusion of ions. Once the light-emitting junction had been formed, the subsequent operation had fast response (microsecond scale or faster) and was no longer diffusion-controlled.