Polymer Electroluminescence Using ac or Reverse dc Biasing

Abstract

Poly(2,5-diheptoxy-p-phenylenevinylene-alt-2,5-dimethoxy-p-phenylenevinylene) (DHDMPPV) exhibits electroluminescence (EL) at room temperature when either a conventional dc forward bias, or a reverse bias, or an ac voltage is applied to the test device, which has a simple ITO (indium tin oxide)/polymer/Al configuration. DHDMPPV is a soluble derivative of poly(p-phenylenevinylene) (PPV) synthesized using a Wittig condensation polymerization. Its EL spectrum lies in the orange region with a maximum intensity at 582 nm and a shoulder at 620 nm. The EL spectra either under a dc forward or reverse bias or on the imposition of an ac voltage are qualitatively identical. The threshold biases are ∼5 V for dc forward bias, ∼6 V for dc reverse bias, and ∼5 V (10 V peak-to-peak) for ac operation, for a polymer emitter film thickness of ca. 80 nm. The EL emissions are bright and relatively stable at room temperature and can be easily seen under normal light conditions. Two other EL-active copolymers were also tested in similar configurations ; no significant dc reverse bias EL was observed. However, blends of DHDMPPV with another EL-active copolymer previously described show the same behavior as DHDMPPV itself ; for example, a 1 :1 weight ratio blend with a green-light-emitting multiblock copolymer shows only an enhanced orange emission from the narrow-band-gap DHDMPPV.