Solid-state polymer light-emitting electrochemical cells have been dev
eloped using thin films of conjugated polymers blended with solid elec
trolytes. The cells contain three parts: the polymer blend films as th
e active medium, and two contact electrodes: indium-tin oxide and alum
inum. When externally biased, the conjugated polymers are p-doped and
n-doped on opposite sides of the polymer layer, and a dynamic light-em
itting p-n junction is formed between the doped regions. The admired s
olid electrolyte provides the dopant counterions and the ionic conduct
ivity necessary for the doping. The p-n junction is dynamic and revers
ible, with an internal built-in potential close to the bandgap of the
redox-active conjugated polymers. Orange, green and blue lights emitte
d from the pn junction have been obtained with turn-on voltage less th
an 3 V and external quantum efficiency higher than 2% photons/electron
. In addition, a two-color light-emitting electrochemical cell has als
o been fabricated based on a bilayer structure consisting of two diffe
rent luminescent polymers. The light-emitting p-n junction can be swit
ched from one layer to another under different bias conditions.