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

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.