Reduction in the threshold voltage of a polymer light-emitting diode by high molecular density on a photo-crosslinking layer

The operation voltage of a polymer light-emitting diode (PLED) is significantly reduced using a photo-crosslinking layer whose thickness is of the order of 10nm. A thin photo-crosslinking layer of a poly(methyl methacrylate)-based photopolymer, coated on the hole injection electrode (anode), was treated with a linearly polarized ultraviolet (LPUV) light. Compared to the unpolarized UV exposure case, for a poly(2-methoxy,5-(2'-ethyl- hexoxy)-1,4-phenylenevinylene) (MEH-PPV) film spin-coated on the LPUV exposed photo-crosslinking layer, the threshold voltage of the PLED was found to be reduced by about 35% and the current density was greatly enhanced. The underlying physical mechanism comes from the increase of the packing density in addition to a partial order of the MEH-PPV on the photo-crosslinking polymer upon the LPUV exposure.