Polymeric light-emitting diode produces reversible voltage-dependent emission

Polymeric light-emitting diode produces reversible voltage-dependent emission

Heterojunction light-emitting diodes (LEDs) formed of a bilayer of semiconducting polymers can produce voltage-tunable emission. Researchers at the University of Rochester (Rochester, NY) developed LEDs consisting of layers of n-type (polyquinoline; PPQ) and p-type (poly(p-phenylenevinylene); PPV) films sandwiched between aluminum and indium tin oxide (ITO) electrodes. The electroluminescence color switches reversibly with applied voltage--at low voltages, the n-type layer generates the emission to produce one color, while at higher voltages, both layers emit, producing a second color.

Several devices with films of varying thickness were produced, which led to generation of different colors. A 40-nm-thick PPQ/25-nm-thick PPV device emitted orange light under an 8- to 10-V forward bias--n-type layer emission--while it emitted green light under 13 to 20 V (both layers emitting). A PPQ (33 nm)/PPV(117 nm) device produced yellow output under a 20-V bias, which turned to green at higher voltages.