THE range of materials now available for polymer-based light-emitting
diodes (LEDs) is such that electroluminescence can be obtained through
out the visible spectrum(1-12). Here we show that, by blending polymer
s with different emission and charge-transport characteristics, LEDs c
an be fabricated in which the emission colour varies as a function of
the operating voltage. This phenomenon arises from the self-organizing
properties of the blends, in which entropy drives phase separation of
the constituent polymers and gives rise to submicrometre-sized domain
s having a range of compositions and emission characteristics. Emissio
n from domains of different composition is controlled by the ease with
which charge is injected, which in turn depends on the applied voltag
e.