Lifetime studies of light-emitting diode structures incorporating polymeric Langmuir–Blodgett films

The operating lifetimes of light-emitting diode structures incorporating poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) Langmuir–Blodgett films are reported. To remove the moisture from the organic layer, a number of post-deposition treatments have been investigated prior to the deposition of the metal top contact. The best external quantum efficiency was found for an indium–tin oxide/MEH-PPV/aluminium structure dried in high vacuum. However, this device possessed a relatively short lifetime. Experiments at constant current and constant voltage revealed that annealing at an elevated temperature could enhance the lifetime. Further improvements were found for devices in which a lithium fluoride layer (≈2 nm) was sandwiched between the aluminium electrode and the polymer layer, and by encapsulating the device with adhesive tape.